- A multi‐objective short‐term optimal operation model for a
cascade system of reservoirs considering the impact on long‐term
- Authors: Bin Xu; Ping‐An Zhong, Zachary Stanko, Yunfa Zhao, William W.‐G. Yeh
Abstract: This paper examines the impact of short‐term operation on long‐term energy production. We propose a multiobjective optimization model for the short‐term, daily operation of a system of cascade reservoirs. The two objectives considered in the daily model are: 1) minimizing the total amount of water released, and 2) maximizing the stored energy in the system. Optimizing short‐term operation without considering its impact on long‐term energy production does not guarantee maximum energy production in the system. Therefore, a major goal of this paper is to identify desirable short‐term operation strategies that, at the same time, optimize long‐term energy production. First, we solve the daily model for one month (30 days) using a non‐dominated genetic algorithm (NSGAII). We then use the non‐dominated solutions obtained by NSGAII to assess the impact on long‐term energy production using a monthly model. We use historical monthly inflows to characterize the inflow variability. We apply the proposed methodology to the Qingjiang cascade system of reservoirs in China. The results show: 1) in average hydrology scenarios, the solution maximizing stored energy produces the most overall long‐term energy production; 2) in moderately wet hydrology scenarios, the solution minimizing water released outperforms the maximizing stored energy solution; and 3) when extremely wet hydrology scenarios are expected, a compromise solution is the best strategy. This article is protected by copyright. All rights reserved.
- Assessment of flow regime alterations over a spectrum of temporal scales
using wavelet‐based approaches
- Authors: Fu‐Chun Wu; Ching‐Fu Chang, Jenq‐Tzong Shiau
Abstract: The full range of natural flow regime is essential for sustaining the riverine ecosystems and biodiversity, yet there are still limited tools available for assessment of flow regime alterations over a spectrum of temporal scales. Wavelet analysis has proven useful for detecting hydrologic alterations at multiple scales via the wavelet power spectrum (WPS) series. The existing approach based on the global WPS (GWPS) ratio tends to be dominated by the rare high‐power flows so that alterations of the more frequent low‐power flows are often underrepresented. We devise a new approach based on individual deviations between WPS (DWPS) that are root‐mean‐squared to yield the global DWPS (GDWPS). We test these two approaches on the three reaches of the Feitsui Reservoir system (Taiwan) that are subjected to different classes of anthropogenic interventions. The GDWPS reveal unique features that are not detected with the GWPS ratios. We also segregate the effects of individual sub‐flow components on the overall flow regime alterations using the sub‐flow GDWPS. The results show that the daily hydropeaking waves below the reservoir not only intensified the flow oscillations at daily scale but most significantly eliminated subweekly flow variability. Alterations of flow regime were most severe below the diversion weir, where the residual hydropeaking resulted in a maximum impact at daily scale while the post‐diversion null flows led to large hydrologic alterations over submonthly scales. The smallest impacts below the confluence reveal that the hydrologic alterations at scales longer than 2 days were substantially mitigated with the joining of the unregulated tributary flows, whereas the daily‐scale hydrologic alteration was retained because of the hydropeaking inherited from the reservoir releases. The proposed DWPS approach unravels for the first time the details of flow regime alterations at these intermediate scales that are overridden by the low‐frequency high‐power flows when the long‐term averaged GWPS are used. This article is protected by copyright. All rights reserved.
- Chute cutoff as a morphological response to stream reconstruction: The
possible role of backwater
- Authors: J.P.C. Eekhout; A.J.F. Hoitink
Abstract: Stream restoration efforts often aim at creating new unconstrained meandering channels without weirs and bank revetments. In reconstructed streams, the initial morphological response of the new streams is often rapid, until a dynamic equilibrium is reached. Here we report on a chute cutoff that occurred within 3 months after realization of a stream restoration project, caused by a plug bar that formed in response to a backwater effect. The temporal evolution of the morphology of both the new and the old channel was monitored over a period of nearly 8 months, including pre‐cutoff conditions. The observations can be separated into three stages. Stage 1 is the initial period leading to cutoff vulnerability, stage 2 is the actual cutoff, and stage 3 is the morphological adjustment in response to the cutoff. In stage 1, a plug bar was deposited in one of the channel bends. Hydrodynamic model results show the location of the plug bar coincides with a region where bed shear stress decreased in downstream direction due to backwater. Longitudinal channel bed profiles show that the channel slope decreased soon after channel reconstruction. Hence, sediment from upstream was available to form the plug bar. After the plug bar was deposited, an embayment formed in the floodplain at a location where the former channel was located (stage 2). The former channel was filled with sediment prior to channel construction. It is likely that the sediment at this location was less consolidated, and therefore, prone to erosion. The chute channel continued to incise and widen into the floodplain and, after 6 months, acted as the main channel, conveying the discharge during the majority of time (stage 3). The cutoff channel gradually continued to fill with sediment, from the moment the plug bar formed until the chute channel incised into the floodplain. Sedimentary successions of the deposited material show upward fining, which is in agreement with observations of chute cutoffs in rivers. Although the artificial setting limits the degree in which the observed processes can be projected on natural rivers, the observations prompt to investigate the role of backwater effects in natural chute initiation. This article is protected by copyright. All rights reserved.
- Abiotic and biotic controls of soil moisture spatio‐temporal
variability and the occurrence of hysteresis
- Authors: Simone Fatichi; Gabriel G. Katul, Valeriy Y. Ivanov, Christoforos Pappas, Athanasios Paschalis, Ada Consolo, Jongho Kim, Paolo Burlando
Abstract: An expression that separates biotic and abiotic controls on the temporal dynamics of the soil moisture spatial coefficient of variation Cv(θ) was explored via numerical simulations using a mechanistic ecohydrological model, Tethys‐Chloris. Continuous soil moisture spatio‐temporal dynamics at an exemplary hillslope domain were computed for six case studies characterized by different climate and vegetation cover and for three configurations of soil properties. It was shown that abiotic controls largely exceed their biotic counterparts in wet climates. Biotic controls on Cv(θ) were found to be more pronounced in Mediterranean climates. The relation between Cv(θ) and spatial mean soil moisture θ¯ was found to be unique in wet locations, regardless of the soil properties. For the case of homogeneous soil texture, hysteretic cycles between Cv(θ) and θ¯ were observed in all Mediterranean climate locations considered here and to a lesser extent in a deciduous temperate forest. Heterogeneity in soil properties increased Cv(θ) to values commensurate with field observations and weakened signatures of hysteresis at all of the studied locations. This finding highlights the role of site‐specific heterogeneities in hiding or even eliminating the signature of climatic and biotic controls on Cv(θ), thereby offering a new perspective on causes of confounding results reported across field experiments. This article is protected by copyright. All rights reserved.
- A framework to identify Pareto‐efficient subdaily environmental flow
constraints on hydropower reservoirs using a grid‐wide power
- Authors: Marcelo A. Olivares; Jannik Haas, Rodrigo Palma‐Behnke, Carlos Benavides
Abstract: Hydrologic alteration due to hydropeaking reservoir operations is a main concern worldwide. Subdaily environmental flow constraints (ECs) on operations can be promising alternatives for mitigating negative impacts. However, those constraints reduce the flexibility of hydropower plants, potentially with higher costs for the power system. To study the economic and environmental efficiency of ECs, this work proposes a novel framework comprising four steps: i) assessment of the current subdaily hydrologic alteration; ii) formulation and implementation of a short‐term, grid‐wide hydrothermal coordination model; iii) design of ECs in the form of maximum ramping rates (MRRs) and minimum flows (MIFs) for selected hydropower reservoirs; and iv) identification of Pareto‐efficient solutions in terms of grid‐wide costs and the Richard‐Baker flashiness index for subdaily hydrologic alteration (SDHA). The framework was applied to Chile's main power grid, assessing 25 EC cases, involving five MIFs and five MRRs. Each case was run for a dry, normal and wet water year type. Three Pareto‐efficient ECs are found, with remarkably small cost increase below 2% and a SDHA improvement between 28% and 90%. While the case involving the highest MIF worsens the flashiness of another basin, the other two have no negative effect on other basins and can be recommended for implementation. This article is protected by copyright. All rights reserved.
- Modeling the Release of E. coli D21g with Transients in Water Content
- Authors: Scott A. Bradford; Yusong Wang, Saeed Torkzaban, Jiri Šimůnek
Abstract: Transients in water content are well known to mobilize colloids that are retained in the vadose zone. However, there is no consensus on the proper model formulation to simulate colloid release during drainage and imbibition. We present a model that relates colloid release to changes in the air‐water interfacial area (Aaw) with transients in water content. Colloid release from the solid‐water interface (SWI) is modeled in two steps. First, a fraction of the colloids on the SWI partitions to the mobile aqueous phase and air‐water interface (AWI) when the Aaw increases during drainage. Second, colloids that are retained on the AWI or at the air‐water‐solid triple line are released during imbibition as the AWI is destroyed. The developed model was used to describe the release of Escherichia coli D21g during cycles of drainage and imbibition under various saturation conditions. Simulations provided a reasonable description of experimental D21g release results. Only two model parameters were optimized to the D21g release data: (i) the cell fraction that was released from the SWI (fr); and (ii) the cell fraction that partitioned from the SWI to the AWI (fawi). Numerical simulations indicated that cell release was proportional to fr and the initial amount of retention on the SWI and AWI. Drainage to a lower water content enhanced cell release, especially during subsequent imbibition, because more bacteria on the SWI were partitioned to the AWI and/or aqueous phase. Imbibition to a larger water content produced greater colloid release because of higher flow rates, and more destruction of the AWI (smaller Aaw). Variation in the value of fawi was found to have a pronounced influence on the amount of cell release in both drainage and imbibition due to changes in the partitioning of cells from the SWI to the aqueous phase and the AWI. This article is protected by copyright. All rights reserved.
- Hyporheic zone hydrologic science: a historical account of its emergence
and a prospectus
- Authors: M. Bayani Cardenas
Abstract: The hyporheic zone, defined by shallow subsurface pathways through river beds and banks beginning and ending at the river, is an integral and unique component of fluvial systems. It hosts myriad hydrologically‐controlled processes that are potentially coupled in complex ways. Understanding these processes and the connections between them is critical since these processes are not only important locally but integrate to impact increasingly larger scale biogeochemical functioning of the river corridor up to the river network scale. Thus, the hyporheic zone continues to be a growing research focus for many hydrologists for more than half the history of Water Resources Research. This manuscript partly summarizes the historical development of hyporheic zone hydrologic science as gleaned from papers published in Water Resources Research, from the birth of the concept of the hyporheic zone as a hydrologic black box (sometimes referred to as transient storage zone), to its adolescent years of being torn between occasionally competing research perspectives of interrogating the hyporheic zone from a surface or subsurface view, to its mature emergence as an interdisciplinary research field that employs the wide array of state‐of‐the‐art tools available to the modern hydrologist. The field is vibrant and moving in the right direction of addressing critical fundamental and applied questions with no clear end in sight in its growth. There are exciting opportunities for scientists that are able to tightly link the allied fields of geology, geomorphology, hydrology, geochemistry and ecology to tackle the many open problems in hyporheic zone science. This article is protected by copyright. All rights reserved.
- Hydraulic controls of in‐stream gravel bar hyporheic exchange and
- Authors: Nico Trauth; Christian Schmidt, Michael Vieweg, Sascha E. Oswald, Jan H. Fleckenstein
Abstract: Hyporheic exchange transports solutes into the subsurface where they can undergo biogeochemical transformations, affecting fluvial water quality and ecology. A three‐dimensional numerical model of a natural in‐stream gravel bar (20 m × 6 m) is presented. Multiple steady state streamflow is simulated with a computational fluid dynamics code that is sequentially coupled to a reactive transport groundwater model via the hydraulic head distribution at the streambed. Ambient groundwater flow is considered by scenarios of neutral, gaining, and losing conditions. The transformation of oxygen, nitrate, and dissolved organic carbon by aerobic respiration and denitrification in the hyporheic zone are modeled, as is the denitrification of groundwater‐borne nitrate when mixed with stream‐sourced carbon. In contrast to fully submerged structures, hyporheic exchange flux decreases with increasing stream discharge, due to decreasing hydraulic head gradients across the partially submerged structure. Hyporheic residence time distributions are skewed in the log‐space with medians of up to 8 h and shift to symmetric distributions with increasing level of submergence. Solute turnover is mainly controlled by residence times and the extent of the hyporheic exchange flow, which defines the potential reaction area. Although streamflow is the primary driver of hyporheic exchange, its impact on hyporheic exchange flux, residence times, and solute turnover is small, as these quantities exponentially decrease under losing and gaining conditions. Hence, highest reaction potential exists under neutral conditions, when the capacity for denitrification in the partially submerged structure can be orders of magnitude higher than in fully submerged structures.
- Hydrological connectivity in river deltas: The first‐order
importance of channel‐island exchange
- Authors: Matthew Hiatt; Paola Passalacqua
Abstract: Deltaic systems are composed of distributary channels and interdistributary islands. While previous work has focused either on the channels or on the islands, here we study the hydrological exchange between channels and islands and point at its important role in delta morphology and ecology. We focus our analysis on Wax Lake Delta in coastal Louisiana (USA) and characterize the surface water component of hydrological connectivity through measurements of water discharge and hydraulic tracer propagation. We find that deltaic islands are zones of significant water flux as 23–54% of the incoming distributary channel flux enters the islands. A calculation of the travel times through a channel‐island complex shows travel times through the islands to be at least 3 times their channel counterparts. A dye release experiment also indicates that travel times in islands are much longer that those within channels as dye remained in the island for the 3.8 day duration of the experiment. Additionally, islands are more sensitive than channels to environmental forces such as tides, which cause flow reversal and thus can increase travel times through the islands. Our work defines the “hydrological network” of a river delta to include not only the distributary channel network but also the interdistributary islands, quantifies the implications of channel‐island hydrological connectivity to travel times through the system, and discusses the relevance of our findings to channel mouth dynamics at the delta front and the potential for denitrification in coastal systems.
- Spin‐up behavior and effects of initial conditions for an integrated
- Authors: Alimatou Seck; Claire Welty, Reed M. Maxwell
Abstract: Initial conditions have been shown to have a strong effect on outputs of surface water models, but their impact on integrated hydrologic models is not well documented. We investigated the effects of initial conditions on an integrated hydrologic model of a 5632 km2 domain in the northeastern U.S. Simulations were run for the year 1980 using four initial conditions spanning a range of average depth to water table, including 1 m (“wet”), 3m, 5m, and 7 m (“dry”) below land surface. Model outputs showed significant effects of initial conditions on basin‐averaged variables such as subsurface storage, surface storage, and surface runoff, with the greatest impact observed on surface storage and runoff. Effects of initial conditions were related to meteorological conditions, with precipitation reducing the effects of initial conditions on surface storage and runoff. Additionally, feedbacks between soil moisture and land‐energy fluxes affected the impacts of initial conditions: higher temperatures magnified the differences in storage, recharge, and discharge among the four initial‐condition scenarios. Ten year recursive runs were conducted for the wet and dry scenarios. Spin‐up times varied by model components and were considerably smaller for land‐surface states and fluxes. Spin‐up for dry initial conditions was slower than for wet initial conditions, indicating longer system memory for dry initial conditions. These variations in persistence of initial conditions should be taken into consideration when designing model initialization approaches. More broadly, this behavior is indicative of increased persistence of the effects of dry years as opposed to wet years in hydrologic systems.
- Developing effective messages about potable recycled water: The importance
of message structure and content
- Authors: J. Price; K. S. Fielding, J. Gardner, Z. Leviston, M. Green
Abstract: Community opposition is a barrier to potable recycled water schemes. Effective communication strategies about such schemes are needed. Drawing on social psychological literature, two experimental studies are presented, which explore messages that improve public perceptions of potable recycled water. The Elaboration‐Likelihood Model of information processing and attitude change is tested and supported. Study 1 (N = 415) premeasured support for recycled water, and trust in government information at Time 1. Messages varied in complexity and sidedness were presented at Time 2 (3 weeks later), and support and trust were remeasured. Support increased after receiving information, provided that participants received complex rather than simple information. Trust in government was also higher after receiving information. There was tentative evidence of this in response to two‐sided messages rather than one‐sided messages. Initial attitudes to recycled water moderated responses to information. Those initially neutral or ambivalent responded differently to simple and one‐sided messages, compared to participants with positive or negative attitudes. Study 2 (N = 957) tested the effectiveness of information about the low relative risks, and/or benefits of potable recycled water, compared to control groups. Messages about the low risks resulted in higher support when the issue of recycled water was relevant. Messages about benefits resulted in higher perceived issue relevance, but did not translate into greater support. The results highlight the importance of understanding people's motivation to process information, and need to tailor communication to match attitudes and stage of recycled water schemes' development.
- Canopy edge flow: A momentum balance analysis
- Authors: Sharon Moltchanov; Yardena Bohbot‐Raviv, Tomer Duman, Uri Shavit
Abstract: Canopy flow models are often dedicated to ideal, infinite, homogenous systems. However, real canopy systems have physical boundaries, where the flow enters and leaves patches of vegetation, generating a complex pressure field and velocity variations. Here we focus our study on the canopy entry region by examining the terms involved in the double (space and time) averaged momentum equations and their relative contribution to the total momentum balance. The estimation of each term is made possible by particle image velocimetry (PIV) measurements in a model canopy constructed of randomly distributed thin glass plates. The instantaneous velocity fields were used to calculate the mean velocities, pressure, drag, Reynolds stresses, and dispersive stresses. It was found that within the entry region, the pressure gradient, the drag forces, and dispersive stresses are the three most significant terms that affect the balance in the streamwise momentum equation. In the vertical direction, the dispersive stresses are also significant and their contribution to the total momentum cannot be ignored. The study shows that dispersive stresses are initially formed around canopy edges; at both the entry region and the canopy top boundary. They start as a sink term, extracting momentum from the flow, and then become a source term that contributes momentum to the flow until they eventually decay at some short penetration distance into the canopy. These results reveal a new understanding on the evolution of momentum within the entry region, necessary in any closure modeling of flow in real canopies.
- A methodology for velocity field measurement in multiphase
high‐pressure flow of CO2 and water in micromodels
- Authors: Farzan Kazemifar; Gianluca Blois, Dimitrios C. Kyritsis, Kenneth T. Christensen
Abstract: This paper presents a novel methodology for capturing instantaneous, temporally and spatially resolved velocity fields in an immiscible multi‐phase flow of liquid/supercritical CO2 and water through a porous micromodel. Of interest is quantifying pore‐scale flow processes relevant to geological CO2 sequestration and enhanced oil recovery, for example, at thermodynamic conditions relevant to geological reservoirs. A previously‐developed two‐color microscopic particle image velocimetry approach based upon proxy fluids [Blois et al. 2015] is adapted to a high‐pressure apparatus, facilitating flow quantification of water interacting with supercritical CO2. This technique simultaneously resolves (in space and time) the aqueous phase velocity field as well as the dynamics of the menisci. The method and the experimental apparatus are detailed, and results are presented to demonstrate its unique capabilities for studying pore‐scale dynamics of CO2‐water interactions. Simultaneous identification of the boundary between the two fluid phases and quantification of the instantaneous velocity field in the aqueous phase provides a step‐change in capability for investigating multi‐phase flow physics at the pore scale at reservoir‐relevant conditions. This article is protected by copyright. All rights reserved.
- Long‐range seasonal streamflow forecasting over the Iberian
Peninsula using large‐scale atmospheric and oceanic information
- Authors: J.M. Hidalgo‐Muñoz; S.R. Gámiz‐Fortis, Y. Castro‐Díez, D. Argüeso, M.J. Esteban‐Parra
Abstract: Identifying the relationship between large‐scale climate signals and seasonal streamflow may provide a valuable tool for long‐range seasonal forecasting in regions under water stress, such as the Iberian Peninsula (IP). The skill of the main teleconnection indices as predictors of seasonal streamflow in the IP was evaluated. The streamflow database used was composed of 382 stations, covering the period 1975‐2008. Predictions were made using a leave‐one‐out cross‐validation approach based on multiple linear regression, combining Variance Inflation Factor and Stepwise Backward selection to avoid multicollinearity and select the best subset of predictors. Predictions were made for four forecasting scenarios, from one to four seasons in advance. The correlation coefficient (RHO), Root Mean Square Error Skill Score (RMSESS) and the Gerrity Skill Score (GSS) were used to evaluate the forecasting skill.
For autumn streamflow, good forecasting skill (RHO>0.5, RMSESS>20%, GSS>0.4) was found for a third of the stations located in the Mediterranean Andalusian Basin, the North Atlantic Oscillation of the previous winter being the main predictor. Also, fair forecasting skill (RHO>0.44, RMSESS>10%, GSS>0.2) was found in stations in the northwestern IP (16 of these located in the Douro and Tagus Basins) with two seasons in advance. For winter streamflow, fair forecasting skill was found for one season in advance in 168 stations, with the Snow Advance Index as the main predictor. Finally, forecasting was poorer for spring streamflow than for autumn and winter, since only 16 stations showed fair forecasting skill in with one season in advance, particularly in north‐western of IP. This article is protected by copyright. All rights reserved.
- Coupled and uncoupled hydrogeophysical inversions using ensemble Kalman
filter assimilation of ERT‐monitored tracer test data
- Authors: Matteo Camporese; Giorgio Cassiani, Rita Deiana, Paolo Salandin, Andrew Binley
Abstract: Recent advances in geophysical methods have been increasingly exploited as inverse modeling tools in groundwater hydrology. In particular, several attempts to constrain the hydrogeophysical inverse problem to reduce inversion errors have been made using time‐lapse geophysical measurements through both coupled and uncoupled (also known as sequential) inversion approaches. Despite the appeal and popularity of coupled inversion approaches, their superiority over uncoupled methods has not been proven conclusively; the goal of this work is to provide an objective comparison between the two approaches within a specific inversion modeling framework based on the ensemble Kalman filter (EnKF). Using EnKF and a model of Lagrangian transport, we compare the performance of a fully coupled and uncoupled inversion method for the reconstruction of heterogeneous saturated hydraulic conductivity fields through the assimilation of ERT‐monitored tracer test data. The two inversion approaches are tested in a number of different scenarios, including isotropic and anisotropic synthetic aquifers, where we change the geostatistical parameters used to generate the prior ensemble of hydraulic conductivity fields. Our results show that the coupled approach outperforms the uncoupled when the prior statistics are close to the ones used to generate the true field. Otherwise, the coupled approach is heavily affected by “filter inbreeding” (an undesired effect of variance underestimation typical of EnKF), while the uncoupled approach is more robust, being able to correct biased prior information thanks to its capability of capturing the solute travel times even in presence of inversion artifacts such as the violation of mass balance. Furthermore, the coupled approach is more computationally intensive than the uncoupled, due to the much larger number of forward runs required by the electrical model. Overall, we conclude that the relative merit of the coupled versus the uncoupled approach cannot be assumed a priori and should be assessed case by case. This article is protected by copyright. All rights reserved.
- Drag force parameters of rigid and flexible vegetal elements
- Authors: John A. Chapman; Bruce N. Wilson, John S. Gulliver
Abstract: This paper compares parameters that characterize vegetation flexibility effects on flow resistance and drag. Drag forces have been measured in a flume for simple cylindrical obstructions of the same shape and size but with different flexibility under several flow conditions. This data set is used to fit drag parameters and to relate their value to flexibility through the Cauchy Number. A formulation is presented where the drag coefficient is evaluated as a function of a new calibration velocity parameter which is related to the elastic modulus of the obstruction. While the use of a Vogel exponent and reference velocity provides a similar response, the reference velocity when used is somewhat nebulous and appears to have a critical impact on the parameter and the drag force calculated. The proposed formulation for drag reduction is more consistently estimated for the range of flexibilities in this study. This article is protected by copyright. All rights reserved.
- Hydroeconomic optimization of integrated water management and transfers
under stochastic surface water supply
- Authors: Tingju Zhu; Guilherme Fernandes Marques, Jay R. Lund
Abstract: Efficient re‐allocation and conjunctive operation of existing water supplies is gaining importance as demands grow, competitions among users intensify, and new supplies become more costly. This paper analyzes the roles and benefits of conjunctive use of surface water and groundwater and market‐based water transfers in an integrated regional water system where agricultural and urban water users coordinate supply and demand management based on supply reliability and economic values of water. Agricultural users optimize land and water use for annual and perennial crops to maximize farm income, while urban users choose short‐term and long‐term water conservation actions to maintain reliability and minimize costs. The temporal order of these decisions is represented in a two‐stage optimization that maximizes the net expected benefits of crop production, urban conservation and water management including conjunctive use and water transfers. Long‐term decisions are in the first stage and short‐term decisions are in a second stage based on probabilities of water availability events. Analytical and numerical analyses are made. Results show that conjunctive use and water transfers can substantially stabilize farmer's income and reduce system costs by reducing expensive urban water conservation or construction. Water transfers can equalize marginal values of water across users, while conjunctive use minimizes water marginal value differences in time. Model results are useful for exploring the integration of different water demands and supplies through water transfers, conjunctive use, and conservation, providing valuable insights for improving system management. This article is protected by copyright. All rights reserved.
- Modeling chloride transport using travel time distributions at Plynlimon,
- Authors: Paolo Benettin; James Kirchner, Andrea Rinaldo, Gianluca Botter
Abstract: Here we present a theoretical interpretation of high‐frequency, high‐quality tracer time series from the Hafren catchment at Plynlimon in mid‐Wales. We make use of the formulation of transport by travel time distributions to model chloride transport originating from atmospheric deposition and compute catchment‐scale travel time distributions. The relevance of the approach lies in the explanatory power of the chosen tools, particularly to highlight hydrologic processes otherwise clouded by the integrated nature of the measured outflux signal. The analysis reveals the key role of residual storages that are poorly visible in the hydrological response, but are shown to strongly affect water quality dynamics. A significant accuracy in reproducing data is shown by our calibrated model. A detailed representation of catchment‐scale travel time distributions has been derived, including the time evolution of the overall dispersion processes (which can be expressed in terms of time‐varying storage sampling functions). Mean computed travel times span a broad range of values (from 80 to 800 days) depending on the catchment state. Results also suggest that, in the average, discharge waters are younger than storage water. The model proves able to capture high‐frequency fluctuations in the measured chloride concentrations, which are broadly explained by the sharp transition between groundwaters and faster flows originating from topsoil layers. This article is protected by copyright. All rights reserved.
- Assimilation of stream discharge for flood forecasting: Updating a
semidistributed model with an integrated data assimilation scheme
- Authors: Yuan Li; Dongryeol Ryu, Andrew W. Western, Q. J. Wang
Abstract: Real‐time discharge observations can be assimilated into flood models to improve forecast accuracy; however, the presence of time lags in the routing process and a lack of methods to quantitatively represent different sources of uncertainties challenge the implementation of data assimilation techniques for operational flood forecasting. To address these issues, an integrated error parameter estimation and lag‐aware data assimilation (IEELA) scheme was recently developed for a lumped model. The scheme combines an ensemble‐based maximum a posteriori (MAP) error estimation approach with a lag‐aware ensemble Kalman smoother (EnKS).
In this study, the IEELA scheme is extended to a semi‐distributed model to provide for more general application in flood forecasting by including spatial and temporal correlations in model uncertainties between sub‐catchments. The result reveals that using a semi‐distributed model leads to more accurate forecasts than a lumped model in an open‐loop scenario. The IEELA scheme improves the forecast accuracy significantly in both lumped and semi‐distributed models, and the superiority of the semi‐distributed model remains in the data assimilation scenario. However, the improvements resulting from IEELA are confined to the outlet of the catchment where the discharge observations are assimilated. Forecasts at “ungauged” internal locations are not improved, and in some instances, even become less accurate. This article is protected by copyright. All rights reserved.
- A thermodynamic interpretation of Budyko and L'vovich formulations of
annual water balance: Proportionality hypothesis and maximum entropy
- Authors: Dingbao Wang; Jianshi Zhao, Yin Tang, Murugesu Sivapalan
Abstract: The paper forms part of the search for a thermodynamic explanation for the empirical Budyko Curve, addressing a long‐standing research question in hydrology. Here, this issue is pursued by invoking the Proportionality Hypothesis underpinning the Soil Conservation Service (SCS) curve number method widely used for estimating direct runoff at the event scale. In this case, the Proportionality Hypothesis posits that the ratio of continuing abstraction to its potential value is equal to the ratio of direct runoff to its potential value. Recently, the validity of the Proportionality Hypothesis has been extended to the partitioning of precipitation into runoff and evaporation at the annual time scale as well. In this case, the Proportionality Hypothesis dictates that the ratio of continuing evaporation to its potential value is equal to the ratio of runoff to its potential value. The Budyko Curve could then be seen as the straightforward outcome of the application of the Proportionality Hypothesis to estimate mean annual water balance. In this paper, we go further and demonstrate that the Proportionality Hypothesis itself can be seen as a result of the application of the thermodynamic principle of Maximum Entropy Production (MEP). In this way, we demonstrate a possible thermodynamic basis for the Proportionality Hypothesis, and consequently for the Budyko Curve. As a further extension, the L'vovich formulation for the two‐stage partitioning of annual precipitation is also demonstrated to be a result of MEP: one for the competition between soil wetting and fast flow during the first stage; another for the competition between evaporation and base flow during the second stage. This article is protected by copyright. All rights reserved.
- Comparison of three dual‐source remote sensing evapotranspiration
models during the MUSOEXE‐12 campaign: Revisit of model physics
- Authors: Yuting Yang; Di Long, Huade Guan, Wei Liang, Craig Simmons, Okke Batelaan
Abstract: Various remote sensing‐based terrestrial evapotranspiration (ET) models have been developed during the past four decades. These models vary in conceptual and mathematical representations of the physics, consequently leading to different performances. Examination of uncertainties associated with limitations in model physics will be useful for model selection and improvement. Here, three dual‐source remote sensing ET models (i.e. the Hybrid dual‐source scheme and Trapezoid framework‐based ET Model (HTEM), the Two‐Source Energy Balance (TSEB) model and the MOD16 ET algorithm) using ASTER images were compared during the MUSOEXE‐12 campaign in the Heihe River Basin in Northwest China, aiming to better understand the differences in model physics that potentially lead to differences in model performance. Model results were firstly compared against observations from a dense network of eddy covariance towers and isotope‐based evaporation (E) and transpiration (T) partitioning. Results show that HTEM outperformed the other two models in simulating ET and its partitioning, whereas MOD16 performed worst (i.e. ET root‐mean‐square errors are 42.3 W/m2 (HTEM), 49.8 W/m2 (TSEB), and 95.3 W/m2 (HTEM)). On to model limitations, HTEM tends to underestimate ET under high advection due mostly to the underestimation of temperatures for the wet edge in its trapezoidal space. For TSEB, large uncertainties occur in determining the initial Priestley‐Taylor coefficient and the iteration procedure for ET partitioning, leading to overestimation/underestimation of T/E in most cases, particularly over sparse vegetation. Primary use of meteorological data for MOD16 does not effectively capture the soil moisture restriction on ET, and therefore results in unreasonable spatial ET patterns. This article is protected by copyright. All rights reserved.
- Mathematical equivalence between time‐dependent single‐rate
and multirate mass transfer models
- Authors: D. Fernàndez‐Garcia; X. Sanchez‐Vila
Abstract: The often observed tailing of tracer breakthrough curves is caused by a multitude of mass transfer processes taking place over multiple scales. Yet, in some cases it is convenient to fit a transport model with a single‐rate mass transfer coefficient that lumps all the non‐Fickian observed behavior. Since mass transfer processes take place at all characteristic times, the single‐rate mass transfer coefficient derived from measurements in the laboratory or in the field vary with time, ω(t). The literature review and tracer experiments compiled by Haggerty et al.  from a number of sites worldwide suggest that the characteristic mass transfer time, which is proportional to ω(t)−1, scales as a power law of the advective and experiment duration. This paper studies the mathematical equivalence between the Multi‐Rate Mass Transfer Model (MRMT) and a time‐dependent single‐rate mass transfer model (t‐SRMT). In doing this, we provide new insights into the previously observed scale‐dependence of mass transfer coefficients. The memory function, g(t), which is the most salient feature of the MRMT model, determines the influence of the past values of concentrations on its present state. We found that the t‐SRMT model can also be expressed by means of a memory function φ(t,τ). In this case though the memory function is non‐stationary, meaning that in general it cannot be written as φ(t‐τ). Nevertheless, the full behavior of the concentrations using a single time‐dependent rate ω(t) is approximately analogous to that of the MRMT model provided that the equality ω(t) = ‐d In g(t)/dt holds and the field capacity is properly chosen. This relationship suggests that when the memory function is a power law, g(t) ∼ t1‐k, the equivalent mass transfer coefficient scales as ω(t) ∼ t−1, nicely fitting without calibration the estimated mass transfer coefficients compiled by Haggerty et al. . This article is protected by copyright. All rights reserved.
- Using the level set method to study the effects of heterogeneity and
anisotropy on hyporheic exchange
- Authors: Cheng Chen; Lingzao Zeng
Abstract: The level set method was used to simulate the interface movement when a conservative solute migrated from stream water to subsurface water, and study the effects of streambed heterogeneity and anisotropy on solute penetration. The level set method is a numerical technique for tracking moving interfaces based on the idea that the interface is a level set curve of a higher‐dimensional function. Numerical simulations were compared to experiments conducted in a recirculating flume. Streambed heterogeneity led to water exchange between multiple bedforms, while in homogeneous streambeds the water exchange was restricted within a single bedform. A thin layer of homogeneous sediments at the top of the heterogeneous streambeds significantly increased the interfacial water influx, resulting in faster solute penetration and stream concentration decrease. Streambed heterogeneity generated horizontal preferential flow paths in the upper part of the bed while decreasing pore water velocities deeper in the bed, which hindered vertical penetration and consequently led to slower stream concentration decrease. Decreasing vertical permeability or increasing horizontal permeability led to slower vertical penetration and stream concentration decrease. Decreasing vertical permeability had a much more significant impact on solute penetration than increasing horizontal permeability, because mass transfer in hyporheic exchange is greatly dominated by vertical advection which depends primarily on the vertical permeability. This study was the first to apply the level set method in the study of hyporheic exchange. The theoretical and numerical methods have important applications in subsurface flow and transport processes. This article is protected by copyright. All rights reserved.
- What does it take to flood the Pampas? Lessons from a decade of strong
- Authors: S. Kuppel; J. Houspanossian, M. D. Nosetto, E. G. Jobbágy
Abstract: While most landscapes respond to extreme rainfalls with increased surface water outflows, very flat and poorly drained ones have little capacity to do this and their most common responses include (i) increased water storage leading to rising water tables and floods, (ii) increased evaporative water losses and, after reaching high levels of storage, (iii) increased liquid water outflows. The relative importance of these pathways was explored in the extensive plains of the Argentine Pampas, where two significant flood episodes (denoted FE1 and FE2) occurred in 2000‐2003 and 2012‐2013. In two of the most flood‐prone areas (Western and Lower Pampa, 60 000 km2 each), surface water cover reached 31 and 19% during FE1 in each subregion, while FE2 covered up to 22 and 10%, respectively. From the spatiotemporal heterogeneity of the flood events, we distinguished slow floods lasting several years when the water table is brought to the surface following sustained precipitation excesses in groundwater‐connected systems (Western Pampa), and "fast" floods triggered by surface water accumulation over the course of weeks to months, typical of poor surface‐groundwater connectivity (Lower Pampa) or when exceptionally‐strong rainfalls overwhelm infiltration capacity. Because of these different hydrological responses, precipitation and evapotranspiration were strongly linked in the Lower Pampa only, while the connection between water fluxes and storage was limited to the Western Pampa. In both regions, evapotranspirative losses were strongly linked to flooded conditions as a regulatory feedback, while liquid water outflows remained negligible. This article is protected by copyright. All rights reserved.
- The impact of reservoir conditions on the residual trapping of carbon
dioxide in Berea sandstone
- Authors: Ben Niu; Ali Al‐Menhali, Samuel C. Krevor
Abstract: The storage of carbon dioxide in deep brine‐filled permeable rocks is an important tool for CO2 emissions mitigation on industrial scales. Residual trapping of CO2 through capillary forces within the pore space of the reservoir is one of the most significant mechanisms for storage security and is also a factor determining the ultimate extent of CO2 migration within the reservoir. In this study we have evaluated the impact of reservoir conditions of pressure, temperature, and brine salinity on the residual trapping characteristic curve of a fired Berea sandstone rock. The observations demonstrate that the initial‐residual characteristic trapping curve is invariant across a wide range of pressure, temperature, and brine salinities and is also the same for CO2‐brine systems as a N2‐water system. The observations were made using a reservoir condition core‐flooding laboratory that included high‐precision pumps, temperature control, the ability to recirculate fluids for weeks at a time, and an X‐ray CT scanner. Experimental conditions covered pressures of 5–20 MPa, temperatures of 25–50°C, and 0–5 mol/kg NaCl brine salinity. A novel coreflooding approach was developed, making use of the capillary end effect to create a large range in initial CO2 saturation (0.15–0.6) in a single coreflood. Upon subsequent flooding with CO2‐equilibriated brine, the observation of residual saturation corresponded to the wide range of initial saturations before flooding resulting in a rapid construction of the initial‐residual curve. For each condition we report the initial‐residual curve and the resulting parameterization of the Land hysteresis models.
- Canopy influence on snow depth distribution in a pine stand determined
from terrestrial laser data
- Authors: J. Revuelto; J.I. López‐Moreno, C. Azorin‐Molina, S.M. Vicente‐Serrano
Abstract: In this study we analyzed the effects of the forest canopy and trunks of a pine stand in the central Spanish Pyrenees on the snow depth (SD) distribution. Using LiDAR technology with a terrestrial laser scanner (TLS), high‐resolution data on the SD distribution were acquired during the 2011–12 and 2012–13 snow seasons, which were two years having very contrasting climatic and snow accumulation conditions. Average SD evolution in open and canopy areas was characterized. Principal component analysis was applied to identify days having similar spatial patterns of SD distribution. There was a clear contrast in the temporal variability of the snowpack in different areas of the forest stand, corresponding generally to beneath the canopy, and in open sites. The canopy and openings showed markedly different accumulation and melting, with higher snow accumulation found in openings. Differences ranged from 14 to 80% reduction (average 49%) in the SD beneath the canopy relative to open sites. The difference in SD between open and canopy areas increased throughout the snow season. The surveyed days were classified in terms of SD distribution, and included days associated with: high SD, low SD, intense melting conditions and periods when the SD distribution was driven by wind conditions. The SD increased with distance from the trunks to a distance of 3.5–4.5 m, coinciding with the average size of the crown of individual trees. This article is protected by copyright. All rights reserved.
- Three‐dimensional versus two‐dimensional bed
form‐induced hyporheic exchange
- Authors: Xiaobing Chen; M. Bayani Cardenas, Li Chen
Abstract: The hyporheic zone is often a critical component of river systems. Hyporheic exchange is generally forced by variation in riverbed topography such as due to bedforms. Most previous research on bedform‐driven hyporheic flow has focused on two‐dimensional (2D) dunes and ripples, while little has been done on their three‐dimensional (3D) counterparts. Here we compared hyporheic exchange and associated metrics for a previously studied pair of corresponding 2D and 3D bedforms. To accomplish this, a series of multiphysics computational fluid dynamics models were conducted both in 2D and 3D with similar open channel Reynolds numbers (Re). Results show that the pressure gradient along the sediment‐water interface is highly sensitive to the spatial structure of bedforms, which consequently determines hyporheic flow dynamics. Hyporheic flux is a function of Re for both 2D and 3D dunes via a power law; however, the equivalent 3D dunes have a higher flux since the 3D form induces more drag. The hyporheic zone depths and volumes are only slightly different with the 3D case having a larger volume. The mean fluid residence times for both cases are related to Re by an inverse power law relationship, with the 3D dune having smaller residence times at moderate to high Re. The effects of increasing flux on residence time in 3D dunes is partly modulated by a slightly increasing hyporheic volume. Our results suggest that a 2D idealization is a reasonable approximation for the more complex 3D situation if local details are unimportant but that development of predictive models for mean fluxes and residence times, which are critical for biogeochemical processes, based on 2D models may be insufficient. This article is protected by copyright. All rights reserved.
- Two‐phase flow properties of a sandstone rock for the CO2/water
system: Core‐flooding experiments, and focus on impacts of
- Authors: JC. Manceau; J. Ma, R. Li, P. Audigane, P. X. Jiang, R. Xu, J. Tremosa, C. Lerouge
Abstract: The two‐phase flow characterization (CO2/water) of a Triassic sandstone core from the Paris Basin, France, is reported in this paper. Absolute properties (porosity and water permeability), capillary pressure, relative permeability with hysteresis between drainage and imbibition, and residual trapping capacities have been assessed at 9 MPa pore pressure and 28 °C (CO2 in liquid state) using a single core‐flooding apparatus associated with magnetic resonance imaging. Different methodologies have been followed to obtain a data‐set of flow properties to be up‐scaled and used in large scale CO2 geological storage evolution modeling tools. The measurements are consistent with the properties of well‐sorted water‐wet porous systems. As the mineralogical investigations showed a non‐negligible proportion of carbonates in the core, the experimental protocol was designed to observe potential impacts on flow properties of mineralogical changes. The magnetic resonance scanning and mineralogical observations indicate mineral dissolution during the experimental campaign, and the core‐flooding results show an increase in porosity and water absolute permeability. The changes in two‐phase flow properties appear coherent with the pore structure modifications induced by the carbonates dissolution but the changes in relative permeability could also be explained by a potential increase of the water‐wet character of the core. Further investigations on the impacts of mineral changes are required with other reactive formation rocks, especially carbonate‐rich ones, because the implications can be significant both for the validity of laboratory measurements and for the outcomes of in situ operations modeling. This article is protected by copyright. All rights reserved.
- Sediment transport and shear stress partitioning in a vegetated flow
- Authors: C. Le Bouteiller; J. G. Venditti
Abstract: Vegetation is a common feature in natural coastal and riverine water ways, interacting with both the water flow and sediment transport. However, the physical processes governing these interactions are still poorly understood, which makes it difficult to predict sediment transport and morphodynamics in a vegetated environment. We performed a simple experiment to study how sediment transport responds to the presence of flexible, single‐blade vegetation and how this response is influenced by the vegetation density. We found that the skin friction and sediment transport are reduced in a plant patch, and that this effect is larger for denser vegetation. We then evaluated several methods to calculate the skin friction in a vegetated flow, which is the key to sediment transport prediction. Among these, the inversion of bedload transport formulas and the Einstein and Banks (1950) methods appeared to produce the most reasonable values of the skin friction. Finally, we suggest using the parameter α, which is the ratio of the skin friction computed by these methods to the total bed shear stress, to make more realistic sediment transport predictions in morphodynamic models. This article is protected by copyright. All rights reserved.
- Water‐quality trading: Can we get the prices of pollution right?
- Authors: Yoshifumi Konishi; Jay S. Coggins, Bin Wang
Abstract: Water‐quality trading requires inducing permit prices that account properly for spatially explicit damage relationships. We compare recent work by Hung and Shaw  and Farrow et al.  for river systems exhibiting branching and nonlinear damages. The Hung‐Shaw scheme is robust to nonlinear damages, but not to hot spots occurring at the confluence of two branches. The Farrow et al. scheme is robust to branching, but not to nonlinear damages. We also compare the two schemes to each other. Neither dominates from a welfare perspective, but the comparison appears to tilt in favor of the Farrow et al. scheme. This article is protected by copyright. All rights reserved.
- Biodegradation of subsurface oil in a tidally influenced sand beach:
Impact of hydraulics and interaction with pore water chemistry
- Authors: Xiaolong Geng; Michel C. Boufadel, Kenneth Lee, Stewart Abrams, Makram Suidan
Abstract: The aerobic biodegradation of oil in tidally influenced beaches was investigated numerically in this work using realistic beach and tide conditions. A numerical model BIOMARUN, coupling a multiple‐Monod kinetic model BIOB to a density‐dependent variably saturated groundwater flow model 2‐D MARUN, was used to simulate the biodegradation of low solubility hydrocarbon and transport processes of associated solute species (i.e., oxygen and nitrogen) in a tidally influenced beach environment. It was found that different limiting factors affect different portions of the beach. In the upper intertidal zone, where the inland incoming nutrient concentration was large (1.2 mg‐N/L), oil biodegradation occurred deeper in the beach (i.e., 0.3 m below the surface). In the mid‐intertidal zone, a reversal was noted where the biodegradation was fast at shallow locations (i.e., 0.1 m below the surface), and it was due to the decrease of oxygen with depth due to consumption, which made oxygen the limiting factor for biodegradation. Oxygen concentration in the mid‐intertidal zone exhibited two peaks as a function of time. One peak was associated with the high tide, when dissolved oxygen laden seawater filled the beach and a second oxygen peak was observed during low tides, and it was due to pore oxygen replenishment from the atmosphere. The effect of the capillary fringe (CF) height was investigated, and it was found that there is an optimal CF for the maximum biodegradation of oil in the beach. Too large a CF (i.e., very fine material) would attenuate oxygen replenishment (either from seawater or the atmosphere), while too small a CF (i.e., very coarse material) would reduce the interaction between microorganisms and oil in the upper intertidal zone due to rapid reduction in the soil moisture at low tide. This article is protected by copyright. All rights reserved.
- Point rainfall statistics for ecohydrological analyses derived from
satellite‐integrated rainfall measurements
- Authors: Manuel del Jesus; Andrea Rinaldo, Ignacio Rodríguez‐Iturbe
Abstract: Satellite rainfall measurements, nowadays commonly available, provide valuable information about the spatial structure of rainfall. In areas with low‐density rain gage networks, or where these networks are non‐existent, satellite rainfall measurements can also provide useful estimates to be used as virtual rain gages. However, satellite and rain gage measurements are statistically different in nature and cannot be directly compared to one another. In the present paper we develop a methodology to downscale satellite rainfall measurements to generate rain‐gage‐equivalent statistics. We apply the methodology to four locations along a strong rainfall gradient in the Kalahari transect, southern Africa, to validate the methodology. We show that the method allows the estimation of point rainfall statistics where only satellite measurements exist. Point rainfall statistics are key descriptors for ecohydrologic studies linking the response of vegetation to rainfall dynamics. This article is protected by copyright. All rights reserved.
- Evaluation of measurement sensitivity and design improvement for time
domain reflectometry penetrometers
- Authors: Tony Liang‐tong Zhan; Qing‐yi Mu, Yun‐min Chen, Han Ke
Abstract: The time domain reflectometry (TDR) penetrometer, which can measure both the apparent dielectric permittivity and the bulk electrical conductivity of soils, is an important tool for the site investigation of contaminated land. This paper presents a theoretical method for evaluating the measurement sensitivity and an improved design of the TDR penetrometer. The sensitivity evaluation method is based on a spatial weighting analysis of the electromagnetic field using a seepage analysis software. This method is used to quantify the measurement sensitivity for the three types of TDR penetrometers reported in literature as well as guide the design improvement of the TDR penetrometer. The improved design includes the use of semicircle‐shaped conductors and the optimization of the conductor diameter. The measurement sensitivity to the targeted medium for the improved TDR penetrometer is evaluated to be greater than those of the three types of TDR penetrometers reported in literature. The performance of the improved TDR penetrometer was demonstrated by conducting an experimental calibration of the probe and penetration tests in a chamber containing a silty soil column. The experimental results demonstrate that the measurements from the improved TDR penetrometer are able to capture the variation in the water content profiles as well as the leachate contaminated soil. This article is protected by copyright. All rights reserved.
- An active heat tracer experiment to determine groundwater velocities using
fiber‐optic cables installed with direct push equipment
- Authors: Mark Bakker; Ruben Caljé, Frans Schaars, Kees‐Jan van der Made, Sander de Haas
Abstract: A new approach is developed to insert fiber optic cables vertically into the ground with direct push equipment. Groundwater temperatures may be measured along the cables with high spatial and temporal resolution using a Distributed Temperature Sensing system. The cables may be inserted up to depths of tens of meters in unconsolidated sedimentary aquifers. The main advantages of the method are that the cables are in direct contact with the aquifer material, the disturbance of the aquifer is minor, and no borehole is needed. This cost‐effective approach may be applied to both passive and active heat tracer experiments. An active heat tracer experiment was conducted to estimate horizontal groundwater velocities in a managed aquifer recharge system in the Netherlands. Six fiber optic cables and a separate heating cable were inserted with a one‐meter spacing at the surface. The heating cable was turned on for 4 days and temperatures were measured during both heating and cooling of the aquifer. Temperature measurements at the heating cable alone were used to estimate the magnitude of the groundwater velocity and the thermal conductivity of the solids. The direction of the velocity and heat capacity of the solids were estimated by including temperature measurements at the other fiber optic cables in the analysis. The latter analysis suffered from the fact that the cables were not inserted exactly vertical. The three‐dimensional position of the fiber optic cables must be measured for future active heat tracer experiments. This article is protected by copyright. All rights reserved.
- CO2 dissolution in the presence of background flow of deep saline aquifers
- Authors: Hamid Emami‐Meybodi; Hassan Hassanzadeh, Jonathan Ennis‐King
Abstract: We study the effect of background flow on the dissolution and transport of carbon dioxide (CO2) during geological storage in saline aquifers, and include the processes of diffusion, advection, and free convection. We develop a semi‐analytical model that captures the evolution of the dissolution in the absence of free convection. Using the semi‐analytical solution, we determine scaling relations for the steady rate of dissolution that follow either or Jst∼Pe depending on the value of Pe/R, where R represents the ratio of the extent of CO2 plume to the aquifer thickness and Pe is the Péclet number. Using direct numerical simulations, we provide detailed behavior of the convective mixing during the dissolution. We establish the criteria for forced and mixed (combined free and forced) convection in aquifers that is governed by the background flow. Accordingly, we provide the scaling relations and Jst∼RaR representing the forced and free convection asymptotes, respectively, where Ra is a Rayleigh number based on aquifer thickness. The results reveal that the background velocity can delay the onset of free convection and can alter the subsequent mixing. This phenomenon is more profound in the systems subject to strong background flows wherein horizontal component of the velocity field generated by background flow hinders the establishments of vertical component of the velocity field. Finally, by applying the proposed relations to several potential storage sites, we demonstrate the screening process in finding aquifers where the background flow exerts an important influence on the dissolution. This article is protected by copyright. All rights reserved.
- Calibrating remotely sensed river bathymetry in the absence of field
measurements: Flow resistance equation‐based imaging of river depths
- Authors: Carl J. Legleiter
Abstract: Remote sensing could enable high‐resolution mapping of long river segments, but realizing this potential will require new methods for inferring channel bathymetry from passive optical image data without using field measurements for calibration. As an alternative to regression‐based approaches, this study introduces a novel framework for Flow REsistance Equation‐Based Imaging of River Depths (FREEBIRD). This technique allows for depth retrieval in the absence of field data by linking a linear relation between an image‐derived quantity X and depth d to basic equations of open channel flow: continuity and flow resistance. One FREEBIRD algorithm takes as input an estimate of the channel aspect (width/depth) ratio A and a series of cross‐sections extracted from the image and returns the coefficients of the X vs. d relation. A second algorithm calibrates this relation so as to match a known discharge Q. As an initial test of FREEBIRD, these procedures were applied to panchromatic satellite imagery and publicly available aerial photography of a clear‐flowing gravel‐bed river. Accuracy assessment based on independent field surveys indicated that depth retrieval performance was comparable to that achieved by direct, field‐based calibration methods. Sensitivity analyses suggested that FREEBIRD output was not heavily influenced by misspecification of A or Q, or by selection of other input parameters. By eliminating the need for simultaneous field data collection, these methods create new possibilities for large‐scale river monitoring and analysis of channel change, subject to the important caveat that the underlying relationship between X and d must be reasonably strong. This article is protected by copyright. All rights reserved.
- How well do CMIP5 climate simulations replicate historical trends and
patterns of meteorological droughts?
- Authors: Nasrin Nasrollahi; Amir AghaKouchak, Linyin Cheng, Lisa Damberg, Thomas J. Phillips, Chiyuan Miao, Kuolin Hsu, Soroosh Sorooshian
Abstract: Assessing the uncertainties and understanding the deficiencies of climate models is fundamental to developing adaptation strategies. The objective of this study is to understand how well Coupled Model Intercomparison‐Phase 5 (CMIP5) climate model simulations replicate ground‐based observations of continental drought areas and their trends. The CMIP5 multi‐model ensemble encompasses the Climatic Research Unit (CRU) ground‐based observations of area under drought at all time‐steps. However, most model members overestimate the areas under extreme drought, particularly in the Southern Hemisphere (SH). Furthermore, the results show that the time series of observations and CMIP5 simulations of areas under drought exhibit more variability in the SH than in the Northern Hemisphere (NH). The trend analysis of areas under drought reveals that the observational data exhibit a significant positive trend at the significance level of 0.05 over all land areas. The observed trend is reproduced by about three‐fourths of the CMIP5 models when considering total land areas in drought. While models are generally consistent with observations at a global (or hemispheric) scale, most models do not agree with observed regional drying and wetting trends. Over many regions, at most 40% of the CMIP5 models are in agreement with the trends of CRU observations. The drying/wetting trend calculated using the 3 months Standardized Precipitation Index (SPI) values show better agreement with the corresponding CRU values than with the observed annual mean precipitation rates. Pixel scale evaluation of CMIP5 models indicates that no single model demonstrates an overall superior performance relative to the other models. This article is protected by copyright. All rights reserved.
- Bayesian model averaging to explore the worth of data for soil‐plant
model selection and prediction
- Authors: Thomas Wöhling; Anneli Schöniger, Sebastian Gayler, Wolfgang Nowak
Abstract: A Bayesian Model Averaging (BMA) framework is presented to evaluate the worth of different observation types and experimental design options for 1) more confidence in model selection and 2) for increased predictive reliability. These two modeling tasks are handled separately, because model selection aims at identifying the most appropriate model with respect to a given calibration data set, while predictive reliability aims at reducing uncertainty in model predictions through constraining the plausible range of both models and model parameters. For that purpose, we pursue an optimal design of measurement framework that is based on BMA and that considers uncertainty in parameters, measurements, and model structures. We apply this framework to select between four crop models (the vegetation components of CERES, SUCROS, GECROS and SPASS), which are coupled to identical routines for simulating soil carbon and nitrogen turnover, soil heat and nitrogen transport, and soil water movement. An ensemble of parameter realizations was generated for each model using Monte‐Carlo simulation. We assess each model's plausibility by determining its posterior weight, which signifies the probability to have generated a given experimental data set. Several BMA analyses were conducted for different data packages with measurements of soil moisture, evapotranspiration (ETa), and leaf area index (LAI). The posterior weights resulting from the different BMA runs were compared to the weight distribution of a reference run with all data types to investigate the utility of different data packages and monitoring design options in identifying the most appropriate model in the ensemble. We found that different (combinations of) data types support different models and none of the four crop models outperforms all others under all data scenarios. The best model discrimination was observed for those data where the competing models disagree the most. The data worth for reducing prediction uncertainty depends on the prediction to be made. LAI data have the highest utility for predicting ETa, while soil moisture data is better for predicting soil water drainage. Our study illustrates, that BMA provides an objective framework for data worth analysis with respect to both model discrimination and model calibration for a wide range of applications. This article is protected by copyright. All rights reserved.
- Global change and the groundwater management challenge
- Authors: Steven M. Gorelick; Chunmiao Zheng
Abstract: With rivers in critical regions already exploited to capacity throughout the world and groundwater overdraft as well as large‐scale contamination occurring in many areas, we have entered an era in which multiple simultaneous stresses will drive water management. Increasingly, groundwater resources are taking a more prominent role in providing freshwater supplies. We discuss the competing fresh groundwater needs for human consumption, food production, energy, and the environment, as well as physical hazards, and conflicts due to transboundary overexploitation. During the past 50 years, groundwater management modeling has focused on combining simulation with optimization methods to inspect important problems ranging from contaminant remediation to agricultural irrigation management. The compound challenges now faced by water planners require a new generation of aquifer management models that address the broad impacts of global change on aquifer storage and depletion trajectory management, land subsidence, groundwater‐dependent ecosystems, seawater intrusion, anthropogenic and geogenic contamination, supply vulnerability, and long‐term sustainability. The scope of research efforts is only beginning to address complex interactions using multi‐agent system models that are not readily formulated as optimization problems and that consider a suite of human behavioral responses. This article is protected by copyright. All rights reserved.
- New ν‐type relative permeability curves for two‐phase
flows through subsurface fractures
- Authors: Noriaki Watanabe; Keisuke Sakurai, Takuya Ishibashi, Yutaka Ohsaki, Tetsuya Tamagawa, Masahiko Yagi, Noriyoshi Tsuchiya
Abstract: Appropriate relative permeability curves for two‐phase flows through subsurface fractures remain unclear. We have conducted decane‐water and nitrogen‐water two‐phase flow experiments and simulations on real variable‐aperture fractures in rocks under confining stress. Experiments have been conducted on fractures for different combinations of rock type (granite or limestone), wettability (contact angle of water: 0° or 90°), and intrinsic fracture permeability (10−11 m2 or 10−10 m2) using different combinations of shear displacement (0 mm or 1 mm) and effective confining stress (1 MPa or 40 MPa). It has been demonstrated that non‐wetting phase relative permeability depends on capillary pressure, except at either a higher contact angle or higher intrinsic permeability (i.e., bigger aperture), where no influence of capillarity is expected from the Young‐Laplace equation. In the absence of an influence of capillarity, relations between wetting and non‐wetting phase relative permeabilities agree with that of the X‐type relative permeability curves. In order to determine the relative permeability curves under the influence of capillarity, the experimental results have been analyzed by two‐phase flow simulations of the aperture distributions of the fractures. It has been revealed that non‐wetting phase relative permeability becomes zero, even at a small wetting phase saturation of approximately 0.3, while wetting phase relative permeability exhibits Corey‐type behavior, resulting in ν‐shaped relative permeability curves. Similar curves have been reported in the literature, but have not been demonstrated for real fractures. It has been revealed that the new ν‐type and traditional X‐type relative permeability curves are appropriate for describing two‐phase flows through subsurface fractures. This article is protected by copyright. All rights reserved.
- Morphodynamics: Rivers beyond steady state
- Authors: M. Church; R. I. Ferguson
Abstract: The morphology of an alluvial river channel affects the movement of water and sediment along it, but in the longer run is shaped by those processes. This interplay has mostly been investigated empirically within the paradigm of Newtonian mechanics. In rivers this has created an emphasis on equilibrium configurations with simple morphology and uniform steady flow. But transient adjustment, whether between equilibrium states or indefinitely, is to be expected in a world in which hydrology, sediment supply, and base level are not fixed. More fundamentally, water flows and all the phenomena that accompany them are inherently unsteady and flows in natural channels are characteristically non‐uniform. The morphodynamics of alluvial river channels is the striking consequence. In this paper we develop the essential connection between the episodic nature of bed material transport and the production of river morphology, emphasizing the fundamental problems of sediment transport, the role of bar evolution in determining channel form, the role of riparian vegetation, and the wide range of timescales for change. As the key integrative exercise, we emphasize the importance of physics‐based modeling of morphodynamics. We note consequences that can be of benefit to society if properly understood. These include the possibility to better be able to model how varying flows drive morphodynamic change, to understand the influence of the sediments themselves on morphodynamics, and to recognize the inherent necessity for rivers that transport bed material to deform laterally. We acknowledge pioneering contributions in WRR and elsewhere that have introduced some of these themes. This article is protected by copyright. All rights reserved.
- Diagnosis of insidious data disasters
- Authors: Jessica D. Lundquist; Nicholas E. Wayand, Adam Massmann, Martyn P. Clark, Fred Lott, Nicoleta C. Cristea
Abstract: Everyone taking field observations has a story of data collection gone wrong, and in most cases, the errors in the data are immediately obvious. A more challenging problem occurs when the errors are insidious, i.e., not readily detectable, and the error‐laden data appear useful for model testing and development. We present two case studies, one related to the water balance in the snow‐fed Tuolumne River, Sierra Nevada, California, combined with modeling using the Distributed Hydrology Soil Vegetation Model (DHSVM); and one related to the energy balance at Snoqualmie Pass, Washington, combined with modeling using the Structure for Unifying Multiple Modeling Alternatives (SUMMA). In the Tuolumne, modeled streamflow in one year was more than twice as large as observed; at Snoqualmie, modeled nighttime surface temperatures were biased by about +10 °C. Both appeared to be modeling failures, until detective work uncovered observational errors. We conclude with a discussion of what these cases teach us about science in an age of specialized research, when one person collects data, a separate person conducts model simulations, and a computer is charged with data quality‐assurance. This article is protected by copyright. All rights reserved.
- Metapopulation capacity of evolving fluvial landscapes
- Authors: Enrico Bertuzzo; Ignacio Rodriguez‐Iturbe, Andrea Rinaldo
Abstract: The form of fluvial landscapes is known to attain stationary network configurations that settle in dynamically accessible minima of total energy dissipation by landscape‐forming discharges. Recent studies have highlighted the role of the dendritic structure of river networks in controlling population dynamics of the species they host and large scale biodiversity patterns. Here, we systematically investigate the relation between energy dissipation, the physical driver for the evolution of river networks, and the ecological dynamics of their embedded biota. To that end we use the concept of metapopulation capacity, a measure to link landscape structures with the population dynamics they host. Technically, metapopulation capacity is the leading eigenvalue λM of an appropriate ‘landscape' matrix subsuming whether a given species is predicted to persist in the long run. λM can conveniently be used to rank different landscapes in terms of their capacity to support viable metapopulations. We study how λM changes in response to the evolving network configurations of spanning trees. Such sequence of configurations is theoretically known to relate network selection to general landscape evolution equations through imperfect searches for dynamically accessible states frustrated by the vagaries of Nature. Results show that the process shaping the metric and the topological properties of river networks, prescribed by physical constraints, leads to a progressive increase in the corresponding metapopulation capacity and therefore on the landscape capacity to support metapopulations – with implications on biodiversity in fluvial ecosystems. This article is protected by copyright. All rights reserved.
- Transport of fluorobenzoate tracers in a vegetated hydrologic control
volume: 1. Experimental results
- Authors: Pierre Queloz; Enrico Bertuzzo, Luca Carraro, Gianluca Botter, Franco Miglietta, P.S.C. Rao, Andrea Rinaldo
Abstract: This paper reports about the experimental evidence collected on the transport of five fluorobenzoate tracers injected under controlled conditions in a vegetated hydrologic volume, a large lysimeter (fitted with load cells, sampling ports and an underground chamber) where two willows prompting large evapotranspiration fluxes had been grown. The relevance of the study lies in the direct and indirect measures of the ways in which hydrologic fluxes, in this case evapotranspiration from the upper surface and discharge from the bottom drainage, sample water and solutes in storage at different times under variable hydrologic forcings. Methods involve the accurate control of hydrologic inputs and outputs and a large number of suitable chemical analyses of water samples in discharge waters. Mass‐extraction from biomass has also been performed ex‐post. The results of the two‐year long experiment established that our initial premises on the tracers' behavior, known to be sorption‐free under saturated conditions which we verified in column leaching tests, were unsuitable as large differences in mass recovery appeared. Issues on reactivity thus arose and were addressed in the paper, in this case attributed to microbial degradation and solute plant uptake. Our results suggest previously unknown features of fluorobenzoate compounds as hydrologic tracers, potentially interesting for catchment studies owing to their suitability for distinguishable multiple injections, and an outlook on direct experimental closures of mass balance in hydrologic transport volumes involving fluxes that are likely to sample differently stored water and solutes. This article is protected by copyright. All rights reserved.
- Transport of fluorobenzoate tracers in a vegetated hydrologic control
volume: 2. Theoretical inferences and modeling
- Authors: Pierre Queloz; Luca Carraro, Paolo Benettin, Gianluca Botter, Andrea Rinaldo, Enrico Bertuzzo
Abstract: A theoretical analysis of transport in a controlled hydrologic volume, inclusive of two willow trees and forced by erratic water inputs, is carried out contrasting the experimental data described in a companion paper. The data refer to the hydrologic transport in a large lysimeter of different fluorobenzoic acids seen as tracers. Export of solute is modeled through a recently developed framework which accounts for non‐stationary travel time distributions where we parametrize how output fluxes (namely, discharge and evapotranspiration) sample the available water ages in storage. The relevance of this work lies in the study of hydrologic drivers of the non‐stationary character of residence and travel time distributions, whose definition and computation shape this theoretical transport study. Our results show that a large fraction of the different behaviors exhibited by the tracers may be charged to the variability of the hydrologic forcings experienced after the injection. Moreover, the results highlight the crucial, and often overlooked, role of evapotranspiration and plant uptake in determining the transport of water and solutes. This application also suggests that the ways evapotranspiration selects water with different ages in storage can be inferred through model calibration contrasting only tracer concentrations in the discharge. A view on upscaled transport volumes like hillslopes or catchments is maintained throughout the paper. This article is protected by copyright. All rights reserved.
- Planform evolution of two anabranching structures in the Upper Peruvian
- Authors: C. E. Frias; J. D. Abad, A. Mendoza, J. Paredes, C. Ortals, H. Montoro
Abstract: We present a study to relate the sinuosity of the main channel and its effect on the dynamics of the secondary channels of anabranching structures. For this purpose, two locations of the Peruvian Amazon River were selected: 1) a site with a medium to high‐sinuosity main channel (MS site: Muyuy, Peru) and 2) a site with a low‐sinuosity main channel (LS site: at the triple boundary between Brazil, Colombia and Peru). The main channels for both the MS and LS anabranching structures have freedom to migrate in the lateral direction, while at least one of their secondary channels' is adjacent to the geological valley. For MS and LS sites, temporal analysis of planform evolution was carried out using 30 years of satellite imagery from which metrics such as width, sinuosity and annual maximum migration rates of main and secondary channels were calculated. Additionally, detailed hydrodynamic and bed morphology field measurements were carried out, and a two‐dimensional shallow water numerical model was developed. For a medium‐ to high‐sinuosity main channel anabranching structure, the secondary channels present a dominant mechanism for reworking the floodplain, while for the low‐sinuosity anabranching structure, the main channel planform dynamics is dominant. Flow velocities along the main and secondary channels for low‐, transition‐, and high‐flow discharges describe that for MS (LS) site, the velocities are much higher along the secondary (main) channels. This article is protected by copyright. All rights reserved.
- Prediction in ungauged estuaries: An integrated theory
- Authors: Hubert H.G. Savenije
Abstract: Many estuaries in the world are ungauged. The International Association of Hydrological Sciences completed its science decade on Prediction in Ungauged Basins (PUB) in 2012. Prediction on the basis of limited data is a challenge in hydrology, but not less so in estuaries, where data on fundamental processes are often lacking. In this paper relatively simple, but science‐based, methods are presented that allow researchers, engineers and water managers to obtain first order estimates of essential process parameters in estuaries, such as the estuary depth, the tidal amplitude, the tidal excursion, the phase lag and the salt water intrusion, on the basis of readily obtainable information, such as topographical maps and tidal tables. These apparently simple relationships are assumed to result from the capacity of freely erodible water bodies to adjust themselves to external drivers and to dissipate the free energy from these drivers as efficiently as possible. Thus it is assumed that these systems operate close to their thermodynamic limit, resulting in predictable patterns that can be described by relatively simple equations. Although still much has to be done to develop an overall physics‐based theory, this does not prevent us from making use of the empirical 'laws' that we observe in alluvial estuaries. This article is protected by copyright. All rights reserved.
- Linking bed morphology changes of two sediment mixtures to sediment
transport predictions in unsteady flows
- Authors: Kevin A. Waters; Joanna Crowe Curran
Abstract: Flume experiments were conducted to measure bed morphology adjustments in sand/gravel and sand/silt sediment mixtures during repeated hydrographs and to link these changes to sediment transport patterns over multiple time scales. Sediment composition and hydrograph flow magnitude greatly influenced channel morphology, which impacted sediment yield, hysteresis, and transport predictions. Bed load yields were larger and more variable for the sand/silt mixture, as gravel in the sand/gravel sediment inhibited grain entrainment, limited bedform growth, and acted to stabilize the bed. Hysteresis patterns varied due to bedform and surface structure adjustments, as well as the stabilizing effect of antecedent low flows. Using half the data set, a dimensionless fractional transport equation was derived based on excess shear stress. Dimensionless reference shear stresses were estimated in two ways: as bulk values from all transport measurements and by applying a separate limb approach in which values were estimated for each limb of each hydrograph. For the other half of the data set, transport predictions with the separate limb approach were more accurate than those from six existing transport equations and the fractional relationship applied with bulk reference shear stresses. Thus, hydrograph limb‐dependent dimensionless reference shear stress links changing bed morphology and sediment transport, providing a parameter to improve transport predictions during individual flood events and in unsteady flow regimes. This approach represents a framework with which to develop site‐specific transport relationships for varying flow regimes, particularly in cases where detailed bed morphology measurements are not feasible and heterogeneous sediment complicates bed structure over time. This article is protected by copyright. All rights reserved.
- Validation of finite water‐content vadose zone dynamics method using
column experiments with a moving water table and applied surface flux
- Authors: Fred L. Ogden; Wencong Lai, Robert C. Steinke, Jianting Zhu
Abstract: Data from laboratory experiments on a 143 cm tall and 14.5 cm diameter column, packed with Wedron sand with varied constant upper boundary fluxes and water table velocities for both falling and rising water tables are used to validate a finite water‐content vadose zone simulation methodology. The one‐dimensional finite water‐content Talbot and Ogden  (T‐O) infiltration and redistribution method was improved to simulate groundwater table dynamic effects, and compared against the numerical solution of the Richards' equation using Hydrus‐1D. Both numerical solutions agreed satisfactorily with time‐series measurements of water content. Results showed similar performance for both methods, with the T‐O method on average having higher Nash‐Sutcliffe efficiencies and smaller absolute biases. Hydrus‐1D was more accurate in predicting de‐ponding times in the case of a falling water table, while Hydrus‐1D and the T‐O method had similar errors in predicted ponding times in the case of a rising water table in 6 of 9 tests. The improved T‐O method was able to predict general features of vadose zone moisture dynamics with moving water table and surface infiltration using an explicit, mass‐conservative formulation. The advantage of an explicit formulation is that it is numerically simple, using forward Euler solution methodology, and is guaranteed to converge and to conserve mass. These properties make the improved T‐O method presented in this paper a robust and computationally efficient alternative to the numerical solution of Richards' equation in hydrological modeling applications involving groundwater table dynamic effects on vadose zone soil moistures. This article is protected by copyright. All rights reserved.
- Potential accumulation of contaminated sediments in a reservoir of a
high‐Andean watershed: Morphodynamic connections with geochemical
- Authors: María Teresa Contreras; Daniel Müllendorff, Pablo Pastén, Gonzalo E. Pizarro, Chris Paola, Cristián Escauriaza
Abstract: Rapid changes due to anthropic interventions in high‐altitude environments, such as the Altiplano region in South America, require new approaches to understand the connections between physical and geochemical processes. Alterations of the water quality linked to the river morphology can affect the ecosystems and human development in the long‐term. The future construction of a reservoir in the Lluta river, located in northern Chile, will change the spatial distribution of arsenic‐rich sediments, which can have significant effects on the lower parts of the watershed. In this investigation we develop a coupled numerical model to predict and evaluate the interactions between morphodynamic changes in the Lluta reservoir [based on the work of Kostic and Parker, 2003a,b], and conditions that can potentially desorb arsenic from the sediments. Assuming that contaminants are mobilized under anaerobic conditions, we calculate the oxygen concentration within the sediments to study the interactions of the delta progradation with the potential arsenic release. This work provides a framework for future studies aimed to analyze the complex connections between morphodynamics and water quality, when contaminant‐rich sediments accumulate in a reservoir. The tool can also help to design effective risk management and remediation strategies in these extreme environments. This article is protected by copyright. All rights reserved.
- Global analysis of approaches for deriving total water storage changes
from GRACE satellites
- Authors: Di Long; Laurent Longuevergne, Bridget R. Scanlon
Abstract: Increasing interest in use of GRACE satellites and a variety of new products to monitor changes in total water storage (TWS) underscores the need to assess the reliability of output from different products. The objective of this study was to assess skills and uncertainties of different approaches for processing GRACE data to restore signal losses caused by spatial filtering based on analysis of 1°×1° grid scale data and in 60 river basins globally. Results indicate that scaling factors from six LSMs, including GLDAS‐1 four models (Noah2.7, Mosaic, VIC, and CLM 2.0), CLM 4.0, and WGHM, are similar over most of humid, sub‐humid, and high‐latitude regions but can differ by up to 100% over arid and semi‐arid basins and areas with intensive irrigation. Temporal variability in scaling factors is generally minor at the basin scale except in arid and semi‐arid regions, but can be appreciable at the 1°×1° grid scale. Large differences in TWS anomalies from three processing approaches (scaling factor, additive, and multiplicative corrections) were found in arid and semi‐arid regions, areas with intensive irrigation, and relatively small basins (e.g., ≤ 200,000km2). Furthermore, TWS anomaly products from gridded data with CLM4.0 scaling factors and the additive correction approach more closely agree with WGHM output than the multiplicative correction approach. This comprehensive evaluation of GRACE processing approaches should provide valuable guidance on applicability of different processing approaches with different climate settings and varying levels of irrigation. This article is protected by copyright. All rights reserved.
- Spatiotemporal decomposition of solute dispersion in watersheds
- Authors: Joakim Riml; Anders Wörman
Abstract: Information about the effect of different dispersion mechanisms on the solute response in watersheds is crucial for understanding the temporal dynamics of many water quality problems. However, to quantify these processes from stream water quality time series may be difficult because the governing mechanisms responsible for the concentration fluctuations span a wide range of temporal and spatial scales. In an attempt to address the quantification problem, we propose a novel methodology that includes a spectral decomposition of the watershed solute response using a distributed solute transport model for the network of transport pathways in surface and sub‐surface water. Closed‐form solutions of the transport problem in both the Laplace and Fourier domains are used to derive formal expressions of: I) the central temporal moments of a solute pulse response, and II) the power spectral response of a solute concentration time series. By evaluating high‐frequency hydrochemical data from the Upper Hafren Watershed, Wales, we linked the watershed dispersion mechanisms to the damping of the concentration fluctuations in different frequency intervals reflecting various environments responsible for the damping. The evaluation of the frequency dependent model parameters indicate that the contribution of the different environments to the concentration fluctuations at the watershed effluent varies with period. For the longest periods (predominantly groundwater transport pathways) we found that the frequency typical transport time of chloride was 100 times longer and that sodium had a 2.5 times greater retardation factor compared with the shortest periods (predominantly shallow groundwater and surface water transport pathways). This article is protected by copyright. All rights reserved.
- Natural gas price uncertainty and the cost effectiveness of hedging
against low hydropower revenues caused by drought
- Authors: Jordan D. Kern; Gregory W. Characklis, Benjamin Foster
Abstract: Prolonged periods of low reservoir inflows (droughts) significantly reduce a hydropower producer's ability to generate both electricity and revenues. Given the capital intensive nature of the electric power industry, this can impact hydropower producers' ability to pay‐down outstanding debt, leading to credit rating downgrades, higher interests rates on new debt, and ultimately, greater infrastructure costs. One potential tool for reducing the financial exposure of hydropower producers to drought is hydrologic index insurance, in particular, contracts structured to pay‐out when stream flows drop below a specified level. An ongoing challenge in developing this type of insurance, however, is minimizing contracts' “basis risk”, that is, the degree to which contract payouts deviate in timing and/or amount from actual damages experienced by policyholders. In this paper, we show that consideration of year‐to‐year changes in the value of hydropower (i.e., the cost of replacing it with an alternative energy source during droughts) is critical to reducing contract basis risk. In particular, we find that volatility in the price of natural gas, a key driver of peak electricity prices, can significantly degrade the performance of index insurance unless contracts are designed to explicitly consider natural gas prices when determining payouts. Results show that a combined index whose value is derived from both seasonal streamflows and the spot price of natural gas yields contracts that exhibit both lower basis risk and greater effectiveness in terms of reducing financial exposure. This article is protected by copyright. All rights reserved.
- What do we mean by sensitivity analysis? The need for comprehensive
characterization of ‘Global’ sensitivity in Earth and
Environmental Systems Models
- Authors: Saman Razavi; Hoshin V. Gupta
Abstract: Sensitivity analysis is an essential paradigm in Earth and Environmental Systems modelling. However, the term ‘sensitivity' has a clear definition, based in partial derivatives, only when specified locally around a particular point (e.g., optimal solution) in the problem space. Accordingly, no unique definition exists for ‘global sensitivity' across the problem space, when considering one or more model responses to different factors such as model parameters or forcings.
A variety of approaches have been proposed for global sensitivity analysis, based on different philosophies and theories, and each of these formally characterizes a different ‘intuitive' understanding of sensitivity. These approaches focus on different properties of the model response at a fundamental level and may therefore lead to different (even conflicting) conclusions about the underlying sensitivities. Here, we revisit the theoretical basis for sensitivity analysis, summarize and critically evaluate existing approaches in the literature, and demonstrate their flaws and shortcomings through conceptual examples. We also demonstrate the difficulty involved in interpreting ‘global' interaction effects, which may undermine the value of exiting interpretive approaches. With this background, we identify several important properties of response surfaces that are associated with the understanding and interpretation of sensitivities in the context of Earth and Environmental System models. Finally, we highlight the need for a new, comprehensive framework for sensitivity analysis that effectively characterizes all of the important sensitivity‐related properties of model response surfaces. This article is protected by copyright. All rights reserved.
- Spatial and temporal characteristics of rainfall in Africa: Summary
statistics for temporal downscaling
- Authors: Armel T. Kaptué; Niall P. Hanan, Lara Prihodko, Jorge A. Ramirez
Abstract: An understanding of rainfall characteristics at multiple spatiotemporal scales is of great importance for hydrological, biogeochemical and land surface modeling studies. In the present study, patterns of rainfall are analyzed over the African continent based on 3‐hourly 0.25° Tropical Rainfall Measuring Mission (TRMM) estimates between 1998 and 2012 to produce monthly statistical summaries. The selected rain event properties are multi‐year means of precipitation total amount (mm), event frequency (number), rate (mm/hr) and duration (hr) calculated independently for each calendar month. Analysis of 3‐hourly and daily events in the 1998‐2012 period suggests that rainfall amount can be summarized using gamma probability density functions. Assuming stationarity, gamma probability density functions of the total depth of 3‐hourly and daily events are estimated and then used for temporal down‐scaling of monthly rainfall estimates (past or future). As a result, we generate 3‐hourly and daily rainfall estimates that are pixel‐wise statistically indistinguishable from the observations while preserving monthly totals. Example scripts are provided that can be used to access monthly statistics and implement down‐scaling using archival (or projected) monthly rainfall estimates. These statistics could also be utilized for the assessment of rainfall from atmospheric models. This article is protected by copyright. All rights reserved.
- Front spreading with nonlinear sorption for oscillating flow
- Authors: D.G. (Gijsbert) Cirkel; S.E.A.T.M. (Sjoerd) van der Zee, J.C.L. (Hans) Meeussen
Abstract: In this paper we consider dispersive and chromatographic mixing at an interface, under alternating flow conditions. In case of a nonreactive or linearly sorbing solute, mixing is in complete analogy with classical dispersion theory. For nonlinear exchange however, oscillating convective flow leads to an alternation of sharpening (Traveling Wave TW) and spreading (Rarefaction Wave RW). As the limiting TW form is not necessarily accomplished at the end of the TW half cycle, the oscillating fronts show gradual continuous spreading that converges to a zero‐convection nonlinear pure diffusion spreading, which is mathematically of quite different nature. This behavior is maintained in case the total (background) concentration differs at both sides of the initial exchange front. This article is protected by copyright. All rights reserved.
- Effects of lateral nitrate flux and instream processes on dissolved
inorganic nitrogen export in a forested catchment: A model sensitivity
- Authors: Laurence Lin; Jackson R. Webster, Taehee Hwang, Lawrence E. Band
Abstract: The importance of terrestrial and aquatic ecosystems in controlling nitrogen dynamics in streams is a key interest of ecologists studying dissolved inorganic nitrogen (DIN) export from watersheds. In this study, we coupled a stream model with a terrestrial ecohydrological model and conducted a global sensitivity analysis to evaluate the relative importance of both ecosystems to nitrogen export. We constructed two scenarios (“normal” and high nitrate loads) to explore conditions under which terrestrial (lateral nitrate flux) or aquatic ecosystems (instream nutrient processes) may be more important in controlling DIN export. In a forest catchment, although the forest ecosystem controls the nitrogen load to streams, sensitivity results suggested that most nitrogen output from the terrestrial ecosystem was taken up by instream microbial immobilization associated with benthic detritus and retained in detritus. Later the immobilized nitrogen was re‐mineralized as DIN. Therefore, the intra‐annual pattern of DIN concentration in the stream was low in fall and became high in spring. Not only was instream microbial immobilization saturated with the high nitrogen load scenario, but also the net effect of immobilization and mineralization on DIN export was minimized because nitrogen cycling between organic and inorganic forms was accelerated. Overall, our linked terrestrial‐aquatic model simulations demonstrated that stream process could significantly affect the amount and timing of watershed nitrogen export when nitrogen export from the terrestrial system is low. However, when nitrogen export from the terrestrial system is high, the effect of stream processes in minimal. This article is protected by copyright. All rights reserved.
- Resolution analysis of tomographic slug test head data:
Two‐dimensional radial case
- Authors: Daniel Paradis; Erwan Gloaguen, René Lefebvre, Bernard Giroux
Abstract: Hydraulic tomography inverse problems, which are solved to estimate aquifer hydraulic properties between wells, are known to be ill‐conditioned and a priori information is often added to regularize numerical inversion of head data. Because both head data and a priori information have effects on the inversed solution, assessing the meaningful information contained in head data alone is required to ensure comprehensive interpretation of inverse solutions, whether they are regularized or not. This study thus aims to assess the amount of information contained in tomographic slug tests head data to resolve heterogeneity in Kh, Kv/Kh and Ss. Therefore, a resolution analysis based on truncated singular value decomposition of the sensitivity matrix with a noise level representative of field measurements is applied using synthetic data reflecting a known littoral aquifer. As an approximation of the hydraulic behavior of a real aquifer system, synthetic tomographic experiments and associated sensitivity matrices are generated using a radial flow model accounting for wellbore storage to simulate slug tests in a plane encompassing a stressed well and an observation well. Although fine‐scale resolution of heterogeneities is limited by the diffusive nature of the groundwater flow equations, inversion of tomographic slug tests head data holds the potential to uniquely resolve coarse‐scale heterogeneity in Kh, Kv/Kh and Ss, as inscribed in the resolution matrix. This implies that tomographic head data can provide key information on aquifer heterogeneity and anisotropy, but that fine‐scale information must be supplied by a priori information to obtain finer details. This article is protected by copyright. All rights reserved.
- A new frequency domain analytical solution of a cascade of diffusive
channels for flood routing
- Authors: Luigi Cimorelli; Luca Cozzolino, Renata Della Morte, Domenico Pianese, Vijay P. Singh
Abstract: Simplified flood propagation models are often employed in practical applications for hydraulic and hydrologic analyses. In this paper, we present a new numerical method for the solution of the Linear Parabolic Approximation (LPA) of the De Saint Venant equations (DSVEs), accounting for the space variation of model parameters and the imposition of appropriate downstream boundary conditions. The new model is based on the analytical solution of a cascade of linear diffusive channels in the Laplace Transform domain. The time domain solutions are obtained using a Fourier series approximation of the Laplace Inversion formula. The new Inverse Laplace Transform Diffusive Flood Routing model (ILTDFR) can be used as a building block for the construction of real time flood forecasting models or in optimization models, because it is unconditionally stable and allows fast and fairly precise computation. This article is protected by copyright. All rights reserved.
- Evapotranspiration based on Equilibrated Relative Humidity (ETRHEQ):
Evaluation over the Continental United States
- Authors: Angela J. Rigden; Guido D. Salvucci
Abstract: A novel method of estimating evapotranspiration (ET), referred to as the ETRHEQ method, is further developed, validated, and applied across the Unites States of America from 1961‐2010. The ETRHEQ method estimates the surface conductance to water vapor transport, which is the key rate‐limiting parameter of typical ET models, by choosing the surface conductance that minimizes the vertical variance of the calculated relative humidity profile averaged over the day. The ETRHEQ method, which was previously tested at five AmeriFlux sites, is modified for use at common weather stations and further validated at twenty AmeriFlux sites that span a wide range of climates and limiting factors. Averaged across all sites, the daily latent heat flux RMSE is ∼26 W·m−2 (or 15%). The method is applied across the United States at 305 weather stations and spatially interpolated using ANUSPLIN software. Gridded annual mean ETRHEQ ET estimates are compared with four datasets, including water balance‐derived ET, machine learning ET estimates based on FLUXNET data, North American Land Data Assimilation System project phase 2 ET, and a benchmark product that integrates 14 global ET datasets, with RMSEs ranging from 8.7 cm·yr−1 to 12.5 cm·yr−1. The ETRHEQ method relies only on data measured at weather stations, an estimate of vegetation height derived from land cover maps, and an estimate of soil thermal inertia. These data requirements allow it to have greater spatial coverage than direct measurements, greater historical coverage than satellite methods, significantly less parameter specification than most land surface models, and no requirement for calibration. This article is protected by copyright. All rights reserved.
- Numerical simulation of the environmental impact of hydraulic fracturing
of tight/shale gas reservoirs on near‐surface groundwater:
Background, base cases, shallow reservoirs, short‐term gas, and
- Authors: Matthew T. Reagan; George J. Moridis, Noel D. Keen, Jeffrey N. Johnson
Abstract: Hydrocarbon production from unconventional resources and the use of reservoir stimulation techniques, such as hydraulic fracturing, has grown explosively over the last decade. However, concerns have arisen that reservoir stimulation creates significant environmental threats through the creation of permeable pathways connecting the stimulated reservoir with shallower fresh‐water aquifers, thus resulting in the contamination of potable groundwater by escaping hydrocarbons or other reservoir fluids. This study investigates, by numerical simulation, gas and water transport between a shallow tight‐gas reservoir and a shallower overlying fresh‐water aquifer following hydraulic fracturing operations, if such a connecting pathway has been created. We focus on two general failure scenarios: 1) communication between the reservoir and aquifer via a connecting fracture or fault and 2) communication via a deteriorated, preexisting nearby well. We conclude that the key factors driving short‐term transport of gas include high permeability for the connecting pathway and the overall volume of the connecting feature. Production from the reservoir is likely to mitigate release through reduction of available free gas and lowering of reservoir pressure, and not producing may increase the potential for release. We also find that hydrostatic tight‐gas reservoirs are unlikely to act as a continuing source of migrating gas, as gas contained within the newly formed hydraulic fracture is the primary source for potential contamination. Such incidents of gas escape are likely to be limited in duration and scope for hydrostatic reservoirs. Reliable field and laboratory data must be acquired to constrain the factors and determine the likelihood of these outcomes. This article is protected by copyright. All rights reserved.
- Toward hyperresolution land surface modeling: The effects of
fine‐scale topography and soil texture on CLM4.0 simulations over
the southwestern U.S
- Authors: R. S. Singh; J.T. Reager, N.L. Miller, J.S. Famiglietti
Abstract: Increasing computational efficiency and the need for improved accuracy are currently driving the development of “hyper‐resolution” land surface models that can be implemented at continental scales with resolutions of 1km or finer. Here, we report research incorporating fine‐scale grid resolutions into the NCAR Community Land Model (CLM v4.0) for simulations at 1‐km, 25‐km, and 100‐km resolution using 1‐km soil and topographic information. Multi‐year model runs were performed over the southwestern United States, including the entire state of California and the Colorado River basin. The results show changes in the total amount of CLM‐modeled water storage, and changes in the spatial and temporal distributions of water in snow and soil reservoirs, as well as changes in surface fluxes and the energy balance. To inform future model progress and continued development needs and weaknesses, we compare simulation outputs to station and gridded observations of model fields. Although the higher grid‐resolution model is not driven by high‐resolution forcing, grid resolution changes alone yield significant improvement (reduction in error) between model outputs and observations, where the RMSE decreases by more than 35%, 36%, 34% and 12% for soil moisture, terrestrial water storage anomaly, sensible heat and snow water equivalent, respectively. As an additional exercise, we performed a 100m‐resolution simulation over a spatial sub‐domain. Those results indicate that parameters such as drainage, runoff, and infiltration are significantly impacted when hillslope scales of ∼100 meters or finer are considered, and we show the ways in which limitations of the current model physics, including no lateral flow between grid cells, may affect model simulation accuracy. This article is protected by copyright. All rights reserved.
- A unified approach for process‐based hydrologic modeling: 1.
- Authors: Martyn P. Clark; Bart Nijssen, Jessica D. Lundquist, Dmitri Kavetski, David E. Rupp, Ross A. Woods, Jim E. Freer, Ethan D. Gutmann, Andrew W. Wood, Levi D. Brekke, Jeffrey R. Arnold, David J. Gochis, Roy M. Rasmussen
Abstract: This work advances a unified approach to process‐based hydrologic modeling to enable controlled and systematic evaluation of multiple model representations (hypotheses) of hydrologic processes and scaling behavior. Our approach, which we term the Structure for Unifying Multiple Modeling Alternatives (SUMMA), formulates a general set of conservation equations, providing the flexibility to experiment with different spatial representations, different flux parameterizations, different model parameter values, and different time stepping schemes. In this paper we introduce the general approach used in SUMMA, detailing the spatial organization and model simplifications, and how different representations of multiple physical processes can be combined within a single modeling framework. We discuss how SUMMA can be used to systematically pursue the method of multiple working hypotheses in hydrology. In particular, we discuss how SUMMA can help tackle major hydrologic modeling challenges, including defining the appropriate complexity of a model, selecting among competing flux parameterizations, representing spatial variability across a hierarchy of scales, identifying potential improvements in computational efficiency and numerical accuracy as part of the numerical solver, and improving understanding of the various sources of model uncertainty. This article is protected by copyright. All rights reserved.
- A unified approach for process‐based hydrologic modeling: 2. Model
implementation and case studies
- Authors: Martyn P. Clark; Bart Nijssen, Jessica D. Lundquist, Dmitri Kavetski, David E. Rupp, Ross A. Woods, Jim E. Freer, Ethan D. Gutmann, Andrew W. Wood, David J. Gochis, Roy M. Rasmussen, David G. Tarboton, Vinod Mahat, Gerald N. Flerchinger, Danny G. Marks
Abstract: This work advances a unified approach to process‐based hydrologic modeling, which we term the Structure for Unifying Multiple Modeling Alternatives (SUMMA). The modeling framework, introduced in the companion paper, uses a general set of conservation equations with flexibility in the choice of process parameterizations (closure relationships) and spatial architecture. This second paper specifies the model equations and their spatial approximations, describes the hydrologic and biophysical process parameterizations currently supported within the framework, and illustrates how the framework can be used in conjunction with multivariate observations to identify model improvements and future research and data needs. The case studies illustrate the use of SUMMA to select among competing modeling approaches based on both observed data and theoretical considerations. Specific examples of preferable modeling approaches include the use of physiological methods to estimate stomatal resistance, careful specification of the shape of the within‐ and below‐canopy wind profile, explicitly accounting for dust concentrations within the snowpack, and explicitly representing distributed lateral flow processes. Results also demonstrate that changes in parameter values can make as much or more difference to the model predictions than changes in the process representation. This emphasizes that improvements in model fidelity require a sagacious choice of both process parameterizations and model parameters. In conclusion, we envisage that SUMMA can facilitate ongoing model development efforts, the diagnosis and correction of model structural errors, and improved characterization of model uncertainty. This article is protected by copyright. All rights reserved.
- Inverse sequential simulation: A new approach for the characterization of
hydraulic conductivities demonstrated on a non‐Gaussian field
- Authors: Teng Xu; J. Jaime Gómez‐Hernández
Abstract: Inverse sequential simulation (iSS) is a new inverse modeling approach for the characterization of hydraulic conductivity fields based on sequential simulation. It is described and demonstrated in a synthetic aquifer with non‐Gaussian spatial features, and compared against the normal‐score ensemble Kalman filter (NS‐EnKF). The new approach uses the sequential simulation paradigm to generate realizations borrowing from the ensemble Kalman filter the idea of using the experimental non‐stationary cross‐covariance between conductivities and piezometric heads computed on an ensemble of realizations. The resulting approach is fully capable of retrieving the non‐Gaussian patterns of the reference field after conditioning on the piezometric heads with results comparable of those obtained by the NS‐EnKF. This article is protected by copyright. All rights reserved.
- A hydrometeorological approach for probabilistic simulation of monthly
soil moisture under bare and crop land conditions
- Authors: Rajib Maity; Sarit Kumar Das
Abstract: This study focuses on the probabilistic estimation of monthly soil moisture variation by considering a) the influence of hydrometeorological forcing to model the temporal variation, and b) the information of Hydrological Soil Groups (HSGs) and Agro‐Climatic Zones (ACZs) to capture the spatial variation. The innovative contributions of this study are – (i) development of a Combined Hydro‐Meteorological (CHM) index to extract the information of different influencing hydrometeorological variables, (ii) consideration of soil‐hydrologic characteristics (through HSGs) and climate regime based zoning for agriculture (through ACZs), and (iii) quantification of uncertainty range of the estimated soil moisture. Usage of Supervised Principal Component Analysis (SPCA) in the development of the CHM index helps to eliminate the “curse of dimensionality”, typically arises in the multivariate analysis. The usage of SPCA also ensures the maximum possible association between the developed CHM index and soil moisture variation. The association between these variables is modelled through their joint distribution which is obtained by using the theory of copula. The proposed approach is also spatially transferable, since the information on HSGs and ACZs are considered. The ‘leave‐one‐out' cross validation (LOO‐CV) approach is adopted for stations belong to a particular HSG to examine the spatial transferability. The simulated soil moisture values are also compared with a few existing soil moisture data sets, derived from different Land Surface Models (LSMs) or retrieved from different satellite based missions. The potential of the proposed approach is found to be promising and even applicable to crop land also, though with a lesser degree of efficiency as compared to bare land conditions. This article is protected by copyright. All rights reserved.
- Impact of viscous fingering and permeability heterogeneity on fluid mixing
in porous media
- Authors: Christos Nicolaides; Birendra Jha, Luis Cueto‐Felgueroso, Ruben Juanes
Abstract: Fluid mixing plays a fundamental role in many natural and engineered processes, including groundwater flows in porous media, enhanced oil recovery, and microfluidic lab‐on‐a‐chip systems. Recent developments have explored the effect of viscosity contrast on mixing, suggesting that the unstable displacement of fluids with different viscosities, or viscous fingering, provides a powerful mechanism to increase fluid–fluid interfacial area and enhance mixing. However, existing studies have not incorporated the effect of medium heterogeneity on the mixing rate. Here, we characterize the evolution of mixing between two fluids of different viscosity in heterogeneous porous media. We focus on a practical scenario of divergent–convergent flow in a quarter five‐spot geometry prototypical of well‐driven groundwater flows. We study by means of numerical simulations the impact of permeability heterogeneity and viscosity contrast on the breakthrough curves and mixing efficiency, and we rationalize the nontrivial mixing behavior that emerges from the competition between the creation of fluid‐fluid interfacial area and channeling. This article is protected by copyright. All rights reserved.
- The influence of water table depth and the free atmospheric state on
convective rainfall predisposition
- Authors: Sara Bonetti; Gabriele Manoli, Jean‐Christophe Domec, Mario Putti, Marco Marani, Gabriel G. Katul
Abstract: A mechanistic model for the soil‐plant system is coupled to a conventional slab representation of the atmospheric boundary layer (ABL) to explore the role of groundwater table (WT) variations and free atmospheric (FA) states on convective rainfall predisposition (CRP) at a Loblolly pine plantation site situated in the lower coastal plain of North Carolina. Predisposition is quantified using the crossing between modeled lifting condensation level (LCL) and convectively grown ABL depth. The LCL‐ABL depth crossing is necessary for air saturation but not sufficient for cloud formation and subsequent convective rainfall occurrence. However, such crossing forms the main template for which all subsequent dynamical processes regulating the formation (or suppression) of convective rainfall operate on. If the feedback between surface fluxes and FA conditions is neglected, a reduction in latent heat flux associated with reduced WT levels is shown to enhance the ABL‐LCL crossing probability. When the soil‐plant system is fully coupled with ABL dynamics thereby allowing feedback with ABL temperature and humidity, FA states remain the leading control on CRP. However, vegetation water stress plays a role in controlling ABL‐LCL crossing when the humidity supply by the FA is within an intermediate range of values. When FA humidity supply is low, cloud formation is suppressed independent of surface latent heat flux. Similarly, when FA moisture supply is high, cloud formation can occur independent of surface latent heat flux. In an intermediate regime of FA moisture supply, the surface latent heat flux controlled by soil water availability can supplement (or suppress) the necessary water vapor leading to reduced LCL and subsequent ABL‐LCL crossing. It is shown that this intermediate state corresponds to FA values around the mode in observed humidity lapse rates γw (between ‐2.5 × 10– 6 and ‐1.5 × 10– 6 kg kg– 1m– 1), suggesting that vegetation water uptake may be controlling CRP at the study site. This article is protected by copyright. All rights reserved.
- High‐resolution experiments on chemical oxidation of DNAPL in
- Authors: Masoud Arshadi; Harihar Rajaram, Russ Detwiler, Trevor Jones
Abstract: Chemical oxidation of dense nonaqueous‐phase liquids (DNAPLs) by permanganate has emerged as an effective remediation strategy in fractured rock. We present high‐resolution experimental investigations in transparent analog variable‐aperture fractures to improve understanding of chemical oxidation of residual entrapped trichloroethylene (TCE) in fractures. Four experiments were performed with different permanganate concentrations, flow rates, and initial TCE phase geometry. The initial aperture field and evolving entrapped‐phase geometry were quantified for each experiment. The integrated mass transfer rate from the TCE phase for all experiments exhibited three time regimes: an early‐time regime with slower mass transfer rates limited by low specific interfacial area; an intermediate‐time regime with higher mass transfer rates resulting from break‐up of large TCE blobs, which greatly increases specific interfacial area; and a late‐time regime with low mass transfer rates due to the deposition of MnO2 precipitates. In two experiments, mass balance analyses suggested that TCE mass removal rates exceeded the maximum upper bound mass removal rates derived by assuming that oxidation and dissolution are the only mechanisms for TCE mass removal. We propose incomplete oxidation by permanganate and TCE solubility enhancement by intermediate reaction products as potential mechanisms to explain this behavior. We also speculate that some intermediate reaction products with surfactant‐like properties may play a role in lowering the TCE‐water interfacial tension, thus causing breakup of large TCE blobs. Our quantitative experimental measurements will be useful in the context of developing accurate computational models for chemical oxidation of TCE in fractures. This article is protected by copyright. All rights reserved.
- Issue Information
- PubDate: 2015-03-13T10:10:22.848135-05:
- The influence of multiyear drought on the annual rainfall‐runoff
relationship: An Australian perspective
- Authors: Margarita Saft; Andrew W. Western, Lu Zhang, Murray C. Peel, Nick J. Potter
Abstract: Most current long‐term (decadal and longer) hydrological predictions implicitly assume that hydrological processes are stationary even under changing climate. However, in practice we suspect that changing climatic conditions may affect runoff generation processes and cause changes in the rainfall‐runoff relationship. In this article we investigate whether temporary but prolonged (i.e. of the order of a decade) shifts in rainfall result in changes in rainfall‐runoff relationships at the catchment scale. Annual rainfall and runoff records from south‐eastern Australia are used to examine whether inter‐decadal climate variability induces changes in hydrological behaviour. We test statistically whether annual rainfall‐runoff relationships are significantly different during extended dry periods, compared with the historical norm. The results demonstrate that protracted drought led to a significant shift in the rainfall‐runoff relationship in ∼44% of the catchment‐dry periods studied. The shift led to less annual runoff for a given annual rainfall, compared with the historical relationship. We explore linkages between cases where statistically significant changes occurred and potential explanatory factors, including catchment properties and characteristics of the dry period (e.g. length, precipitation anomalies). We find that long‐term drought is more likely to affect transformation of rainfall to runoff in drier, flatter, and less forested catchments. Understanding changes in the rainfall‐runoff relationship is important for accurate streamflow projections and to help develop adaptation strategies to deal with multiyear droughts. This article is protected by copyright. All rights reserved.
- Estimation of soil salinity in a drip irrigation system by using joint
inversion of multicoil electromagnetic induction measurements
- Authors: Khan Zaib Jadoon; Davood Moghadas, Aurangzeb Jadoon, Thomas M. Missimer, Samir K. Al‐Mashharawi, Matthew F. McCabe
Abstract: Low frequency electromagnetic induction (EMI) is becoming a useful tool for soil characterization due to its fast measurement capability and sensitivity to soil moisture and salinity. In this research, a new EMI system (the CMD mini‐Explorer) is used for sub‐surface characterization of soil salinity in a drip irrigation system via a joint inversion approach of multi‐configuration EMI measurements. EMI measurements were conducted across a farm where Acacia trees are irrigated with brackish water. In‐situ measurements of vertical bulk electrical conductivity (σb) were recorded in different pits along one of the transects to calibrate the EMI measurements and to compare with the modeled electrical conductivity (σ) obtained by the joint inversion of multi‐configuration EMI measurements. Estimates of σ were then converted into the universal standard of soil salinity measurement (i.e. electrical conductivity of a saturated soil paste extract – ECe). Soil apparent electrical conductivity (ECa) was repeatedly measured with the CMD mini‐Explorer to investigate the temperature stability of the new system at a fixed location, where the ambient air temperature increased from 26 °C to 46 °C. Results indicate that the new EMI system is very stable in high temperature environments, especially above 40 °C, where most other approaches give unstable measurements. In addition, the distribution pattern of soil salinity is well estimated quantitatively by the joint inversion of multi‐component EMI measurements. The approach of joint inversion of EMI measurements allows for the quantitative mapping of the soil salinity distribution pattern and can be utilized for the management of soil salinity. This article is protected by copyright. All rights reserved.
- Estimating bankfull discharge and depth in ungauged estuaries
- Authors: Jacqueline Isabella Anak Gisen; Hubert H.G. Savenije
Abstract: It is difficult to measure river discharge accurately in an estuary, and particularly in the region where the tidal flow dominates over the river discharge. River discharge is important for the morphology and hydrodynamics of estuaries as it influences the salt intrusion process, tidal dynamics, fresh water supply (water resources management) and the occurrence of floods. Here, we try to derive river regime characteristics from the seaward end: the estuary. It is found that there are empirical relationships that link the geometry of an estuary to its river regime, which can be used to estimate river discharge characteristics with the least of data available. The aims of this study are: 1) to derive empirical relations between geometrical characteristics of estuaries and the bankfull discharge; 2) to explore a physical explanation for this relation; and 3) to estimate the bankfull discharge in estuaries. The physical connection between an estuary and its river regime is found by combining estuary shape analysis, tidal dynamic analysis and Lacey's hydraulic geometry theory. The relationships found between the estuary depth, width and bankfull river discharge have been tested in 23 estuaries around the world (including 7 recently surveyed estuaries). From the analysis, it shows that the depth of an estuary is a function of the bankfull flood discharge to the power of 1/3, which is in agreement with Lacey's formula. This finding not only provides a method to estimate estuary depth, it also allows estimating flood discharge characteristics from readily available estuary shape indicators. This article is protected by copyright. All rights reserved.
- Optimizing water resources management in large river basins with
integrated surface water‐groundwater modeling: A
- Authors: Bin Wu; Yi Zheng, Xin Wu, Yong Tian, Feng Han, Jie Liu, Chunmiao Zheng
Abstract: Integrated surface water‐groundwater modeling can provide a comprehensive and coherent understanding on basin‐scale water cycle, but its high computational cost has impeded its application in real‐world management. This study developed a new surrogate‐based approach, SOIM (Surrogate‐based Optimization for Integrated surface water‐groundwater Modeling), to incorporate the integrated modeling into water management optimization. Its applicability and advantages were evaluated and validated through an optimization research on the conjunctive use of surface water (SW) and groundwater (GW) for irrigation in a semi‐arid region in northwest China. GSFLOW, an integrated SW‐GW model developed by USGS, was employed. The study results show that, due to the strong and complicated SW‐GW interactions, basin‐scale water saving could be achieved by spatially optimizing the ratios of groundwater use in different irrigation districts. The water‐saving potential essentially stems from the reduction of non‐beneficial evapotranspiration from the aqueduct system and shallow groundwater, and its magnitude largely depends on both water management schemes and hydrological conditions. Important implications for water resources management in general include: first, environmental flow regulation needs to take into account inter‐annual variation of hydrological conditions, as well as spatial complexity of SW‐GW interactions; and second, to resolve water use conflicts between upper stream and lower stream, a system approach is highly desired to reflect ecological, economic and social concerns in water management decisions. Overall, this study highlights that surrogate‐based approaches like SOIM represent a promising solution to filling the gap between complex environmental modeling and real‐world management decision‐making. This article is protected by copyright. All rights reserved.
- Modification of the Local Cubic Law of fracture flow for weak inertia,
tortuosity, and roughness
- Authors: Lichun Wang; M. Bayani Cardenas, Donald T. Slottke, Richard A. Ketcham, John M. Sharp
Abstract: The classical Local Cubic Law (LCL) generally overestimates flow through real fractures. We thus developed and tested a modified LCL (MLCL) which takes into account local tortuosity and roughness, and works across a low range of local Reynolds Numbers. The MLCL is based on (1) modifying the aperture field by orienting it with the flow direction, and (2) correcting for local roughness changes associated with local flow expansion/contraction. In order to test the MLCL, we compared it with direct numerical simulations with the Navier‐Stokes equations using real and synthetic three‐dimensional rough‐walled fractures, previous corrected forms of the LCL, and experimental flow tests. The MLCL performed well and the effective errors (δ) in volumetric flow rate range from ‐3.4% to 13.4% with an arithmetic mean of δ (< δ >) equal to 3.7%. The MLCL is more accurate than previous modifications of the LCL. We also investigated the error associated with applying the Cubic Law (CL) while utilizing modified aperture field. The δ from the CL ranges from ‐14.2% to 11.2%, with a slightly higher < δ >=6.1% than the MLCL. The CL with the modified aperture field considering local tortuosity and roughness may also be sufficient for predicting the hydraulic properties of rough fractures. This article is protected by copyright. All rights reserved.
- The use of discharge perturbations to understand in situ vegetation
resistance in wetlands
- Authors: A. M. Wasantha Lal; M. Zaki Moustafa, Walter M. Wilcox
Abstract: The ability to better quantify resistance to water flow exerted by vegetation is receiving increased attention due to ongoing worldwide efforts to restore natural vegetation communities in the wetlands and use of vegetation for environmental benefits in streams and wetlands. In south Florida, vegetation resistance affects discharge through shallow wetlands of the Everglades and projects under way in the system to restore remaining natural systems. A more detailed knowledge of the flow dynamics in these wetlands is required to improve modeling of these systems that supports restoration and management efforts.
The goal of this investigation is to understand the flow dynamics and the vegetation resistance within a 3km by 7km area in the Everglades referred to as STA‐3/4 Cell 3A. Methods are developed to demonstrate the use of analytical solutions of partial differential equations (PDEs) and inverse methods to obtain bulk and spatially‐varying resistance parameters. To achieve this goal, a field test was conducted using sinusoidal discharge disturbances capable of creating water water waves in the stormwater treatment area (STAs). The discharges, wave speeds and the wave attenuation rates from the test are used to develop graphical and empirical functions expressing discharge in terms of water depth and energy slope. The empirical functions developed are power‐law type, and different functions are developed for different depths.
The results show that the Manning's equation is not applicable for wetlands with thick emergent vegetation, as well as the difficulty of applying a single power‐law type expression for vegetation resistance over a wide range of depths and energy slopes without errors. This is partly due to the existence of multiple flow regimes and different power exponents over depth and energy slopes in these regimes. Results show that the flow regime at low depths is similar to porous media flow, and the flow regime at higher depths is more turbulent. This article is protected by copyright. All rights reserved.
- Global sensitivity analysis of the radiative transfer model
- Authors: Maheshwari Neelam; Binayak P. Mohanty
Abstract: With the recently launched Soil Moisture Active Passive (SMAP) mission, it is very important to have a complete understanding of the radiative transfer model for better soil moisture retrievals and to direct future research and field campaigns in areas of necessity. Because natural systems show great variability and complexity with respect to soil, land cover, topography, precipitation, there exist large uncertainties and heterogeneities in model input factors. In this paper, we explore the possibility of using global sensitivity analysis (GSA) technique to study the influence of heterogeneity and uncertainties in model inputs on zero order radiative transfer (ZRT) model and to quantify interactions between parameters. GSA technique is based on decomposition of variance and can handle non‐linear and non‐monotonic functions. We direct our analyses towards growing agricultural fields of corn and soybean in two different regions, Iowa, U.S.A (SMEX02) and Winnipeg, Canada (SMAPVEX12). We noticed that, there exists a spatio‐temporal variation in parameter interactions under different soil moisture and vegetation conditions. Radiative Transfer Model (RTM) behaves more non‐linearly in SMEX02 and linearly in SMAPVEX12, with average parameter interactions of 14% in SMEX02 and 5% in SMAPVEX12. Also parameter interactions increased with vegetation water content (VWC) and roughness conditions. Interestingly, soil moisture shows an exponentially decreasing sensitivity function whereas parameters such as root mean square height (RMS height) and vegetation water content show increasing sensitivity with 0.05 v/v increase in soil moisture range. Overall, considering the SMAPVEX12 fields to be water rich environment (due to higher observed SM) and SMEX02 fields to be energy rich environment (due to lower SM and wide ranges of TSURF), our results indicate that first order as well as interactions between the parameters change with water and energy rich environments. This article is protected by copyright. All rights reserved.
- Soil hydrology: Recent methodological advances, challenges, and
- Authors: H. Vereecken; J.A. Huisman, H.J. Hendricks Franssen, N. Brüggemann, H.R. Bogena, S. Kollet, M. Javaux, J van der Kruk, J. Vanderborght
Abstract: Technological and methodological progress is essential to improve our understanding of fundamental processes in natural and engineering sciences. In this paper, we will address the potential of new technological and methodological advancements in soil hydrology to move forward our understanding of soil water related processes across a broad range of scales. We will focus on advancements made in quantifying root water uptake processes, subsurface lateral flow, and deep drainage at the field and catchment scale, respectively. We will elaborate on the value of establishing a science‐driven network of hydrological observatories to test fundamental hypotheses, to study organizational principles of soil hydrologic processes at catchment scale, and to provide data for the development and validation of models. Finally, we discuss recent developments in data assimilation methods, which provide new opportunities to better integrate observations and models and to improve predictions of the short‐term evolution of hydrological processes. This article is protected by copyright. All rights reserved.
- Contextual and socio‐psychological factors in predicting habitual
cleaning of water storage containers in rural Benin
- Authors: Andrea Stocker; Hans‐Joachim Mosler
Abstract: Recontamination of drinking water occurring between water collection at the source and the point of consumption is a current problem in developing countries. The household drinking water storage container is one source of contamination and should therefore be cleaned regularly. First, the present study investigated contextual factors that stimulate or inhibit the development of habitual cleaning of drinking water storage containers with soap and water. Second, based on the Risk, Attitudes, Norms, Abilities and Self‐regulation (RANAS) Model of behavior, the study aimed to determine which socio‐psychological factors should be influenced by an intervention to promote habitual cleaning. In a cross‐sectional study, 905 households in rural Benin were interviewed by structured face‐to‐face interviews. A forced‐entry regression analysis was used to determine potential contextual factors related to habitual cleaning. Subsequently, a hierarchical regression was conducted with the only relevant contextual factor entered in the first step (R2 = 6.7%) and the socio‐psychological factors added in the second step (R2 = 62.5%). Results showed that households using a clay container for drinking water storage had a significantly weaker habit of cleaning their water storage containers with soap and water than did households using other types of containers (β = ‐.10). The most important socio‐psychological predictors of habitual cleaning were commitment (β = .35), forgetting (β = ‐.22), and self‐efficacy (β = .14). The combined investigation of contextual and socio‐psychological factors proved beneficial in terms of developing intervention strategies. Possible interventions based on these findings are recommended. This article is protected by copyright. All rights reserved.
- Occurrence of seawater intrusion overshoot
- Authors: Leanne K. Morgan; Mark Bakker, Adrian D. Werner
Abstract: A number of numerical modeling studies of transient sea‐level rise (SLR) and seawater intrusion (SI) in flux‐controlled aquifer systems have reported an overshoot phenomenon, whereby the freshwater‐saltwater interface temporarily extends further inland than the eventual steady‐state position. Recently, physical sand tank modeling has shown overshoot to be a physical process. In this paper, we have carried out numerical modeling of SLR‐SI to demonstrate that overshoot can occur at the field‐scale within unconfined aquifers. This result is contrary to previous conclusions drawn from a restricted number of cases. In addition, we show that SI overshoot is plausible under scenarios of gradual sea‐level rise that are consistent with conditions predicted by the Intergovernmental Panel for Climate Change. Overshoot was found to be largest in flux‐controlled unconfined aquifers characterised by low freshwater flux, high specific yield, and large inland extent. These conditions result in longer timeframes for the aquifer to reach new steady‐state conditions following SLR, and the extended period prior to re‐equilibration of the groundwater flow field produces more extensive overshoot. This article is protected by copyright. All rights reserved.
- Lowland fluvial phosphorus altered by dams
- Authors: Jianjun Zhou; Man Zhang, Binliang Lin, Pingyu Lu
Abstract: Dams affect ecosystems, but their physical link to the variations in fluvial fluxes and downstream ecological consequences are inadequately understood. After estimating the current effects of the Three Gorges project and other reservoirs upstream on the Yangtze River on the fluvial phosphorus (P) in the middle and lower Yangtze River [Zhou et al 2013], we further investigated the long‐term effects of dams on the fluvial regimes of P and P enriched sediment (PES). Simultaneously measured P distributions with sediment size (PDSS) from the Three Gorges Reservoir (TGR) proved that the areal density of particulate P (PP) bound on graded sediment can be measured using the surface area concentration of the total sediment. A PDSS relationship is obtained and the selective transport and long‐term sedimentation of P are simulated using a non‐uniform suspended sediment model, which incorporates the PDSS formula. The computations revealed that a reservoir would significantly lower the downstream availability of P in the dry season and promote high pulses of P in summer when the reservoir is flushed as sedimentation accumulates. As a result, the P buffering and replenishing mechanism in the pristine ecosystem from upstream supplies and local re‐suspension are permanently eliminated when a regulating reservoir is built upstream. This change is irreversible if reservoir regulation continues. Changes could potentially aggravate the existing P‐limitation, decrease the water's ability to adjust nutrient/pollutant fluctuations, accumulate a greater surplus of carbon and nitrogen and even exacerbate blooms in favorable conditions. This article is protected by copyright. All rights reserved.
- Blended near‐optimal alternative generation, visualization, and
interaction for water resources decision making
- Authors: David E. Rosenberg
Abstract: State‐of‐the‐art systems analysis techniques focus on efficiently finding optimal solutions. Yet an optimal solution is optimal only for the modelled issues and managers often seek near‐optimal alternatives that address un‐modelled objectives, preferences, limits, uncertainties, and other issues. Early on, Modelling to Generate Alternatives (MGA) formalized near‐optimal as performance within a tolerable deviation from the optimal objective function value and identified a few maximally‐different alternatives that addressed select un‐modelled issues. This paper presents new stratified, Monte Carlo Markov Chain sampling and parallel coordinate plotting tools that generate and communicate the structure and extent of the near‐optimal region to an optimization problem. Interactive plot controls allow users to explore region features of most interest. Controls also streamline the process to elicit un‐modelled issues and update the model formulation in response to elicited issues. Use for an example, single‐objective, linear water quality management problem at Echo Reservoir, Utah identifies numerous and flexible practices to reduce the phosphorus load to the reservoir and maintain close‐to‐optimal performance. Flexibility is upheld by further interactive alternative generation, transforming the formulation into a multi‐objective problem, and relaxing the tolerance parameter to expand the near‐optimal region. Compared to MGA, the new blended tools generate more numerous alternatives faster, more fully show the near‐optimal region, and help elicit a larger set of un‐modelled issues. This article is protected by copyright. All rights reserved.
- A direct simulation demonstrating the role of spacial heterogeneity in
determining anomalous diffusive transport
- Authors: Vaughan R. Voller
Abstract: Typically the spreading length scale in a diffusion process increases in time as l∼tn,n=12. In the presence of spatial heterogeneities however, so called anomalous diffusion can occur, here the time exponent n≠
12. The objective of this paper is to present a numerical simulation that directly demonstrates the link between spacial heterogeneity and anomalous diffusion. The simulation is of the infiltration of a fluid into a horizontal unit area Hele‐Shaw cell in which the permeability at specified locations, laid out as a Sierpinski fractal carpet pattern, can differ from the nominal value K = 1. When there is no permeability contrast, the fluid infiltration has a diffusion‐like behavior, i.e., the filled plan‐form area increases in time as F=Atn,n=
12. When there is a permeability contrast, however, although the evolution of infiltration still follows a power‐law, anomalous behavior is observed; sub‐diffusive (n<
12) when K
12) when K > 1. These anomalous behaviors persist, even when the permeability contrast is only imposed over the largest sized fractal pattern element. But, if the pattern over which the permeability contrast is imposed has a sub‐domain length scale (obtained by filling in the largest sized element in the Sierpinski carpet with the smaller sized elements), normal diffusion n=
12 behavior is recovered. In the special case that the imposed permeability in the pattern elements are obstacles, K≪1, an approximate model, directly relating the coefficient and exponent in the infiltration power‐law to the porosity and fractal dimension of the carpet pattern, is derived and validated. This article is protected by copyright. All rights reserved.
- On the formation of multiple local peaks in breakthrough curves
- Authors: Erica R. Siirila‐Woodburn; Xavier Sanchez‐Vila, Daniel Fernàndez‐Garcia
Abstract: The analysis of breakthrough curves (BTCs) is of interest in hydrogeology as a way to parameterize and explain processes related to anomalous transport. Classical BTCs assume the presence of a single peak in the curve, where the location and size of the peak and the slope of the receding limb has been of particular interest. As more information is incorporated into BTCs (for example, with high frequency data collection, supercomputing efforts), it is likely that classical definitions of BTC shapes will no longer be adequate descriptors for contaminant transport problems. We contend that individual BTCs may display multiple local peaks depending on the hydrogeologic conditions and the solute travel distance. In such cases classical definitions should be reconsidered. In this work, the presence of local peaks in BTCs is quantified from high‐resolution numerical simulations in synthetic fields with a particle tracking technique and a kernel density estimator to avoid either overly jagged or smoothed curves that could mask the results. Individual BTCs from three‐dimensional heterogeneous hydraulic conductivity fields with varying combinations of statistical anisotropy, heterogeneity models, and local dispersivity are assessed as a function of travel distance. The number of local peaks, their corresponding slopes, and a transport connectivity index, are shown to strongly depend on statistical anisotropy and travel distance. Results show that the choice of heterogeneity model also affects the frequency of local peaks, but the slope is less sensitive to model selection. We also discuss how solute shearing and re‐routing can be determined from local peak quantification. This article is protected by copyright. All rights reserved.
- An empirical vegetation correction for soil water content quantification
using cosmic ray probes
- Authors: R. Baatz; H.R. Bogena, H.‐J. Hendricks Franssen, J.A. Huisman, C. Montzka, H. Vereecken
Abstract: Cosmic‐ray probes are an emerging technology to continuously monitor soil water content at a scale significant to land surface processes. However, the application of this method is hampered by its susceptibility to the presence of aboveground biomass. Here, we present a simple empirical framework to account for moderation of fast neutrons by aboveground biomass in the calibration. The method extends the N0‐calibration function and was developed using an extensive data set from a network of ten cosmic‐ray probes located in the Rur catchment, Germany. The results suggest a 0.9% reduction in fast neutron intensity per 1kg of dry aboveground biomass per m2 or per 2kg of biomass water equivalent per m2. We successfully tested the novel vegetation correction using temporary cosmic‐ray probe measurements along a strong gradient in biomass due to deforestation, and using the COSMIC, and the hmf‐method as independent soil water content retrieval algorithms. The extended N0‐calibration function was able to explain 95% of the overall variability in fast neutron intensity. This article is protected by copyright. All rights reserved.
- Estimating hydraulic conductivity of fractured rocks from
high‐pressure packer tests with an Izbash's law‐based
- Authors: Yi‐Feng Chen; Shao‐Hua Hu, Ran Hu, Chuang‐Bing Zhou
Abstract: High pressure packer test (HPPT) is an enhanced constant head packer test for characterizing the permeability of fractured rocks under high‐pressure groundwater flow conditions. The interpretation of the HPPT data, however, remains difficult due to the transition of flow conditions in the conducting structures and the hydraulic fracturing‐induced permeability enhancement in the tested rocks. In this study, a number of HPPTs were performed in the sedimentary and intrusive rocks located at 450m depth in central Hainan Island. The obtained Q−P curves were divided into a laminar flow phase (I), a non‐Darcy flow phase (II) and a hydraulic fracturing phase (III). The critical Reynolds number for the deviation of flow from linearity into phase II was 25 − 66. The flow of phase III occurred in sparsely to moderately fractured rocks, and was absent at the test intervals of perfect or poor intactness. The threshold fluid pressure between phases II and III was correlated with RQD and the confining stress. An Izbash's law‐based analytical model was employed to calculate the hydraulic conductivity of the tested rocks in different flow conditions. It was demonstrated that the estimated hydraulic conductivity values in phases I and II are basically the same and are weakly dependent on the injection fluid pressure, but it becomes strongly pressure‐dependent as a result of hydraulic fracturing in phase III. The hydraulic conductivity at different test intervals of a borehole is remarkably enhanced at highly fractured zone or contact zone, but within a rock unit of weak heterogeneity, it decreases with the increase of depth. This article is protected by copyright. All rights reserved.
- A hybrid pore‐scale and continuum‐scale model for solute
diffusion, reaction and biofilm development in porous media
- Authors: Youneng Tang; Albert J. Valocchi, Charles J. Werth
Abstract: It is a challenge to up‐scale solute transport in porous media for multi‐species bio‐kinetic reactions because of incomplete mixing within the elementary volume and because biofilm growth can change porosity and affect pore‐scale flow and diffusion. To address this challenge, we present a hybrid model that couples pore‐scale sub‐domains to continuum‐scale sub‐domains. While the pore‐scale sub‐domains involving significant biofilm growth and reaction are simulated using pore‐scale equations, the other subdomains are simulated using continuum‐scale equations to save computational time. The pore‐scale and continuum‐scale subdomains are coupled using a mortar method to ensure continuity of solute concentration and flux at the interfaces. We present results for a simplified two‐dimensional system, neglect advection, and use dual Monod kinetics for solute utilization and biofilm growth. The results based on the hybrid model are consistent with the results based on a pore‐scale model for three test cases that cover a wide range of Damköhler (Da = reaction rate/diffusion rate) numbers for both homogeneous (spatially periodic) and heterogeneous pore structures. We compare results from the hybrid method with an up‐scaled continuum model and show that the latter is valid only for cases of small Damköhler numbers, consistent with other results reported in the literature. This article is protected by copyright. All rights reserved.
- Influence of Surfactants on Water Flow and Solute Transport
- Authors: Ahmet Karagunduz; Michael H. Young, Kurt D. Pennell
Abstract: Surfactants can reduce soil water retention by changing the surface tension of water and the contact angle between the liquid and solid phases. As a result, water flow and solute transport in unsaturated soil may be altered in the presence of surfactants. In this study, the effects of a representative non‐ionic surfactant, Triton X‐100, on coupled water flow and non‐reactive solute transport during unsaturated flow conditions were evaluated. Batch reactor experiments were conducted to measure the surfactant sorption characteristics, while unsaturated transport experiments were performed in columns packed with 40‐270 mesh Ottawa sand at five initial water contents. Following the introduction of surfactant solution, the rate of water percolation through the sand increased, however, this period of rapid water drainage was followed by decreased water percolation due to the reduction in soil water content and the corresponding decrease in unsaturated hydraulic conductivity behind the surfactant front. The observed changes in water percolation occurred sequentially, and resulted in faster non‐reactive solute transport than was observed in the absence of surfactant. A one‐dimensional mathematical model accurately described coupled water flow, surfactant and solute transport under most experimental conditions. Differences between model predictions and experimental data were observed in the column study performed at the lowest water content (0.115cm3/cm3), which was attributed to surfactant adsorption at the air‐water interface. These findings demonstrate the potential influence of surfactants additives on unsaturated water flow and solute transport in soils, and demonstrate a methodology to couple these processes in a predictive modeling tool. This article is protected by copyright. All rights reserved.
- Enhanced nonlinearity interval mapping scheme for high performance
simulation‐optimization of watershed‐scale BMP placement
- Authors: Rui Zou; John Riverson, Yong Liu, Ryan Murphy, Youn Sim
Abstract: Integrated continuous simulation‐optimization models can be effective predictors of a process‐based responses for cost‐benefit optimization of best management practices (BMPs) selection and placement. However, practical application of simulation‐optimization model is computationally prohibitive for large‐scale systems. This study proposes an enhanced Nonlinearity Interval Mapping Scheme (NIMS) to solve large‐scale watershed simulation‐optimization problems several orders of magnitude faster than other commonly‐used algorithms. An efficient interval response coefficient (IRC) derivation method was incorporated into the NIMS framework to overcome a computational bottleneck. The proposed algorithm was evaluated using a case study watershed in the Los Angeles County Flood Control District. Using a continuous simulation watershed/stream‐transport model, Loading Simulation Program in C++ (LSPC), three nested in‐stream compliance points (CP)—each with multiple Total Maximum Daily Loads (TMDL) targets—were selected to derive optimal treatment levels for each of the 28 subwatersheds, so that the TMDL targets at all the CP were met with the lowest possible BMP implementation cost. Genetic Algorithm (GA) and NIMS were both applied and compared. The results showed that the NIMS took 11 iterations (about 11 minutes) to complete with the resulting optimal solution having a total cost of $67.2 million, while each of the multiple GA executions took 21 to 38 days to reach near optimal solutions. The best solution obtained among all the GA executions compared had a minimized cost of $67.7 million—marginally higher, but approximately equal to that of the NIMS solution. The results highlight the utility for decision making in large‐scale watershed simulation‐optimization formulations. This article is protected by copyright. All rights reserved.
- The value of multiple dataset calibration versus model complexity for
improving the performance of hydrological models in mountain catchments
- Authors: David Finger; Marc Vis, Matthias Huss, Jan Seibert
Abstract: The assessment of snow, glacier and rainfall runoff contribution to discharge in mountain streams is of major importance for an adequate water resource management. Such contributions can be estimated via hydrological models, provided that the modeling adequately accounts for snow and glacier melt, as well as rainfall runoff. We present a multi‐dataset calibration approach to estimate runoff composition using hydrological models with three levels of complexity. For this purpose the code of the conceptual runoff model HBV‐light was enhanced to allow calibration and validation of simulations against glacier mass balances, satellite‐derived snow cover area and measured discharge. Three levels of complexity of the model were applied to glacierized catchments in Switzerland, ranging from 39km2 to 103km2. The results indicate that all three observational datasets are reproduced adequately by the model, allowing an accurate estimation of the runoff composition in the three mountain streams. However, calibration against only runoff leads to unrealistic snow and glacier melt rates. Based on these results we recommend using all three observational datasets in order to constrain model parameters and compute snow, glacier and rain contributions. Finally, based on the comparison of model performance of different complexities we postulate that the availability and use of different datasets to calibrate hydrological models might be more important than model complexity to achieve realistic estimations of runoff composition. This article is protected by copyright. All rights reserved.
- Comment on “Hydraulic fracturing in faulted sedimentary basins:
Numerical simulation of potential long‐term contamination of shallow
- Authors: Samuel A. Flewelling; Manu Sharma
- Reply to comment by Flewelling and Sharma on “Hydraulic fracturing
in faulted sedimentary basins: Numerical simulation of potential
contamination of shallow aquifers over long time scales”
- Authors: René Lefebvre; Tom Gleeson, Jeffrey M. McKenzie, Claire Gassiat
- Multiscale solute transport upscaling for a three‐dimensional
hierarchical porous medium
- Authors: Mingkan Zhang; Ye Zhang
Abstract: A laboratory‐generated hierarchical, fully heterogeneous aquifer model (FHM) provides a reference for developing and testing an upscaling approach that integrates large‐scale connectivity mapping with flow and transport modeling. Based on the FHM, three hydrostratigraphic models (HSMs) that capture lithological (static) connectivity at different resolutions are created, each corresponding to a sedimentary hierarchy. Under increasing system lnK variances (0.1, 1.0, 4.5), flow upscaling is first conducted to calculate equivalent hydraulic conductivity for individual connectivity (or unit) of the HSMs. Given the computed flow fields, an instantaneous, conservative tracer test is simulated by all models. For the HSMs, two upscaling formulations are tested based on the advection‐dispersion equation (ADE), implementing space‐ versus time‐dependent macrodispersivity. Comparing flow and transport predictions of the HSMs against those of the reference model, HSMs capturing connectivity at increasing resolutions are more accurate, although upscaling errors increase with system variance. Results suggest: (1) by explicitly modeling connectivity, an enhanced degree of freedom in representing dispersion can improve the ADE‐based upscaled models by capturing non‐Fickian transport of the FHM; (2) when connectivity is sufficiently resolved, the type of data conditioning used to model transport becomes less critical. Data conditioning, however, is influenced by the prediction goal; (3) when aquifer is weakly‐to‐moderately heterogeneous, the upscaled models adequately capture the transport simulation of the FHM, despite the existence of hierarchical heterogeneity at smaller scales. When aquifer is strongly heterogeneous, the upscaled models become less accurate because lithological connectivity cannot adequately capture preferential flows; (4) three‐dimensional transport connectivities of the hierarchical aquifer differ quantitatively from those analyzed for two‐dimensional systems. This article is protected by copyright. All rights reserved.
- Impact of interfacial tension on residual CO2 clusters in porous sandstone
- Authors: Fei Jiang; Takeshi Tsuji
Abstract: We develop a numerical simulation that uses the lattice Boltzmann method to directly calculate the characteristics of residual nonwetting‐phase clusters to quantify capillary trapping mechanisms in real sandstone. For this purpose, a digital‐rock‐pore model reconstructed from micro‐CT‐scanned images of Berea sandstone is filtered and segmented into a binary file. The residual‐cluster distribution is generated following simulation of the drainage and imbibition processes. The characteristics of the residual cluster in terms of size distribution, major length, interfacial area, and sphericity are investigated under conditions of different interfacial tension (IFT). Our results indicate that high interfacial tension increases the residual saturation and leads to a large size distribution of residual clusters. However, low interfacial tension results in a larger interfacial area, which is beneficial for dissolution and reaction processes during geological carbon storage. Analysis of the force balance acting on the residual clusters demonstrates that trapping stability is higher in high interfacial tension case, and the interfacial tension should be a controlling factor for the trapping stability in addition to the pore geometry and connectivity. The proposed numerical method can handle the complex displacement of multicomponent systems in porous media. By using this method, we can obtain residual‐cluster distributions under different conditions for optimizing the storage capacity of carbon‐storage projects. This article is protected by copyright. All rights reserved.
- Transient response of Salix cuttings to changing water level regimes
- Authors: L. Gorla; C. Signarbieux, P. Turberg, A. Buttler, P. Perona
Abstract: Sustainable water management requires an understanding of the effects of flow regulation on riparian eco‐morphological processes. We investigated the transient response of Salix viminalis by examining the effect of water level regimes on its above and below‐ground biomass. Four sets of Salix cuttings, three juveniles (in the first growing season) and one mature (one year old), were planted and initially grown under the same water‐level regime for one month. We imposed three different water level regime treatments representing natural variability, a seasonal trend with no peaks, and minimal flow (characteristic of hydropower) consisting of a constant water level and natural flood peaks. We measured sap flux, stem water potential, photosynthesis, growth parameters, and final root architecture. The mature cuttings were not affected by water table dynamics, but the juveniles displayed causal relationships between the changing water regime, plant growth, and root distribution during a two‐month transient period. For example, a 50% drop in mean sap flux corresponded with a −1.5 Mpa decrease in leaf water potential during the first day after the water regime was changed. In agreement with published field observations, the cuttings concentrated their roots close to the mean water table of the corresponding treatment, allowing survival under altered conditions and resilience to successive stress events. Juvenile development was strongly impacted by the minimum flow regime, leading to more than 60% reduction of both above‐ and below‐ground biomass, with respect to the other treatments. Hence, we suggest avoiding minimum flow regimes where Salix restoration is prioritized. This article is protected by copyright. All rights reserved.
- Towards Understanding Non‐stationarity in Climate and Hydrology
through Tree‐ring proxy records
- Authors: Saman Razavi; Amin Elshorbagy, Howard Wheater, David Sauchyn
Abstract: Natural proxy records of hydroclimatic behaviour, such as tree‐ring chronologies, are a rich source of information of past climate‐driven non‐stationarities in hydrologic variables. In this study, we investigate tree‐ring chronologies that demonstrate significant correlations with streamflows, with the objective of identifying the spatiotemporal patterns and extents of non‐stationarities in climate and hydrology, which are essentially representations of past “climate changes”. First‐ and second‐order non‐stationarities are of particular interest in this study. As a prerequisite, we develop a methodology to assess the consistency and credibility of a regional network of tree‐ring chronologies as proxies for hydrologic regime. This methodology involves a cluster analysis of available tree‐ring data to understand and evaluate their dependence structure, and a regional temporal‐consistency plot to assess the consistency of different chronologies over time. The major headwater tributaries of the Saskatchewan River basin (SaskRB), the main source of surface water in the Canadian Prairie Provinces, are used as the case study. Results indicate that stationarity might never have existed in the hydrology of the region, as the statistical properties of annual paleo‐hydrologic proxy records across the basin, i.e., the mean and autocorrelation structure, have consistently undergone significant changes (non‐stationarities) at different points in the history of the region. The spatial pattern of the changes in the mean statistic has been variable with time, indicating a time‐varying cross‐correlation structure across the tributaries of the SaskRB. Conversely, the changes in the autocorrelation structure across the basin have been in harmony over time. The results demonstrate that the 89‐year period of observational record in this region is a poor representation of the long‐term properties of the hydrologic regime, and shorter periods, e.g., 30 year periods, are by no means representative. This paper highlights the need to broaden the understanding of hydrologic characteristics in any basin beyond the limited observational records, as an improved understanding is essential for more reliable assessment and management of available water resources. This article is protected by copyright. All rights reserved.
- Toward the camera rain gauge
- Authors: P. Allamano; A. Croci, F. Laio
Abstract: We propose a novel technique based on the quantitative detection of rain intensity from images, i.e. from pictures taken in rainy conditions. The method is fully analytical and based on the fundamentals of camera optics. A rigorous statistical framing of the technique allows one to obtain the rain rate estimates in terms of expected values and associated uncertainty. We show that the method can be profitably applied to real rain events and we obtain promising results with errors of the order of ±25% . A precise quantification of the method's accuracy will require a more systematic and long‐term comparison with benchmark measures. The significant step forward with respect to standard rain gauges resides in the possibility to retrieve measures at very high temporal resolution (e.g., 30 measures per minute) at a very low cost. Perspective applications include the possibility to dramatically increase the spatial density of rain observations by exporting the technique to crowdsourced pictures of rain acquired with cameras and smartphones. This article is protected by copyright. All rights reserved.
- Seasonal hydrologic responses to climate change in the Pacific Northwest
- Authors: Julie A. Vano; Bart Nijssen, Dennis P. Lettenmaier
Abstract: Increased temperatures and changes in precipitation will result in fundamental changes in the seasonal distribution of streamflow in the Pacific Northwest and will have serious implications for water resources management. To better understand local impacts of regional climate change, we conducted model experiments to determine hydrologic sensitivities of annual, seasonal, and monthly runoff to imposed annual and seasonal changes in precipitation and temperature. We used the Variable Infiltration Capacity (VIC) land‐surface hydrology model applied at 1/16° latitude‐longitude spatial resolution over the Pacific Northwest (PNW), a scale sufficient to support analyses at the hydrologic unit code eight (HUC‐8) basin level. These experiments resolve the spatial character of the sensitivity of future water supply to precipitation and temperature changes by identifying the seasons and locations where climate change will have the biggest impact on runoff. The PNW exhibited a diversity of responses, where transitional (intermediate elevation) watersheds experience the greatest seasonal shifts in runoff in response to cool season warming. We also developed a methodology that uses these hydrologic sensitivities as basin‐specific transfer functions to estimate future changes in long‐term mean monthly hydrographs directly from climate model output of precipitation and temperature. When principles of linearity and superposition apply, these transfer functions can provide feasible first‐order estimates of the likely nature of future seasonal streamflow change without performing downscaling and detailed model simulations. This article is protected by copyright. All rights reserved.
- Thermodynamics in the hydrologic response: Travel time formulation and
application to Alpine catchments
- Authors: F. Comola; B. Schaefli, A. Rinaldo, M. Lehning
Abstract: This paper presents a spatially‐explicit model for hydro‐thermal response simulations of Alpine catchments, accounting for advective and non‐advective energy fluxes in stream networks characterized by arbitrary degrees of geomorphological complexity. The relevance of the work stems from the increasing scientific interest concerning the impacts of the warming climate on water resources management and temperature‐controlled ecological processes. The description of the advective energy fluxes is cast in a travel time formulation of water and energy transport, resulting in a closed form solution for water temperature evolution in the soil compartment. The application to Alpine catchments hinges on the boundary conditions provided by the fully‐distributed and physically‐based snow model Alpine3D. The performance of the simulations is illustrated by comparing modeled and measured hydrographs and thermographs at the outlet of the Dischma catchment (45km2) in the Swiss Alps. The Monte Carlo calibration shows that the model is robust and that a simultaneous fitting of streamflow and stream temperature reduces the uncertainty in the hydrological parameters estimation. The calibrated model also provides a good fit to the measurements in the validation period, suggesting that it could be employed for predictive applications, both for hydrological and ecological purposes. The temperature of the subsurface flow, as described by the proposed travel time formulation, proves warmer than the stream temperature during winter and colder during summer. Finally, the spatially‐explicit results of the model during snowmelt show a notable hydro‐thermal spatial variability in the river network, owing to the small spatial correlation of infiltration and meteorological forcings in Alpine regions. This article is protected by copyright. All rights reserved.
- Multiple regression and inverse moments improve the characterization of
the spatial scaling behavior of daily streamflows in the southeast United
- Authors: William H. Farmer; Thomas M. Over, Richard M. Vogel
Abstract: Understanding the spatial structure of daily streamflow is essential for managing freshwater resources, especially in poorly‐gaged regions. Spatial scaling assumptions are common in flood frequency prediction (e.g., index‐flood method) and the prediction of continuous streamflow at ungaged sites (e.g. drainage‐area ratio), with simple scaling by drainage area being the most common assumption. In this study, scaling analyses of daily streamflow from 173 streamgages in the southeastern US resulted in three important findings. First, the use of only positive integer moment orders, as has been done in most previous studies, captures only the probabilistic and spatial scaling behavior of flows above an exceedance probability near the median; negative moment orders (inverse moments) are needed for lower streamflows. Second, assessing scaling by using drainage area alone is shown to result in a high degree of omitted‐variable bias, masking the true spatial scaling behavior. Multiple regression is shown to mitigate this bias, controlling for regional heterogeneity of basin attributes, especially those correlated with drainage area. Previous univariate scaling analyses have neglected the scaling of low‐flow events and may have produced biased estimates of the spatial scaling exponent. Third, the multiple regression results show that mean flows scale with an exponent of one, low flows scale with spatial scaling exponents greater than one, and high flows scale with exponents less than one. The relationship between scaling exponents and exceedance probabilities may be a fundamental signature of regional streamflow. This signature may improve our understanding of the physical processes generating streamflow at different exceedance probabilities. This article is protected by copyright. All rights reserved.
- Prolonged river water pollution due to variable‐density flow and
solute transport in the riverbed
- Authors: Guangqiu Jin; Hongwu Tang, Ling Li, D. A. Barry
Abstract: A laboratory experiment and numerical modeling were used to examine effects of density gradients on hyporheic flow and solute transport under the condition of a solute pulse input to a river with regular bedforms. Relatively low density gradients due to an initial salt pulse concentration of 1.55kg m−3 applied in the experiment were found to modulate significantly the pore‐water flow and solute transport in the riverbed. Such density gradients increased downward flow and solute transport in the riverbed by factors up to 1.6. This resulted in a 12.2% increase in the total salt transfer from the water column to the riverbed over the salt pulse period. As the solute pulse passed, the effect of the density gradients reversed, slowing down the release of the solute back to the river water by a factor of 3.7. Numerical modeling indicated that these density effects intensified as salt concentrations in the water column increased. Simulations further showed that the density gradients might even lead to unstable flow and result in solute fingers in the bed of large bedforms. The slow release of solute from the bed back to the river led to a long tail of solute concentration in the river water. These findings have implications for assessment of impact of pollution events on river systems, in particular, long‐term effects on both the river water and riverbed due to the hyporheic exchange. This article is protected by copyright. All rights reserved.
- From analytical solutions of solute transport equations to
multidimensional time‐domain random walk (TDRW) algorithms
- Authors: Jacques Bodin
Abstract: In this study, new multi‐dimensional time‐domain random walk (TDRW) algorithms are derived from approximate one‐dimensional (1‐D), two‐dimensional (2‐D), and three‐dimensional (3‐D) analytical solutions of the advection‐dispersion equation and from exact 1‐D, 2‐D, and 3‐D analytical solutions of the pure‐diffusion equation. These algorithms enable the calculation of both the time required for a particle to travel a specified distance in a homogeneous medium and the mass recovery at the observation point, which may be incomplete due to 2‐D or 3‐D transverse dispersion or diffusion. The method is extended to heterogeneous media, represented as a piecewise collection of homogeneous media. The particle motion is then decomposed along a series of intermediate checkpoints located on the medium interface boundaries. The accuracy of the multi‐dimensional TDRW method is verified against (i) exact analytical solutions of solute transport in homogeneous media and (ii) finite‐difference simulations in a synthetic 2‐D heterogeneous medium of simple geometry. The results demonstrate that the method is ideally suited to purely diffusive transport and to advection‐dispersion transport problems dominated by advection. Conversely, the method is not recommended for highly dispersive transport problems because the accuracy of the advection‐dispersion TDRW algorithms degrades rapidly for a low Péclet number, consistent with the accuracy limit of the approximate analytical solutions. The proposed approach provides a unified methodology for deriving multi‐dimensional time‐domain particle equations and may be applicable to other mathematical transport models, provided that appropriate analytical solutions are available. This article is protected by copyright. All rights reserved.
- Model averaging methods to merge operational statistical and dynamic
seasonal streamflow forecasts in Australia
- Authors: Andrew Schepen; Q.J. Wang
Abstract: The Australian Bureau of Meteorology produces statistical and dynamic seasonal streamflow forecasts. The statistical and dynamic forecasts are similarly reliable in ensemble spread; however skill varies by catchment and season. Therefore, it may be possible to optimize forecasting skill by weighting and merging statistical and dynamic forecasts. Two model averaging methods are evaluated for merging forecasts for 12 locations. The first method, Bayesian model averaging (BMA), applies averaging to forecast probability densities (and thus cumulative probabilities) for a given forecast variable value. The second method, quantile model averaging (QMA), applies averaging to forecast variable values (quantiles) for a given cumulative probability (quantile fraction).
BMA and QMA are found to perform similarly in terms of overall skill scores and reliability in ensemble spread. Both methods improve forecast skill across catchments and seasons. However, when both the statistical and dynamical forecasting approaches are skillful but produce, on special occasions, very different event forecasts, the BMA merged forecasts for these events can have unusually wide and bi‐modal distributions. In contrast, the distributions of the QMA merged forecasts for these events are narrower, uni‐modal and generally more smoothly shaped, and are potentially more easily communicated to and interpreted by the forecast users. Such special occasions are found to be rare. However, every forecast counts in an operational service, and therefore the occasional contrast in merged forecasts between the two methods may be more significant than the indifference shown by the overall skill and reliability performance. This article is protected by copyright. All rights reserved.
- Experimental study on effects of geologic heterogeneity in enhancing
dissolution trapping of supercritical CO2
- Authors: Elif Agartan; Luca Trevisan, Abdullah Cihan, Jens Birkholzer, Quanlin Zhou, Tissa H. Illangasekare
Abstract: Dissolution trapping is one of the primary mechanisms that enhance the storage security of supercritical carbon dioxide (scCO2) in saline geologic formations. ScCO2, when dissolved in formation brine, produces an aqueous solution that is denser than brine, which leads to convective mixing driven by gravitational instabilities. Convective mixing can enhance the dissolution of CO2 and thus contribute to stable trapping of dissolved CO2. However, in the presence of geologic heterogeneities, diffusive mixing may also contribute to dissolution trapping. The effects of heterogeneity on mixing and its contribution to stable trapping are not well understood. The goal of this experimental study is to investigate the effects of geologic heterogeneity on mixing and stable trapping of dissolved CO2. The homogeneous and heterogeneous media experiments were conducted in a two‐dimensional test tank with various packing configurations using surrogates for scCO2 (water) and brine (propylene glycol) under ambient pressure and temperature conditions. The results show that the density‐driven flow in heterogeneous formations may not always cause significant convective mixing especially in layered systems containing low permeability zones. In homogeneous formations, density‐driven fingering enhances the storage in the deeper parts of the formation and also the contact between the host rock and dissolved CO2 for the potential mineralization. On the other hand, for layered systems, dissolved CO2 becomes immobilized in low permeability zones with low diffusion rates, which reduces the risk of leakage through any fault or fracture. Both cases contribute to the permanence of the dissolve plume in the formation. This article is protected by copyright. All rights reserved.
- Effects of tidal fluctuations and spatial heterogeneity on mixing and
spreading in spatially heterogeneous coastal aquifers
- Authors: María Pool; Vincent E. A. Post, Craig T. Simmons
Abstract: We study the combined effect of heterogeneity in the hydraulic conductivity field and tidal oscillations on the three‐dimensional dynamics of seawater intrusion in coastal aquifers. We focus on the quantification of its impact on solute mixing and spreading of the freshwater‐seawater interface. Three‐dimensional Monte Carlo realizations of log‐normally distributed permeability fields were performed, and for each realization, numerical variable density flow and solute transport simulations were conducted. Mixing is characterized by the spatial moments of concentration. The enhanced solute mixing is quantified by an effective dispersion coefficient. The simulations show that heterogeneity produces an inland movement of the toe location along with a significant widening of the transition zone, which is linearly proportional to the product of the arithmetic mean of the correlation lengths in the three spatial dimensions (λa) and the permeability field variance . We find that once tidal oscillations are included, as the degree of heterogeneity increases, the combined effect of heterogeneity and tidal oscillations on mixing and spreading of the interface reduces. This is explained by the fact that an increase in the log‐permeability variance induces an increase in both the effective permeability and the spatial connectivity, which implies a more uniform hydraulic response to tidal forcing and, as a result, the degree of mixing decreases. This study also identifies that the mixing behavior induced by tidal oscillations in heterogeneous coastal aquifers is controlled by the effective tidal mixing number which depends on the amplitude, the period, the storativity and the effective horizontal permeability. This article is protected by copyright. All rights reserved.
- Impact of errors in the downwelling irradiances on simulations of snow
water equivalent, snow surface temperature, and the snow energy balance
- Authors: Karl E. Lapo; Laura M. Hinkelman, Mark S. Raleigh, Jessica D. Lundquist
Abstract: The forcing irradiances irradiances (downwelling shortwave and longwave irradiances) are the primary drivers of snowmelt; however, in complex terrain, few observations, the use of estimated irradiances, and the influence of topography and elevation all lead to uncertainties in these radiative fluxes. The impact of uncertainties in the forcing irradiances on simulations of snow is evaluated in idealized modeling experiments. Two snow models of contrasting complexity, the Utah Energy Balance Model (UEB) and the Snow Thermal Model (SNTHERM), are forced with irradiances with prescribed errors of the structure and magnitude representative of those found in methods for estimating the downwelling irradiances. Relatively modest biases have substantial impacts on simulated snow water equivalent (SWE) and surface temperature (Ts) across a range of climates, whereas random noise at the daily scale has a negligible effect on modeled SWE and Ts. Shortwave biases have a smaller SWE impact, due to the influence of albedo, and Ts impact, due to their diurnal cycle, compared to equivalent longwave biases. Warmer sites exhibit greater sensitivity to errors when evaluated using SWE, while colder sites exhibit more sensitivity as evaluated using Ts.
The two models displayed different sensitivity and responses to biases. The stability feedback in the turbulent fluxes explains differences in Ts between models in the negative longwave bias scenarios. When the models diverge during melt events, differences in the turbulent fluxes and internal energy change of the snow are found to be responsible. From this analysis we suggest model evaluations use Ts in addition to SWE. This article is protected by copyright. All rights reserved.
- A hierarchical Bayesian regional model for nonstationary precipitation
extremes in Northern California conditioned on tropical moisture exports
- Authors: Scott Steinschneider; Upmanu Lall
Abstract: Warm, moist, and longitudinally confined tropical air masses are being linked to some of the most extreme precipitation and flooding events in the mid‐latitudes. The inter‐annual frequency and intensity of such atmospheric rivers (ARs), or tropical moisture exports (TMEs), are connected to the risk of extreme precipitation events in areas where moisture convergence occurs. This study presents a non‐stationary, regional frequency analysis of precipitation extremes in Northern California that is conditioned on the inter‐annual variability of TMEs entering the region. Parameters of a multi‐site peaks‐over‐threshold model are allowed to vary conditional on the integrated moisture delivery from TMEs over the area. Parameters are also related to time‐invariant, local characteristics to facilitate regionalization to ungaged sites. The model is developed and calibrated in a hierarchical Bayesian framework to support partial pooling and enhance regionalization skill. The model is cross‐validated along with two alternative, increasingly parsimonious formulations to assess the additional skill provided by the covariates. Climate diagnostics are also used to better understand the instances where TMEs fail to explain variations in rainfall extremes to provide a path forward for further model improvement. The modeling structure is designed to link seasonal forecasting and long‐term projections of TMEs directly to regional models of extremes used for risk estimation. Results suggest that the inclusion of TME‐based information greatly improves the characterization of extremes, particularly for their frequency of occurrence. Diagnostics indicate that the model could be further improved by considering an index for frontal systems as an additional covariate. This article is protected by copyright. All rights reserved.
- An efficient and guaranteed stable numerical method for continuous
modeling of infiltration and redistribution with a shallow dynamic water
- Authors: Wencong Lai; Fred L. Ogden, Robert C. Steinke, Cary A. Talbot
Abstract: We have developed a one‐dimensional numerical method to simulate infiltration and redistribution in the presence of a shallow dynamic water table. This method builds upon the Green‐Ampt infiltration with Redistribution (GAR) model (Ogden and Saghafian, 1997) and incorporates features from the Talbot‐Ogden (T‐O) infiltration and redistribution method (Talbot and Ogden, 2008) in a discretized moisture content domain. The redistribution scheme is more physically meaningful than the capillary weighted redistribution scheme in the T‐O method. Groundwater dynamics are considered in this new method instead of hydrostatic groundwater front. It is also computationally more efficient than the T‐O method. Motion of water in the vadose zone due to infiltration, redistribution and interactions with capillary groundwater are described by ordinary differential equations. Numerical solutions to these equations are computationally less expensive than solutions of the highly non‐linear Richards' (1931) partial differential equation. We present results from numerical tests on 11 soil types using multiple rain pulses with different boundary conditions, with and without a shallow water table and compare against the numerical solution of Richards' Equation (RE). Results from the new method are in satisfactory agreement with RE solutions in term of ponding time, de‐ponding time, infiltration rate and cumulative infiltrated depth. The new method, which we call “GARTO” can be used as an alternative to the RE for 1‐D coupled surface and groundwater models in general situations with homogeneous soils with dynamic water table. The GARTO method represents a significant advance in simulating groundwater surface water interactions because it very closely matches the RE solution while being computationally efficient, with guaranteed mass conservation, and no stability limitations that can affect RE solvers in the case of a near‐surface water table. This article is protected by copyright. All rights reserved.
- Exploring storage and runoff generation processes for urban flooding
through a physically based watershed model
- Authors: B. K. Smith; J. A. Smith, M. L. Baeck, A. J. Miller
Abstract: A physically based model of the 14 km2 Dead Run watershed in Baltimore County, MD was created to test the impacts of detention basin storage and soil storage on the hydrologic response of a small urban watershed during flood events. The Dead Run model was created using the Gridded Surface Subsurface Hydrologic Analysis (GSSHA) algorithms and validated using U.S. Geological Survey stream gaging observations for the Dead Run watershed and 5 sub‐basins over the largest 21 warm season flood events during 2008‐2012. Removal of the model detention basins resulted in a median peak discharge increase of 11% and a detention efficiency of 0.5, which was defined as the percent decrease in peak discharge divided by percent detention controlled area. Detention efficiencies generally decreased with increasing basin size. We tested the efficiency of detention basin networks by focusing on the “drainage network order,” akin to the stream order but including storm drains, streams, and culverts. The detention efficiency increased dramatically between first order detention and second order detention but was similar for second and third order detention scenarios. Removal of the soil compacted layer, a common feature in urban soils, resulted in a 7% decrease in flood peak discharges. This decrease was statistically similar to the flood peak decrease caused by existing detention. Current soil storage within the Dead Run watershed decreased flood peak discharges by a median of 60%. Numerical experiment results suggested that detention basin storage and increased soil storage have the potential to substantially decrease flood peak discharges. This article is protected by copyright. All rights reserved.
- Adaptive, multiobjective optimal sequencing approach for urban water
supply augmentation under deep uncertainty
- Authors: Eva H.Y. Beh; Holger R. Maier, Graeme C. Dandy
Abstract: Optimal long‐term sequencing and scheduling play an important role in many water resources problems. The optimal sequencing of urban water supply augmentation options is one example of this. In this paper, an adaptive, multi‐objective optimal sequencing approach for urban water supply augmentation under deep uncertainty is introduced. As part of the approach, optimal long‐term sequence plans are updated at regular intervals and trade‐offs between the robustness and flexibility of the solutions that have to be fixed at the current time and objectives over entire planning horizon are considered when selecting the most appropriate course of action. The approach is demonstrated for the sequencing of urban water supply augmentation options for the southern Adelaide water supply system for two assumed future realities. The results demonstrate the utility of the proposed approach, as it is able to identify optimal sequences that perform better than those obtained using static approaches. This article is protected by copyright. All rights reserved.
- Relating reactive solute transport to hierarchical and multiscale
sedimentary architecture in a Lagrangian‐based transport model: 1.
Time‐dependent effective retardation factor
- Authors: Mohamad Reza Soltanian; Robert W. Ritzi, Chao Cheng Huang, Zhenxue Dai
Abstract: This series of papers addresses the transport of reactive solutes in groundwater. In part 1, the time‐dependent effective retardation factor, R˜eff(t), of reactive solutes undergoing equilibrium sorption is linked to hierarchical stratal architecture using a Lagrangian‐based transport model. The model is based on hierarchical expressions of the spatial covariance of the log distribution coefficient, Ξ=In(Kd), and the spatial cross‐covariance between Ξ and the log permeability,Y=In(k). The spatial correlation structure in these covariance expressions is the probability of transitioning across strata types of different scales, and they are parameterized by independent and quantifiable physical attributes of sedimentary architecture including univariate statistics for Y, Ξ, and proportions and facies lengths. Nothing is assumed about Y‐Ξ point correlation; it is allowed to differ by facies type. The duration of the time‐dependent change in R˜eff(t) is a function of the effective ranges of the cross‐transition probability structures (i.e. the ranges of indicator correlation structures) for each scale of stratal architecture. The plume velocity and the effective retardation stabilize at a large‐time limit after the plume centroid has traveled a distance that encompasses the effective ranges of these cross‐transition probability structures. The well‐documented perchloroethene (PCE) tracer test at the Borden research site is used to illustrate the model. The model gives a viable explanation for the observed PCE plume deceleration, and thus the observed R˜eff(t) can be explained by the process of linear equilibrium sorption and the heterogeneity in k and Kd. In part 2 (Soltanian et al., under review), reactive plume dispersion, as quantified by the particle displacement variance is linked to stratal architecture using a Lagrangian‐based transport model. This article is protected by copyright. All rights reserved.
- Relating sorptive solute transport to hierarchical and multiscale
sedimentary architecture in a Lagrangian‐based transport model: 2.
Particle displacement variance
- Authors: Mohamad Reza Soltanian; Robert W. Ritzi, Chao Cheng Huang, Zhenxue Dai
Abstract: This series of papers addresses the transport of sorbing solutes in groundwater. In part 2, plume dispersion, as quantified by the particle displacement variance, X11R(t), is linked to hierarchical sedimentary architecture using a Lagrangian‐based transport model. This allows for a fundamental understanding of how dispersion arises from the hierarchical architecture of sedimentary facies, and allows for a quantitative decomposition of dispersion into facies‐related contributions at different scales within the hierarchy. As in part 1, the plume behavior is assumed to be controlled by linear‐equilibrium sorption and the heterogeneity in both the log permeability, Y = ln(k), and the log distribution coefficient, Ξ = ln(Kd). Heterogeneity in Y and Ξ arises from sedimentary processes and is structured by the consequent sedimentary architecture. Our goal is to understand the basic science of the dispersion process at this very fundamental level. The spatial auto‐ and cross covariances for the relevant attributes are linear sums of terms corresponding to the probability of transitioning across stratal facies types defined at different scales. Unlike previous studies that used empirical relationships for the spatial covariances, here the model parameters are developed from independent measurements of physically quantifiable attributes of the stratal architecture (i.e. proportions and lengths of facies types, and univariate statistics for Y and Ξ). Nothing is assumed about Y‐Ξ point correlation; it is allowed to differ by facies type. However, it is assumed that Y and Ξ variance is small but meaningful, and that pore‐scale dispersion is negligible. The time‐dependent spreading rate is a function of the effective ranges of the cross‐transition probability structures (i.e. the ranges of indicator correlation structures) for each relevant scale of stratal hierarchy. As in part 1, the well‐documented perchloroethene (PCE) tracer test at the Borden research site is used to illustrate the model. The model was parameterized with univariate statistics for Y, Ξ of (PCE), and proportions and lengths of lithologic facies types defined at two scales within a two‐level hierarchical classification, as given by Ritzi et al. . The model gives a viable explanation for the observed PCE plume dispersion, and thus X11R(t) can be explained by the process of linear equilibrium sorption and the heterogeneity in k and Kd. The results quantitatively show that the k ‐ Kd cross‐correlation, though small, and varied by facies type, can significantly impact the particle displacement variance. Furthermore, by quantitatively decomposing the dispersion into facies‐related contributions, we gain the fundamental insight that that the time‐dependent rate of spreading is mostly defined by the cross‐transition probability correlation structure imparted by the proportions and sizes of the larger‐scale facies types. This article is protected by copyright. All rights reserved.
- Prediction of Glossosoma biomass spatial distribution in Valley Creek by
field measurements and a three‐dimensional turbulent
open‐channel flow model
- Authors: M. Morris; M. Haji Mohammadi, S. Day, M. Hondzo, F. Sotiropoulos
Abstract: The fluid flow environment associated with high Glossosoma abundance is predicted by large‐eddy simulation of a natural turbulent open‐channel flow. The spatial distribution of Glossosoma was depicted by high resolution physical variables described by fluid flow and streambed topography. Variogram analysis of the streambed topography revealed a characteristic length scale of the streambed of the order 0.2m over which bed roughness height was correlated. Flow simulation output was spatially and temporally averaged over the streambed characteristic length scale and linked to Glossosoma spatial density. A dimensionless scaling relationship between Glossosoma spatial distribution and streamwise velocity averaged in the longitudinal and transverse direction, spatial velocity fluctuation, and spanwise vorticity from the computational fluid dynamics simulation output explained 79% of the variation in observed dimensionless Glossosoma spatial density. The analysis demonstrated that computational fluid mechanics and high resolution bed topography could be instrumental in predicting benthic macroinvertebrate spatial distribution in streams and rivers. This article is protected by copyright. All rights reserved.
- Toward a true spatial model evaluation in distributed hydrological
modeling: Kappa statistics, Fuzzy theory, and EOF analysis benchmarked by
the human perception and evaluated against a modeling case study
- Authors: Julian Koch; Karsten Høgh Jensen, Simon Stisen
Abstract: The hydrological modeling community is aware that the validation of distributed hydrological models has to move beyond aggregated performance measures, like hydrograph assessment by means of Nash‐Suitcliffe efficiency towards a true spatial model validation. Remote sensing facilitates continuous data and can be measured on a similar spatial scale as the predictive scale of the hydrological model thereby it can serve as suitable data for the spatial validation. The human perception is often described as a very reliable and well trained source for pattern comparison, which this study wants to exploit. A web‐based survey that is interpreted based on approximately 200 replies reflects the consensus of the human perception on map comparisons of a reference map and 12 synthetic perturbations. The resulting similarity ranking can be used as a reference to benchmark various spatial performance metrics. This study promotes Fuzzy theory as a suitable approach because it considers uncertainties related to both location and value in the simulated map. Additionally, an EOF‐analysis (Empirical Orthogonal Function) is conducted to decompose the map comparison into its similarities and dissimilarities. A modeling case study serves to further examine the metrics capability to assess the goodness of fit between simulated and observed land surface temperature maps. The EOF‐analysis unambiguously identifies a systematic depth to groundwater table related model deficiency. Kappa statistic extended by Fuzziness is a suitable and commonly applied measure for map comparison. However, its apparent bias sensitivity limits it's capability as a diagnostic tool to detect the distinct deficiency. This article is protected by copyright. All rights reserved.
- Impacts of rainfall spatial variability on hydrogeological response
- Authors: Gonzalo Sapriza‐Azuri; Jorge Jódar, Vicente Navarro, Luit Jan Slooten, Jesús Carrera, Hoshin V. Gupta
Abstract: There is currently no general consensus on how the spatial variability of rainfall impacts and propagates through complex hydrogeological systems. Most studies to date have focused on the effects of rainfall spatial variability (RSV) on river discharge, while paying little attention to other important aspects of system response. Here, we study the impacts of RSV on several responses of a hydrological model of an overexploited system. To this end, we drive a spatially distributed hydrogeological model for the semi‐arid Upper Guadiana basin in central Spain with stochastic daily rainfall fields defined at three different spatial resolutions (fine → 2.5km x 2.5km, medium → 50km x50km, large → lumped). This enables us to investigate how (i) RSV at different spatial resolutions, and (ii) rainfall uncertainty, are propagated through the hydrogeological model of the system. Our results demonstrate that RSV has a significant impact on the modeled response of the system, by specifically affecting groundwater recharge and runoff generation, and thereby propagating through to various other related hydrological responses (river discharge, river‐aquifer exchange, groundwater levels). These results call into question the validity of management decisions made using hydrological models calibrated or forced with spatially lumped rainfall. This article is protected by copyright. All rights reserved.
- Anomalous solute transport in saturated porous media: Relating transport
model parameters to electrical and nuclear magnetic resonance properties
- Authors: Ryan D. Swanson; Andrew Binley, Kristina Keating, Samantha France, Gordon Osterman, Frederick D. Day‐Lewis, Kamini Singha
Abstract: The advection‐dispersion equation (ADE) fails to describe commonly observed non‐Fickian solute transport in saturated porous media, necessitating the use of other models such as the dual‐domain mass transfer (DDMT) model. DDMT model parameters are commonly calibrated via curve fitting, providing little insight into the relation between effective parameters and physical properties of the medium. There is a clear need for material characterization techniques that can provide insight into the geometry and connectedness of pore spaces related to transport model parameters. Here, we consider proton nuclear magnetic resonance (NMR), direct‐current (DC) resistivity, and complex conductivity (CC) measurements for this purpose, and assess these methods using glass beads as a control and two different samples of the zeolite clinoptilolite, a material that demonstrates non‐Fickian transport due to intragranular porosity. We estimate DDMT parameters via calibration of a transport model to column‐scale solute tracer tests, and compare NMR, DC resistivity, CC results, which reveal that grain size alone does not control transport properties and measured geophysical parameters; rather, volume and arrangement of the pore space play important roles. NMR cannot provide estimates of more‐ and less‐mobile pore volumes in the absence of tracer tests because these estimates depend critically on the selection of a material‐ and flow‐ dependent cutoff time. Increased electrical connectedness from DC resistivity measurements are associated with greater mobile pore space determined from transport model calibration. CC was hypothesized to be related to length scales of mass transfer, but the CC response is unrelated to DDMT. This article is protected by copyright. All rights reserved.
- Impact of space‐time mesh adaptation on solute transport modeling in
- Authors: Bahman Esfandiar; Giovanni Porta, Simona Perotto, Alberto Guadagnini
Abstract: We implement a space‐time grid adaptation procedure to efficiently improve the accuracy of numerical simulations of solute transport in porous media in the context of model parameter estimation. We focus on the Advection Dispersion Equation (ADE) for the interpretation of non‐reactive transport experiments in laboratory‐scale heterogeneous porous media. When compared to a numerical approximation based on a fixed space‐time discretization, our approach is grounded on a joint automatic selection of the spatial grid and the time step to capture the main (space‐time) system dynamics. Spatial mesh adaptation is driven by an anisotropic recovery‐based error estimator which enables us to properly select the size, shape and orientation of the mesh elements. Adaptation of the time step is performed through an ad‐hoc local reconstruction of the temporal derivative of the solution via a recovery‐based approach. The impact of the proposed adaptation strategy on the ability to provide reliable estimates of the key parameters of an ADE model is assessed on the basis of experimental solute breakthrough data measured following tracer injection in a non‐uniform porous system. Model calibration is performed in a Maximum Likelihood (ML) framework upon relying on the representation of the ADE solution through a generalized Polynomial Chaos Expansion (gPCE). Our results show that the proposed anisotropic space‐time grid adaptation leads to ML parameter estimates and to model results of markedly improved quality when compared to classical inversion approaches based on a uniform space‐time discretization. This article is protected by copyright. All rights reserved.
- Estimating freshwater flows from tidally affected hydrographic data
- Authors: D.E. Pagendam; D.B. Percival
Abstract: De‐tiding end‐of‐catchment flow data is an important step in determining the total volumes of freshwater (and associated pollutant loads) entering the ocean. We examine three approaches for separating freshwater and tidal flows from tidally‐affected data: (i) a simple low‐pass Butterworth filter (BWF); (ii) a robust, harmonic analysis with Kalman smoothing (RoHAKS) which is a novel approach introduced in this paper; and (iii) dynamic harmonic regression (DHR). Using hydrographic data collected in the Logan River, Australia over a period of 452 days, we judge the accuracy of the three methods based on three criteria: consistency of freshwater flows with upstream gauges; consistency of total discharge volumes with the raw data over the event; and minimal upstream flow. A simulation experiment shows that RoHAKS outperforms both BWF and DHR on a number of criteria. In addition RoHAKS enjoys a computational advantage over DHR in speed and use of freely available software. This article is protected by copyright. All rights reserved.
- Dynamic connectivity in a fluvial network for identifying hot spots of
- Authors: Jonathan A. Czuba; Efi Foufoula‐Georgiou
Abstract: Dynamical processes occurring on the hierarchical branching structure of a river network tend to heterogeneously distribute fluxes on the network, often concentrating them into “clusters,” i.e., places of excess flux accumulation. Here, we put forward the hypothesis that places in the network predisposed (due to process dynamics and network topology) to accumulate excess sediment over a considerable river reach and over a considerable period of time reflect locations where a local imbalance in sediment flux may occur thereby highlighting a susceptibility to potential fluvial geomorphic change. We develop a dynamic connectivity framework which uses the river network structure and a simplified Lagrangian transport model to trace fluxes through the network and integrate emergent “clusters” through a cluster persistence index (CPI). The framework was applied to sand transport in the Greater Blue Earth River Network in the Minnesota River Basin. Three hotspots of fluvial geomorphic change were defined as locations where high rates of channel migration were observed and places of high CPI coincided with two of these hotspots of possibly sediment‐driven change. The third hotspot was not identified by high CPI, but instead is believed to be a hotspot of streamflow‐driven change based on additional information and the fact that high bed shear stress coincided with this hotspot. The proposed network‐based dynamic connectivity framework has the potential to place dynamical processes occurring at small scales into a network context to understand how reach‐scale changes cascade into network‐scale effects, useful for informing the large‐scale consequences of local management actions. This article is protected by copyright. All rights reserved.
- Evaluating observational methods to quantify snow duration under diverse
- Authors: Susan E. Dickerson‐Lange; James A. Lutz, Kael A. Martin, Mark S. Raleigh, Rolf Gersonde, Jessica D. Lundquist
Abstract: Forests cover almost 40% of the seasonally snow‐covered regions in North America. However, operational snow networks are located primarily in forest clearings, and optical remote sensing cannot see through tree canopies to detect forest snowpack. Due to the complex influence of the forest on snowpack duration, ground observations in forests are essential. We therefore consider the effectiveness of different strategies to observe snow‐covered area under forests. At our study location in the Pacific Northwest, we simultaneously deployed fiber‐optic cable, stand‐alone ground temperature sensors, and time‐lapse digital cameras in three diverse forest treatments: control second‐growth forest, thinned forest, and forest gaps (one tree height in diameter). We derived fractional snow‐covered area and snow duration metrics from the co‐located instruments to assess optimal spatial resolution and sampling configuration, and snow duration differences between forest treatments. The fiber‐optic cable and the cameras indicated that mean snow duration was 8 days longer in the gap plots than in the control plots (p
- Variational Lagrangian data assimilation in open channel networks
- Authors: Qingfang Wu; Andrew Tinka, Kevin Weekly, Jonathan Beard, Alexandre M. Bayen
Abstract: This article presents a data assimilation method in a tidal system, where data from both Lagrangian drifters and Eulerian flow sensors were fused to estimate water velocity. The system is modeled by first‐order, hyperbolic partial differential equations subject to periodic forcing. The estimation problem can then be formulated as the minimization of the difference between the observed variables and model outputs, and eventually provide the velocity and water stage of the hydrodynamic system. The governing equations are linearized and discretized using an implicit discretization scheme, resulting in linear equality constraints in the optimization program. Thus, the flow estimation can be formed as a optimization problemming problem and efficiently solved.
The effectiveness of the proposed method was substantiated by a large‐scale field experiment in the Sacramento‐San Joaquin River Delta in California. A fleet of 100 sensors developed at the University of California, Berkeley were deployed in Walnut Grove, CA to collect a set of Lagrangian data, a time‐series of positions as the sensors moved through the water. Measurements were also taken from Eulerian sensors in the region, provided by the United States Geological Survey. It is shown that the proposed method can effectively integrate Lagrangian and Eulerian measurement data, resulting in a suited estimation of the flow variables within the hydraulic system. This article is protected by copyright. All rights reserved.
- Probabilistic precipitation rate estimates with ground‐based radar
- Authors: Pierre‐Emmanuel Kirstetter; Jonathan J. Gourley, Yang Hong, Jian Zhang, Saber Moazamigoodarzi, Carrie Langston, Ami Arthur
Abstract: The uncertainty structure of radar quantitative precipitation estimation (QPE) is largely unknown at fine spatiotemporal scales near the radar measurement scale. By using the WSR‐88D radar network and gauge datasets across the conterminous US, an investigation of this subject has been carried out within the framework of the NOAA/NSSL ground radar‐based Multi‐Radar Multi‐Sensor (MRMS) QPE system. A new method is proposed and called PRORATE for Probabilistic QPE using Radar Observations of Rate And Typology Estimates. Probability distributions of precipitation rates are computed instead of deterministic values using a model quantifying the relation between radar reflectivity and the corresponding “true” precipitation. The probabilistic model acknowledges the uncertainty arising from many factors operative at the radar measurement scale and from the correction algorithm. Ensembles of reflectivity‐to‐precipitation rate relationships accounting explicitly for precipitation typology were derived at a 5‐min/1‐km scale. This approach conditions probabilistic quantitative precipitation estimates (PQPE) on the precipitation rate and type. The model components were estimated on the basis of a 1‐year‐long data sample over the CONUS. This PQPE model provides the basis for precipitation probability maps and the generation of radar precipitation ensembles. Maps of the precipitation exceedance probability for specific thresholds (e.g. precipitation return periods) are computed. Precipitation probability maps are accumulated to the hourly time scale and compare favorably to the deterministic QPE. As an essential property of precipitation, the impact of the temporal correlation on the hourly accumulation is examined. This approach to PQPE can readily apply to other systems including space‐based passive and active sensor algorithms. This article is protected by copyright. All rights reserved.
- North American precipitation isotope (δ18O) zones revealed in time
series modeling across Canada and northern United States
- Authors: Delavau C; Chun K.P, Stadnyk T, Birks S.J, Welker J.M.
Abstract: Delineating spatial patterns of precipitation isotopes (“isoscapes”) is becoming increasingly important to understand the processes governing the modern water isotope cycle and their application to migration forensics, climate proxy interpretation, and ecohydrology of terrestrial systems. However, the extent to which these patterns can be empirically predicted across Canada and the northern United States of America (USA) has not been fully articulated, in part due to a lack of time‐series precipitation isotope data for major regions of North America. In this study, we use multiple linear regressions of CNIP, GNIP and USNIP observations alongside climatological variables, teleconnection indices, and geographic indicators to create empirical models that predict the δ18O of monthly precipitation (δ18Oppt) across Canada and the northern USA. Five regionalization approaches are used to separate the study domain into isotope zones to explore the effect of spatial grouping on model performance. Stepwise regression‐derived parameterizations quantified by permutation testing indicate the significance of precipitable water content and latitude as predictor variables. Within the Canadian Arctic and eastern portion of the study domain, models from all regionalizations capture the inter‐ and intra‐annual variability of δ18Oppt. The Pacific coast and northwestern portions of the study domain show less agreement between models and poorer model performance, resulting in higher uncertainty in simulations throughout these regions. Long‐term annual average δ18Oppt isoscapes are generated, highlighting the uncertainty in the regionalization approach as it compounds over time. Additionally, monthly time‐series simulations are presented at various locations, and model structure uncertainty and 90% bootstrapped prediction bounds are detailed for these predictions. This article is protected by copyright. All rights reserved.
- Hydraulic effects on nitrogen removal in a tidal spring‐fed river
- Authors: Robert T. Hensley; Matthew J. Cohen, Larry V. Korhnak
Abstract: Hydraulic properties such as stage and residence time are important controls on riverine N removal. In most rivers, these hydraulic properties vary with stochastic precipitation forcing, but in tidal rivers, hydraulics variation occurs on a predictable cycle. In Manatee Springs, a highly productive, tidally influenced spring‐fed river in Florida, we observed significant reach‐scale N removal that varied in response to tidally‐driven variation in hydraulic properties as well as sunlight‐driven variation in assimilatory uptake. After accounting for channel residence time and stage variation, we partitioned the total removal signal into assimilatory (i.e., plant uptake) and dissimilatory (principally denitrification) pathways. Assimilatory uptake was strongly correlated with primary production and ecosystem C:N was concordant with tissue stoichiometry of the dominant autotrophs. The magnitude of N removal was broadly consistent in magnitude with predictions from models (SPARROW and RivR‐N). However, contrary to model predictions, the highest removal occurred at the lowest values of τ/d (residence time divided by depth), which occurred at low tide. Removal efficiency also exhibited significant counterclockwise hysteresis with incoming versus outgoing tides. This behavior is best explained by the sequential filling and draining of transient storage zones such that water that has spent the longest time in the storage zone, and thus had the most time for N removal, drains back into the channel at the end of an outgoing tide, concurrent with shortest channel residence times. Capturing this inversion of the expected relationship between channel residence time and N removal highlights the need for non‐steady‐state reactive transport models. This article is protected by copyright. All rights reserved.
- Temporal responses of groundwater‐surface water exchange to
successive storm events
- Authors: Marina Dudley‐Southern; Andrew Binley
Abstract: Groundwater‐surface water exchange within the hyporheic zone is widely recognized as a key mechanism controlling the fate of nutrients within catchments. In gaining river systems, groundwater‐surface water interactions are constrained by upwelling groundwater but there is increasing evidence that a rapid rise in river stage during storm events can result in a temporary reversal of vertical hydraulic gradients, leading to surface water infiltration into the subsurface and supply of surface‐borne reactive solutes to this biogeochemically active interface. At a UK study site, using logged hydraulic heads in the surface water, riverbed and riverbanks and logged electrical conductivity at multiple depths in the riverbed we show that storm events can lead to a temporary reversal of vertical hydraulic gradient with mixing evident up to 30cm beneath the riverbed. Cross‐channel variability is evident, with the center of the channel consistently having shorter reversals of hydraulic gradient, compared to the channel margins. The direction of shallow subsurface riverbank flow at the site is also reactive to storm events, temporarily aligning with the surface flow direction and then reverting back to pre‐event conditions. Such a transition of flow paths during events is also likely to lead to expansion of lateral hyporheic exchange. This study provides evidence that storm events can be a key driver of enhanced hyporheic exchange in gaining river systems, which may support nutrient reactions beyond the duration of event‐driven change. Our observations demonstrate the dynamic nature of the hyporheic zone, which should be considered when evaluating its biogeochemical function. This article is protected by copyright. All rights reserved.
- Blending satellite‐based snow depth products with in situ
- Authors: Yuqiong Liu; Christa D. Peters‐Lidard, Sujay V. Kumar, Kristi R. Arsenault, David M. Mocko
Abstract: In snowmelt‐driven river systems, it is critical to enable reliable predictions of the spatio‐temporal variability in seasonal snowpack to support local and regional water management. Previous studies have shown that assimilating satellite‐station blended snow depth datasets can lead to improved snow predictions, which however do not always translate into improved streamflow predictions, especially in complex mountain regions. In this study, we explore how the existing optimal interpolation‐based blending strategy [Liu et al., 2013] can be enhanced to reduce biases in satellite snow depth products for improving streamflow predictions. Two major new considerations are explored, including: 1) incorporating terrain aspect and 2) incorporating areal snow coverage information. The methodology is applied to the bias reduction of the Advanced Microwave Scanning Radiometer for the Earth Observing System (AMSR‐E) snow depth estimates, which are then assimilated into the Noah land surface model via the ensemble Kalman Filtering (EnKF) for streamflow predictions in the Upper Colorado River Basin. Our results indicate that using only observations from low‐elevation stations such as the Global Historical Climatology Network (GHCN) in the bias correction can lead to underestimation in streamflow, while using observations from high‐elevation stations (e.g., the Snow Telemetry (SNOTEL) network) along with terrain aspect is critically important for achieving reliable streamflow predictions. Additionally incorporating areal snow coverage information from the Moderate Resolution Imaging Spectroradiometer (MODIS) can slightly improve the streamflow results further. This article is protected by copyright. All rights reserved.
- Ecohydrologic role of solar radiation on landscape evolution
- Authors: Omer Yetemen; Erkan Istanbulluoglu, J. Homero Flores‐Cervantes, Enrique R. Vivoni, Rafael L. Bras
Abstract: Solar radiation has a clear signature on the spatial organization of ecohydrologic fluxes, vegetation patterns and dynamics, and landscape morphology in semiarid ecosystems. Existing landscape evolution models (LEMs) do not explicitly consider spatially‐explicit solar radiation as model forcing. Here, we improve an existing LEM to represent coupled processes of energy, water, and sediment balance for semiarid fluvial catchments. To ground model predictions a study site is selected in central New Mexico where hillslope aspect has a marked influence on vegetation patterns and landscape morphology. Model predictions are corroborated using limited field observations in central NM and other locations with similar conditions. We design a set of comparative LEM simulations to investigate the role of spatially‐explicit solar radiation on landscape ecohydro‐geomorphic development under different uplift scenarios. Aspect‐ and network‐controls were identified as the two main drivers of soil moisture and vegetation organization on the landscape. Landscape‐scale and long‐term implications of these short‐term ecohdrologic patterns emerged in modeled landscapes. As north facing slopes (NFS) get steeper by continuing uplift they support erosion‐resistant denser vegetation cover which leads to further slope steepening until erosion and uplift attains a dynamic equilibrium. Conversely, on north facing slopes (SFS), as slopes grow with uplift, increased solar radiation exposure with slope supports sparser biomass and shallower slopes. At the landscape scale, these differential erosion processes lead to asymmetric development of catchment forms, consistent with regional observations. Understanding of ecohydro‐geomorphic evolution will improve to assess the impacts of past and future climates on landscape response and morphology. This article is protected by copyright. All rights reserved.
- Analysis of subsurface storage and streamflow generation in urban
- Authors: Aditi S. Bhaskar; Claire Welty
Abstract: Subsurface storage as a regulator of streamflow was investigated as an explanation for the large proportion of pre‐event water observed in urban streams during storm events. We used multiple lines of inquiry to explore the relationship between pre‐event water proportion, subsurface storage, and streamflow under storm conditions. First, we used a three‐dimensional model of integrated subsurface and surface flow and solute transport to simulate an idealized hillslope to perform model‐based chemical hydrograph separation of storm flow. Second, we employed simple dynamical systems analysis to derive the relationship between subsurface storage and streamflow for three Baltimore, Maryland watersheds (3.8 to 14km2 in area) along an urban‐to‐rural gradient. Last, we applied chemical hydrograph separation to high‐frequency specific conductance data in nested urban watersheds (∼50% impervious surface cover) in Dead Run, Baltimore County, Maryland. Unlike the importance of antecedent subsurface storage observed in some systems, we found that rainfall depth and not subsurface storage was the primary control on pre‐event water proportion in both field observations and hillslope numerical experiments. Field observations showed that antecedent stream base flow did not affect pre‐event water proportion or streamflow values under storm conditions. Hillslope model results showed that the relationship between streamflow values under storm conditions and subsurface storage was clockwise hysteretic. The simple dynamical systems approach showed that stream base flow in the most urbanized of three watersheds exhibited the largest sensitivity to changes in storage. This work raises questions about the streamflow generation mechanisms by which pre‐event water dominates urban storm hydrographs, and the shifts between mechanisms in rural and urban watersheds. This article is protected by copyright. All rights reserved.
- Functional error modeling for uncertainty quantification in hydrogeology
- Authors: L. Josset; D. Ginsbourger, I. Lunati
Abstract: Approximate models (proxies) can be employed to reduce the computational costs of estimating uncertainty. The price to pay is that the approximations introduced by the proxy model can lead to a biased estimation. To avoid this problem and ensure a reliable uncertainty quantification, we propose to combine Functional Data Analysis and Machine Learning to build error models that allow us to obtain an accurate prediction of the exact response without solving the exact model for all realizations. We build the relationship between proxy and exact model on a learning set of geostatistical realizations for which both exact and approximate solvers are run. Functional principal components analysis (FPCA) is used to investigate the variability in the two sets of curves and reduce the dimensionality of the problem while maximizing the retained information. Once obtained, the error model can be used to predict the exact response of any realization on the basis of the sole proxy response. This methodology is purpose‐oriented as the error model is constructed directly for the quantity of interest, rather than for the state of the system. Also, the dimensionality reduction performed by FPCA allows a diagnostic of the quality of the error model to assess the informativeness of the learning set and the fidelity of the proxy to the exact model. The possibility of obtaining a prediction of the exact response for any newly generated realization suggests that the methodology can be effectively used beyond the context of uncertainty quantification, in particular for Bayesian inference and optimization. This article is protected by copyright. All rights reserved.
- Evaluating the complementary relationship of evapotranspiration in the
alpine steppe of the Tibetan Plateau
- Authors: Ning Ma; Yinsheng Zhang, Jozsef Szilagyi, Yanhong Guo, Jianqing Zhai, Haifeng Gao
Abstract: The complementary relationship (CR) of evapotranspiration allows the estimation of the actual evapotranspiration rate (ETa) of the land surface using only routine meteorological data, which is of great importance in the Tibetan Plateau (TP) due to its sparse observation network. With the highest in‐situ automatic climate observation system in a typical semi‐arid alpine steppe region of the TP, the wind function of Penman was replaced by one based on the Monin‐Obukhov Similarity theory for calculating the potential evapotranspiration rate (ETp); the Priestley‐Taylor coefficient, α, was estimated using observations in wet days; and the slope of the saturation vapor pressure curve was evaluated at an estimate of the wet surface temperature, provided the latter was smaller than the actual air temperature. A symmetric CR was obtained between the observed daily actual and potential evapotranspiration. Local calibration of the parameter value (in this order) is key to obtaining a symmetric CR: α, wet environment air temperature (Twea), and wind function. Also, present symmetric CR contradicts previous research that used default parameter values for claiming an asymmetric CR in arid and semi‐arid regions of the TP. The effectiveness of estimating the daily ETa via symmetric CR was greatly improved when local calibrations were implemented. At the same time, an asymmetric CR was found between the observed daily ETa and pan evaporation rates (Epan), both for D20 above‐ground and E601B sunken pans. The daily ETa could also be estimated by coupling the Epan of D20 above‐ground and/or E601B sunken pan through CR. The former provided good descriptors for observed ETa, while the latter still tended to overestimate it to some extent. This article is protected by copyright. All rights reserved.
- Testing a simple model of gas bubble dynamics in porous media
- Authors: Jorge A. Ramirez; Andy J. Baird, Tom J. Coulthard, J. Michael Waddington
Abstract: Bubble dynamics in porous media are of great importance in industrial and natural systems. Of particular significance is the impact that bubble‐related emissions (ebullition) of greenhouse gases from porous media could have on global climate (e.g., wetland methane emissions). Thus predictions of future changes in bubble storage, movement and ebullition from porous media are needed. Methods exist to predict ebullition using numerical models, but all existing models are limited in scale (spatial and temporal) by high computational demands or represent porous media simplistically. A suitable model is needed to simulate ebullition at scales beyond individual pores or relatively small collections (< 10−4 m3) of connected pores. Here we present a cellular automaton model of bubbles in porous media that addresses this need. The model is computationally efficient, and could be applied over large spatial and temporal extent without sacrificing fine scale detail. We test this cellular automaton model against a physical model and find a good correspondence in bubble storage, bubble size and ebullition between both models. It was found that porous media heterogeneity alone can have a strong effect on ebullition. Furthermore, results from both models suggest that the frequency distributions of number of ebullition events per time and the magnitude of bubble loss are strongly right skewed, which partly explains the difficulty in interpreting ebullition events from natural systems. This article is protected by copyright. All rights reserved.
- Breakthrough curve moments scaling in hyporheic exchange
- Authors: A. Bellin; D. Tonina, A. Marzadri
Abstract: The interaction between stream flow and bedforms creates an uneven distribution of near‐bed energy heads, which is the driving force of hyporheic exchange. Owing to the large disparity of advection characteristic times in the stream and within the hyporheic zone, solute mass exchange is often modeled by considering the latter as an immobile region. In a recent contribution Gónzalez‐Pinzón et al.  showed that existing models employing this hypothesis are structurally inconsistent with the scaling revealed by the analysis of 384 breakthrough curves collected in 44 stream across five continents. Motivated by this result, we analyze the scaling characteristics of a model that we recently developed by combining the analytical solution of the advective flow within the hyporheic zone with a Lagrangian solute transport model. Results show that similarly to the experimental data our model predicts breakthrough curves showing a constant skewness, irrespective of the stream size, and that the scaling of the first three moments observed by Gónzalez‐Pinzón et al.  is also respected. Moreover, we propose regression curves that relate the first three moments of the residence time distribution with the alternate bar dimensionless depth (), a quantity that is easily measurable in the field. The connection between BTC moments and opens new possibilities for modeling transport processes at the catchment scale. This article is protected by copyright. All rights reserved.
- Challenges in modeling unstable two‐phase flow experiments in porous
- Authors: Andrea Ferrari; Joaquin Jimenez‐Martinez, Tanguy Le Borgne, Yves Méheust, Ivan Lunati
Abstract: The simulation of unstable invasion patterns in porous media flow is very challenging because small perturbations are amplified, so that slight differences in geometry or initial conditions result in significantly different invasion structures at later times. We present a detailed comparison of pore‐scale simulations and experiments for unstable primary drainage in porous micromodels. The porous media consist of Hele‐Shaw cells containing cylindrical obstacles. By means of soft lithography, we have constructed two experimental flow cells, with different degrees of heterogeneity in the grain size distribution. As the defending (wetting) fluid is the most viscous, the interface is destabilized by viscous forces, which promote the formation of preferential flow paths in the form of a branched finger structure. We model the experiments by solving the Navier‐Stokes equations for mass and momentum conservation in the discretized pore space and employ the Volume of Fluid (VOF) method to track the evolution of the interface. We test different numerical models (a 2D vertical integrated model and a full‐3D model) and different initial conditions, studying their impact on the simulated spatial distributions of the fluid phases. To assess the ability of the numerical model to reproduce unstable displacement, we compare several statistical and deterministic indicators. We demonstrate the impact of three main sources of error: i) the uncertainty on the pore space geometry, ii) the fact that the initial phase configuration cannot be known with an arbitrarily small accuracy, and iii) three dimensional effects. Although the unstable nature of the flow regime leads to different invasion structures due to small discrepancies between the experimental setup and the numerical model, a pore‐by‐pore comparison shows an overall satisfactory match between simulations and experiments. Moreover, all statistical indicators used to characterized the invasion structures are in excellent agreement. This validates the modeling approach, which can be used to complement experimental observations with information about quantities that are difficult or impossible to measure, such as the pressure and velocity fields in the two fluid phases. This article is protected by copyright. All rights reserved.
- Pore‐scale and multiscale numerical simulation of flow and transport
in a laboratory‐scale column
- Authors: Timothy D. Scheibe; William A. Perkins, Marshall C. Richmond, Matthew I. McKinley, Pedro D. J. Romero‐Gomez, Mart Oostrom, Thomas W. Wietsma, John A. Serkowski, John M. Zachara
Abstract: Pore‐scale models are useful for studying relationships between fundamental processes and phenomena at larger (i.e., Darcy) scales. However, the size of domains that can be simulated with explicit pore‐scale resolution is limited by computational and observational constraints. Direct numerical simulation of pore‐scale flow and transport is typically performed on millimeter‐scale volumes at which X‐ray computed tomography (XCT), often used to characterize pore geometry, can achieve micrometer resolution. In contrast, laboratory experiments that measure continuum properties are typically performed on decimeter‐scale columns. At this scale, XCT resolution is coarse (tens to hundreds of micrometers) and prohibits characterization of small pores and grains. We performed simulations of pore‐scale processes over a decimeter‐scale volume of natural porous media with a wide range of grain sizes, and compared to results of column experiments using the same sample. Simulations were conducted using high‐performance codes executed on a supercomputer. Two approaches to XCT image segmentation were evaluated, a binary (pores and solids) segmentation and a ternary segmentation that resolved a third category (porous solids with pores smaller than the imaged resolution). We used a multiscale Stokes‐Darcy simulation method to simulate the combination of Stokes flow in large open pores and Darcy‐like flow in porous solid regions. Flow and transport simulations based on the binary segmentation were inconsistent with experimental observations because of overestimation of large connected pores. Simulations based on the ternary segmentation provided results that were consistent with experimental observations, demonstrating our ability to successfully model pore‐scale flow over a column‐scale domain. This article is protected by copyright. All rights reserved.
- A Bayesian kriging approach for blending satellite and ground
- Authors: Andrew Verdin; Balaji Rajagopalan, William Kleiber, Chris Funk
Abstract: Drought and flood management practices require accurate estimates of precipitation. Gauge observations, however, are often sparse in regions with complicated terrain, clustered in valleys, and of poor quality. Consequently, the spatial extent of wet events is poorly represented. Satellite‐derived precipitation data is an attractive alternative, though it also tends to underestimate the magnitude of wet events due to its dependency on retrieval algorithms and the indirect relationship between satellite infrared observations and precipitation intensities. Here, we offer a Bayesian kriging approach for blending precipitation gauge data and the Climate Hazards Group Infrared Precipitation (CHIRP) satellite‐derived precipitation estimate for Central America and Colombia. First, the gauge observations are modeled as a linear function of satellite‐derived estimates and any number of other variables – for this research we include elevation. Prior distributions are defined for all model parameters and the posterior distributions are obtained simultaneously via Markov chain Monte Carlo (MCMC) sampling. The posterior distributions of these parameters are required for spatial estimation, and thus are obtained prior to implementing the spatial kriging model. This functional framework is applied to model parameters obtained by sampling from the posterior distributions, and the residuals of the linear model are subject to a spatial kriging model. Consequently, the posterior distributions and uncertainties of the blended precipitation estimates are obtained. We demonstrate this method by applying it to pentadal and monthly total precipitation fields during 2009. The model's performance and its inherent ability to capture wet events are investigated. It is shown that this blending method significantly improves upon the satellite‐derived estimates and is also competitive in its ability to represent wet events. This procedure also provides a means to estimate a full conditional distribution of the “true” observed precipitation value at each grid cell. This article is protected by copyright. All rights reserved.
- Three‐dimensional quantification of soil hydraulic properties using
X‐ray Computed Tomography and image‐based modeling
- Authors: Saoirse R. Tracy; Keith R. Daly, Craig J. Sturrock, Neil M. J. Crout, Sacha J. Mooney, Tiina Roose
Abstract: We demonstrate the application of a high‐resolution X‐ray Computed Tomography (CT) method to quantify water distribution in soil pores under successive reductive drying. We focus on the wet end of the water release characteristic (WRC) (0 to ‐75 kPa) to investigate changes in soil water distribution in contrasting soil textures (sand and clay) and structures (sieved and field structured), to determine the impact of soil structure on hydraulic behaviour. The 3D structure of each soil was obtained from the CT images (at a 10 µm resolution). Stokes equations for flow were solved computationally for each measured structure to estimate hydraulic conductivity. The simulated values obtained compared extremely well with the measured saturated hydraulic conductivity values. By considering different sample sizes we were able to identify that the smallest possible representative sample size which is required to determine a globally valid hydraulic conductivity. This article is protected by copyright. All rights reserved.
- Untangling the effects of urban development on subsurface storage in
- Authors: Aditi S. Bhaskar; Claire Welty, Reed M. Maxwell, Andrew J. Miller
Abstract: The impact of urban development on surface flow has been studied extensively over the last half century, but effects on groundwater systems are still poorly understood. Previous studies of the influence of urban development on subsurface storage have not revealed any consistent pattern, with results showing increases, decreases, and negligible change in groundwater levels. In this paper we investigated the effects of four key features that impact subsurface storage in urban landscapes. These include reduced vegetative cover, impervious surface cover, infiltration and inflow (I&I) of groundwater and stormwater into wastewater pipes, and other anthropogenic recharge and discharge fluxes including water supply pipe leakage and well and reservoir withdrawals. We applied the integrated groundwater‐surface water‐land surface model ParFlow.CLM to the Baltimore metropolitan area. We compared the base case (all four features) to simulations in which an individual urban feature was removed. For the Baltimore region, the effect of infiltration of groundwater into wastewater pipes had the greatest effect on subsurface storage (I&I decreased subsurface storage 11.1% relative to precipitation minus evapotranspiration after one year), followed by the impact of water supply pipe leakage and lawn irrigation (combined anthropogenic discharges and recharges led to a 7.4% decrease) and reduced vegetation (1.9% increase). Impervious surface cover led to a small increase in subsurface storage (0.56% increase) associated with decreased groundwater discharge as baseflow. The change in subsurface storage due to infiltration of groundwater into wastewater pipes was largest despite the smaller spatial extent of surface flux modifications, compared to other features. This article is protected by copyright. All rights reserved.
- A faster numerical scheme for a coupled system modeling soil erosion and
- Authors: M.‐H. Le; S. Cordier, C. Lucas, O. Cerdan
Abstract: Overland flow and soil erosion play an essential role in water quality and soil degradation. Such processes, involving the interactions between water flow and the bed sediment, are classically described by a well‐established system coupling the shallow water equations and the Hairsine‐Rose model. Numerical approximation of this coupled system requires advanced methods to preserve some important physical and mathematical properties; in particular the steady states and the positivity of both water depth and sediment concentration. Recently, finite volume schemes based on Roe's solver have been proposed by Heng et al.  and Kim et al.  for one and two‐dimensional problems. In their approach, an additional and artificial restriction on the time step is required to guarantee the positivity of sediment concentration. This artificial condition can lead the computation to be costly when dealing with very shallow flow and wet/dry fronts. The main result of this paper is to propose a new and faster scheme for which only the CFL condition of the shallow water equations is sufficient to preserve the positivity of sediment concentration. In addition, the numerical procedure of the erosion part can be used with any well‐balanced and positivity preserving scheme of the shallow water equations. The proposed method is tested on classical benchmarks and also on a realistic configuration. This article is protected by copyright. All rights reserved.
- Multiporosity flow in fractured low‐permeability rocks
- Authors: Kristopher L. Kuhlman; Bwalya Malama, Jason E. Heath
Abstract: A multiporosity extension of classical double and triple porosity fractured rock flow models for slightly compressible fluids is presented. The multiporosity model is an adaptation of the multirate solute transport model of Haggerty and Gorelick  to viscous flow in fractured rock reservoirs. It is a generalization of both pseudo‐steady‐state and transient interporosity flow double porosity models. The model includes a fracture continuum and an overlapping distribution of multiple rock matrix continua, whose fracture‐matrix exchange coefficients are specified through a discrete probability mass function. Semi‐analytical cylindrically symmetric solutions to the multiporosity mathematical model are developed using the Laplace transform to illustrate its behavior. The multiporosity model presented here is conceptually simple, yet flexible enough to simulate common conceptualizations of double and triple porosity flow. This combination of generality and simplicity makes the multiporosity model a good choice for flow in low‐permeability fractured rocks. This article is protected by copyright. All rights reserved.
- A data‐driven approach to develop physically sound predictors:
Application to depth‐averaged velocities on flows through submerged
arrays of rigid cylinders
- Authors: R. O. Tinoco; E. B. Goldstein, G. Coco
Abstract: We use a machine learning approach to seek an accurate, physically sound predictor, to estimate the mean velocity for open‐channel flow when submerged arrays of rigid cylinders (model vegetation) are present. A genetic programming routine is used to find a robust relationship between relevant properties of the model vegetation and flow parameters. We use published data from laboratory experiments covering a broad range of conditions to obtain an equation that matches the performance of other predictors from recent literature in terms of accuracy, while showing a less complex structure. We also investigate how different criteria for data selection, as well as the size of the data set used to train the algorithm, influences the accuracy of the resulting predictors. Our results show that a proper use of Machine‐Learning techniques does not only provide empirical correlations, but can yield physically sound models as representative of the physical processes involved. We provide a clear, thorough example of the application of GP, its advantages and shortcomings, to encourage the use of data‐driven techniques as part of the data analysis process, and to address common misconceptions of machine learning as simple correlation techniques or physically senseless statistical analysis. This article is protected by copyright. All rights reserved.
- Comment on “Objective extraction of channel heads from
high‐resolution topographic data” by F. J. Clubb, S. M. Mudd,
D. T. Milodowski, M. D. Hurst, and L. J. Slater
- Authors: Paola Passalacqua; Efi Foufoula‐Georgiou
- Reply to comment by P. Passalacqua and E. Foufoula‐Georgiou on
“Objective extraction of channel heads from high‐resolution
- Authors: Fiona Clubb; Simon Mudd, David Milodowski
- Water and entrapped air redistribution in heterogeneous sand sample:
Quantitative neutron imaging of the process
- Authors: Michal Snehota; Vladimira Jelinkova, Martina Sobotkova, Jan Sacha, Peter Vontobel, Jan Hovind
Abstract: Saturated flow in soil with the occurrence of preferential flow often exhibits temporal changes of saturated hydraulic conductivity even during the time scale of a single infiltration event. These effects, observed in a number of experiments done mainly on heterogeneous soils, are often attributed to the changing distribution of water and air in the sample. We have measured the variation of the flow rates during the steady state stage of the constant head ponded infiltration experiment conducted on a packed sample composed of three different grades of sand. The experiment was monitored by quantitative neutron imaging, which provided information about the spatial distribution of water in the sample. Measurements were taken during i) the initial stages of infiltration by neutron radiography and ii) during the steady state flow by neutron tomography. A gradual decrease of the hydraulic conductivity has been observed during the first four hours of the infiltration event.
A series of neutron tomography images taken during the quasi‐steady state stage showed the trapping of air bubbles in coarser sand. Furthermore, the water content in the coarse sand decreased even more while the water content in the embedded fine sand blocks gradually increased. The experimental results support the hypothesis that the effect of the gradual hydraulic conductivity decrease is caused by entrapped air redistribution and the build‐up of bubbles in preferential pathways. The trapped air thus restricts the preferential flow pathways and causes lower hydraulic conductivity. This article is protected by copyright. All rights reserved.
- Hydrophobic organic contaminant transport property heterogeneity in the
- Authors: Richelle M. Allen‐King; Indra Kalinovich, David F. Dominic, Guohui Wang, Reid Polmanteer, Dana Divine
Abstract: We determined that the spatial heterogeneity in aquifer properties governing the reactive transport of volatile organic contaminants is defined by the arrangement of lithofacies. We measured permeability (k) and perchloroethene sorption distribution coefficient (Kd) for lithofacies that we delineated for samples from the Canadian Forces Base Borden Aquifer. We compiled existing data and collected 57 new cores to characterize a 30 m section of the aquifer near the test location of Mackay et al. . The k and Kd were measured for samples taken at six elevations from all cores to create a data set consisting of nearly 400 co‐located measurements. Through analysis of variance (corrected for multiple comparisons), we determined that the twelve originally mapped lithofacies could be grouped into five relatively distinct chemohydrofacies that capture the variability of both transport properties. The mean of ln k by lithofacies was related to the grain size and the variance was relatively consistent. In contrast, both the mean and variance of ln Kd were greater for more poorly sorted lithofacies, which were also typically more coarse‐grained. Half of the aquifer sorption capacity occurred in the three highest‐sorbing lithofacies but comprised only 20% of its volume. The model of the aquifer that emerged is that of discontinuous scour‐fill deposits of medium sand, generally characterized by greater Kd and k, within laterally extensive fine‐ to very‐fine grained sands of lower Kd and k. Our findings demonstrate the importance of considering source rock composition, transport and deposition processes when constructing conceptual models of chemohydrofacies. This article is protected by copyright. All rights reserved.
- A physically based surface resistance model for evaporation from bare
- Authors: Chenming Zhang; Ling Li, David Lockington
Abstract: The resistance to vapor transfer across the soil‐air interface, termed surface resistance, plays an important role in determining the evaporation rate from unsaturated bare soils. A physically based analytical model is developed to describe the surface resistance under varying liquid water saturation. When the vaporization plane remains in the topmost soil layer (TSL), the model considers the vapor transport through the external diffusive layer (EDL), and the hydraulic connection between the capillary water in the TSL and underneath water source for evaporation. When the vaporization plane develops below the TSL, the model predicts the surface resistance by taking into account the development of the dry soil layer, the major barrier for vapor transport at the this soil‐drying stage. With the consideration of the soil pore size distribution, the model is applicable to different soil types. The model was validated against six sets of laboratory experiments on the drying process of initially water‐saturated soil columns under non‐isothermal conditions. These experiments were conducted using different soil types and/or heat intensities above the soil surface. The model was found to perform well over intermediate and low liquid water saturation ranges while underestimating the surface resistance for the high liquid water saturation range. The results suggest that the model overall represents reasonably well the processes underlying the vapor transfer across the soil‐air interface. Future model improvement may be gained by considering the hydraulic connection between the capillary water and film water in the TSL. This article is protected by copyright. All rights reserved.
- Predicting DNAPL mass discharge and contaminated site longevity
probabilities—Conceptual model and high‐resolution stochastic
- Authors: J Koch; W. Nowak
Abstract: Improper storage and disposal of non‐aqueous‐phase liquids (NAPLs) has resulted in widespread contamination of the subsurface, threatening the quality of groundwater as a freshwater resource. The high frequency of contaminated sites and the difficulties of remediation efforts demand rational decisions based on a sound risk assessment. Due to sparse data and natural heterogeneities, this risk assessment needs to be supported by appropriate predictive models with quantified uncertainty. This study proposes a physically and stochastically coherent model concept to simulate and predict crucial impact metrics for DNAPL contaminated sites, such as contaminant mass discharge and DNAPL source longevity. To this end, aquifer parameters and the contaminant source architecture are conceptualized as random space functions. The governing processes are simulated in a three‐dimensional, highly‐resolved, stochastic, and coupled model that can predict probability density functions of mass discharge and source depletion times. While it is not possible to determine whether the presented model framework is sufficiently complex or not, we can investigate whether and to which degree the desired model predictions are sensitive to simplifications often found in the literature. By testing four commonly made simplifications, we identified aquifer heterogeneity, groundwater flow irregularity, uncertain and physically‐based contaminant source zones, and their mutual interlinkages as indispensable components of a sound model framework. This article is protected by copyright. All rights reserved.
- Projected changes in snowfall extremes and interannual variability of
snowfall in the western U.S
- Authors: A. C. Lute; J. T. Abatzoglou, K. C. Hegewisch
Abstract: Projected warming will have significant impacts on snowfall accumulation and melt, with implications for water availability and management in snow‐dominated regions. Changes in snowfall extremes are confounded by projected increases in precipitation extremes. Downscaled climate projections from 20 global climate models were bias corrected to montane Snowpack Telemetry stations across the western United States to assess mid‐21st century changes in the mean and variability of annual snowfall water equivalent (SFE) and extreme snowfall events, defined by the 90th percentile of cumulative 3‐day SFE amounts. Declines in annual SFE and number of snowfall days were projected for all stations. Changes in the magnitude of snowfall event quantiles were sensitive to historical winter temperature. At climatologically cooler locations, such as in the Rocky Mountains, changes in the magnitude of snowfall events mirrored changes in the distribution of precipitation events, with increases in extremes and less change in more moderate events. By contrast, declines in snowfall event magnitudes were found for all quantiles in warmer locations. Common to both warmer and colder sites was a relative increase in the magnitude of snowfall extremes compared to annual SFE and a larger fraction of annual SFE from snowfall extremes. The coefficient of variation of annual SFE increased up to 80% in warmer montane regions due to projected declines in snowfall days and the increased contribution of snowfall extremes to annual SFE. In addition to declines in mean annual SFE, more frequent low snowfall years and less frequent high snowfall years were projected for every station. This article is protected by copyright. All rights reserved.
- On the upscaling of mass transfer rate expressions for interpretation of
source zone partitioning tracer tests
- Authors: Ali Boroumand; Linda M. Abriola
Abstract: Analysis of partitioning tracer tests conducted in dense nonaqueous phase liquid (DNAPL) source zones relies on conceptual models that describe mass exchange between the DNAPL and aqueous phases. Such analysis, however, is complicated by the complex distribution of entrapped DNAPL mass and formation heterogeneity. Due to parameter uncertainty in heterogeneous regions and the desire to reduce model complexity, the effect of mass transfer limitations is, thus, often neglected, and an equilibrium‐based model is typically used to interpret test results. This work explores the consequences of that simplifying assumption on test data interpretation and develops an alternative upscaled modeling approach to quantify effective mass transfer rates. To this end, a series of partitioning tracer tests is numerically simulated in heterogeneous two‐dimensional PCE‐DNAPL source zones, representative of a range of hydraulic conductivity and DNAPL mass distribution characteristics. The effective mass transfer coefficient corresponding to each test is determined by fitting an upscaled model to the simulated data, and regression analysis is performed to explore the correlation between various source zone metrics and the effective mass transfer coefficient. Results suggest that vertical DNAPL spreading, Reynolds number, pool fraction, and the effective organic phase saturation are the most significant parameters controlling tracer partitioning rates. Finally, a correlation for prediction of the effective (upscaled) mass transfer coefficient is proposed and verified using existing experimental data. The developed upscaled model incorporates the influence of physical heterogeneity on the rate of tracer partitioning and, thus, can be used for the estimation of source zone mass distribution characteristics from tracer test results. This article is protected by copyright. All rights reserved.
- Determining groundwater‐surface water exchange from temperature time
series: Combining a local polynomial method with a maximum likelihood
- Authors: G. Vandersteen; U. Schneidewind, C. Anibas, C. Schmidt, P. Seuntjens, O. Batelaan
Abstract: The use of temperature‐time series measured in streambed sediments as input to coupled water flow and heat transport models has become standard when quantifying vertical groundwater‐surface water exchange fluxes. We develop a novel methodology, called LPML, to estimate the parameters for 1D water flow and heat transport by combining a local polynomial (LP) signal processing technique with a maximum likelihood (ML) estimator. The LP method is used to estimate the frequency response functions (FRFs) and their uncertainties between the streambed top and several locations within the streambed from measured temperature‐time series data. Additionally, we obtain the analytical expression of the FRFs assuming a pure sinusoidal input. The estimated and analytical FRFs are used in an ML estimator to deduce vertical groundwater‐surface water exchange flux and its uncertainty as well as information regarding model quality. The LPML method is tested and verified with the heat transport models STRIVE and VFLUX. We demonstrate that the LPML method can correctly reproduce a priori known fluxes and thermal conductivities and also show that the LPML method can estimate averaged and time‐variable fluxes from periodic and non‐periodic temperature records. The LPML method allows for a fast computation of exchange fluxes as well as model and parameter uncertainties from many temperature sensors. Moreover, it can utilize a broad frequency spectrum beyond the diel signal commonly used for flux calculations. This article is protected by copyright. All rights reserved.
- Drivers of atmospheric nitrate processing and export in forested
- Authors: Lucy A. Rose; Stephen D. Sebestyen, Emily M. Elliott, Keisuke Koba
Abstract: Increased deposition of reactive atmospheric N has resulted in the nitrogen saturation of many forested catchments worldwide. Isotope‐based studies from multiple forest sites report low proportions (mean = ~10%) of unprocessed atmospheric nitrate in streams during baseflow, regardless of N deposition or nitrate export rates. Given similar proportions of atmospheric nitrate in baseflow across a variety of sites and forest types, it is important to address the post‐depositional drivers and processes that affect atmospheric nitrate transport and fate within catchments. In a meta‐analysis of stable isotope‐based studies, we examined the influence of methodological, biological, and hydrologic drivers on the export of atmospheric nitrate from forests. The δ18O‐NO3‐ values in stream waters may increase, decrease, or not change with increasing discharge during stormflow conditions, and δ18O‐NO3‐ values are generally higher in stormflow than baseflow. However, δ18O‐NO3‐ values tended to increase with increasing baseflow discharge at all sites examined. To explain these differences, we present a conceptual model of hydrologic flowpath characteristics (e.g., saturation overland flow versus subsurface stormflow) and the influence of topography on landscape‐stream hydrologic connectivity and delivery of unprocessed atmospheric nitrate to streams. Methodological biases resulting from differences in sampling frequency and stable isotope analytical techniques may further influence the perceived degree of unprocessed atmospheric nitrate export. Synthesis of results from numerous isotope‐based studies shows that small proportions of unprocessed atmospheric nitrate are common in baseflow. However, hydrologic, topographic, and methodological factors are important drivers of actual or perceived elevated contributions of unprocessed atmospheric nitrate to streams. This article is protected by copyright. All rights reserved.
- A bivariate extension of the Hosking and Wallis
goodness‐of‐fit measure for regional distributions
- Authors: T. R. Kjeldsen; I. Prosdocimi
Abstract: This study presents a bivariate extension of the goodness‐of‐fit measure for regional frequency distributions developed by Hosking and Wallis  for use with the method of L‐moments. Utilising the approximate joint normal distribution of the regional L‐skewness and L‐kurtosis, a graphical representation of the confidence region on the L‐moment diagram can be constructed as an ellipsoid. Candidate distributions can then be accepted where the corresponding theoretical relationship between the L‐skewness and L‐kurtosis intersects the confidence region, and the chosen distribution would be the one that minimises the Mahalanobis distance measure. Based on a set of Monte Carlo simulations it is demonstrated that the new bivariate measure generally selects the true population distribution more frequently than the original method. Results are presented to show that the new measure remains robust when applied to regions where the level of inter‐site correlation is at a level found in real world regions. Finally the method is applied to two different case studies involving annual maximum peak flow data from Italian and British catchments to identify suitable regional frequency distributions. This article is protected by copyright. All rights reserved.
- Hydroclimatic variables and acute gastro‐intestinal illness in
British Columbia, Canada: A time series analysis
- Authors: L.P. Galway; D.M. Allen, M.W. Parkes, L. Li, T.K. Takaro
Abstract: Using epidemiologic time‐series analysis, we examine associations between three hydroclimatic variables (temperature, precipitation, and streamflow) and waterborne acute gastro‐intestinal illness (AGI) in two communities in the province of British Columbia (BC), Canada. The communities were selected to represent the major hydroclimatic regimes that characterize BC: rainfall‐dominated and snowmelt‐dominated. Our results show that the number of monthly cases of AGI increased with increasing temperature, precipitation, and streamflow in the same month in the context of a rainfall‐dominated regime, and with increasing streamflow in the previous month in the context of a snowfall‐dominated regime. These results suggest that hydroclimatology plays a role in driving the occurrence and variability of AGI in these settings. Further, this study highlights that the nature and magnitude of the effects of hydroclimatic variability on AGI are different in the context of a snowfall‐dominated regime versus a rainfall‐dominated regimes. We conclude by proposing that the watershed may be an appropriate context for enhancing our understanding of the complex linkages between hydroclimatic variability and waterborne illness in the context of a changing climate. This article is protected by copyright. All rights reserved.
- Upstream water resource management to address downstream pollution
concerns: A policy framework with application to the Nakdong River basin
in South Korea
- Authors: Taeyeon Yoon; Charles Rhodes, Farhed A. Shah
Abstract: An empirical framework for assisting with water quality management is proposed that relies on open‐source hydrologic data. Such data are measured periodically at fixed water stations and commonly available in time‐series form. To fully exploit the data, we suggest that observations from multiple stations should be combined into a single long‐panel data set, and an econometric model developed to estimate upstream management effects on downstream water quality. Selection of the model's functional form and explanatory variables would be informed by rating curves, and idiosyncrasies across and within stations handled in an error term by testing contemporary correlation, serial correlation, and heteroskedasticity. Our proposed approach is illustrated with an application to the Nakdong River Basin in South Korea. Three alternative policies to achieve downstream BOD level targets are evaluated: upstream water treatment, greater dam discharge, and development of a new water source. Upstream water treatment directly cuts off incoming pollutants, thereby presenting the smallest variation in its downstream effects on BOD levels. Treatment is advantageous when reliability of water quality is a primary concern. Dam discharge is a flexible tool, and may be used strategically during a low‐flow season. We consider development of a new water corridor from an extant dam as our third policy option. This turns out to be the most cost‐effective way for securing lower BOD levels in the downstream target city. Even though we consider a relatively simple watershed to illustrate the usefulness of our approach, it can be adapted easily to analyze more complex upstream‐downstream issues. This article is protected by copyright. All rights reserved.
- Impact of velocity correlation and distribution on transport in fractured
media: Field evidence and theoretical model
- Authors: Peter K. Kang; Tanguy Le Borgne, Marco Dentz, Olivier Bour, Ruben Juanes
Abstract: Flow and transport through fractured geologic media often leads to anomalous (non‐Fickian) transport behavior, the origin of which remains a matter of debate: whether it arises from variability in fracture permeability (velocity distribution), connectedness in the flow paths through fractures (velocity correlation), or interaction between fractures and matrix. Here we show that this uncertainty of distribution‐ vs. correlation‐controlled transport can be resolved by combining convergent and push‐pull tracer tests because flow reversibility is strongly dependent on velocity correlation, whereas late‐time scaling of breakthrough curves is mainly controlled by velocity distribution. We build on this insight, and propose a Lagrangian statistical model that takes the form of a continuous time random walk (CTRW) with correlated particle velocities. In this framework, velocity distribution and velocity correlation are quantified by a Markov process of particle transition times that is characterized by a distribution function and a transition probability. Our transport model accurately captures the anomalous behavior in the breakthrough curves for both push‐pull and convergent flow geometries, with the same set of parameters. Thus, the proposed correlated CTRW modeling approach provides a simple yet powerful framework for characterizing the impact of velocity distribution and correlation on transport in fractured media. This article is protected by copyright. All rights reserved.
- Agricultural virtual water flows within U.S.
- Authors: Qian Dang; Xiaowen Lin, Megan Konar
Abstract: Trade plays an increasingly important role in the global food system, which is projected to be strained by population growth, economic development, and climate change. For this reason, there has been a surge of interest in the water resources embodied in international trade, referred to as ‘global virtual water trade’. In this paper, we present a comprehensive assessment of virtual water flows within the USA, a country with global importance as a major agricultural producer and trade power. This is the first study of domestic virtual water flows based upon intra‐national food transfer empirical data and it provides insight into how the properties of virtual water transfers vary across scales. We find that the volume of virtual water flows within the USA is equivalent to 51\% of international flows, which is slightly higher than the USA food value and mass shares, due to the fact that water‐intensive meat commodities comprise a much larger fraction of food transfers within the USA. The USA virtual water flow network is more social, homogeneous, and equitable than the global virtual water trade network, although it is still not perfectly equitable. Importantly, a core group of U.S. States is central to the network structure, indicating that both domestic and international trade may be vulnerable to disruptive climate or economic shocks in these U.S. States. This article is protected by copyright. All rights reserved.
- Bayesian inversion of Mualem‐van Genuchten parameters in a
multilayer soil profile: A data‐driven, assumption‐free
- Authors: Matthew W. Over; Ute Wollschläger, Carlos Andres Osorio‐Murillo, Yoram Rubin
Abstract: This paper introduces a hierarchical simulation and modeling framework that allows for inference and validation of the likelihood function in Bayesian inversion of vadose zone hydraulic properties. The likelihood function or its analogs (objective functions and likelihood measures) are commonly assumed to be multivariate Gaussian in form, however this assumption is not possible to verify without a hierarchical simulation and modeling framework. In this paper, we present the necessary statistical mechanisms for utilizing the hierarchical framework. We apply the hierarchical framework to the inversion of the vadose zone hydraulic properties within a multi‐layer soil profile conditioned on moisture content observations collected in the uppermost four layers. The key result of our work is that the goodness‐of‐fit validated likelihood function form provides empirical justification for the assumption of multivariate Gaussian likelihood functions in past and future inversions at similar sites. As an alternative, the likelihood function needs not be assumed to follow a parametric statistical distribution and can be computed directly using non‐parametric methods. The non‐parametric methods are considerably more computationally demanding and to demonstrate this approach we present a smaller dimension synthetic case study of evaporation from a soil column. The main drawback of our work is the increased computational expense of the inversion. This article is protected by copyright. All rights reserved.