- Wavelet‐based multiscale performance analysis: An approach to assess
and improve hydrological models
- Authors: Maheswaran Rathinasamy; Rakesh Khosa, Jan Adamowski, Sudheer ch, Partheepan G, Jatin Anand, Boini Narsimlu
Pages: n/a - n/a
Abstract: The temporal dynamics of hydrological processes are spread across different time timescales and, as such, the performance of hydrological models cannot be estimated reliably from global performance measures that assign a single number to the fit of a simulated time series to an observed reference series. Accordingly, it is important to analyze model performance at different time scales. Wavelets have been used extensively in the area of hydrological modeling for multi‐scale analysis, and have been shown to be very reliable and useful in understanding dynamics across timescales and as these evolve in time. In this paper, a wavelet based multi‐scale performance measure for hydrological models is proposed and tested (i.e. Multiscale Nash Sutcliffe Criteria, and Multiscale Normalized Root Mean Square Error). The main advantage of this method is that it provides a quantitative measure of model performance across different timescales. In the proposed approach, model and observed time series are decomposed using the Discrete Wavelet Transform (known as the a trous wavelet transform), and performance measures of the model are obtained at each time scale.The applicability of the proposed method was explored using various case studies ‐ both real as well as synthetic. The synthetic case studies included various kinds of errors (e.g., timing error, under and over prediction of high and low flows) in outputs from a hydrologic model. The real time case studies investigated in this study included simulation results of both the process based Soil Water Assessment Tool (SWAT) model, as well as statistical models, namely the Coupled Wavelet‐Volterra (WVC), Artificial Neural Network (ANN), and Auto Regressive Moving Average (ARMA) methods. For the SWAT model, data from Wainganga and Sind Basin (India) was used, while for the Wavelet Volterra, ANN and ARMA models, data from the Cauvery River Basin (India) and Fraser River (Canada) were used. The study also explored the effect of the choice of the wavelets in multi‐scale model evaluation. It was found that the proposed wavelet based performance measures, namely the MNSC (Multiscale Nash Sutcliffe Criteria) and MNRMSE (Multiscale Normalized Root Mean Square Error), are a more reliable measure than traditional performance measures such as the Nash Sutcliffe Criteria (NSC), Root Mean Square Error (RMSE), and Normalized Root Mean Square Error (NRMSE). Further, the proposed methodology can be used to: i) compare different hydrological models (both physical and statistical models), and ii) help in model calibration. This article is protected by copyright. All rights reserved.
- Regional frequency analysis conditioned on large‐scale atmospheric
or oceanic fields
- Authors: Benjamin Renard; Upmanu Lall
Pages: n/a - n/a
Abstract: Many studies report that hydrologic regimes are modulated by large‐scale modes of climate variability such as the El Niño Southern Oscillation (ENSO) or the North Atlantic Oscillation (NAO). Climate‐informed frequency analysis models have therefore been proposed to condition the distribution of hydrologic variables on climate indices. However, standard climate indices may be poor predictors in some regions. This paper therefore describes a regional frequency analysis framework that conditions the distribution of hydrologic variables directly on atmospheric or oceanic fields, as opposed to predefined climate indices.
This framework is based on a 2‐level probabilistic model describing both climate and hydrologic data. The climate dataset (predictor) is typically a time series of atmospheric of oceanic fields defined on a grid over some area, while the hydrologic dataset (predictand) is typically a regional dataset of station data (e.g. annual average flow at several gauging stations). A Bayesian estimation framework is used, so that a natural quantification of uncertainties affecting hydrologic predictions is available.
A case study aimed at predicting the number of autumn flood events in 16 catchments located in Mediterranean France using geopotential heights at 500 hPa over the North‐Atlantic region is presented. The temporal variability of hydrologic data is shown to be associated with a particular spatial pattern in the geopotential heights. A cross‐validation experiment indicates that the resulting probabilistic climate‐informed predictions are skillful: their reliability is acceptable and they are much sharper than predictions based on standard climate indices and baseline predictions that ignore climate information. This article is protected by copyright. All rights reserved.
- A conceptual model of people's vulnerability to floods
- Authors: Luca Milanesi; Marco Pilotti, Roberto Ranzi
Pages: n/a - n/a
Abstract: Hydraulic risk maps provide the baseline for land use and emergency planning. Accordingly, they should convey clear information on the potential physical implications of the different hazards to the stakeholders. This paper presents a vulnerability criterion focused on human stability in a flow specifically devised for rapidly evolving floods where life, before than economic values, might be threatened. The human body is conceptualized as a set of cylinders and its stability to slipping and toppling is assessed by forces and moments equilibrium. Moreover, a depth threshold to consider drowning is assumed. In order to widen its scope of application, the model takes the destabilizing effect of local slope (so far disregarded in the literature) and fluid density into account. The resulting vulnerability classification could be naturally subdivided in three levels (low, medium and high) that are limited by two stability curves for children and adults respectively. In comparison with the most advanced literature conceptual approaches, the proposed model is weakly parameterised and the computed thresholds fit better the available experimental data sets. A code that implements the proposed algorithm is provided. This article is protected by copyright. All rights reserved.
- Retrieval of river discharge solely from satellite imagery and
at‐many‐stations hydraulic geometry: Sensitivity to river form
and optimization parameters
- Authors: Colin J. Gleason; Laurence C. Smith, Jinny Lee
Pages: n/a - n/a
Abstract: Knowledge of river discharge is critically important for water resource management, climate modeling, and improved understanding of the global water cycle, yet discharge is poorly known in much of the world. Remote sensing holds promise to mitigate this gap, yet current approaches for quantitative retrievals of river discharge require in situ calibration or a priori knowledge of river hydraulics, limiting their utility in unmonitored regions. Recently, Gleason and Smith  demonstrated discharge retrievals within 20‐30% of in situ observations solely from Landsat TM satellite images through discovery of a river‐specific geomorphic scaling phenomenon termed at‐many‐stations hydraulic geometry (AMHG). This paper advances the AMHG discharge retrieval approach via additional parameter optimizations and validation on 34 gauged rivers spanning a diverse range of geomorphic and climatic settings. Sensitivity experiments reveal that discharge retrieval accuracy varies with river morphology, reach averaging procedure, and optimization parameters. Quality of remotely sensed river flow widths is also important. Recommended best practices include a proposed global parameter set for use when a priori information is unavailable. Using this global parameterization, AMHG discharge retrievals are successful for most investigated river morphologies (median RRMSE 33% of in situ gauge observations), except braided rivers (median RRMSE 74%), rivers having low at‐a‐station hydraulic geometry b exponents (reach‐averaged b < 0.1, median RRMSE 86%), and arid rivers having extreme discharge variability (median RRMSE >1000%). Excluding such environments, 26‐41% RRMSE agreement between AMHG discharge retrievals and in‐situ gauge observations suggests AMHG can meaningfully address global discharge knowledge gaps solely from repeat satellite imagery. This article is protected by copyright. All rights reserved.
- An error analysis of the Budyko hypothesis for assessing the contribution
of climate change to runoff
- Authors: Hanbo Yang; Dawen Yang, Qingfang Hu
Pages: n/a - n/a
Abstract: Many previous studies have evaluated the hydrologic response to climate change using the first‐order approximation (first‐order Taylor expansion) of the Mezentsev‐Choudhury‐Yang equation (formulating the Budyko hypothesis), which has a parameter n representing catchment characteristics. However, no studies have paid attention to the error due to the first‐order approximation. This study therefore estimates this error to improve the theoretical framework for assessing the contribution of climate change to runoff based on the Budyko hypothesis. Specifically, the error increases when precipitation (P) decreases and potential evaporation (E0) increases, and n increases. Therefore, increasing P or decreasing E0 lead to an underestimate of the climatic contributions, while a decreasing P or increasing E0 lead to an overestimate. In addition, we suggest a new method to accurately estimate the contribution of climate change to runoff. This article is protected by copyright. All rights reserved.
- Morphodynamics of river‐influenced back‐barrier tidal basins:
The role of landscape and hydrodynamic settings
- Authors: Z. Zhou; G. Coco, M. Jiménez, M. Olabarrieta, M. van der Wegen, I. Townend
Pages: n/a - n/a
Abstract: We investigate the morphodynamics of river‐influenced barrier basins numerically, with a particular emphasis on the effects of landscape and hydrodynamic settings. The simulated morphologies are qualitatively comparable to natural systems (e.g., tidal inlets along the East Coast of the U.S.). Model results suggest that the basin morphology is governed by the relative importance of tidal and fluvial forcing which is reflected, to the first‐order approximation, in the ratio (rQ) between the mean tidal and river discharge. In agreement with empirical knowledge, the model indicates that riverine influence can be neglected when rQ is larger than 20. On the other hand, the river may dominate when rQ is smaller than 5.
Pronounced differences in morphodynamic evolution are observed for different landscape settings (i.e., initial basin bathymetries and river inflow locations), indicating their fundamental importance in governing the evolution of barrier basins. Model results also show that the addition of a river tends to compensate the flood dominance in the tidal basin. Overall, the river flow has limited influence on the volumetric change of tidal flats, while it plays a more important role in determining the depth of the tidal channels and the size of the ebb delta. The riverine sediment source appears to be more important in shaping the basin morphology when the fluvial forcing is stronger. Last, we show that the presence of a large river in a tidal inlet system influences the performance of the widely‐adopted relation between tidal prism and inlet cross‐sectional area. This article is protected by copyright. All rights reserved.
- Relating relative hydraulic and electrical conductivity in the unsaturated
- Authors: Chloe Mawer; Rosemary Knight, Peter K. Kitanidis
Pages: n/a - n/a
Abstract: Numerical modeling was used to generate pore‐scale structures with different structural properties. They were partially saturated according to wetting and drainage regimes using morphological operations for a range of saturations. The hydraulic and electrical conductivities of the resulting partially saturated grain packs were numerically computed to produce relative hydraulic conductivity versus saturation and relative electrical conductivity versus saturation curves. The relative hydraulic conductivities were then compared to the relative electrical conductivities for the same saturations and it was found that relative hydraulic conductivity could be expressed as relative electrical conductivity to a power law exponent, β. This exponent β was not correlated to porosity, specific surface area, or tortuosity. It did change according to whether the soil was wetting or draining. However, a β value of 2.1 reproduced relative hydraulic conductivity from relative electrical conductivity with little added error. The effects of surface conduction on the observed power‐law relationship due to either low fluid electrical conductivity or increased clay content were analyzed. The relationship was found to hold for fluid conductivities typical of groundwater and for clay content of less than 5% if the clays were layered perpendicular to electrical flow. The relationship breaks down for electrical flow parallel to clay layers, which makes the choice of electrode arrangement important in cases where clay may be present. This relationship can be used with secondary pressure or saturation data to characterize a soil's hydraulic conductivity curve. This article is protected by copyright. All rights reserved.
- The daily mean zero‐flux plane during soil‐controlled
evaporation: A Green's function approach
- Authors: Wilfried Brutsaert
Pages: n/a - n/a
Abstract: A solution is presented of the linearized Richards equation with inclusion of gravity and with appropriate boundary conditions describing the combined soil‐controlled surface evaporation and the downward infiltration, following the application of a given amount of precipitation or irrigation. This solution is shown to agree with available field measurements, namely the evolution with time of the zero‐flux plane depth and of the rate of soil‐controlled evaporation from the bare soil surface. The problem is solved by means of the Green's function method; the result is general enough to be also applicable to flow problems in linear soils with boundary conditions substantially different from the ones considered herein.
- A modified Holly‐Preissmann scheme for simulating sharp
- Authors: Zhao‐wei Liu; De‐jun Zhu, Yong‐can Chen, Zhi‐gang Wang
Pages: n/a - n/a
Abstract: RIV1Q is the stand‐alone water quality program of CE‐QUAL‐RIV1, a hydraulic and water quality model developed by U.S. Army Corps of Engineers Waterways Experiment Station. It utilizes an operator‐splitting algorithm and the advection term in governing equation is treated using the explicit two‐point, fourth‐order accurate, Holly‐Preissmann scheme, in order to preserve numerical accuracy for advection of sharp gradients in concentration. In the scheme, the spatial derivative of the transport equation, where the derivative of velocity is included, is introduced to update the first derivative of dependent variable. In the stream with larger cross sectional variation, steep velocity gradient can be easily found and should be estimated correctly. In the original version of RIV1Q, however, the derivative of velocity is approximated by a finite difference which is first‐order accurate. Its leading truncation error leads to the numerical error of concentration which is related with the velocity and concentration gradients and increases with the decreasing Courant number. The simulation may also be unstable when a sharp velocity drop occurs. In the present paper, the derivative of velocity is estimated with a modified second‐order accurate scheme and the corresponding numerical error of concentration decreases. Additionally, the stability of the simulation is improved. The modified scheme is verified with a hypothetical channel case and the results demonstrate that satisfactory accuracy and stability can be achieved even when the Courant number is very low. Finally, the applicability of the modified scheme is discussed.
- Quantifying stream thermal regimes at multiple scales: Combining thermal
infrared imagery and stationary stream temperature data in a novel
- Authors: Shane J. Vatland; Robert E. Gresswell, Geoffrey C. Poole
Pages: n/a - n/a
Abstract: Accurately quantifying stream thermal regimes can be challenging because stream temperatures are often spatially and temporally heterogeneous. In this study, we present a novel modeling framework that combines stream temperature data sets that are continuous in either space or time. Specifically, we merged the fine spatial resolution of thermal infrared (TIR) imagery with hourly data from 10 stationary temperature loggers in a 100 km portion of the Big Hole River, MT, USA. This combination allowed us to estimate summer thermal conditions at a relatively fine spatial resolution (every ~100 m of stream length) over a large extent of stream (~100 km of stream) during the warmest part of the summer. Rigorous evaluation, including internal validation, external validation with spatially continuous instream temperature measurements collected from a Langrangian frame of reference, and sensitivity analyses, suggests the model was capable of accurately estimating longitudinal patterns in summer stream temperatures for this system (validation RMSEs < 1 °C). Results revealed considerable spatial and temporal heterogeneity in summer stream temperatures and highlighted the value of assessing thermal regimes at relatively fine spatial and temporal scales. Preserving spatial and temporal variability and structure in abiotic stream data provides a critical foundation for understanding the dynamic, multiscale habitat needs of mobile stream organisms. Similarly, enhanced understanding of spatial and temporal variation in dynamic water quality attributes, including temporal sequence and spatial arrangement, can guide strategic placement of monitoring equipment that will subsequently capture variation in environmental conditions directly pertinent to research and management objectives.
- Modeling and Mitigating Natural Hazards: Stationarity is Immortal!
- Authors: Alberto Montanari; Demetris Koutsoyiannis
Pages: n/a - n/a
Abstract: Environmental change is a reason of relevant concern as it is occurring at an unprecedented pace and might increase natural hazards. Moreover, it is deemed to imply a reduced representativity of past experience and data on extreme hydroclimatic events. The latter concern has been epitomized by the statement that “stationarity is dead”. Setting up policies for mitigating natural hazards, including those triggered by floods and droughts, is an urgent priority in many countries, which implies practical activities of management, engineering design and construction. These latter necessarily need to be properly informed and therefore the research question on the value of past data is extremely important. We herein argue that there are mechanisms in hydrological systems that are time invariant, which may need to be interpreted through data inference. In particular, hydrological predictions are based on assumptions which should include stationarity, as any hydrological model, including deterministic and non‐stationary approaches, is affected by uncertainty and therefore should include a random component that is stationary. Given that an unnecessary resort to non‐stationarity may imply a reduction of predictive capabilities, a pragmatic approach, based on the exploitation of past experience and data is a necessary prerequisite for setting up mitigation policies for environmental risk.
- Inverse modeling of geochemical and mechanical compaction in sedimentary
basins through Polynomial Chaos Expansion
- Authors: G. Porta; L. Tamellini, V. Lever, M. Riva
Pages: n/a - n/a
Abstract: We present an inverse modeling procedure for the estimation of model parameters of sedimentary basins subject to compaction driven by mechanical and geochemical processes. We consider a sandstone basin whose dynamics are governed by a set of unknown key quantities. These include geophysical and geochemical system attributes as well as pressure and temperature boundary conditions. We derive a reduced (or surrogate) model of the system behavior based on generalized Polynomial Chaos Expansion (gPCE) approximations, which are directly linked to the variance‐based Sobol indices associated with the selected uncertain model parameters. Parameter estimation is then performed within a Maximum Likelihood (ML) framework. We then study the way the ML inversion procedure can benefit from the adoption of anisotropic polynomial approximations (a‐gPCE) in which the surrogate model is refined only with respect to selected parameters according to an analysis of the nonlinearity of the input‐output mapping, as quantified through the Sobol sensitivity indices. Results are illustrated for a one‐dimensional setting involving quartz cementation and mechanical compaction in sandstones. The reliability of gPCE and a‐gPCE approximations in the context of the inverse modeling framework is assessed. The effects of (a) the strategy employed to build the surrogate model, leading either to a gPCE or a‐gPCE representation, and (b) the type and quality of calibration data on the goodness of the parameter estimates is then explored.
- Integration of altimetric lake levels and GRACE gravimetry over Africa:
Inferences for terrestrial water storage change 2003–2011
- Authors: P Moore; S D P Williams
Pages: n/a - n/a
Abstract: Terrestrial water storage (TWS) change for 2003‐2011 is estimated over Africa from GRACE gravimetric data. The signatures from change in water of the major lakes are removed by utilising kernel functions with lake heights recovered from retracked ENVISAT satellite altimetry. In addition, the contribution of gravimetric change due to soil moisture and biomass is removed from the total GRACE signal by utilising the GLDAS land surface model. The residual TWS time series, namely ground water and the surface waters in rivers, wetlands and small lakes, are investigated for trends and the seasonal cycle using linear regression. Typically, such analyses assume the data is temporally uncorrelated but this has been shown to lead to erroneous inferences in related studies concerning the linear rate and acceleration. In this study, we utilise autocorrelation and investigate the appropriate stochastic model. The results show the proper distribution of TWS change and identify the spatial distribution of significant rates and accelerations. The effect of surface water in the major lakes is shown to contribute significantly to the trend and seasonal variation in TWS in the lake basin. Lake Volta, a managed reservoir in Ghana, is seen to have a contribution to the linear trend that is a factor of three greater than that of Lake Victoria despite having a surface area one eighth of that of Lake Victoria. Analysis also shows the confidence levels of the deterministic trend and acceleration identifying areas where the signatures are most likely due to a physical deterministic cause and not simply stochastic variations.
- Sub‐second pore scale displacement processes and relaxation dynamics
in multiphase flow
- Authors: Ryan T. Armstrong; Holger Ott, Apostolos Georgiadis, Alex Schwing, Steffen Berg, Maja Rücker
Pages: n/a - n/a
Abstract: With recent advances at X‐ray micro‐computed tomography (μCT) synchrotron beam lines, it is now possible to study pore‐scale flow in porous rock under dynamic flow conditions. The collection of 4 dimensional data allows for the direct 3D visualization of fluid‐fluid displacement in porous rock as a function of time. However, even state‐of‐the‐art fast‐μCT scans require between one and a few seconds to complete and the much faster fluid movement occurring during that time interval is manifested as imaging artifacts in the reconstructed 3D volume. We present an approach to analyze the 2D radiograph data collected during fast‐μCT to study the pore‐scale displacement dynamics on the time scale of 40 milliseconds which is near the intrinsic time scale of individual Haines jumps. We present a methodology to identify the time intervals at which pore scale displacement events in the observed field of view occur and hence, how reconstruction intervals can be chosen to avoid fluid‐movement induced reconstruction artifacts. We further quantify the size, order, frequency, and location of fluid‐fluid displacement at the millisecond time scale. We observe that after a displacement event, the pore scale fluid distribution relaxes to (quasi‐) equilibrium in cascades of pore‐scale fluid re‐arrangements with an average relaxation time for the whole cascade between 0.5 and 2.0 seconds. These findings help to identify the flow regimes and intrinsic time and length scales relevant to fractional flow. While the focus of the work is in the context of multiphase flow, the approach could be applied to many different μCT applications where morphological changes occur at a time scale less than that required for collecting a μCT scan.
- Improved Bayesian Multi‐modeling: Integration of Copulas and
Bayesian Model Averaging
- Authors: Shahrbanou Madadgar; Hamid Moradkhani
Pages: n/a - n/a
Abstract: Bayesian Model Averaging (BMA) is a popular approach to combine hydrologic forecasts from individual models, and characterize the uncertainty induced by model structure. In the original form of BMA, the conditional probability density function (PDF) of each model is assumed to be a particular probability distribution (e.g. Gaussian, gamma, etc.). If the predictions of any hydrologic model do not follow certain distribution, a data transformation procedure is required prior to model averaging. Moreover, it is strongly recommended to apply BMA on unbiased forecasts, whereas it is sometimes difficult to effectively remove bias from the predictions of complex hydrologic models. To overcome these limitations, we develop an approach to integrate a group of multivariate functions, the so‐called copula functions, into BMA. Here, we introduce a copula‐embedded BMA (Cop‐BMA) method that relaxes any assumption on the shape of conditional PDFs. Copula functions have a flexible structure and do not restrict the shape of posterior distributions. Furthermore, copulas are effective tools in removing bias from hydrologic forecasts. To compare the performance of BMA with Cop‐BMA, they are applied to hydrologic forecasts from different rainfall‐runoff and land‐surface models. We consider the streamflow observation and simulations for ten river basins provided by the Model Parameter Estimation Experiment (MOPEX) project. Results demonstrate that the predictive distributions are more accurate and reliable, less biased, and more confident with small uncertainty after Cop‐BMA application. It is also shown that the post‐processed forecasts have better correlation with observation after Cop‐BMA application.
- Issue Information
- Pages: i - vi
- A new method for analysis of variance of the hydraulic and reactive
attributes of aquifers as linked to hierarchical and multiscaled
- Authors: Mohamad Reza Soltanian; Robert W. Ritzi
Pages: n/a - n/a
Abstract: This technical note presents a useful methodology for studying how the variance of hydraulic and/or reactive attributes of an aquifer are linked to the multi‐scaled and hierarchical sedimentary architecture of the aquifer. A new recursive equation is derived which quantitatively describes how the variance is related to sedimentary facies defined at all scales across an entire stratal hierarchy. As compared to prior published equations that emphasize differences in means among facies populations within a hierarchical level, it emphasizes differences across levels. Because of the hierarchical relationships among the terms of the equation, we find it to be useful for conducting a holistic analysis of the relative contributions to the variance arising from all facies types defined across all scales. The methodology is demonstrated using appropriate field data, and is shown to be useful in defining parsimonious classification systems.
- Soil water storage, rainfall, and runoff relationships in a tropical dry
- Authors: Kegan K. Farrick; Brian A. Branfireun
Pages: n/a - n/a
Abstract: In forested catchments the exceedance of rainfall and antecedent water storage thresholds is often required for runoff generation, yet to our knowledge these threshold relationships remain undescribed in tropical dry forest catchments. We therefore identified the controls of streamflow activation and the timing and magnitude of runoff in a tropical dry forest catchment near the Pacific coast of central Mexico. During a 52 day transition phase from the dry to wet season, soil water movement was dominated by vertical flow which continued until a threshold soil moisture content of 26% was reached at 100 cm below the surface. This satisfied a 162 mm storage deficit and activated streamflow, likely through lateral subsurface flow pathways. High antecedent soil water conditions were maintained during the wet phase but had a weak influence on stormflow. We identified a threshold value of 289 mm of summed rainfall and antecedent soil water needed to generate >4 mm of stormflow per event. Above this threshold, stormflow response and magnitude was almost entirely governed by rainfall event characteristics and not antecedent soil moisture conditions. Our results show that over the course of the wet season in tropical dry forests the dominant controls on runoff generation changed from antecedent soil water and storage to the depth of rainfall.
- A method for characterizing desiccation‐induced consolidation and
permeability loss of organic soils
- Authors: Chelsea L. Arnold; Teamrat A. Ghezzehei
Pages: n/a - n/a
Abstract: A new method was developed to measure soil consolidation by capillary suction in organic soils. This method differs from previous methods of measuring soil consolidation in that no external load is utilized and only the forces generated via capillary suction consolidate the soil matrix. This limits the degree of consolidation that can occur, but gives a more realistic ecological perspective on the response of organic soils to desiccation in the field. This new method combines the principles behind a traditional triaxial cell (for measurements of volume change), a pressure plate apparatus, (to facilitate drainage by capillary suction), and the permeameter, (to measure saturated hydraulic conductivity), and allows for simultaneous desaturation of the soil while monitoring desiccation induced volume change in the soil. This method also enables detection of historic limit of dryness. The historic limit of dryness is a novel concept that is unique to soils that have never experienced drying since their formation. It is fundamentally equivalent to the pre‐compression stress of externally loaded soils. This method is particularly important for forecasting structural and hydrologic changes that may occur in soils that were formed in very wet regimes (e.g., wet meadows at the foot of persistent snow packs and permafrost peats) as they respond to a changing climate.
- Hybrid modeling and receding horizon control of sewer networks
- Authors: Bernat Joseph‐Duran; Carlos Ocampo‐Martinez, Gabriela Cembrano
Pages: n/a - n/a
Abstract: In this work, a control‐oriented sewer network model is presented based on a hybrid linear modeling framework. The model equations are described independently for each network element, thus allowing the model to be applied to a broad class of networks. A parameter calibration procedure using data obtained from simulation software that solves the physically based model equations is described and validation results are given for a case study. Using the control model equations, an optimal control problem to minimize flooding and pollution is formulated to be solved by means of mixed‐integer linear or quadratic programming. A receding horizon control strategy based on this optimal control problem is applied to the case study using the simulation software as a virtual reality. Results of this closed‐loop simulation tests show the effectiveness of the proposed approach in fulfilling the control objectives while complying with physical and operational constraints.
- Sensitivity of snowpack storage to precipitation and temperature using
spatial and temporal analog models
- Authors: Charles H. Luce; Viviana Lopez‐Burgos, Zachary Holden
Pages: n/a - n/a
Abstract: Empirical sensitivity analyses are important for evaluation of the effects of a changing climate on water resources and ecosystems. Although mechanistic models are commonly applied for evaluation of climate effects for snowmelt, empirical relationships provide a first‐order validation of the various postulates required for their implementation. Previous studies of empirical sensitivity for April 1 snow water equivalent (SWE) in the western United States were developed by regressing interannual variations in SWE to winter precipitation and temperature. This offers a temporal analog for climate change, positing that a warmer future looks like warmer years. Spatial analogs are used to hypothesize that a warmer future may look like warmer places, and are frequently applied alternatives for complex processes, or states/metrics that show little interannual variability (e.g. forest cover). We contrast spatial and temporal analogs for sensitivity of April 1 SWE and the mean residence time of snow (SRT) using data from 524 Snowpack Telemetry (SNOTEL) stations across the western US. We built relatively strong models using spatial analogs to relate temperature and precipitation climatology to snowpack climatology (April 1 SWE, R2=0.87, and SRT, R2=0.81). Although the poorest temporal analog relationships were in areas showing the highest sensitivity to warming, spatial analog models showed consistent performance throughout the range of temperature and precipitation. Generally, slopes from the spatial relationships showed greater thermal sensitivity than the temporal analogs, and high elevation stations showed greater vulnerability using a spatial analog than shown in previous modeling and sensitivity studies. The spatial analog models provide a simple perspective to evaluate potential futures and may be useful in further evaluation of snowpack with warming.
- An integrated modeling framework for exploring flow regime and water
quality changes with increasing biofuel crop production in the US Corn
- Authors: Mary A. Yaeger; Mashor Housh, Ximing Cai, Murugesu Sivapalan
Pages: n/a - n/a
Abstract: To better address the dynamic interactions between human and hydrologic systems, we develop an integrated modeling framework that employs a System of Systems optimization model to emulate human development decisions which are then incorporated into a watershed model to estimate the resulting hydrologic impacts. The two models are run interactively to simulate the co‐evolution of coupled human‐nature systems, such that reciprocal feedbacks between hydrologic processes and human decisions (i.e., human impacts on critical low flows and hydrologic impacts on human decisions on land and water use) can be assessed. The framework is applied to a Midwestern US agricultural watershed, in the context of proposed biofuels development. This operation is illustrated by projecting three possible future co‐evolution trajectories, two of which use dedicated biofuel crops to reduce annual watershed nitrate export while meeting ethanol production targets. Imposition of a primary external driver (biofuel mandate) combined with different secondary drivers (water quality targets) results in highly nonlinear and multi‐scale responses of both the human and hydrologic systems, including multiple tradeoffs, impacting the future co‐evolution of the system in complex, heterogeneous ways. The strength of the hydrologic response is sensitive to the magnitude of the secondary driver; 45% nitrate reduction target leads to noticeable impacts at the outlet, while a 30% reduction leads to noticeable impacts that are mainly local. The local responses are conditioned by previous human hydrologic modifications and their spatial relationship to the new biofuel development, highlighting the importance of past co‐evolutionary history in predicting future trajectories of change.
- Model selection on solid ground: Rigorous comparison of nine ways to
evaluate Bayesian model evidence
- Authors: Anneli Schöniger; Thomas Wöhling, Luis Samaniego, Wolfgang Nowak
Pages: n/a - n/a
Abstract: Bayesian model selection or averaging objectively ranks a number of plausible, competing conceptual models based on Bayes' theorem. It implicitly performs an optimal tradeoff between performance in fitting available data and minimum model complexity. The procedure requires determining Bayesian model evidence (BME), which is the likelihood of the observed data integrated over each model's parameter space. The computation of this integral is highly challenging because it is as high‐dimensional as the number of model parameters. Three classes of techniques to compute BME are available, each with its own challenges and limitations: 1) Exact and fast analytical solutions are limited by strong assumptions. 2) Numerical evaluation quickly becomes unfeasible for expensive models. 3) Approximations known as information criteria (ICs) such as the AIC, BIC, or KIC (Akaike, Bayesian or Kashyap information criterion, respectively) yield contradicting results with regard to model ranking. Our study features a theory‐based intercomparison of these techniques. We further assess their accuracy in a simplistic synthetic example where for some scenarios an exact analytical solution exists. In more challenging scenarios, we use a brute‐force Monte Carlo integration method as reference. We continue this analysis with a real‐world application of hydrological model selection. This is a first‐time benchmarking of the various methods for BME evaluation against true solutions. Results show that BME values from ICs are often heavily biased and that the choice of approximation method substantially influences the accuracy of model ranking. For reliable model selection, bias‐free numerical methods should be preferred over ICs whenever computationally feasible.
- Functional approach to exploring climatic and landscape controls of runoff
generation: 1. Behavioral constraints on runoff volume
- Authors: Hong‐Yi Li; Murugesu Sivapalan, Fuqiang Tian, Ciaran Harman
Pages: n/a - n/a
Abstract: Inspired by the Dunne diagram, the climatic and landscape controls on the partitioning of annual runoff into its various components (Hortonian and Dunne overland flow and subsurface stormflow) are assessed quantitatively, from a purely theoretical perspective. A simple distributed hydrologic model has been built sufficient to simulate the effects of different combinations of climate, soil and topography on the runoff generation processes. The model is driven by a sequence of simple hypothetical precipitation events, for a large combination of climate and landscape properties, and hydrologic responses at the catchment scale are obtained through aggregation of grid‐scale responses. It is found, firstly, that the water balance responses, including relative contributions of different runoff generation mechanisms, could be related to a small set of dimensionless similarity parameters. These capture the competition between the wetting, drying, storage and drainage functions underlying the catchment responses, and in this way provide a quantitative approximation of the conceptual Dunne diagram. Secondly, only a subset of all hypothetical catchment/climate combinations is found to be “behavioral”, in terms of falling sufficiently close to the Budyko curve, describing mean annual runoff as a function of climate aridity. Furthermore, these behavioral combinations are mostly consistent with the qualitative picture presented in the Dunne diagram, indicating clearly the commonality between the Budyko curve and the Dunne diagram. These analyses also suggest clear inter‐relationships amongst the “behavioral” climate, soil and topography parameter combinations, implying these catchment properties may be constrained to be co‐dependent in order to satisfy the Budyko curve.
- Functional approach to exploring climatic and landscape controls on runoff
generation: 2. Timing of runoff storm response
- Authors: Hong‐Yi Li; Murugesu Sivapalan
Pages: n/a - n/a
Abstract: Hortonian overland flow, Dunne overland flow and subsurface stormflow are the three most dominant mechanisms contributing to both the volume and timing of streamflow in headwater catchments. In this paper, guided by the Dunne diagram, we explore the impacts of climate, soil and topography on estimated probability distributions of the travel times of each of these three runoff components. In each case, these are expressed in terms of the Connected Instantaneous Response Functions (CIRF) and account for the dynamics of their individual partial effective contributing areas that retain the connectivity to the outlet (instead of the whole catchment area). A spatially distributed hydrological model is used to derive the CIRFs numerically under multiple combinations of climate, soil and topographic properties. The mean travel times and dimensionless forms of the CIRFs (i.e., scaled by their respective mean travel times) are used to examine both advective and dispersive aspects of catchment's runoff routing response. It is found that the CIRFs, upon non‐dimensionalization, collapsed to common characteristic shapes, which could be explained in terms of the relative contributions of hillslope and channel network flows, and the size of runoff contributing areas. The contributing areas, particularly for the Dunne overland flow, are themselves found to be governed by the competition between drainage of and recharge to the water table, and could be explained by a dimensionless drainage index which quantifies this competition. The study also reveals simple indicators based on landscape properties that can explain the magnitude of travel times in different catchments.
- Heterogeneity‐Enhanced Gas Phase Formation in Shallow Aquifers
During Leakage of CO2‐Saturated Water from Geologic Sequestration
- Authors: Michael Plampin; Rune Lassen, Toshihiro Sakaki, Mark Porter, Rajesh Pawar, Karsten H. Jensen, Tissa Illangasekare
Pages: n/a - n/a
Abstract: A primary concern for geologic carbon storage is the potential for leakage of stored carbon dioxide (CO2) into the shallow subsurface where it could degrade the quality of groundwater and surface water. In order to predict and mitigate the potentially negative impacts of CO2 leakage, it is important to understand the physical processes that CO2 will undergo as it moves through naturally heterogeneous porous media formations. Previous studies have shown that heterogeneity can enhance the evolution of gas phase CO2 in some cases, but the conditions under which this occurs have not yet been quantitatively defined, nor tested through laboratory experiments. This study quantitatively investigates the effects of geologic heterogeneity on the process of gas phase CO2 evolution in shallow aquifers through an extensive set of experiments conducted in a column that was packed with layers of various test sands. Soil moisture sensors were utilized to observe the formation of gas phase near the porous media interfaces. Results indicate that the conditions under which heterogeneity controls gas phase evolution can be successfully predicted through analysis of simple parameters, including the dissolved CO2 concentration in the flowing water, the distance between the heterogeneity and the leakage location, and some fundamental properties of the porous media. Results also show that interfaces where a less permeable material overlies a more permeable material affect gas phase evolution more significantly than interfaces with the opposite layering.
- Technological change in irrigated agriculture in a semi‐arid region
- Authors: Jean‐Marc Philip; Julio Sánchez‐Chóliz, Cristina Sarasa
Pages: n/a - n/a
Abstract: Technological change plays a decisive role in irrigated agriculture, which is particularly challenging in semi‐arid regions. The main objective of this paper is to assess four kinds of alternative technological improvements aimed at dealing with future water availability, especially in the case of extreme events like drought. We evaluate these technologies for a better understanding of what form should be applied in irrigated agriculture in a context of limits on natural resources.
We develop a dynamic computable general equilibrium (CGE) model, whose production structure distinguishes between rainfed and irrigated crops, and between a variety of irrigated crops. Land use changes are also evaluated. As well as technological change, we consider the Water Framework Directive (EC 2000/60), which establishes water cost recovery as a key goal. Thus, we assess strategies that combine irrigation water pricing strategies and improved technology. Our results show that policy strategies that focus on fostering technical progress can mitigate the long‐term economic effects of downward trends in water supplies, even in drought years. The study also confirms that the absence of price volatility achieved through a water pricing strategy could improve the sustainable use of water.
- Heat and mass transport during a groundwater replenishment trial in a
highly heterogeneous aquifer
- Authors: Simone Seibert; Henning Prommer, Adam Siade, Brett Harris, Mike Trefry, Michael Martin
Pages: n/a - n/a
Abstract: Changes in subsurface temperature distribution resulting from the injection of fluids into aquifers may impact physiochemical and microbial processes as well as basin resource management strategies. We have completed a two year field trial in a hydrogeologically and geochemically heterogeneous aquifer below Perth, Western Australia in which highly treated wastewater was injected for large‐scale groundwater replenishment. During the trial, chloride and temperature data were collected from conventional monitoring wells and by time‐lapse temperature logging. We used a joint inversion of these solute tracer and temperature data to parameterize a numerical flow and multi‐species transport model and to analyze the solute and heat propagation characteristics that prevailed during the trial. The simulation results illustrate that while solute transport is largely confined to the most permeable lithological units, heat transport was also affected by heat exchange with lithological units that have a much lower hydraulic conductivity. Heat transfer by heat conduction was found to significantly influence the complex temporal and spatial temperature distribution, especially with growing radial distance and in aquifer sequences with a heterogeneous hydraulic conductivity distribution. We attempted to estimate spatially varying thermal transport parameters during the data inversion to illustrate the anticipated correlations of these parameters with lithological heterogeneities, but estimates could not be uniquely determined on the basis of the collected data.
- Channeling, channel density and mass recovery in aquifer transport, with
application to the MADE experiment
- Authors: A. Fiori
Pages: n/a - n/a
Abstract: Channeling effects in heterogeneous formations are studied through a new quantity denoted as channel density a (x,t). Focusing on advection only, a (x,t) is defined as the relative number of streamtubes (or channels) containing solute between x and x + dx at a given time t, regardless of the mass that they carry. The channel density generally differs from the widely employed longitudinal mass distribution m(x,t), and their difference increases with time and the degree of heterogeneity. The difference between a and m reflects the nonuniformity of mass distribution relative to the plume geometry. In particular, the “fast” channels typically carry a larger fraction of mass than their share in their relative volume, which in turn can be rather small. Detecting such channels by a network of monitoring wells may be a challenging task, which might explain the poor solute recovery of some field experiments at increasing times. After application of the proposed concepts to the simple case of stratified formations, we model the channel density and mass distribution pertaining to the MADE experiment, which exhibited poor mass recovery at large times. The results presented in this study emphasize the possible channeling effects at MADE and the general difficulty in sampling the leading edge of the plume, which in turn may contain a significant fraction of the plume mass.
- Root zone salinity and sodicity under seasonal rainfall due to feedback of
decreasing hydraulic conductivity
- Authors: S.E.A.T.M. van der Zee; S.H.H. Shah, R.W. Vervoort
Pages: n/a - n/a
Abstract: Soil sodicity, where the soil cation exchange complex is occupied for a significant fraction by Na+, may lead to vulnerability to soil structure deterioration. With a root zone flow and salt transport model, we modeled the feedback effects of salt concentration (C) and exchangeable sodium percentage (ESP) on saturated hydraulic conductivity Ks(C,ESP) for different groundwater depths and climates, using the functional approach of McNeal . We assume that a decrease of Ks is practically irreversible at a time scale of decades. Representing climate with a Poisson rainfall process, the feedback hardly affects salt and sodium accumulation compared with the case that feedback is ignored. However, if salinity decreases, the much more buffered ESP stays at elevated values, while Ks decreases. This situation may develop if rainfall has a seasonal pattern where drought periods with accumulation of salts in the root zone alternate with wet rainfall periods in which salts are leached. Feedback that affects both drainage/leaching and capillary upwards flow from groundwater, or only drainage, leads to opposing effects. If both fluxes are affected by sodicity induced degradation, this leads to reduced salinity (C) and sodicity (ESP), which suggests that the system dynamics and feedback oppose further degradation. Experiences in the field point in the same direction.
- The influence of spatially variable stream hydraulics on reach scale
solute transport modeling
- Authors: Noah M. Schmadel; Bethany T. Neilson, Justin E. Heavilin, David K. Stevens, Anders Wörman
Pages: n/a - n/a
Abstract: Within the context of reach scale transient storage modeling, there is limited understanding of how best to establish reach segment lengths that represent the effects of spatially variable hydraulic and geomorphic channel properties. In this paper, we progress this understanding through the use of channel property distributions derived from high‐resolution imagery that are fundamental for hydraulic routing. We vary the resolution of reach segments used in the model representation and investigate the minimum number necessary to capture spatially variable influences on downstream predictions of solute residence time probability density functions while sufficiently representing the observed channel property distributions. We also test if the corresponding statistical moments of the predictions provide comparable results and, therefore, a method for establishing appropriate reach segment lengths. We find that the predictions and the moment estimates begin to represent the majority of the variability at reach segment lengths coinciding with distances where observed channel properties are spatially correlated. With this approach, reach scales where the channel properties no longer significantly change predictions can be established, which provides a foundation for more focused transient storage modeling efforts.
- Sensitivity of fluvial sediment source apportionment to mixing model
assumptions: A Bayesian model comparison
- Authors: Richard J. Cooper; Tobias Krueger, Kevin M. Hiscock, Barry G. Rawlins
Pages: n/a - n/a
Abstract: Mixing models have become increasingly common tools for apportioning fluvial sediment load to various sediment sources across catchments using a wide variety of Bayesian and frequentist modelling approaches. In this study, we demonstrate how different model setups can impact upon resulting source apportionment estimates in a Bayesian framework via a one‐factor‐at‐a‐time (OFAT) sensitivity analysis. We formulate 13 versions of a mixing model, each with different error assumptions and model structural choices, and apply them to sediment geochemistry data from the River Blackwater, Norfolk, UK, to apportion suspended particulate matter (SPM) contributions from three sources (arable topsoils, road verges and subsurface material) under baseflow conditions between August 2012 and August 2013. Whilst all 13 models estimate subsurface sources to be the largest contributor of SPM (median ~76%), comparison of apportionment estimates reveal varying degrees of sensitivity to changing priors, inclusion of covariance terms, incorporation of time‐variant distributions and methods of proportion characterization. We also demonstrate differences in apportionment results between a full and an empirical Bayesian setup, and between a Bayesian and a frequentist optimization approach. This OFAT sensitivity analysis reveals that mixing model structural choices and error assumptions can significantly impact upon sediment source apportionment results, with estimated median contributions in this study varying by up to 21% between model versions. Users of mixing models are therefore strongly advised to carefully consider and justify their choice of model structure prior to conducting sediment source apportionment investigations.
- On consumers’ attitudes and willingness to pay for improved drinking
water quality and infrastructure
- Authors: Eftila Tanellari; Darrell Bosch, Kevin Boyle, Elton Mykerezi
Pages: n/a - n/a
Abstract: This paper examines the determinants of consumers’ willingness to pay for improvement programs for three drinking water issues: water quality, pinhole leaks in home plumbing infrastructure and aging public infrastructure. The research is based on a mail survey of consumers in Northern Virginia and the Maryland suburbs of Washington D.C. The analysis focuses on the relationship between information, risk perceptions and willingness to pay. An alternative specific conditional logit model is used to model consumers’ willingness to pay for improvements. Results indicate that the willingness to pay for any of the programs is negatively affected by the cost of the proposed improvement. Consumers’ risk perceptions, the external information provided in the survey and whether they read the annual report from their water utility affect consumers’ willingness to pay for improvement programs.
- Incorporating spatial dependence in regional frequency analysis
- Authors: Zhuo Wang; Jun Yan, Xuebin Zhang
Pages: n/a - n/a
Abstract: The efficiency of regional frequency analysis (RFA) is undermined by intersite dependence, which is usually ignored in parameter estimation. We propose a spatial index flood model where marginal generalized extreme value distributions are joined by an extreme‐value copula characterized by a max‐stable process for the spatial dependence. The parameters are estimated with a pairwise likelihood constructed from bivariate marginal generalized extreme value distributions. The estimators of model parameters and return levels can be more efficient than those from the traditional index flood model when the max‐stable process fits the intersite dependence well. Through simulation, we compared the pairwise likelihood method with an L‐moment method and an independence likelihood method under various spatial dependence models and dependence levels. The pairwise likelihood method was found to be the most efficient in mean squared error if the dependence model was correctly specified. When the dependence model was misspecified within the max‐stable models, the pairwise likelihood method was still competitive relative to the other two methods. When the dependence model was not a max‐stable model, the pairwise likelihood method led to serious bias in estimating the shape parameter and return levels, especially when the dependence was strong. In an illustration with annual maximum precipitation data from Switzerland, the pairwise likelihood method yielded remarkable reduction in the standard errors of return level estimates in comparison to the L‐moment method.
- Comments on “Capabilities and limitations of tracing spatial
temperature patterns by fiber‐optic distributed temperature
sensing” by L. Rose, S. Krause, and N. J. Cassidy
- Authors: Francisco Suárez
Pages: n/a - n/a
- Impact of biofilm‐induced heterogeneities on solute transport in
- Authors: Kone T; Golfier F, Orgogozo L, Oltéan C, Lefèvre E, Block J.C, Buès M.A.
Pages: n/a - n/a
Abstract: In subsurface systems, biofilm may degrade organic or organo‐metallic pollutants contributing to natural attenuation and soil bioremediation techniques. This increase of microbial activity leads to change the hydrodynamic properties of aquifers. The purpose of this work was to investigate the influence of biofilm‐induced heterogeneities on solute transport in porous media and more specifically on dispersivity. We pursued this goal by (i) monitoring both spatial concentration fields and solute breakthrough curves from conservative tracer experiments in a biofilm‐supporting porous medium, (ii) characterizing in situ the changes in biovolume and visualizing the dynamics of the biological material at the meso‐scale. A series of experiments was carried out in a flow‐cell system (60 cm3) with a silica sand (Φ = 50‐70 mesh) as solid carrier and Shewanella oneidensis MR‐1 as bacterial strain. Biofilm growth was monitored by image acquisition with a digital camera. The biofilm volume fraction was estimated through tracer experiments with the Blue Dextran macromolecule as in size‐exclusion chromatography, leading to a fair picture of the biocolonization within the flow‐cell. Biofilm growth was achieved in the whole flow‐cell in 29 days and up to 50% of void space volume was plugged. The influence of biofilm maturation on porous medium transport properties was evaluated from tracer experiments using Brilliant Blue FCF An experimental correlation was found between effective (i.e., non‐biocolonized) porosity and biofilm‐affected dispersivity. Comparison with values given by the theoretical model of Taylor and Jaffe [1990b] yields a fair agreement.
- Comparison of passive microwave and modeled estimates of total watershed
SWE in the continental United States
- Authors: Carrie M. Vuyovich; Jennifer M. Jacobs, Steven F. Daly
Pages: n/a - n/a
Abstract: In the United States, a dedicated system of snow measurement stations and snowpack modeling products are available to estimate the snow water equivalent (SWE) throughout the winter season. In other regions of the world that depend on snowmelt for water resources, snow data can be scarce, and these regions are vulnerable to drought or flood conditions. Even in the U.S., water resource management is hampered by limited snow data in certain regions, as evident by the 2011 Missouri Basin flooding due in large part to the significant Plains snowpack. Satellite data could potentially provide important information in under‐sampled areas. This study compared the daily AMSR‐E and SSM/I SWE products over nine winter seasons to spatially distributed, modeled output SNODAS summed over 2100 watersheds in the conterminous U.S. Results show large areas where the passive microwave retrievals are highly correlated to the SNODAS data, particularly in the northern Great Plains and southern Rocky Mountain regions. However, the passive microwave SWE is significantly lower than SNODAS in heavily forested areas, and regions that typically receive a deep snowpack. The best correlations are associated with basins in which maximum annual SWE is less than 200 mm, and forest fraction is less than 20%. Even in many watersheds with poor correlations between the passive microwave data and SNODAS maximum annual SWE values, the overall pattern of accumulation and ablation did show good agreement and therefore may provide useful hydrologic information on melt timing and season length.
- Ecogeomorphic feedbacks and flood loss of riparian tree seedlings in
meandering channel experiments
- Authors: Li Kui; John Stella, Anne Lightbody, Andrew C. Wilcox
Pages: n/a - n/a
Abstract: During floods, fluvial forces interact with riparian plants to influence evolution of river morphology and floodplain plant community development. Understanding of these interactions, however, is constrained by insufficient precision and control of drivers in field settings, and insufficient realism in laboratory studies. We completed a novel set of flume experiments using woody seedlings planted on a sandbar within an outdoor meandering stream channel. We quantified effects on local sedimentation and seedling loss to scour and burial across realistic ranges of woody plant morphologies (Populus versus Tamarix species), densities (240 plants m‐2 versus 24 m‐2), and sediment supply (equilibrium versus deficit). Sedimentation was higher within Tamarix patches than Populus patches, reflecting Tamarix‘s greater crown frontal area and lower maximum crown density. Plant dislodgement occurred rarely (1% of plants) and was induced in plants with shorter roots. Complete burial was most frequent for small Tamarix that occurred at high densities. Burial risk decreased 3% for Populus and 13% for Tamarix for every centimeter increment in stem height, and was very low for plants >50 cm tall. These results suggest that Tamarix are proportionally more vulnerable than Populus when small (
- Pore‐scale study of dissolution‐induced changes in hydrologic
properties of rocks with binary minerals
- Authors: Li Chen; Qinjun Kang, Hari S. Viswanathan, Wen‐Quan Tao
Pages: n/a - n/a
Abstract: A pore‐scale numerical model for reactive transport processes based on the Lattice Boltzmann method is used to study the dissolution‐induced changes in hydrologic properties of a fractured medium and a porous medium. The solid phase of both media consists of two minerals, and a structure reconstruction method called quartet structure generation set is employed to generate the distributions of both minerals. Emphasis is put on the effects of undissolved minerals on the changes of permeability and porosity under different Peclet and Damkohler numbers. The simulation results show porous layers formed by the undissolved mineral remain behind the dissolution reaction front. Due to the large flow resistance in these porous layers, the permeability increases very slowly or even remains at a small value although the porosity increases by a large amount. Besides, due to the heterogeneous characteristic of the dissolution, the chemical, mechanical and hydraulic apertures are very different from each other. Further, simulations in complex porous structures demonstrate that the existence of the porous layers of the nonreactive mineral suppresses the wormholing phenomena observed in the dissolution of mono‐mineralic rocks.
- Empirical assessment of theory for bankfull characteristics of alluvial
- Authors: S. M. Trampush; S. Huzurbazar, B. McElroy
Pages: n/a - n/a
Abstract: We compiled a dataset of 541 bankfull measurements of alluvial rivers (see supplemental material) and used Bayesian linear regression to examine empirical and theoretical support for the hypothesis that alluvial channels adjust to a predictable condition of basal shear stress as a function of sediment transport mode. An empirical closure based on channel slope, bankfull channel depth, and median grain size is proposed and results in the scaling of bankfull Shields stress with the inverse square root of particle Reynolds number. The empirical relationship is sufficient for purposes of quantifying paleohydraulic conditions in ancient alluvial channels. However, it is not currently appropriate for application to alluvial channels on extraterrestrial bodies because it depends on constant‐valued, Earth‐based coefficients.
- Statistical emulation of streamflow projections from a distributed
hydrological model: Application to CMIP3 and CMIP5 climate projections for
British Columbia, Canada
- Authors: Markus A. Schnorbus; Alex J. Cannon
Pages: n/a - n/a
Abstract: A recent hydrological impacts study in British Columbia, Canada used an ensemble of 23 climate change simulations to assess potential future changes in streamflow. These Coupled Model Intercomparison Project, phase 3 (CMIP3) simulations were statistically downscaled and used to drive the Variable Infiltration Capacity (VIC) hydrology model over several watersheds. Due to computational restrictions, the 23 member VIC ensemble is a subset of the full 136‐member CMIP3 archive. Extending the VIC ensemble to cover the full range of uncertainty represented by CMIP3, and incorporating the latest generation CMIP5 ensembles, poses a considerable computing challenge. Thus, we extend the VIC ensemble using a computationally‐efficient statistical emulation model, which approximates the combined output of the two‐step process of statistical downscaling and hydrologic modeling, trained with the 23 member VIC ensemble. Regularized multiple linear regression links projected changes in monthly temperature and precipitation with projected changes in monthly streamflow over the Fraser and Peace River watersheds. Following validation, the statistical emulator is forced with the full suite of CMIP3 and CMIP5 climate change projections. The 23 member VIC ensemble has a smaller spread than the full ensemble; however, both ensembles provide the same consensus estimate of monthly streamflow change. Qualitatively, CMIP5 shows a similar streamflow response as CMIP3 for snow‐dominated hydrologic regimes. However, by end‐century the CMIP5 worst‐case RCP8.5 has a larger impact than CMIP3 A2. This work also underscores the advantage of using emulation to rapidly identify those future extreme projections that may merit further study using more computationally‐demanding process‐based methods.
- The influence of geomorphology on large wood dynamics in a
low‐gradient headwater stream
- Authors: Dixon Simon J; Sear David. A.
Pages: n/a - n/a
Abstract: Understanding large wood dynamics is critical for a range of disciplines including flood risk management, ecology and geomorphology. Despite the importance of wood in rivers, our understanding of the mobility of large wood remains limited. In this study individual pieces of large wood were tagged and surveyed over a 32 month period within a third and fourth order lowland forest river. Individual pieces of wood were found to be highly mobile, with 75% of pieces moving during the survey period, and a maximum transport distance of 5.6km. Multivariate analyses of data from this study and two other published studies identified dimensionless wood length as the important factor in explaining likelihood of movement. A length threshold of 2.5 channel widths is identified for near functional immobility, with few pieces above this size moving. In addition, for this study, wood type, branching complexity, location and dimensionless wood diameter were found to be important in determining mobility only for sinuous reaches with readily inundated floodplains. Where logjams persist over multiple years they were shown to be reworked, with component pieces being transported away and replaced by newly trapped pieces. The findings of this study have implications for river management and restoration. The high mobility observed in this study demonstrates that only very large pieces of wood of length greater than 2.5 channel widths should be considered functionally immobile. For pieces of wood of length less than the channel width the possibility of high rates of mobility and long transport distances should be anticipated.
- Fine suspended sediment and water budgets for a large, seasonally dry
tropical catchment: Burdekin River catchment, Queensland, Australia
- Authors: Zoë T. Bainbridge; Stephen E. Lewis, Scott G. Smithers, Petra M. Kuhnert, Brent L. Henderson, Jon E. Brodie
Pages: n/a - n/a
Abstract: The Burdekin River catchment (˜130,400 km2) is a seasonally‐dry tropical catchment located in north‐east Queensland, Australia. It is the single largest source of suspended sediment to the Great Barrier Reef (GBR). Fine sediments are a threat to ecosystems on the GBR where they contribute to elevated turbidity (reduced light), sedimentation stress and potential impacts from the associated nutrients. Suspended sediment data collected over a five‐year period were used to construct a catchment‐wide sediment source and transport budget. The Bowen River tributary was identified as the major source of end‐of‐river suspended sediment export, yielding an average of 530 t km‐2 yr‐1 during the study period. Sediment trapping within a large reservoir (1.86 million ML) and the preferential transport of clays and fine silts downstream of the structure were also examined. The data reveal that the highest clay and fine silt loads ‐ which are of most interest to environmental managers of the GBR ‐ are not always sourced from areas that yield the largest total suspended sediment load (i.e. all size fractions). Our results demonstrate the importance of incorporating particle size into catchment sediment budget studies undertaken to inform management decisions to reduce downstream turbidity and sedimentation. Our data on sediment source, reservoir influence and sub‐catchment and catchment yields will improve understandings of sediment dynamics in other tropical catchments, particularly those located in seasonally wet‐dry tropical savannah/semi‐arid climates. The influence of climatic variability (e.g. drought/wetter periods) on annual sediment loads within large seasonally‐dry tropical catchments is also demonstrated by our data.
- Regional patterns of interannual variability of catchment water balances
across the continental U.S.: A Budyko framework
- Authors: Alejandra M. Carmona; Murugesu Sivapalan, Mary A. Yaeger, Germán Poveda
Pages: n/a - n/a
Abstract: Patterns of inter‐annual variability of the annual water balance are explored using data from 190 MOPEX catchments across the continental United States. This analysis has led to the derivation of a quantitative, dimensionless, Budyko‐type framework to characterize the observed inter‐annual variability of annual water balances. The resulting model is expressed in terms of a humidity index that measures the competition between water and energy availability at the annual time scale, and a similarity parameter (α) that captures the net effects of other short‐term climate features and local landscape characteristics. This application of the model to the 190 study catchments revealed the existence of space‐time symmetry between spatial (between‐catchment) variability and general trends in the temporal (between‐year) variability of the annual water balances. The MOPEX study catchments were classified into eight similar catchment groups on the basis of magnitudes of the similarity parameter α. Interesting regional trends of α across the continental U.S. were brought out through identification of similarities between the spatial positions of the catchment groups with the mapping of distinctive ecoregions that implicitly take into account common climatic and vegetation characteristics. In this context, this study has introduced a deep sense of similarity that is evident in observed space‐time variability of water balances that also reflect the co‐dependence and co‐evolution of climate and landscape properties.
- Effects of grain angularity on NaCl precipitation in porous media during
- Authors: Mansoureh Norouzi Rad; Nima Shokri
Pages: n/a - n/a
Abstract: Three dimensional pore‐scale analysis was carried out using X‐ray micro‐tomography to investigate the effects of grain angularity on NaCl precipitation dynamics and patterns during evaporation from saline porous media. To do so, quartz sand and glass beads with similar average particle size and porosity were used enabling us to constrain the effects of grain angularity on NaCl precipitation since the glass beads were spherical and smooth whereas the sand consisted of irregularly shaped grains. Presence of angularity resulted in different pore sizes and shapes influencing the dynamics of evaporation and NaCl precipitation. Our results demonstrate that the preferential evaporation exclusively in fine pores at the surface of porous media results in discrete efflorescence. We observed a higher cumulative NaCl precipitation in the case of glass beads at the early stages of precipitation due to the presence of a fewer number of evaporation sites at the surface. This phenomenon resulted in formation of a thicker and more discrete NaCl crust at the surface of glass beads compared to sand grains. Also, computed water saturation profiles reveals formation of a wider unsaturated zone above the receding drying front in the case of sand compared to glass beads due to the presence of finer pores affecting the capillary flow though the partially wet zone. Our results provide new insights regarding the effects of grain angularity on NaCl precipitation dynamics and patterns during evaporation from saline porous media.
- Parameterization of mires in a numerical weather prediction model
- Authors: Alla Yurova; Mikhail Tolstykh, Mats Nilsson, Andrey Sirin
Pages: n/a - n/a
Abstract: Mires (peat‐accumulating wetlands) occupy 8.1 percent of Russian territory and are especially numerous in the Western Siberian Lowlands, where they can significantly modify atmospheric heat and water balances. They also influence air temperatures and humidity in the boundary layers closest to the earth's surface. The purpose of our study was to incorporate the influence of mires into the SL‐AV numerical weather prediction model, which is used operationally in the Hydrometeorological Center of Russia. This was done by adjusting the multilayer soil component (by modifying the peat thermal conductivity in the heat diffusion equation and reformulating the lower boundary condition for Richard's equation), and reformulating both the evapotranspiration and runoff from mires. When evaporation from mires was incorporated into the SL‐AV model, the latent heat flux in the areas dominated by mires increased strongly, resulting in surface cooling and hence reductions in the sensible heat flux and outgoing terrestrial long‐wave radiation. Presented results show that including mires significantly decreased the bias and RMSE of predictions of temperature and relative humidity 2 m above the ground for lead times of 12, 36 and 60 h from 00 hours Coordinated Universal Time (evening conditions), but did not eliminate the bias in forecasts for lead times of 24, 48 and 72 h (morning conditions) in Siberia. Different parameterizations of mire evapotranspiration are also compared.
- Subsurface lateral preferential flow network revealed by time‐lapse
ground‐penetrating radar in a hillslope
- Authors: Li Guo; Jin Chen, Henry Lin
Pages: n/a - n/a
Abstract:  Subsurface lateral preferential flow (LPF) has been observed to contribute substantially to hillslope and catchment runoff. However, the complex nature of LPF and the lack of an appropriate investigation method have hindered direct LPF observation in the field. Thus, the initiation, persistence, and dynamics of LPF networks remain poorly understood. This study explored the application of time‐lapse ground‐penetrating radar (GPR) together with an artificial infiltration to shed light on the nature of LPF and its dynamics in a hillslope. Based on the enhanced field experimental set‐up and carefully‐refined GPR data post‐processing algorithms, we developed a new protocol to reconstruct LPF networks with centimeter resolution. This is the first time that a detailed LPF network and its dynamics have been revealed non‐invasively along a hillslope. Real‐time soil water monitoring and field soil investigation confirmed the locations of LPF in the deeper BC horizon mapped by time‐lapse GPR surveys. Our results indicated the following: 1) Increased spatial variations of radar signals after infiltration suggested heterogeneous soil water changes within the studied soil, which reflected the generation and dynamics of LPF; 2) Two types of LPF networks were identified, the network at the location of soil permeability contrasts and that formed via a series of connected preferential flow paths; and 3) The formation and distribution of LPF networks were influenced by antecedent soil water condition. Overall, this study demonstrates clearly that carefully designed time‐lapse GPR surveys with enhanced data post‐processing offer a practical and nondestructive way of mapping LPF networks in the field, thereby providing a potentially significant enhancement in our ability to study complex subsurface flow processes across the landscape.
- CO2 wettability of seal and reservoir rocks and the implications for
- Authors: Stefan Iglauer; C.H. Pentland, A. Busch
Pages: n/a - n/a
Abstract: We review the literature data published on the topic of CO2 wettability of storage and seal rocks. We first introduce the concept of wettability and explain why it is important in the context of carbon geo‐sequestration (CGS) projects, and review how it is measured. This is done to raise awareness of this parameter in the CGS community, which, as we show later on in this text, may have a dramatic impact on structural and residual trapping of CO2. These two trapping mechanisms would be severely and negatively affected in case of CO2‐wet storage and/or seal rock.
Overall, at the current state‐of‐the‐art, a substantial amount of work has been completed, and we find that:
sandstone and limestone, plus pure minerals such as quartz, calcite, feldspar and mica are strongly water wet in a CO2‐water system.
oil‐wet limestone, oil‐wet quartz or coal is intermediate‐wet or CO2‐wet in a CO2‐water system.
the contact angle alone is insufficient for predicting capillary pressures in reservoir or seal rocks.
the current contact angle data has a large uncertainty.
solid theoretical understanding on a molecular level of rock‐CO2‐brine interactions is currently limited.
in an ideal scenario all seal and storage rocks in CGS formations are tested for their CO2‐wettability.
achieving representative subsurface conditions (especially in terms of the rock surface) in the laboratory is of key importance but also very challenging.
- Reply to comment by L. Rose, S. Krause, and N.J. Cassidy on
“Capabilities and limitations of tracing spatial temperature
patterns by fiber‐optic distributed temperature sensing”
- Authors: S. Krause; L. Rose, N.J. Cassidy
Pages: n/a - n/a
- Dynamic aspects of soil water availability for isohydric plants: Focus on
root hydraulic resistances
- Authors: V. Couvreur; J. Vanderborght, X. Draye, M. Javaux
Pages: n/a - n/a
Abstract: Soil water availability for plant transpiration is a key concept in agronomy. The objective of this study is to revisit this concept and discuss how it may be affected by processes locally influencing root hydraulic properties. A physical limitation to soil water availability in terms of maximal flow rate available to plant leaves (Qavail) is defined. It is expressed for isohydric plants, in terms of plant‐centered variables and properties (the equivalent soil water potential sensed by the plant, Ψs eq the root system equivalent conductance, Krs; and a threshold leaf water potential, Ψleaflim). The resulting limitation to plant transpiration is compared to commonly used empirical stress functions. Similarities suggest that the slope of empirical functions might correspond to the ratio of Krs to the plant potential transpiration rate. The sensitivity of Qavail to local changes of root hydraulic conductances in response to soil matric potential is investigated using model simulations. A decrease of radial conductances when the soil dries induces earlier water stress, but allows maintaining higher night plant water potentials and higher Qavail during the last week of a simulated one‐month drought. In opposition, an increase of radial conductances during soil drying provokes an increase of hydraulic redistribution and Qavail at short term. This study offers a first insight on the effect of dynamic local root hydraulic properties on soil water availability. By better understanding complex interactions between hydraulic processes involved in soil‐plant hydrodynamics, better prospects on how root hydraulic traits mitigate plant water stress might be achieved.
- Assimilation of SMOS‐derived soil moisture in a fully integrated
hydrological and soil‐vegetation‐atmosphere transfer model in
- Authors: Marc‐Etienne Ridler; Henrik Madsen, Simon Stisen, Simone Bircher, Rasmus Fensholt
Pages: n/a - n/a
Abstract: Real surface soil moisture retrieved from the Soil Moisture and Ocean Salinity (SMOS) satellite is downscaled and assimilated in a fully integrated hydrological and soil‐vegetation‐atmosphere transfer (MIKE SHE SW‐ET) model using a bias aware Ensemble Transform Kalman Filter (Bias‐ETKF). Satellite‐derived soil moisture assimilation in a catchment scale model is typically restricted by two challenges: 1) passive microwave is too coarse for direct assimilation, and 2) the data tends to be biased. The solution proposed in this study is to disaggregate the SMOS bias using a higher resolution land cover classification map that was derived from Landsat thermal images. Using known correlations between SMOS bias and vegetation type, the assimilation filter is adapted to calculate biases online, using an initial bias estimate.
Real SMOS derived soil moisture is assimilated in a pre‐calibrated catchment model in Denmark. The objective it to determine if any additional gains can be achieved by SMOS surface soil moisture assimilation beyond the optimized model. A series of assimilation experiments were designed to (1) determine how effectively soil moisture corrections propagate downward in the soil column, (2) compare the efficacy of in‐situ versus SMOS assimilation, and (3) determine how soil moisture assimilation affects fluxes and discharge in the catchment.
We find that assimilation of SMOS improved R2 soil moisture correlations in the upper 5cm compared to a network of 30 in‐situ sensors for most land cover classes. Assimilation also brought modest gains in R2 at 25 cm depth, but slightly degraded the correlation at 50 cm depth. Assimilation overcorrected discharge peaks.
- Controls on mixing‐dependent denitrification in hyporheic zones
induced by riverbed dunes: A steady state modeling study
- Authors: Erich T. Hester; Katie I. Young, Mark A. Widdowson
Pages: n/a - n/a
Abstract: The hyporheic zone is known to attenuate contaminants originating from surface water, yet the ability of the hyporheic zone to attenuate contaminants in upwelling groundwater plumes as they exit to surface water is less understood. We used MODFLOW and SEAM3D to simulate hyporheic flow cells induced by riverbed dunes and upwelling GW together with mixing‐dependent denitrification of an upwelling nitrate (NO3‐) plume. Our base case modeled labile dissolved organic carbon (DOC) and dissolved oxygen (DO) advecting from surface water, and DO and NO3‐ advecting from groundwater, typical of certain agricultural areas. We conducted sensitivity analyses that showed mixing‐dependent denitrification in the hyporheic zone increased with increasing hydraulic conductivity (K), decreasing lower bottom flux, and increasing DOC in surface water or NO3‐ in groundwater. Surface water DOC, groundwater NO3‐ and K were the most sensitive parameters affecting mixing‐dependent denitrification. Non‐mixing‐dependent denitrification also occurred when there was surface water NO3‐, and its magnitude was often greater than mixing‐dependent denitrification. Nevertheless, mixing‐dependent reactions provide functions that non‐mixing‐dependent reactions cannot, with potential for hyporheic zones to attenuate upwelling NO3‐ plumes, depending on geomorphic, hydraulic and biogeochemical conditions. Stream and river restoration efforts may be able to increase mixing‐dependent reactions by promoting natural processes that promote bedform creation and augment labile carbon sources.
- Exploration of discrepancy between radar and gauge rainfall estimates
driven by wind fields
- Authors: Qiang Dai; Dawei Han
Pages: n/a - n/a
Abstract: Due to the fact that weather radar is prone to several sources of errors, it is acknowledged that adjustment against ground observations such as rain gauges is crucial for radar measurement. Spatial matching of precipitation patterns between radar and rain gauge is a significant premise in radar bias corrections. It is a conventional way to construct radar‐gauge pairs based on their vertical locations. However, due to the wind effects, the raindrops observed by the radar do not always fall vertically to the ground, and the raindrops arriving at the ground may not all be caught by the rain gauge. This study proposes a fully‐formulated scheme to numerically simulate the movement of raindrops in a three‐dimensional wind field in order to adjust the wind‐induced errors. The Brue catchment (135 sq. km) in Southwest England covering 28 radar pixels and 49 rain gauges is an experimental catchment, where the radar central beam height varies between 500‐700 meters. 20 typical events (with durations of 6˜36 hours) are chosen to assess the correlation between hourly radar and gauge rainfall surfaces. It is found that for most events, the improved rates of correlation coefficients are greater than 10%, and nearly half of the events increase by 20%. With the proposed method, except four events, all the event‐averaged correlation values are greater than 0.5. This work is the first study to tackle both wind effects on radar and rain gauges, which could be considered as one of the essential components in processing radar observational data in its hydrometeorological applications.
- Comparison of satellite rainfall estimates in the Blue Nile basin: lowland
plain versus highland mountain
- Authors: Mekonnen Gebremichael; Menberu M. Bitew, Feyera A. Hirpa, Gebrehiwot N. Tesfay
Pages: n/a - n/a
Abstract: The demand for accurate satellite rainfall products is increasing particularly in Africa where ground‐based data are mostly unavailable, timely inaccessible, or unreliable. In this study, the accuracy of three widely‐used, near‐global, high‐resolution satellite rainfall products (CMORPH, TMPA‐RT v7, TMPA‐RP v7), with a spatial resolution of 0.25° and a temporal resolution of 3 hours, is assessed over the Blue Nile River Basin, a basin characterized by complex terrain and tropical monsoon. The assessment is made using relatively‐dense experimental networks of rain gauges deployed at two, 0.25°×0.25°, sites that represent contrasting topographic features: the lowland plain (mean elevation of 719 m.a.s.l.) site and the highland mountain (mean elevation of 2268 m.a.s.l.). The investigation period covers the summer seasons of 2012 and 2013. Compared to the highland mountain site, the lowland plain site exhibits marked extremes of rain intensity, higher rain intensity, lower frequency of rain occurrence, and smaller seasonal rainfall accumulation. All the satellite products considered tend to overestimate the mean rainfall rate at the lowland plain site, but underestimate it at the highland mountain site. The satellite products miss more rainfall at the highland mountain site than at the lowland plain site. The satellite products underestimate the heavy rain rates at both sites. Both sites have uncertainty (root mean square error) values greater than 100% for 3 hour accumulations of less than 5 mm, or daily accumulations of less than 10 mm, and the uncertainty values decrease with increasing rainfall accumulation. Among the satellite products, CMORPH suffers from a large positive bias at the lowland plain site, and TMPA‐RP and TMPA‐RT miss a large number of rainfall events that contribute nearly half of the total rainfall at the lowland plain.
- Lattice Boltzmann methods applied to large‐scale
three‐dimensional virtual cores constructed from digital optical
borehole images of the karst carbonate biscayne aquifer in southeastern
- Authors: Michael C. Sukop; Kevin J. Cunningham
Pages: n/a - n/a
Abstract: Digital optical borehole images at approximately 2‐mm vertical resolution and borehole caliper data were used to create three‐dimensional renderings of the distribution of (1) matrix porosity and (2) vuggy megaporosity for the karst carbonate Biscayne aquifer in southeastern Florida. The renderings based on the borehole data were used as input into Lattice Boltzmann methods to obtain intrinsic permeability estimates for this extremely transmissive aquifer, where traditional aquifer test methods may fail due to very small drawdowns and non‐Darcian flow that can reduce apparent hydraulic conductivity. Variogram analysis of the borehole data suggests a nearly isotropic rock structure at lag lengths up to the nominal borehole diameter. A strong correlation between the diameter of the borehole and the presence of vuggy megaporosity in the data set led to a bias in the variogram where the computed horizontal spatial autocorrelation is strong at lag distances greater than the nominal borehole size. Lattice Boltzmann simulation of flow across a 0.4 × 0.4 × 17 m (2.72 m3 volume) parallel‐walled column of rendered matrix and vuggy megaporosity indicates a high hydraulic conductivity of 53 m s‐1. This value is similar to previous Lattice Boltzmann calculations of hydraulic conductivity in smaller limestone samples of the Biscayne aquifer. The development of simulation methods that reproduce dual‐porosity systems with higher resolution and fidelity and that consider flow through horizontally longer renderings could provide improved estimates of the hydraulic conductivity and help to address questions about the importance of scale.
- Analytical approximation for the recession of a sloping aquifer
- Authors: W.L. Hogarth; L. Li, D.A. Lockington, F. Stagnitti, M.B. Parlange, D.A. Barry, T.S. Steenhuis, J.‐Y. Parlange
Pages: n/a - n/a
Abstract: An approximation is obtained for the recession of a sloping aquifer. The analytical approximation can provide a useful tool to analyze data and obtain physical properties of the aquifer. In contrast to the case of a horizontal aquifer, when plotting the time derivative of the flux vs. the flux on a log scale, the result shows that the flux derivative reaches a minimum value and that the curve can have a slope of unity as often observed. Illustration of the application of the analytical results to the Mahantango Creek data is also discussed.
- Multisensor earth observations to characterize wetlands and malaria
epidemiology in Ethiopia
- Authors: Alemayehu Midekisa; Gabriel Senay, Michael C Wimberly
Pages: n/a - n/a
Abstract: Malaria is a major global public health problem, particularly in sub‐Saharan Africa. The spatial heterogeneity of malaria can be affected by factors such as hydrological processes, physiography, and land cover patterns. Tropical wetlands, for example, are important hydrological features that can serve as mosquito breeding habitats. Mapping and monitoring of wetlands using satellite remote sensing can thus help to target interventions aimed at reducing malaria transmission. The objective of this study was to map wetlands and other major land cover types in the Amhara Region of Ethiopia and to analyze district‐level associations of malaria and wetlands across the region. We evaluated three random forests classification models using remotely sensed topographic and spectral data based on Shuttle Radar Topographic Mission (SRTM) and Landsat TM/ETM+ imagery respectively. The model that integrated data from both sensors yielded more accurate land cover classification than single‐sensor models. The resulting map of wetlands and other major land cover classes had an overall accuracy of 93.5 percent. Topographic indices and sub‐pixel level fractional cover indices contributed most strongly to the land cover classification. Further, we found strong spatial associations of percent area of wetlands with malaria cases at the district‐level across the dry, wet, and fall seasons. Overall, our study provided the most extensive map of wetlands for the Amhara region and documented spatiotemporal associations of wetlands and malaria risk at a broad regional level. These findings can assist public health personnel in developing strategies to effectively control and eliminate malaria in the region.
- Comment on “Assessing invertebrate assemblages in the subsurface
zone of stream sediments (0–15 cm deep) using a hyporheic
sampler” by Marie‐José Dole‐Olivier et al.
- Authors: James W. Roy; Serban Danielescu
Pages: n/a - n/a
- Reply to comment by James W. Roy and Serban Danielescu on “Assessing
invertebrate assemblages in the subsurface zone of stream sediments
(0–15 cm deep) using a hyporheic sampler”
- Authors: M.‐J. Dole‐Olivier; Diana M. P. Galassi, Pierre Marmonier
Pages: n/a - n/a
- Simultaneous measurement of unfrozen water content and ice content in
frozen soil using Gamma ray attenuation and TDR
- Authors: Xiaohai Zhou; Jian Zhou, Wolfgang Kinzelbach, Fritz Stauffer
Pages: n/a - n/a
Abstract: The freezing temperature of water in soil is not constant, but varies over a range determined by soil texture. Consequently, the amounts of unfrozen water and ice change with temperature in frozen soil, which in turn affects hydraulic, thermal and mechanical properties of frozen soil. In this paper, an Am‐241 gamma ray source and TDR were combined to measure unfrozen water content and ice content in frozen soil simultaneously. The gamma ray attenuation was used to determine total water content. The TDR was used to determine the dielectric constant of the frozen soil. Based on a four‐phase mixing model, the amount of unfrozen water content in the frozen soil could be determined. The ice content was inferred by the difference between total water content and unfrozen water content. The gamma ray attenuation and the TDR were both calibrated by a gravimetric method. Water contents measured both by gamma ray attenuation and TDR separately in an unfrozen silt column under infiltration were compared and showed that the two methods have the same accuracy and response to changes of water content. Unidirectional column freezing experiments were performed to apply the combined method of gamma ray attenuation and TDR for measuring unfrozen water content and ice content. The measurement error of the gamma ray attenuation and TDR was, around 0.02 m3/m3 and 0.01 m3/m3, respectively. The overestimation of unfrozen water in frozen soil by TDR alone was quantified and found to depend on the amount of ice content. The higher the ice content, the larger the overestimation. The study confirmed that the combined method could accurately determine unfrozen water content and ice content in frozen soil. The results of soil column freezing experiments indicated that total water content distribution is affected by available pore space and the freezing front advance rate. It was found that there is similarity between soil water characteristic curve and soil freezing characteristic curve of variably‐saturated soil. Unfrozen water content is independent of total water content and affected only by temperature when the freezing point is reached.
- Wind‐driven nearshore sediment resuspension in a deep lake during
- Authors: Kristin E. Reardon; Fabián A. Bombardelli, Patricio A. Moreno‐Casas, Francisco J. Rueda, S. Geoffrey Schladow
Pages: n/a - n/a
Abstract: Ongoing public concern over declining water quality at Lake Tahoe, California‐Nevada (USA) led to an investigation of wind‐driven nearshore sediment resuspension that combined field measurements and modeling. Field data included: wind speed and direction, vertical profiles of water temperature and currents, nearbed velocity, lakebed sediment characteristics, and suspended sediment concentration and particle size distribution. Bottom shear stress was computed from ADV‐measured nearbed velocity data, adapting a turbulent kinetic energy method to lakes, and partitioned according to its contributions attributed to wind‐waves, mean currents, and random motions. When the total shear stress exceeded the critical shear stress, the contribution to overall shear stress was about 80 percent from wind‐waves and 10 percent each from mean currents and random motions. Therefore, wind‐waves were the dominant mechanism resulting in sediment resuspension as corroborated by simultaneous increases in shear stress and total measured sediment concentration. The wind‐wave model STWAVE was successfully modified to simulate wind‐wave induced sediment resuspension for viscous‐dominated flow typical in lakes. Previous lake applications of STWAVE have been limited to special instances of fully‐turbulent flow. To address the validity of expressions for sediment resuspension in lakes, sediment entrainment rates were found to be well represented by a modified 1991 García and Parker formula. Lastly, in situ measurements of suspended sediment concentration and particle size distribution revealed that the predominance of fine particles (by particle count) that most negatively impact clarity was unchanged by wind‐related sediment resuspension. Therefore, we cannot assume that wind‐driven sediment resuspension contributes to Lake Tahoe's declining nearshore clarity.
- The pronounced seasonality of global groundwater recharge
- Authors: Scott Jasechko; S. Jean Birks, Tom Gleeson, Yoshihide Wada, Peter J. Fawcett, Zachary D. Sharp, Jeffrey J. McDonnell, Jeffrey M. Welker
Pages: n/a - n/a
Abstract: Groundwater recharged by meteoric water supports human life by providing 2 billion people with drinking water and by supplying 40% of cropland irrigation. Whilst annual groundwater recharge rates are reported in many studies, fewer studies have explicitly quantified intra‐annual (i.e., seasonal) differences in groundwater recharge. Understanding seasonal differences in the fraction of precipitation that recharges aquifers is important for predicting annual groundwater rates under changing seasonal precipitation and evapotranspiration regimes in a warming climate, for accurately interpreting isotopic proxies in paleoclimate records, and for understanding linkages between ecosystem productivity and groundwater recharge. Here we determine seasonal differences in the groundwater recharge ratio, defined here as the ratio of groundwater recharge to precipitation, at 54 globally‐distributed locations on the basis of 18O/16O and 2H/1H ratios in precipitation and groundwater. Our analysis shows that arid and temperate climates have wintertime groundwater recharge ratios that are consistently higher than summertime groundwater recharge ratios, while tropical groundwater recharge ratios are at a maximum during the wet season. The isotope‐based recharge ratio seasonality is consistent with monthly outputs from a global hydrological model (PCR‐GLOBWB) for most, but not all locations. The pronounced seasonality in groundwater recharge ratios shown in this study signifies that, from the point of view of predicting future groundwater recharge rates, a unit change in winter (temperate and arid regions) or wet season (tropics) precipitation will result in a greater change to the annual groundwater recharge rate than the same unit change to summer or dry season precipitation.
- A climate change range‐based method for estimating robustness for
water resources supply
- Authors: Sarah Whateley; Scott Steinschneider, Casey Brown
Pages: n/a - n/a
Abstract: Many water planning and operations decisions are affected by climate uncertainty. Given concerns about the effects of uncertainty on the outcomes of long term decisions, many water planners seek adaptation alternatives that are robust given a wide range of possible climate futures. However, there is no standardized paradigm for quantifying robustness in the water sector. This study uses a new framework for assessing the impact of future climate change and uncertainty on water supply systems and defines and demonstrates a new metric for quantifying climate robustness. The metric is based on the range of climate change space over which an alternative provides acceptable performance. The metric is independent of assumptions regarding future climate; however, GCM‐based (or other) climate projections can be used to create a “climate‐informed” version of the metric. The method is demonstrated for a water supply system in the Northeast US to evaluate the additional robustness that can be attained through optimal operational changes, by comparing optimal reservoir operations with current reservoir operations. Results show the additional robustness gained through adaptation. They also reveal the additional insight regarding robust adaptation gained from the decision‐scaling approach that would not be discerned using a GCM projection based analysis.
- Influence of the sediment supply texture on morphological adjustments in
gravel bed rivers
- Authors: Ferrer‐Boix Carles; Hassan Marwan A.
Pages: n/a - n/a
Abstract: The role played by the texture of the sediment supply on channel bed adjustments in gravel‐bed rivers is poorly understood. To address this issue, an experimental campaign has been designed. Flume experiments lasting 96 hours in a 9‐m long, 0.60 m‐wide have been performed with different sand‐gravel mixtures as feed textures. The response of the surface texture has been found to be highly dependent on the grain size distribution of the feed. When the feed texture included gravel, the finest fractions of the sediment supply infiltrate beneath the surface. Conversely, sand remains on the surface when the feed texture lacks gravel. This different textural response becomes obscured when water discharge increases. Further, the sediment transport rate approaches the feed rate differently depending on the content of gravel in the feed texture. When a small proportion of gravel is part of the feed texture, bedload transport rate asymptotically approaches the feed rate. However, when a significant fraction of gravel is part of the feed grain size distribution, bedload transport rate approaches the feed rate by following an oscillatory path. These findings have been verified in terms of a one‐dimensional numerical model. This modelling reveals that the higher the differences in mobility among the grain sizes contained in the feed texture, the more evident is the non‐asymptotic transient trend towards equilibrium.
- Monitoring reservoir storage in South Asia from multisatellite remote
- Authors: Shuai Zhang; Huilin Gao, Bibi S. Naz
Pages: n/a - n/a
Abstract: Reservoir storage information is essential for accurate flood monitoring and prediction. South Asia, however, is dominated by international river basins where communications among neighboring countries about reservoir storage and management are extremely limited. A suite of satellite observations were combined to achieve high quality estimation of reservoir storage and storage variations in South Asia from 2000 to 2012. The approach used water surface area estimations from the Moderate Resolution Imaging Spectroradiometer (MODIS) vegetation indices product and the area‐elevation relationship to estimate reservoir storage. The surface elevation measurements were from the Geoscience Laser Altimeter System (GLAS) on board the Ice, Cloud and land Elevation Satellite (ICESat). In order to improve the accuracy of water surface area estimations for relatively small reservoirs, a novel classification algorithm was developed. In this study, storage information was retrieved for a total of 21 reservoirs, which represents 28% of the integrated reservoir capacity in South Asia. The satellite‐based reservoir elevation and storage were validated by gauge observations over five reservoirs. The storage estimates were highly correlated with observations (i.e., correlation coefficients larger than 0.9), with normalized root mean square error (NRMSE) ranging from 9.51% to 25.20%. Uncertainty analysis was also conducted for the remotely sensed storage estimations. For the parameterization uncertainty associated with surface area retrieval, the storage mean relative error was 3.90%. With regard to the uncertainty introduced by ICESat/GLAS elevation measurements, the storage mean relative error was 0.67%.
- The blue water footprint and land use of biofuels from algae
- Authors: P.W. Gerbens‐Leenes; L. Xu, G.J. DeVries, A.Y. Hoekstra
Pages: n/a - n/a
Abstract: Biofuels from microalgae are potentially important sources of liquid renewable energy. Algae are not yet produced on a large scale, but research shows promising results. This study assesses the blue water footprint (WF) and land use of algae‐based biofuels. It combines the WF concept with an energy balance approach to determine the blue WF of net energy. The study considers open ponds and closed photobioreactors (PBRs). All systems have a positive energy balance, ranging from output‐input ratios between 1.13 and 1.98. This study shows that the WF of algae‐based biofuels lies between 8 and 193 m3 per GJ net energy provided. The land use of micro‐algal biofuels ranges from 20 to 200 m2 per GJ net energy. For a scenario in which algae‐based biofuels provide 3.5% of the transportation fuels in the European Union in 2030, the system with the highest land productivity needs 3,300 km2 to produce the 850 PJ per year. Producing all algae‐based biofuels through the system with the highest water productivity, would lead to a blue WF of 7 Gm3 per year, which is equivalent to 15% of the present blue WF in the EU28. However, a complete transition to algae‐based transportation fuels will cause increased competition over water and land resources.
- Multimodel Bayesian analysis of groundwater data worth
- Authors: Liang Xue; Dongxiao Zhang, Alberto Guadagnini, Shlomo P. Neuman
Pages: n/a - n/a
Abstract: We explore the way in which uncertain descriptions of aquifer heterogeneity and groundwater flow impact one's ability to assess the worth of collecting additional data. We do so on the basis of Maximum Likelihood Bayesian Model Averaging (MLBMA) by accounting jointly for uncertainties in geostatistical and flow model structures and parameter (hydraulic conductivity) as well as system state (hydraulic head) estimates, given uncertain measurements of one or both variables. Previous description of our approach was limited to geostatistical models based solely on hydraulic conductivity data. Here we implement the approach on a synthetic example of steady state flow in a two‐dimensional random log hydraulic conductivity field with and without recharge by embedding an inverse stochastic moment solution of groundwater flow in MLBMA. A moment‐equations‐based geostatistical inversion method is utilized to circumvent the need for computationally expensive numerical Monte Carlo simulations. The approach is compatible with either deterministic or stochastic flow models and consistent with modern statistical methods of parameter estimation, admitting but not requiring prior information about the parameters. It allows but does not require approximating lead predictive statistical moments of system states by linearization while updating model posterior probabilities and parameter estimates on the basis of potential new data both before and after such data are actually collected.
- Coupled estimation of surface heat fluxes and vegetation dynamics from
remotely sensed land surface temperature and fraction of
photosynthetically active radiation
- Authors: S. M. Bateni; D. Entekhabi, S. Margulis, F. Castelli, L. Kergoat
Pages: n/a - n/a
Abstract: Remotely sensed Land Surface Temperature (LST) and Fraction of Photosynthetically Active Radiation absorbed by vegetation (FPAR) are assimilated respectively into the Surface Energy Balance (SEB) equation and a Vegetation Dynamics Model (VDM) in order to estimate surface fluxes and vegetation dynamics. The problem is posed in terms of three unknown and dimensionless parameters: (1) neutral bulk heat transfer coefficient, which scales the sum of turbulent heat fluxes, (2) soil and canopy evaporative fractions that characterize partitioning among the turbulent heat fluxes over soil and vegetation, and (3) specific leaf area, which captures seasonal phenology and vegetation dynamics.
The model is applied over the Gourma site in Mali, the northern region of the West African Monsoon (WAM) domain. The application of the model over the Gourma site shows that space‐borne LST observations can be used to constrain the SEB equation and obtain its key two unknown parameters (i.e., neutral bulk heat transfer coefficient and evaporative fraction). We demonstrate that the spatial patterns of estimated neutral bulk heat transfer coefficient and evaporative fraction resemble respectively those of independently observed vegetation index and soil moisture. The framework also yields estimates of surface energy balance components. The daily sensible, latent, and ground heat flux estimates at the Agoufou site that is located in the south of the Gourma region have respectively a root‐mean‐square error (RMSE) of 53.6, 34.4, and 45.1 Wm‐2. The daily sensible heat flux estimates at the Bamba site which is located in the north of the Gourma domain have a RMSE of 42.6 Wm‐2. The results also show that remotely sensed FPAR observations can constrain the VDM and retrieve its main unknown parameter (specific leaf area) over large‐scale domains without costly in situ measurements. The results indicate that the estimated specific leaf area values vary reasonably with the expected influential environmental variables such as precipitation, air temperature, and solar radiation. Assimilating FPAR observations into the VDM can also provide an estimate of Leaf Area Index (LAI) dynamics. The estimated LAI values are comparable in magnitude, spatial pattern and temporal evolution with satellite retrievals.
- Stochastic temporal disaggregation of monthly precipitation for regional
gridded data sets
- Authors: Stephan Thober; Juliane Mai, Matthias Zink, Luis Samaniego
Pages: n/a - n/a
Abstract: Weather generators are used for spatio‐temporal downscaling of climate model outputs (e.g., precipitation and temperature) to investigate the impact of climate change on the hydrological cycle. In this study, a multiplicative random cascade model is proposed for the stochastic temporal disaggregation of monthly to daily precipitation fields, which is designed to be applicable to grids of any spatial resolution and extent. The proposed method uses stationary distribution functions that describe the partitioning of precipitation throughout multiple temporal scales (e.g., weekly and bi‐weekly scale). Moreover, it explicitly considers the intensity and spatial covariance of precipitation in the disaggregation procedure, but requires no assumption about the temporal relationship and spatial isotropy of precipitation fields. A split sampling test is conducted on a high‐resolution (i.e., 4×4 km2 grid) daily precipitation data set over Germany (≈ 357 000 km2) to assess the performance of the proposed method during future periods. The proposed method has proven to consistently reproduce distinctive location dependent precipitation distribution functions with biases less than 5% during both a calibration and evaluation period. Furthermore, extreme precipitation amounts and the spatial and temporal covariance of the generated fields are comparable to those of the observations. Consequently, the proposed temporal disaggregation approach satisfies the minimum conditions for a precipitation generator aiming at the assessment of hydrological response to climate change at regional and continental scales or for generating seamless predictions of hydrological variables.
- Enhancement of rainfall and runoff upstream from irrigation location in a
climate model of West Africa
- Authors: Eun‐Soon Im; Elfatih A. B. Eltahir
Pages: n/a - n/a
Abstract: This study investigates the impact of potential medium‐scale irrigation (about 60,000km2) on the climate of West Africa using the MIT Regional Climate Model. We find that irrigation at this scale induces an atmospheric response similar to that of large‐scale irrigation (about 400,000km2) which was considered in our previous theoretical study. While the volume of water needed for large‐scale irrigation is about 230‐270 km3, the medium‐scale irrigation requires about 50 km3, and the annual flow of the Niger river in the relevant section is about 70 km3. The remote response of rainfall distribution to local irrigation exhibits a significant sensitivity to the latitudinal location of irrigation. The nature of this response is such that irrigation from the Niger River around latitude 18oN induces significant increase in rainfall of order 100% in the upstream sources of the Niger River and results in significant increase in runoff of order 50%. This additional runoff can potentially be collected by the river network and delivered back towards the irrigation area. By selecting the location of irrigation carefully, the positive impacts of irrigation on rainfall distribution can be maximized. The approach of using a regional climate model to investigate the impact of location and size of irrigation schemes, explored in this study, may be the first step in incorporating land‐atmosphere interactions in the design of location and size of irrigation projects. However, this theoretical approach is still in early stages of development and further research is needed before any practical application in water resources planning.
- Probabilistic collocation method for strongly nonlinear problems: 2.
Transform by displacement
- Authors: Qinzhuo Liao; Dongxiao Zhang
Pages: n/a - n/a
Abstract:  The probabilistic collocation method (PCM) is widely used for uncertainty quantification and sensitivity analysis. In paper 1 of this series, we demonstrated that the PCM may provide inaccurate results when the relation between the random input parameter and the model response is strongly nonlinear, and presented a location‐based transformed PCM (xTPCM) to address this issue, relying on the transform between response and location. However, the xTPCM is only applicable for one‐dimensional problems, and two‐ or three‐dimensional problems in homogeneous media. In this paper, we propose a displacement‐based transformed PCM (dTPCM), which is valid in two‐ or three‐dimensional problems in heterogeneous media. In the PCM, we first select collocation points and run model/simulator to obtain response, and then approximate the response by polynomial construction. Whereas in the dTPCM, we apply motion analysis to transform the response to displacement. That is, the response field is now represented by the displacement field. Next, we approximate the displacement instead of the response by polynomial, since the displacement is more linear to the input parameter than the response. Finally, we randomly generate a sufficient number of displacement samples and transform them back to obtain response samples to estimate statistical properties. Through multiphase flow and solute transport examples, we demonstrate that the dTPCM provides much more accurate statistics than does the PCM, and requires considerably less computer time than does the Monte Carlo (MC) method.
- A two stage Bayesian stochastic optimization model for cascaded hydropower
systems considering varying uncertainty of flow forecasts
- Authors: Wei Xu; Chi Zhang, Yong Peng, Guangtao Fu, Huicheng Zhou
Pages: n/a - n/a
Abstract: This paper presents a new Two Stage Bayesian Stochastic Dynamic Programming (TS‐BSDP) model for real time operation of cascaded hydropower systems to handle varying uncertainty of inflow forecasts from Quantitative Precipitation Forecasts. In this model, the inflow forecasts are considered as having increasing uncertainty with extending lead time, thus the forecast horizon is divided into two periods: the inflows in the first period are assumed to be accurate, and the inflows in the second period assumed to be of high uncertainty. Two operation strategies are developed to derive hydropower operation policies for the first and the entire forecast horizon using TS‐BSDP. In this paper, the newly developed model is tested on China's Hun River cascade hydropower system and is compared with three popular stochastic dynamic programming models. Comparative results show that the TS‐BSDP model exhibits significantly improved system performance in terms of power generation and system reliability due to its explicit and effective utilization of varying degrees of inflow forecast uncertainty. The results also show that the decision strategies should be determined considering the magnitude of uncertainty in inflow forecasts. Further, this study confirms the previous finding that the benefit in hydropower generation gained from the use of a longer horizon of inflow forecasts is diminished due to higher uncertainty and further reveals that the benefit reduction can be substantially mitigated through explicit consideration of varying magnitudes of forecast uncertainties in the decision making process.
- Filling in the gaps: Inferring spatially distributed precipitation from
gauge observations over complex terrain
- Authors: Ben Livneh; Jeff Deems, Dominik Schneider, Joseph Barsugli, Noah Molotch
Pages: n/a - n/a
Abstract: In recent decades, computational hydrology has trended towards higher‐resolution distributed models of the land surface. The accuracy of these models is limited, by uncertainty in distributed precipitation forcings. In this research, different precipitation distribution schemes were compared through inter‐station transfer experiments, as well as within a distributed hydrologic model applied at 30%, for RSWE products using the MSS techniques versus MBP (< 15%). Overall, PRISM performed best using MSS, while RSWE performed best using MBP.
- Modeling the effect of rainfall intensity on soil‐water nutrient
exchange in flooded rice paddies and implications for nitrate fertilizer
runoff to the Oita River in Japan
- Authors: Makoto Higashino; Heinz G Stefan
Pages: n/a - n/a
Abstract: This paper examines the effect of rainfall intensity on nutrient exchange at the soil‐water interface of rice paddy fields and the implications to nitrate runoff to the Oita River. The Oita River Basin on Kyushu Island in Japan covers 650 km2 of which 11% are used for agriculture (rice). During the monsoon season in June/July, the heavily fertilized paddy fields are flooded and large amounts of NO3‐N are discharged to the Oita River. A model has been developed for the NO3‐N release in the rice paddy fields. The model focuses on the effect of rainfall intensity. It assumes that in addition to increased surface runoff and infiltration the monsoon rain enhances pore water flow and cause nitrate release from the soil by dynamic pressure fluctuations at the soil/water interface. The magnitude of NO3‐N release from paddy fields is described by the simulated soil/water exchange velocity (W) which increases with rising rainfall intensity and hydraulic conductivity, and is on the order of 10‐2 to 10‐6 cm/s. When the river flow rises due to precipitation (monsoon), the NO3‐N load rises almost proportionately to the river discharge, and with little delay. Measured unit NO3‐N loads in the Oita River per day and m2 of paddy fields, were nearly proportional to precipitation intensity R (R1.042) and so were modeled unit NO3‐N release rates in the paddy fields (R1.095). This results suggests that raindrop induced pumping is an important if not crucial process that enhances NO3‐N runoff from rice paddy fields. An implication is the nutrient loading of surface water bodies may grow as the occurrence of extreme rainfall events increases with climate change.
- Sensitivity of reference evapotranspiration to changes in meteorological
parameters in Spain (1961–2011)
- Authors: Sergio M. Vicente‐Serrano; Cesar Azorin‐Molina, Arturo Sanchez‐Lorenzo, Jesús Revuelto, Enrique Morán‐Tejeda, Juan I. López‐Moreno, Francisco Espejo
Pages: n/a - n/a
Abstract: This study analyzes changes in monthly reference evapotranspiration (ETo) by use of the Penman‐Monteith equation and data from 46 meteorological stations in Spain from 1961 to 2011. Over the 51 year study period, there were trends for increasing average ETo during all months and annually at most of the individual meteorological stations. Sensitivity analysis of ETo to changes in meteorological variables was conducted by increasing and decreasing an individual climate variable holding the other variables constant. Sensitivity analysis indicated that relative humidity, wind speed, and maximum temperature had stronger effects on ETo than sunshine duration and minimum temperature. This suggests that aerodynamic component has more importance than radiative component to determine the atmospheric evaporative demand in Spain. The analysis showed a dominant latitudinal spatial gradient in the ETo relative changes across the 46 meteorological observatories, mainly controlled by the increasing available solar energy southward. In addition, the role of different meteorological variables on ETo is influenced by the average climatology at each observatory. ETo trends are mainly explained by the decrease in relative humidity and the increase in maximum temperature since the 1960s, particularly during the summer months. The physical mechanisms that explain ETo sensitivity to the different physical variables and current ETo trends are discussed in detail.
- Understanding the hydrologic sources and sinks in the Nile Basin using
multisource climate and remote sensing data sets
- Authors: Gabriel B. Senay; Naga Manohar Velpuri, Stefanie Bohms, Yonas Demissie, Mekonnen Gebremichael
Pages: n/a - n/a
Abstract: In this study, we integrated satellite‐drived precipitation, and modeled evapotranspiration data (2000–2012) to describe spatial variability of hydrologic sources and sinks in the Nile Basin. Over 2000–2012 period, four out of eleven countries (Ethiopia, Tanzania, Kenya, and Uganda) in the Nile Basin showed a positive water balance while three downstream countries (South Sudan, Sudan, and Egypt) showed a negative balance. Gravity Recovery and Climate Experiment (GRACE) mass deviation in storage data analysis showed that at annual time‐scales, the Nile Basin shows storage change is substantial while over longer time periods, it is minimal (
- Pore‐scale dynamics underlying the motion of drainage fronts in
- Authors: Franziska Moebius; Dani Or
Pages: n/a - n/a
Abstract: Fluid displacement fronts in porous media exhibit a peculiar duality; the seemingly regular macroscopic motion of the front is propelled by numerous and irregular pore scale interfacial jumps. These pore scale events shape emergent front morphology, affect phase entrapment behind a front, and are likely important for colloidal mobilization and solute dispersion at the front. We present an experimental study focusing on drainage fluid front invasion dynamics through sintered glass beads using a high‐speed camera and rapid capillary pressure measurements to resolve pore scale invasion events over a wide range of boundary conditions (flow rates and gravitational influences). We distinguished three types of “pores”: geometrical pores deduced from image analyses; individual pore invasion volumes imaged during displacement; and pore volumes deduced from capillary pressure fluctuations during constant withdrawal rates. The resulting pore volume distributions were remarkably similar for slow drainage rates. Invaded pore volumes were not affected by gravitational forces, however with increased viscous forces (higher displacement rates) the fraction of small invaded volumes increased. Capillary pressure fluctuations were exponentially distributed in agreement with findings from previous studies [Måløy et al., 1992]. The distribution of pressure fluctuations exhibited a distinct cutoff concurrent with the onset of simultaneous invasion events. The study highlights the different manifestation of “pores” and their sensitivity to external (macroscopic) boundary conditions. The remarkable similarity of geometrical and pressure‐deduced pore spaces offers opportunities for deducing pore size distribution dynamically.
- Why do households invest in sanitation in rural Benin: Health, wealth, or
- Authors: Elena Gross; Isabel Günther
Pages: n/a - n/a
Abstract: Seventy percent of the rural population in sub‐Saharan Africa does not use adequate sanitation facilities. In rural Benin, as much as 95 percent of the population does not use improved sanitation. By analyzing a representative sample of 2000 rural households, this paper explores why households remain without latrines. Our results show that wealth and latrine prices play the most decisive role for sanitation demand and ownership. At current income levels, sanitation coverage will only increase to 50 percent if costs for construction are reduced from currently $190 USD to $50 USD per latrine. Our analysis also suggests that previous sanitation campaigns, which were based on prestige and the allure of a modern lifestyle as motives for latrine construction, have had no success in increasing sanitation coverage. Moreover, improved public health, which is the objective of public policies promoting sanitation, will not be effective at low sanitation coverage rates. Fear at night, especially of animals, and personal harassment, are stated as the most important motivational factors for latrine ownership and the intention to build one. We therefore suggest changing the message of sanitation projects and introduce new low cost technologies into rural markets, otherwise marketing strategies will continue to fail in increasing sanitation demand.
- Use of 7Be measurements to estimate rates of soil loss from cultivated
land: Testing a new approach applicable to individual storm events
occurring during an extended period
- Authors: Paolo Porto; Des E. Walling
Pages: n/a - n/a
Abstract: The techniques available for documenting rates and patterns of soil redistribution in the landscape have many limitations and the value of those employing fallout radionuclides (FRNs), including caesium‐137 (137Cs) and excess lead‐210 (210Pbex) is being increasingly recognised. However, the use of 137Cs and 210Pbex measurements is best suited to investigations of longer‐term soil redistribution rates (i.e., ca. 50‐60 years for 137Cs and ca. 100 years for 210Pbex). For many purposes, a much shorter timeframe is required. Beryllium‐7 (7Be), another FRN (half‐life 53 days), offers the potential to document soil redistribution associated with individual events or short periods of heavy rainfall. However, existing approaches for using 7Be possess important limitations related to both the timing of the study period and its length. This contribution reports the development of a new approach that permits estimation of the soil loss associated with all individual events or short periods of rainfall occurring during a study period extending over a year or more. The approach was validated using data collected from an erosion plot located in southern Italy. The close agreement of the measured and estimated values demonstrates the validity of the new approach which has the potential to greatly increase the scope for using 7Be measurements to document short‐term soil redistribution associated with individual storm events.
- Organizing groundwater regimes and response thresholds by soils: A
framework for understanding runoff generation in a headwater catchment
- Authors: John P. Gannon; Scott W. Bailey, Kevin J. McGuire
Pages: n/a - n/a
Abstract: A network of shallow groundwater wells in a headwater catchment at the Hubbard Brook Experimental Forest in New Hampshire, USA was used to investigate the hydrologic behavior of five distinct soil morphological units. The soil morphological units were hypothesized to be indicative of distinct water table regimes. Water table fluctuations in the wells were characterized by their median and interquartile range of depth, proportion of time water table was present in the solum, and storage‐discharge behavior of subsurface flow. Statistically significant differences in median, interquartile range, and presence of water table were detected among soil units. Threshold responses were identified in storage‐discharge relationships of subsurface flow, with thresholds varying among soil units. These results suggest that soil horizonation is indicative of distinct groundwater flow regimes. The spatial distribution of water table across the catchment showed variably connected/disconnected active areas of runoff generation in the solum. The spatial distribution of water table and therefore areas contributing to stormflow is complex and changes depending on catchment storage.
- The concept of dual‐boundary forcing in land
surface‐subsurface interactions of the terrestrial hydrologic and
- Authors: M. Rahman; M. Sulis, S. Kollet
Pages: n/a - n/a
Abstract: Terrestrial hydrological processes interact in a complex, non‐linear fashion. It is important to quantify these interactions to understand the overall mechanisms of the coupled water and energy cycles. In this study, the concept of a dual boundary forcing is proposed that connects the variability of atmospheric (upper boundary) and subsurface (lower boundary) processes to the land surface mass and energy balance components. According to this concept, the space‐time patterns of land surface mass and energy fluxes can be explained by the variability of the dominating boundary condition for the exchange processes, which is determined by moisture and energy availability. A coupled subsurface‐land surface model is applied on the Rur catchment, Germany, to substantiate the proposed concept. Spectral and geostatistical analysis on the observations and model results show the coherence of different processes at various space‐time scales in the hydrological cycle. The spectral analysis shows that atmospheric radiative forcing generally drives the variability of the land surface energy fluxes at the daily time scale, while influence of subsurface hydrodynamics is significant at monthly to multi‐month time scales under moisture limited conditions. The geostatistical analysis demonstrates that atmospheric forcing and groundwater control the spatial variability of land surface processes under energy and moisture limited conditions, respectively. These results suggest that under moisture limited conditions, groundwater influences the variability of the land surface mass and energy fluxes. Under energy limited conditions, on the contrary, variability of land surface processes can be explained by atmospheric forcing alone.
- Freshwater lenses as archive of climate, groundwater recharge, and
hydrochemical evolution—Insights from depth‐specific water
- Authors: Georg J. Houben; Paul Koeniger, Jürgen Sültenfuß
Pages: n/a - n/a
Abstract: The age stratification of a freshwater lens on the island of Langeoog, Germany, was reconstructed through depth‐specific sampling and groundwater dating using the tritium‐helium method. The stratification is strongly affected by the land use and resulting differences in recharge rates. Infiltration at the dune tops is significantly lower than in the valleys, due to repellency of the dry sand. Dune valleys contribute up to four times more groundwater recharge per area than other areas. Housing development in dune areas might therefore significantly decrease the available fresh groundwater. The freshwater column shows a distinct increase of stable isotope values with decreasing depths. Hence, the freshwater lens contains a climate archive which reflects changing environmental conditions at the time of recharge. Combined with tritium‐helium dating, this pattern could be matched to climate records which show an increase of the temperature at the time of recharge and rainfall rates during the last 50 years. The spatial and temporal development of water chemistry during the passage through the lens follows a marked pattern from a sodium and chloride‐dominated rain water of low conductivity to a more mineralized sodium bicarbonate water type, caused by dissolution of carbonate shells close to the surface and subsequent ion exchange of calcium for sodium in the deeper parts.
- A dynamical system model of eco‐geomorphic response to landslide
- Authors: Colin P. Stark; Paola Passalacqua
Pages: n/a - n/a
Abstract: Vegetated landscapes form through the interactions of ecologic and geomorphic processes. These interactions are generally slow and steady, but they are occasionally the subject of abrupt disturbance. In humid uplands, for example, landslides episodically disrupt forest growth and regolith development, suddenly mobilize soil, regolith and bedrock, and facilitate runoff‐driven erosion by abruptly removing canopy protection over a wide area. Here we model such an environment as a stochastically perturbed dynamical system whose simplified low dimensionality makes its eco‐geomorphic interactions easier to explore and understand. The model captures some of the spatial variability across a catchment by treating an ensemble of sub‐catchments: in each, aggregated biomass and regolith co‐evolve as a two‐dimensional dynamical system subject to episodic disturbance by slope failure. This co‐evolution gives rise to a notional stable equilibrium between regolith and biomass, but one that is not achieved in practice where landslide disturbance is significant. Instead, a catchment‐scale, ensemble‐average state arises in which higher storm frequency entails thinner regolith, less biomass, and weaker canopy protection against runoff erosion. The model makes the counter‐intuitive prediction that, as rainfall‐triggered landslides become more frequent, their contribution to the erosion of weathered bedrock will diminish as the role from storm runoff erosion rises. In some environments apparently dominated by landsliding, the model predicts that runoff erosion may be more important in the removal of regolith and fine sediment from hillslopes than mass wasting.
- Relationships between stream nitrate concentration and spatially
distributed snowmelt in high‐elevation catchments of the western
- Authors: Danielle Perrot; Noah P. Molotch, Mark W. Williams, Steven M. Jepsen, James O. Sickman
Pages: n/a - n/a
Abstract: This study compares stream nitrate (NO3‐ ) concentrations to spatially distributed snowmelt in two alpine catchments, the Green Lakes Valley, Colorado (GLV4) and Tokopah Basin, California (TOK). A snow water equivalent reconstruction model and Landsat 5 and 7 snow cover data were used to estimate daily snowmelt at 30‐m spatial resolution in order to derive indices of new snowmelt areas (NSAs). Estimates of NSA were then used to explain the NO3‐ flushing behavior for each basin over a 12 year period (1996‐2007). To identify the optimal method for defining NSAs and elucidate mechanisms underlying catchment NO3‐ flushing, we conducted a series of regression analyses using multiple thresholds of snowmelt based on temporal and volumetric metrics. NSA indices defined by volume of snowmelt (e.g. snowmelt ≤ 30 cm) rather than snowmelt duration (e.g. snowmelt ≤ 9 days) were the best predictors of stream NO3‐ concentrations. The NSA indices were better correlated with stream NO3‐ concentration in TOK (average R2=0.68) versus GLV4 (average R2=0.44). Positive relationships between NSA and stream NO3‐ concentration were observed in TOK with peak stream NO3‐ concentration occurring on the rising limb of snowmelt. Positive and negative relationships between NSA and stream NO3‐concentration were found in GLV4 with peak stream NO3‐ concentration occurring as NSA expands. Consistent with previous works, the contrasting NO3‐ flushing behavior suggests that streamflow in TOK was primarily influenced by overland flow and shallow subsurface flow, whereas GLV4 appeared to be more strongly influenced by deeper subsurface flowpaths.
- Dual‐domain mass transfer parameters from electrical hysteresis:
Theory and analytical approach applied to laboratory, synthetic streambed,
and groundwater experiments
- Authors: Martin A. Briggs; Frederick D. Day‐Lewis, John B. Ong, Judson W. Harvey, John W. Lane
Pages: n/a - n/a
Abstract: Models of dual‐domain mass transfer (DDMT) are used to explain anomalous aquifer transport behavior such as the slow release of contamination and solute tracer tailing. Traditional tracer experiments to characterize DDMT are performed at the flowpath scale (meters), which inherently incorporates heterogeneous exchange processes; hence, estimated “effective” parameters are sensitive to experimental design (i.e., duration and injection velocity). Recently, electrical geophysical methods have been used to aid in the inference of DDMT parameters because, unlike traditional fluid sampling, electrical methods can directly sense less‐mobile solute dynamics and can target specific points along subsurface flowpaths. Here, we propose an analytical framework for graphical parameter inference based on a simple petrophysical model explaining the hysteretic relation between measurements of bulk and fluid conductivity arising in the presence of DDMT at the local scale. Analysis is graphical and involves visual inspection of hysteresis patterns to (1) determine the size of paired mobile and less‐mobile porosities, and (2) identify the exchange rate coefficient through simple curve fitting. We demonstrate the approach using laboratory column experimental data, synthetic streambed experimental data, and field tracer‐test data. Results from the analytical approach compare favorably with results from calibration of numerical models and also independent measurements of mobile and less‐mobile porosity. We show that localized electrical hysteresis patterns resulting from diffusive exchange are independent of injection velocity, indicating that repeatable parameters can be extracted under varied experimental designs, and these parameters represent the true intrinsic properties of specific volumes of porous media of aquifers and hyporheic zones.
- Using observations and a distributed hydrologic model to explore runoff
thresholds linked with mesquite encroachment in the Sonoran Desert
- Authors: Nicole A. Pierini; Enrique R. Vivoni, Agustin Robles‐Morua, Russell L. Scott, Mark A. Nearing
Pages: n/a - n/a
Abstract: Woody‐plant encroachment is a worldwide phenomenon with implications on the hydrologic cycle at the catchment scale that are not well understood. In this study, we use observations from two small semiarid watersheds in southern Arizona that have been encroached by the velvet mesquite tree and apply a distributed hydrologic model to explore runoff threshold processes experienced during the North American monsoon. The paired watersheds have similar soil and meteorological conditions, but vary considerably in terms of vegetation cover (mesquite, grass, bare soil) and their proportions with one basin having undergone mesquite removal in 1974. Long‐term observations from the watersheds exhibit changes in runoff production over time, such that the watershed with more woody plants currently has less runoff for small rainfall events, more runoff for larger events, and a larger runoff ratio during the study periods (summers 2011 and 2012). To explain this observation, we first test the distributed model, parameterized with high‐resolution (1 m) terrain and vegetation distributions, against continuous data from an environmental sensor network, including an eddy covariance tower, soil moisture and temperature profiles in different vegetation types, and runoff observations. We find good agreement between the model and observations for simultaneous water and energy states and fluxes over a range of measurement scales. We then identify that the areal fraction of grass (bare soil) cover determines the runoff response for small (large) rainfall events due to the dominant controls of antecedent wetness (hydraulic conductivity). These model‐derived mechanisms explain how woody plants have differential effects on runoff in semiarid basins depending on precipitation event sizes.
- Storm‐event flow pathways in lower coastal plain forested watersheds
of the southeastern U.S.
- Authors: Michael P. Griffin; Timothy J. Callahan, Vijay M. Vulava, Thomas M. Williams
Pages: n/a - n/a
Abstract: The landscape of the coastal plain of the southeastern United States is rapidly changing due to urbanization and climate‐change‐related impacts. In addition to the forecasted population increase, this region could experience significant changes in precipitation patterns making watershed management very challenging. In order to establish baseline data, storm‐event flow pathways were studied in three lower coastal plain (LCP) forested lowland watersheds of the southeastern United States between 2010‐2011. Two of the watersheds had clay loam sub‐soils while the third had sandy soils throughout the profile. Stream flow and water samples from water‐table wells, piezometers, lysimeters, and rain gauges were analyzed for ion concentrations; ion trends were assessed using principal components analysis; and chemical hydrograph separation was performed for nine storm events using end member mixing analysis. End members consisted of lower concentration rainwater; a near‐stream source (riparian or streambed groundwater); and a distant or deep groundwater source. Storm‐event stream water on the clayey sites was composed primarily of rainwater (45‐67% by volume) and shallow groundwater (21‐55%), with small inputs from deep (below the clay‐rich soil horizon) groundwater (0.2‐21%). At the sandy site, a greater proportion was groundwater (56‐61%), with smaller inputs from rainwater (28‐33%) and soil water (6‐16%). Dry antecedent soil moisture (ASM) conditions and larger storms resulted in greater rainwater contribution at the clayey sites. Shallow groundwater was an important contributor even in dry ASM conditions, perhaps due to the high specific retention of the soils. The results from this study will inform researchers about stormwater routing in forested, shallow water table watersheds and provide land managers with baseline data as they plan stormwater mitigation practices.
- Combining the bulk transfer formulation and surface renewal analysis for
estimating the sensible heat flux without involving the parameter
- Authors: F. Castellví; P. Gavilán, M.P. González
Pages: n/a - n/a
Abstract: The single‐source bulk transfer formulation (based on the Monin ‐ Obukhov Similarity Theory, MOST) has been used to estimate the sensible heat flux, H, in the framework of remote sensing over homogeneous surfaces (HMOST). The latter involves the canopy parameter kB‐1, which is difficult to parameterise. Over short and dense grass at a site influenced by regional advection of sensible heat flux, HMOST with kB‐1 =2 (i.e., the value recommended) correlated strongly with the H measured using the Eddy Covariance, EC, method, HEC. However, it overestimated HEC by 50% under stable conditions for samples showing a local air temperature gradient larger than the measurement error, 0.4 Km‐1. Combining MOST and Surface Renewal analysis, three methods of estimating H that avoid kB‐1 dependency have been derived. These new expressions explain the variability of H versus u* (Tc ‐ T(z)), where u* is the friction velocity, Tc is the radiometric surface temperature and T(z) is the air temperature at height, z. At two measurement heights, the three methods performed excellently. One of the methods developed required the same readily/commonly available inputs as HMOST due to the fact that the ratio between (Tc ‐ T(z)) and the ramp amplitude was found fairly constant under stable and unstable cases. Over homogeneous canopies, at a site influenced by regional advection of sensible heat flux, the methods proposed are an alternative to the traditional bulk transfer method because they are reliable, exempt of calibration against the EC method, and are comparable or identical in cost of application. It is suggested that the methodology may be useful over bare soil and sparse vegetation.
- Numerical study of evaporation‐induced salt accumulation and
precipitation in bare saline soils: Mechanism and feedback
- Authors: Chenming Zhang; Ling Li, David Lockington
Pages: n/a - n/a
Abstract:  Evaporation from bare saline soils in coastal wetlands causes salt precipitation in the form of efflorescence and subflorescence. However, it is not clear how much the precipitated salt in turn affects the water transport in the soil and hence the evaporation rate. We hypothesized that efflorescence exerts a mulching resistance to evaporation, while subflorescence reduces the pore space for water vapor to move through the soil. A numerical model is developed to simulate the transport of water, solute and heat in the soil, and resulting evaporation and salt precipitation with the hypothesized feedback mechanism incorporated. The model was applied to simulate four evaporation experiments in soil columns with and without a fixed shallow water table, and was found to replicate well the experimental observations. The simulated results indicated that as long as the hydraulic connection between the near surface soil layer and the water source in the interior soil layer exists, vaporization occurs near the surface and salt precipitates exclusively as efflorescence. When such hydraulic connection is absent, the vaporization plane develops downward and salt precipitates as subflorescence. Being more substantial in quantity, efflorescent affects more significantly evaporation than subflorescence during the soil‐drying process. Different evaporation stages based on the location of the vaporization plane and the state of salt accumulation can be identified for characterizing the process of evaporation from bare saline soils with or without a fixed shallow water table.
- Seeking genericity in the selection of parameter sets: Impact on
hydrological model efficiency
- Authors: Vazken Andréassian; François Bourgin, Ludovic Oudin, Thibault Mathevet, Charles Perrin, Julien Lerat, Laurent Coron, Lionel Berthet
Pages: n/a - n/a
Abstract: This paper evaluates the use of a small number of generalist parameter sets as an alternative to classical calibration. Here parameter sets are considered generalist when they yield acceptable performance on a large number of catchments.
We tested the genericity of an initial collection of 106 parameter sets sampled in the parameter space for the four‐parameter GR4J rainfall‐runoff model. A short‐list of 27 generalist parameter sets was obtained as a good compromise between model efficiency and length of the short‐list.
A different data set was used for an independent evaluation of a calibration procedure, in which the search for an optimum parameter set is only allowed within this short‐list. In validation mode, the performance obtained is inferior to that of a classical calibration, but when the amount of data available for calibration is reduced, the generalist parameter sets become progressively more competitive, with better results for calibration series shorter than 1 year.
- Lateral subsurface stormflow and solute transport in a forested hillslope:
A combined measurement and modeling approach
- Authors: Hanne Laine‐Kaulio; Soile Backnäs, Tuomo Karvonen, Harri Koivusalo, Jeffrey J. McDonnell
Pages: n/a - n/a
Abstract: Preferential flow dominates water movement and solute transport in boreal forest hillslopes. However, only a few model applications to date have accounted for preferential flow at forest sites. Here, we present a parallel and coupled simulation of flow and transport processes in the preferential flow domain and soil matrix of a forested hillslope section in Kangaslampi, Finland, using a new, three‐dimensional, physically‐based dual‐permeability model. Our aim is to simulate lateral subsurface stormflow and solute transport at the slope during a chloride tracer experiment, and to investigate the role of preferential flow in the tracer transport. The model was able to mimic the observed tracer transport during tracer irrigation, but overestimated the dilution velocity of the tracer plume in the highly conductive soil horizons near the soil surface after changing the irrigation to tracer‐free water. According to the model, 140 times more chloride was transported downslope in the preferential flow domain than in the soil matrix during the tracer irrigation. The simulations showed, together with reference simulations with a traditional one pore domain model, that a two pore domain approach was required to simulate the observed flow and transport event. The event was characterized by the transmissivity feedback phenomenon and controlled by preferential flow mechanisms, in particular by lateral by‐pass flow. According to our results, accounting for the slow‐ and fast‐flow domains of soil, as well as the water and solute exchange between the domains, is essential for a successful simulation of flow and solute transport in preferential flow dominated hillslopes.
- Runoff sources and flow paths in a partially burned, upland boreal
catchment underlain by permafrost
- Authors: Koch Joshua C; Kikuchi Colin P, Wickland Kimberly P, Schuster Paul
Pages: n/a - n/a
Abstract: Boreal soils in permafrost regions contain vast quantities of frozen organic material that is released to terrestrial and aquatic environments via subsurface flowpaths as permafrost thaws. Longer flowpaths may allow chemical reduction of solutes, nutrients, and contaminants, with implications for greenhouse gas emissions and aqueous export. Predicting boreal catchment runoff is complicated by soil heterogeneities related to variability in active layer thickness, soil type, fire history, and preferential flow potential. By coupling measurements of permeability, infiltration potential, and water chemistry with a stream chemistry end member mixing model, we tested the hypothesis that organic soils and burned slopes are the primary sources of runoff, and that runoff from burned soils is greater due to increased hydraulic connectivity. Organic soils were more permeable than mineral soils, and 25% of infiltration moved laterally upon reaching the organic‐mineral soil boundary on unburned hillslopes. A large portion of the remaining water infiltrated into deeper, less permeable soils. In contrast, burned hillslopes displayed poorly defined soil horizons, allowing rapid, mineral‐rich runoff through preferential pathways at various depths. On the catchment scale, mineral/organic runoff ratios averaged 1.6 and were as high as 5.2 for an individual storm. Our results suggest that burned soils are the dominant source of water and solutes reaching the stream in summer, whereas unburned soils may provide longer term storage and residence times necessary for production of anaerobic compounds. These results are relevant to predicting how boreal catchment drainage networks and stream export will evolve given continued warming and altered fire regimes.
- Micromodel study of two‐phase flow under transient conditions:
Quantifying effects of specific interfacial area
- Authors: N. K. Karadimitriou; S. M. Hassanizadeh, V. Joekar‐Niasar, P. J. Kleingeld
Pages: n/a - n/a
Abstract: Recent computational studies of two‐phase flow suggest that the role of fluid‐fluid interfaces should be explicitly included in the capillarity equation as well as equations of motion of phases. The aim of this study has been to perform experiments where transient movement of interfaces can be monitored and to determine interfacial variables and quantities under transient conditions. We have performed two‐phase flow experiments in a transparent micro‐model. Specific interfacial area is defined, and calculated from experimental data, as the ratio of the total area of interfaces between two phases per unit volume of the porous medium. Recent studies have shown that all drainage and imbibition data points for capillary pressure, saturation, and specific interfacial area fall on a unique surface. But, up to now, almost all micro‐model studies of two‐phase flow have dealt with quasi‐static or steady‐state flow conditions. Thus, only equilibrium properties have been studied.
We present the first study of two‐phase flow in an elongated PDMS micro‐model under transient conditions with high temporal and spatial resolutions. We have established that different relationships between capillary pressure, saturation, and specific interfacial area are obtained under steady‐state and transient conditions. The difference between the surfaces depends on the capillary number. Furthermore, we use our experimental results to obtain average (macro scale) velocity of fluid‐fluid interfaces and the rate of change of specific interfacial area as a function of time and space. Both terms depend on saturation nonlinearly but show a linear dependence on the rate of change of saturation. We also determine macro‐scale material coefficients that appear in the equation of motion of fluid‐fluid interfaces. This is the first time that these parameters are determined experimentally.
- Exact versus Dupuit interface flow in anisotropic coastal aquifers
- Authors: M. Bakker
Pages: n/a - n/a
Abstract: The Dupuit solution for interface flow towards the coast in a confined aquifer is compared to a new exact solution, which is obtained with the Hodograph method and conformal mapping. The position of the toe of the interface is a function of two dimensionless parameters: the ratio of the hydraulic gradient upstream of the interface where flow is one‐dimensional over the dimensionless density difference, and the ratio of the horizontal hydraulic conductivity over the vertical hydraulic conductivity. The Dupuit interface, which neglects resistance to vertical flow, is a very accurate approximation of the exact interface for isotropic aquifers. The difference in the position of the toe between the exact and Dupuit solutions increases when the vertical anisotropy increases. For highly anistropic aquifers, it is proposed to add an effective resistance layer along the bottom of the sea in Dupuit models. The resistance of the layer is chosen such that the head in the Dupuit model is equal to the head in the exact solution upstream of the interface where flow is one‐dimensional.
- Sensitivity of power functions to aggregation: Bias and uncertainty in
radar rainfall retrieval
- Authors: M. G. Sassi; H. Leijnse, R. Uijlenhoet
Pages: n/a - n/a
Abstract: Rainfall retrieval using weather radar relies on power functions between radar reflectivity Z and rain rate R. The nonlinear nature of these relations complicates the comparison of rainfall estimates employing reflectivities measured at different scales. Transforming Z into R using relations that have been derived for other scales results in a bias and added uncertainty. We investigate the sensitivity of Z‐R relations to spatial and temporal aggregation using high‐resolution reflectivity fields for five rainfall events. Existing Z‐R relations were employed to investigate the behavior of aggregated Z‐R relations with scale, the aggregation bias and the variability of the estimated rain rate. The prefactor and the exponent of aggregated Z‐R relations systematically diverge with scale, showing a break that is event‐dependent in the temporal domain and nearly constant in space. The systematic error associated with the aggregation bias at a given scale can become of the same order as the corresponding random error associated with intermittent sampling. The bias can be constrained by including information about the variability of Z within a certain scale of aggregation, and is largely captured by simple functions of the coefficient of variation of Z. Several descriptors of spatial and temporal variability of the reflectivity field are presented, to establish the links between variability descriptors and resulting aggregation bias. Prefactors in Z‐R relations can be related to multi‐fractal properties of the rainfall field. We find evidence of scaling breaks in the structural analysis of spatial rainfall with aggregation.
- Patterns of local and nonlocal water resource use across the western U.S.
determined via stable isotope intercomparisons
- Authors: Stephen P. Good; Casey D. Kennedy, Jeremy C. Stalker, Lesley A. Chesson, Luciano O. Valenzuela, Melanie M. Beasley, James R. Ehleringer, Gabriel. J. Bowen
Pages: n/a - n/a
Abstract: In the western United States, the mis‐match between public water demands and natural water availability necessitates large inter‐basin transfers of water as well as groundwater mining of fossil aquifers. Here, we identify probable situations of non‐local water use in both space and time based on isotopic comparisons between tap waters and potential water resources within hydrologic basins. Our approach, which considers evaporative enrichment of heavy isotopes during storage and distribution, is used to determine the likelihood of local origin for 612 tap water samples collected from across the western United States. We find that 64% of samples are isotopically distinct from precipitation falling within the local hydrologic basin, a proxy for groundwater with modern recharge, and 31% of samples are isotopically distinct from estimated surface water found within the local basin. Those samples inconsistent with local water sources, which we suggest are likely derived from water imported from other basins or extracted from fossil‐aquifers, are primarily clustered in southern California, the San Francisco Bay area, and central Arizona. Our isotope‐based estimates of non‐local water use are correlated with both hydro‐geomorphic and socio‐economic properties of basins, suggesting that these factors exert a predictable influence on the likelihood that non‐local waters are used to supply tap water. We use these basin properties to develop a regional model of non‐local water resource use that predicts (r2=0.64) isotopically inferred patterns and allows assessment of total inter‐basin transfer and/or fossil aquifer extraction volumes across the western United States.
- A parameter estimation framework for multiscale Kalman smoother algorithm
in precipitation data fusion
- Authors: Shugong Wang; Xu Liang
Pages: n/a - n/a
Abstract: A new effective parameter estimation approach is presented for the Multiscale Kalman Smoother (MKS) algorithm. As demonstrated, it shows promising potentials in deriving better data products involving sources from different spatial scales and precisions. The proposed approach employs a multi‐objective parameter estimation framework, which includes three multi‐objective estimation schemes (MO schemes), rather than using the conventional maximum likelihood scheme (ML scheme), to estimate the MKS parameters. Unlike the ML scheme, the MO schemes are not built on strict statistical assumptions related to prediction errors and observation errors, rather, they directly associate the fused data of multiple scales with multiple objective functions. In the MO schemes, objective functions are defined to facilitate consistency among the fused data at multiple scales and the input data at their original scales as well in terms of spatial patterns and magnitudes. Merits of the new approach are evaluated through a Monte Carlo experiment and a series of comparison analyses using synthetic precipitation data that contain noises which follow either the multiplicative error model or the additive error model. Our results show that the MKS fused precipitation performs better using the MO framework. Improvements are particularly significant for the fused precipitation associated with fine spatial resolutions. This is due mainly to the adoption of more criteria and constraints in the MO framework. The weakness of the original ML scheme, arising from its blindly putting more weights into the data associated with finer resolutions, is circumvented in the proposed new MO framework.
- Improving the surface‐ground water interactions in the Community
Land Model: Case study in the Blue Nile Basin
- Authors: Di D. Wu; Emmanouil N. Anagnostou, Guiling Wang, Semu Moges, Matteo Zampieri
Pages: n/a - n/a
Abstract: Soil moisture is a key water cycle parameter known to interact with atmospheric processes. Arguably, land surface models that simulate land surface processes and surface fluxes to the atmosphere do not capture adequately the spatial variability of soil moisture, particularly over areas with complex topography. In this study, version 3.5 of the Community Land Model (CLM3.5) is applied with a new parameterization in an effort to correct the spatial bias of soil moisture and understand the consequential effects on the simulated water cycle fluxes and states in the Blue Nile basin. This parameterization accounts for a groundwater recharge term from surface water, a process that is not included in CLM, providing an effective two‐way interaction scheme between rivers and groundwater. Using satellite soil moisture data, this parameterized term is shown to have a positive correlation to contributing area, defined at each model grid cell and representing the number of grid cells that drain to that local grid cell. With the new parameterization applied to CLM, soil moisture, soil temperature, evapotranspiration flux, water table depth, and vegetation water content all showed significant differences from the control CLM run (without the parameterization) at or above the 95% confidence level. The differences in the spatial distribution of these variables are expected to affect precipitation simulations from regional climate modeling. As the Blue Nile is a region that has one of the greatest inter‐annual and seasonal precipitation variability globally, the ability to predict this variability is essential for optimal reservoir operations including buffering of water resources during times of drought.
- Spatially variable water table recharge and the hillslope hydrologic
response: Analytical solutions to the linearized hillslope Boussinesq
- Authors: David N. Dralle; Gabrielle F.S. Boisramé, Sally E. Thompson
Pages: n/a - n/a
Abstract: The linearized hillslope Boussinesq equation, introduced by Brutsaert, describes the dynamics of saturated, subsurface flow from hillslopes with shallow, unconfined aquifers. In this paper, we use a new analytical technique to solve the linearized hillslope Boussinesq equation to predict water table dynamics and hillslope discharge to channels. The new solutions extend previous analytical treatments of the linearized hillslope Boussinsq equation to account for the impact of spatiotemporal heterogeneity in water table recharge. The results indicate that the spatial character of recharge may significantly alter both steady‐state subsurface storage characteristics and the transient hillslope hydrologic response, depending strongly on similarity measures of controls on the subsurface flow dynamics. Additionally, we derive new analytical solutions for the linearized hillslope‐storage Boussinesq equation and explore the interaction effects of recharge structure and hillslope morphology on water storage and baseflow recession characteristics. A theoretical recession analysis, for example, demonstrates that decreasing the relative amount of downslope recharge has a similar effect as increasing hillslope convergence. In general, the theory suggests that recharge heterogeneity can serve to diminish or enhance the hydrologic impacts of hillslope morphology.
- Channel evolution after dam removal in a poorly sorted sediment mixture:
Experiments and numerical model
- Authors: Carles Ferrer‐Boix; Juan Pedro Martín‐Vide, Gary Parker
Pages: n/a - n/a
Abstract: Dam removal is commonly used for river restoration. However, there are still some uncertainties associated with dam removal, mainly related to the sediment transport rates released downstream from the deposit that had previously filled the impoundment. This research studies the physical response to dam removal in the antecedent deposit by answering the following questions: a) how does an initial channel excavated into the deposit evolve, and b) what is the time distribution of the material released during the early stages of the process. These goals are achieved by an experimental campaign using a poorly sorted mixture of sediment in the antecedent deposit. The research shows that for the given conditions of our experiments, the rate at which the sediment is released depends on the height of the removed dam, the water discharge and the maximum potential volume of sediment to be eroded. This investigation provides new insights of the width evolution when the sediment is composed of a poorly‐sorted mixture. This evolution is linked to the bed degradation rates: channel narrows during a rapid incisional phase, and subsequently widens when bed degradation rates decrease. Channel width changes propagate upstream as a convection‐like perturbation associated with a kinematic wave starting at the location of the antecedent dam. These features are modeled through a new numerical model accounting for mixtures. More specifically, a set of equations has been derived for the variation of bed elevation, channel bottom width and bed grain size distribution, that when solved numerically, describe the observed channel processes.
- Falling head ponded infiltration in the nonlinear limit
- Authors: D. Triadis
Pages: n/a - n/a
Abstract: The Green and Ampt infiltration solution represents only an extreme example of behaviour within a larger class of very nonlinear, delta function diffusivity soils. The mathematical analysis of these soils is greatly simplified by the existence of a sharp wetting front below the soil surface. Solutions for more realistic delta function soil models have recently been presented for infiltration under surface saturation without ponding.
After general formulation of the problem, solutions for a full suite of delta function soils are derived for ponded surface water depleted by infiltration. Exact expressions for the cumulative infiltration as a function of time, or the drainage time as a function of the initial ponded depth may take implicit or parametric forms, and are supplemented by simple asymptotic expressions valid for small times, and small and large initial ponded depths.
As with surface saturation without ponding, the Green–Ampt model overestimates the effect of the soil hydraulic conductivity. At the opposing extreme a low‐conductivity model is identified that also takes a very simple mathematical form and appears to be more accurate than the Green–Ampt model for larger ponded depths. Between these two, the nonlinear limit of Gardner's soil is recommended as a physically valid first approximation. Relative discrepancies between different soil models are observed to reach a maximum for intermediate values of the dimensionless initial ponded depth, and in general are smaller than for surface saturation without ponding.
- Nonlinear time series modeling of unconfined groundwater head
- Authors: T. J. Peterson; A. W. Western
Pages: n/a - n/a
Abstract: This paper presents a nonlinear transfer function noise model for time‐series modeling of unconfined groundwater hydrographs. The motivation for its development was that existing groundwater time‐series models were unable to simulate large recharge events and multi‐year droughts. This was because existing methods do not partition rainfall to runoff and do not account for nonlinear soil water drainage. To account for these nonlinear processes, a vertically integrated soil moisture module was added to an existing transfer function noise model. The soil moisture module has a highly flexible structure that allowed 84 different forms to be built. Application of the time‐series model requires numerical calibration of parameters for the transfer functions, noise model and, for the nonlinear models, the soil moisture module. This was undertaken using the Covariance Matrix Adaptation Evolutionary Strategy (CMA‐ES) global calibration scheme. However, reproducible calibration to the global optima was challenging and a number of modifications were required to the transfer function noise model. In trialling the 84 nonlinear models and 2 linear models, each was applied to eleven observation bores within a paired catchment study area in Great Western, Victoria, Australia. In comparison with existing groundwater hydrograph time‐series models, the proposed nonlinear time‐series model performed significantly better at all observation bores during calibration and evaluation periods. Both the linear and nonlinear models were also used to quantify the impact of re‐vegetation within the paired catchment; however, results were inconclusive, which is likely due to time‐series data for the state of the re‐vegetation being unavailable. By analyzing the application of 84 nonlinear models to each bore, an optimal structure for the soil moisture module was identified. It is unlikely, however, that this model structure would be appropriate for all climates and geologies. To encourage further investigations, open‐source code for the highly flexible groundwater time‐series modeling framework is available and we invite others to develop new models.
- Physical context for theoretical approaches to sediment transport
magnitude‐frequency analysis in alluvial channels
- Authors: Joel Sholtes; Kevin Werbylo, Brian Bledsoe
Pages: n/a - n/a
Abstract: Theoretical approaches to magnitude‐frequency analysis (MFA) of sediment transport in channels couple continuous flow probability density functions (PDFs) with power law flow‐sediment transport relations (rating curves) to produce closed‐form equations relating MFA metrics such as the effective discharge, Qeff, and fraction of sediment transported by discharges greater than Qeff, f+, to statistical moments of the flow PDF and rating curve parameters. These approaches have proven useful in understanding the theoretical drivers behind the magnitude and frequency of sediment transport. However, some of their basic assumptions and findings may not apply to natural rivers and streams with more complex flow‐sediment transport relationships or management and design scenarios, which have finite time horizons. We use simple numerical experiments to test the validity of theoretical MFA approaches in predicting the magnitude and frequency of sediment transport. Median values of Qeff and f+ generated from repeated, synthetic, finite flow series diverge from those produced with theoretical approaches using the same underlying flow PDF. The closed‐form relation for f+ is a monotonically‐increasing function of flow variance. However, using finite flow series, we find that f+ increases with flow variance to a threshold that increases with flow record length. By introducing a sediment entrainment threshold, we present a physical mechanism for the observed diverging relationship between Qeff and flow variance in fine and coarse bed channels. Our work shows that through complex and threshold‐driven relationships sediment transport mode, channel morphology, flow variance, and flow record length all interact to influence estimates of what flow frequencies are most responsible for transporting sediment in alluvial channels.
- Use of an entropy‐based metric in multiobjective calibration to
improve model performance
- Authors: I.G. Pechlivanidis; B. Jackson, H. McMillan, H. Gupta
Pages: n/a - n/a
Abstract: Parameter estimation for hydrological models is complicated for many reasons, one of which is the arbitrary emphasis placed, by most traditional measures of fit, on various magnitudes of the model residuals. Recent research has called for the development of robust diagnostic measures that provide insights into which model structural components and/or data may be inadequate. In this regard, the flow duration curve (FDC) represents the historical variability of flow and is considered to be an informative signature of catchment behavior. Here we investigate the potential of using the recently developed conditioned entropy difference metric (CED) in combination with the Kling‐Gupta efficiency (KGE). The CED respects the static information contained in the flow frequency distribution (and hence the FDC), but does not explicitly characterize temporal dynamics. The KGE reweights the importance of various hydrograph components (correlation, bias, variability) in a way that has been demonstrated to provide better model calibrations than the commonly used Nash‐Sutcliffe efficiency, while being explicitly time‐sensitive. We employ both measures within a multi‐objective calibration framework and achieve better performance over the full range of flows than obtained by single‐criteria approaches, or by the common multi‐objective approach that uses log‐transformed and un‐transformed data to balance fitting of low‐ and high‐flow periods. The investigation highlights the potential of CED to complement KGE (and vice versa) during model identification. It is possible that some of the complementarity is due to CED representing more information from moments > 2 than KGE or other common metrics. We therefore suggest that an interesting way forward would be to extend KGE to include higher moments, i.e. use different moments as multiple criteria.
- Evolving many‐objective water management to exploit exascale
- Authors: Patrick M. Reed; David Hadka
Pages: n/a - n/a
Abstract: This study contributes one of the largest parallel scalability experiments ever attempted within the water resources literature to date, encompassing 2000 years of computational time. A severely challenging multiobjective benchmark problem focusing on urban water portfolio planning under uncertainty in the Lower Rio Grande Valley (LRGV) is used to demonstrate that a multi‐master variant of the Borg multiobjective evolutionary algorithm (MOEA) can be used efficiently on more than 524,288 compute cores. The scalability of the multi‐master Borg MOEA enables users to compress up to 20 years of computational work into 20 minutes of actual wall‐clock time. Beyond these temporal efficiency gains, metric‐based statistical assessments of solution quality show that the multi‐master Borg MOEA dramatically enhances the effectiveness and reliability of the algorithm's auto‐adaptive search features. Theoretical algorithmic analysis shows that the multi‐master Borg MOEA could maintain high levels of parallel scalability on future exascale computing platforms (i.e., millions of compute cores). These results mark a fundamental expansion of the scope, computational demands, and difficulties that can be addressed in multiobjective water resources applications.
- Willingness to pay and willingness to work for improvements of municipal
and community‐managed water services
- Authors: William F. Vásquez
Pages: n/a - n/a
Abstract: This study investigates household preferences, in labor time and monetary terms, for improved water services in Guatemala using sequential contingent valuation questions. The household survey was implemented in areas served by municipal and community‐managed systems, which allows for comparing household preferences under those governance approaches. Results show that respondents with municipal services are willing to pay a substantial increase (more than 200%) in their water bills for reliable supplies of safe drinking water. They are also willing to work approximately 19 hours per month for such improved services when labor hours are proposed as the payment vehicle. In contrast, households with community‐managed services are not willing to pay or work for service improvements, even though they report to be quite dissatisfied with current services. Policy implications are discussed.
- Spatially implemented Bayesian network model to assess environmental
impacts of water managementmain
- Authors: Ryan R. Morrison; Mark C. Stone
Pages: n/a - n/a
Abstract: Bayesian networks (BNs) have become a popular method of assessing environmental impacts of water management. However, spatial attributes that influence ecological processes are rarely included in BN models. We demonstrate the benefits of combining two‐dimensional hydrodynamic and BN modeling frameworks to explicitly incorporate the spatial variability within a system. The impacts of two diversion scenarios on riparian vegetation recruitment at the Gila River, New Mexico, USA, were evaluated using a coupled modeling framework. We focused on five individual sites in the Upper Gila Basin. Our BN model incorporated key ecological drivers based on the “recruitment box” conceptual model, including the timing of seed availability, floodplain inundation, river recession rate, and groundwater depths. Results indicated that recruitment potential decreased by more than 20% at some locations within each study site, relative to existing conditions. The largest impacts occurring along dynamic fluvial landforms, such as side channel and sand bars. Reductions in recruitment potential varied depending on the diversion scenario. Our unique approach allowed us to evaluate recruitment consequences of water management scenarios at a fine spatial scale, which not only helped differentiate impacts at distinct channel locations, but was useful for informing stakeholders of possible ecological impacts. Our findings also demonstrate that minor changes to river flow may have large ecological implications.
- Using a Bayesian hierarchical model to improve Lake Erie cyanobacteria
- Authors: Daniel R. Obenour; Andrew D. Gronewold, Craig A. Stow, Donald Scavia
Pages: n/a - n/a
Abstract: The last decade has seen a dramatic increase in the size of western Lake Erie cyanobacteria blooms, renewing concerns over phosphorus loading, a common driver of freshwater productivity. However, there is considerable uncertainty in the phosphorus load‐bloom relationship, because of other biophysical factors that influence bloom size, and because the observed bloom size is not necessarily the true bloom size, owing to measurement error. In this study, we address these uncertainties by relating late‐summer bloom observations to spring phosphorus load within a Bayesian modeling framework. This flexible framework allows us to evaluate three different forms of the load‐bloom relationship, each with a particular combination of statistical error distribution and response transformation. We find that a novel implementation of a gamma error distribution, along with an untransformed response, results in a model with relatively high predictive skill and realistic uncertainty characterization, when compared to models based on more common statistical formulations. Our results also underscore the benefits of a hierarchical approach that enables assimilation of multiple sets of bloom observations within the calibration processes, allowing for more thorough uncertainty quantification and explicit differentiation between measurement and model error. Finally, in addition to phosphorus loading, the model includes a temporal trend component indicating that Lake Erie has become increasingly susceptible to large cyanobacteria blooms over the study period (2002‐2013). Results suggest that current phosphorus loading targets will be insufficient for reducing the intensity of cyanobacteria blooms to desired levels, so long as the lake remains in a heightened state of bloom susceptibility.
- A drought index accounting for snow
- Authors: Maria Staudinger; Kerstin Stahl, Jan Seibert
Pages: n/a - n/a
Abstract: The Standardized Precipitation Index (SPI) is the most widely used index to characterize droughts that are related to precipitation deficiencies. However, the SPI does not always deliver the relevant information for hydrological drought management particularly in snow influenced catchments. If precipitation is temporarily stored as snow, then there is a significant difference between meteorological and hydrological drought because the delayed release of melt water to the stream. We introduce an extension to the SPI, the Standardized Snow Melt and Rain Index (SMRI), that accounts for rain and snow melt deficits, which effectively influence streamflow. The SMRI can be derived without snow data, using temperature and precipitation to model snow. The value of the new index is illustrated for seven Swiss catchments with different degrees of snow influence. In particular for catchments with a larger component of snowmelt in runoff generation, the SMRI was found to be a worthwhile complementary index to the SPI to characterize streamflow droughts.
- Relating soil specific surface area, water film thickness, and water vapor
- Authors: Tairone Paiva Leão; Markus Tuller
Pages: n/a - n/a
Abstract: Estimation of soil specific surface area (SSA) and dry‐end water vapor adsorption are important for porous media characterization and for prediction of water and vapor fluxes in arid environments. The objective of the presented study was to model water adsorption, film thickness and SSA based on t‐curve theory originally developed for N2 adsorption. Data from 21 source soils with clay contents ranging from 0.6 to 52.2%, was used to estimate specific surface area based on water retention, a t‐curve type method, the linear prediction method and a simplified monolayer method. The water retention and the t‐curve methods were found to be mathematically analogous and were among the most accurate with regard to correlation coefficient (r = 0.97) and root mean square error (RMSE = 11.36 x 103 m2/kg) when compared to measurements obtained with the standard ethylene glycol monoethyl ether (EGME) method. The corrected t‐curve method significantly overestimated SSA when compared to EGME data. Comparison of all considered methods with N2‐BET (BET) measurements disclosed lower correlation coefficients. For soil studies, the vapor adsorption in conjunction with the t‐curve or water retention methods should be preferred for SSA estimation as they show much higher correlation with soil clay content and EGME measurements.
- Embedded resource accounting for coupled natural‐human systems: An
application to water resource impacts of the western U.S. electrical
- Authors: Benjamin L. Ruddell; Elizabeth A. Adams, Richard Rushforth, Vincent C. Tidwell
Pages: n/a - n/a
Abstract: In complex coupled natural‐human systems (CNH), multitype networks link social, environmental, and economic systems with flows of matter, energy, information, and value. Embedded Resource Accounting (ERA) is a systems analysis framework that includes the indirect connections of a multitype CNH network. ERA is conditioned on perceived system boundaries, which may vary according to the accountant's Point of View. Both direct and indirect impacts are implicit whenever two subnetworks interact in such a system; the ratio of two subnetworks' impacts is the Embedded Intensity. For trade in the services of water, this is understood as the indirect component of a Water Footprint, and as ‘virtual water' trade. ERA is a generalization of Input‐Output, Footprint, and Substance Flow methods, and is a type of Life Cycle Analysis.
This paper presents results for the water and electrical energy system in the Western U.S. This system is dominated by California, which outsources the majority of its Water Footprint of electrical energy. Electricity trade increases total water consumption for electricity production in the Western U.S. by 15%, and shifts water use to water‐stressed Colorado River Basin States. A systemic under‐accounting for Water Footprints occurs because State‐level processes discount a portion of the Water Footprint occurring outside of the State boundary.
- Spatial periodicity in bed form‐scale solute and thermal transport
models of the hyporheic zone
- Authors: Tariq Laattoe; Adrian D. Werner, Vincent E.A. Post
Pages: n/a - n/a
Abstract: Spatially periodic solute boundaries force symmetry across a model domain by ensuring that concentrations and concentration gradients are identical at the same location on opposite boundaries. They have been used in multiple publications on a hyporheic zone model of a single ripple or dune style bedform, including variable density flow and reactive transport variants. We evaluate simulations of multi‐bedform models without imposing spatially periodic transport to demonstrate that non‐physical solute distributions arise from the periodic solute transport assumption. That is, the flow field within the single bedform model leads to a transport scenario that violates the forced symmetry of periodic solute boundary conditions, culminating in a physically unrealistic solute distribution. Our results show that lack of symmetry between boundaries is a function of the vertical concentration gradient and two dimensionless parameters characterizing the hyporheic and underflow flow regimes, and the solute exchange between them. We assess the error associated with the spatially periodic assumption based on an analysis of solute fluxes across the lateral bedform model boundaries. While the focus is on steady‐state concentration distributions, the implications for transient solute transport models are also discussed. We conclude that periodic solute transport boundary conditions should be applied only to bedform models that have minimal vertical dispersive and diffusive solute transfer. This includes gaining systems and tracers such as temperature, for which a temporally‐periodic flux reversal occurs across the top boundary.
- A hidden Markov model combined with climate indices for multidecadal
- Authors: C. Bracken; B. Rajagopalan, E. Zagona
Pages: n/a - n/a
Abstract: Hydroclimate time series often exhibit very low year‐to‐year autocorrelation while showing prolonged wet and dry epochs reminiscent of regime shifting behavior. Traditional stochastic time series models cannot capture the regime shifting features thereby misrepresenting the risk of prolonged wet and dry periods, consequently impacting management and planning efforts. Upper Colorado River Basin (UCRB) annual flow series highlights this clearly. To address this, a simulation framework is developed using a Hidden Markov (HM) model in combination with large scale climate indices that drive multidecadal variability. We demonstrate this on the UCRB flows and show that the simulations are able to capture the regime features by reproducing the multidecadal spectral features present in the data where a basic HM model without climate information cannot.
- Pore by pore capillary pressure measurements using X‐ray
microtomography at reservoir conditions: Curvature, snap‐off, and
remobilization of residual CO2
- Authors: Matthew Andrew; Branko Bijeljic, Martin J. Blunt
Pages: n/a - n/a
Abstract: X‐ray microtomography was used to image the shape and size of residual ganglia of supercritical CO2 at resolutions of 3.5 µm and 2 µm and at representative subsurface conditions of temperature and pressure. The capillary pressure for each ganglion was found by measuring the curvature of the CO2‐brine interface, while the pore structure was parameterised using distance maps of the pore‐space. The formation of the residual clusters by snap‐off was examined by comparing the ganglion capillary pressure to local pore topography. The capillary pressure was found to be inversely proportional to the radius of the largest restriction (throat) surrounding the ganglion, which validates the imbibition mechanisms used in pore‐network modelling. The potential mobilization of residual ganglia was assessed using a reformulation of both the capillary (Ncmacro) and Bond numbers (Nbmacro), rigorously based on a balance of pore‐scale forces, with the majority of ganglia remobilized at Ncmacro around 1. Buoyancy forces were found to be small in this system (Nbmacro
- Quantitative generalizations for catchment sediment yield following forest
- Authors: James C. Bathurst; Andrés Iroumé
Pages: n/a - n/a
Abstract: Published data for temperate forests across the world are analyzed to investigate the potential for generalized quantitative expressions of catchment sediment yield impact in the years immediately following logging. Such generalizations would be useful in a variety of forestry and engineering tasks and would aid the spread of knowledge amongst both relevant professionals and new students. Data were assembled for paired catchment studies (51 catchments including 16 controls) that enabled the post‐logging sediment yield impact to be compared with both the pre‐logging period and an undisturbed control catchment, using a specially defined relative response factor. Three categories of impact were derived: low‐moderate, high and very high, defined by specific ranges in the maximum value of the relative response factor. The maximum increase in specific sediment yield (in t km‐2 yr‐1) following logging is one order of magnitude above the control sediment yield at both the annual and storm event scales, at least under normal circumstances of Best Management Practice. There is no apparent relationship between sediment yield and the proportion of catchment logged, at least at the general scale. A cumulative probability distribution for the year in which the maximum post‐logging sediment yield occurs, shows the majority of cases falling in the first two years. These generalizations refer to the broad response to logging as a function of ground disturbance, for example by logging technique, roads and burning. Although limited to order‐of‐magnitude quantification, they provide a basis for first estimates and for a general appreciation of an impact problem.
- Fully integrated modeling of surface‐subsurface solute transport and
the effect of dispersion in tracer hydrograph separation
- Authors: Jessica E. Liggett; Adrian D. Werner, Brian D. Smerdon, Daniel Partington, Craig T. Simmons
Pages: n/a - n/a
Abstract: Tracer hydrograph separation has been widely applied to identify streamflow components, often indicating that pre‐event water comprises a large proportion of stream water. Previous work using numerical modeling suggests that hydrodynamic mixing in the subsurface inflates the pre‐event water contribution to streamflow when derived from tracer‐based hydrograph separation. This study compares the effects of hydrodynamic dispersion, both within the subsurface and at the surface‐subsurface boundary, on the tracer‐based pre‐event water contribution to streamflow. Using a fully integrated surface‐subsurface code, we simulate two hypothetical 2D hillslopes with surface‐subsurface solute exchange represented by different solute transport conceptualizations (i.e. advective and dispersive conditions). Results show that when surface‐subsurface solute transport occurs via advection only, the pre‐event water contribution from the tracer‐based separation agrees well with the hydraulically determined value of pre‐event water from the numerical model, despite dispersion occurring within the subsurface. In this case, subsurface dispersion parameters have little impact on the tracer‐based separation results. However, the pre‐event water contribution from the tracer‐based separation is larger when dispersion at the surface‐subsurface boundary is considered. This work demonstrates that dispersion within the subsurface may not always be a significant factor in apparently large pre‐event water fluxes over a single rainfall event. Instead, dispersion at the surface‐subsurface boundary may increase estimates of pre‐event water contribution. This work also shows that solute transport in numerical models is highly sensitive to the representation of the surface‐subsurface interface. Hence, models of catchment‐scale solute dynamics require careful treatment and sensitivity testing of the surface‐subsurface interface to avoid misinterpretation of real‐world physical processes.
- Parameterization of training images for aquifer 3‐D facies modeling,
integrating geological interpretations and statistical inference
- Authors: Sanjeev Kumar Jha; Alessandro Comunian, Gregoire Mariethoz, Bryce F. J. Kelly
Pages: n/a - n/a
Abstract: We develop a stochastic approach to construct channelized 3D geological models constrained to borehole measurements as well as geological interpretation. The methodology is based on simple 2D geologist‐provided sketches of fluvial depositional elements, which are extruded in the 3rd dimension. Multiple‐point geostatistics (MPS) is used to impair horizontal variability to the structures by introducing geometrical transformation parameters. The sketches provided by the geologist are used as elementary training images, whose statistical information is expanded through randomized transformations. We demonstrate the applicability of the approach by applying it to modeling a fluvial valley filling sequence in the Maules Creek catchment, Australia. The facies models are constrained to borehole logs, spatial information borrowed from an analogue and local orientations derived from the present‐day stream networks. The connectivity in the 3D facies models is evaluated using statistical measures and transport simulations. Comparison with a statistically equivalent variogram‐based model shows that our approach is more suited for building 3D facies models that contain structures specific to the channelized environment and which have a significant influence on the transport processes.
- Coupling ground and airborne geophysical data with upscaling techniques
for regional groundwater modeling of heterogeneous aquifers: Case study of
a sedimentary aquifer intruded by volcanic dykes in Northern Ireland
- Authors: Neil Edwin Matthew Dickson; Jean‐Christophe Comte, Jennifer McKinley, Ulrich Ofterdinger
Pages: n/a - n/a
Abstract: In highly heterogeneous aquifer systems, conceptualization of regional groundwater flow models frequently results in the generalization or negligence of aquifer heterogeneities, both of which may result in erroneous model outputs. The calculation of equivalence related to hydrogeological parameters and applied to upscaling provides a means of accounting for measurement scale information but at regional scale. In this study, the Permo‐Triassic Lagan Valley strategic aquifer in Northern Ireland is observed to be heterogeneous, if not discontinuous, due to sub‐vertical trending low‐permeability Tertiary dolerite dykes. Interpretation of ground and aerial magnetic surveys produces a deterministic solution to dyke locations. By measuring relative permeabilities of both the dykes and the sedimentary host rock, equivalent directional permeabilities, that determine anisotropy calculated as a function of dyke density, are obtained. This provides parameters for larger scale equivalent blocks, which can be directly imported to numerical groundwater flow models. Different conceptual models with different degrees of upscaling are numerically tested and results compared to regional flow observations. Simulation results show that the upscaled permeabilities from geophysical data allow one to properly account for the observed spatial variations of groundwater flow, without requiring artificial distribution of aquifer properties. It is also found that an intermediate degree of upscaling, between accounting for mapped field scale dykes and accounting for one regional anisotropy value (maximum upscaling) provides results the closest to the observations at the regional scale.
- Solute transport in low‐heterogeneity sand boxes: The role of
correlation length and permeability variance
- Authors: Peyman Heidari; Li Li
Pages: n/a - n/a
Abstract: This work examines how heterogeneity structure, in particular correlation length, controls flow and solute transport. We used two‐dimensional (2D) sand boxes (21.9 cm by 20.6 cm) and four modeling approaches, including 2D Advection‐Dispersion Equation (ADE) with explicit heterogeneity structure, 1D ADE with average properties, and non‐local Continuous Time Random Walk (CTRW) and fractional ADE (fADE). The goal is to answer two questions: 1) How and to what extent does correlation length control effective permeability and breakthrough curves (BTC)? 2) Which model can best reproduce data under what conditions? Sand boxes were packed with the same 20% (v/v) fine and 80% (v/v) coarse sands in three patterns that differ in correlation length. The Mixed cases contained uniformly distributed fine and coarse grains. The Four‐zone and One‐zone cases had four and one square fine zones, respectively. A total of 7 experiments were carried out with permeability variance of 0.10 (LC), 0.22 (MC), and 0.43 (HC). Experimental data show that the BTC curves depend strongly on correlation length, especially in the HC cases. The HC One‐zone (HCO) case shows distinct breakthrough steps arising from fast advection in the coarse zone, slow advection in the fine zone, and slow diffusion, while the LCO and MCO BTCs do not exhibit such behavior. With explicit representation of heterogeneity structure, 2D ADE reproduces BTCs well in all cases. CTRW reproduces temporal moments with smaller deviation from data than fADE in all cases except HCO, where fADE has the lowest deviation.
- A new formulation for steady multiaquifer flow; an analytic element for
piecewise constant infiltration
- Authors: O.D.L. Strack; Taha Namazi
Pages: n/a - n/a
Abstract: This paper contains a new formulation for infiltration inside domains bounded by polygons and its application to problems of steady multi‐aquifer flow, using the Dupuit‐Forchheimer approximation and assuming vertical flow in the separating layers. An alternative formulation is presented for leaky aquifer systems where infiltration or extraction is given. Existing formulations of multi‐aquifer flow involve a system of equations that must be solved for the heads in the aquifers. These formulations are abstract, and the relation between the parameters in the solution and physical quantities is hidden. The formulation in the paper aims at linking the system of equations to physical quantities; we have done this in two ways. First, we formulate the problem in terms of leakage potentials, related directly to the leakage through the leaky layers. Second, we introduce the concept of ‘equilibrated leakage’; leakage that is either the result of infiltration, or of some disturbance in the flow pattern, such as that caused by a well. The leakage through the leaky layers tends to some constant value far from a disturbance, e.g., a well, or the boundary of an area of constant infiltration. This concept of equilibrated leakage is useful in practice and helps in understanding the distribution of leakage; we explain this in detail in the paper. The study of problems of steady flow in leaky aquifer systems is inspired by problems of groundwater sustainability, where the overall distribution of flow over long periods of time is important, rather than detailed information.
- Effect of hydrophobicity on colloid transport during two‐phase flow
in a micromodel
- Authors: Qiulan Zhang; S.M. Hassanizadeh, B. Liu, J. F. Schijven, N.K. Karadimitriou
Pages: n/a - n/a
Abstract: The goal of this research was to investigate the difference in behavior of hydrophilic and hydrophobic colloids during transport in two‐phase flow, in general, and their attachment and remobilization characters, in particular. Experiments were performed in a hydrophobic Polydimethylsiloxane (PDMS) micro‐model. Water and fluorinert‐FC43 were used as the two immiscible liquids. Given the fact that PDMS is a hydrophobic material, fluorinert was the wetting phase and water was the non‐wetting phase in this micro‐model. As model colloids, we used hydrophilic polystyrene carboxylate‐modified microspheres (dispersible in water) and hydrophobic fluorous‐modified silica microspheres (dispersible in fluorinert) in separate experiments. Using a confocal laser scanning microscope, we directly observed fluid distribution and colloid movement within pores of the micro‐model. We also obtained concentration breakthrough curves by measuring the fluorescent intensities in the outlet of the micro‐model.
The breakthrough curves during steady‐state flow showed that the colloid attachment rate is inversely related to the background saturation of the fluid in which the colloids were dispersed. Our visualization results showed that the enhanced attachment of hydrophilic colloids at lower water saturations was due to the retention at the fluorinert‐water interface and fluorinert‐water‐solid contact lines. This effect was observed to be much less in the case of hydrophobic colloids (dispersed in fluorinert). In order to explain the colloids behavior, we calculated interaction potential energies of colloids with PDMS surfaces, fluid‐fluid interfaces, and fluid‐fluid‐solid contact lines. Also, balance of forces that control colloid, including DLVO, hydrodynamic, and surface tension forces, were determined. Our calculations showed that there is a stronger repulsive energy barrier between hydrophobic colloids and fluorinert‐water interface and solid‐fluid interface, compared with the hydrophilic colloids. Moreover, hydrophobic colloids were seen to aggregate due to strong attractive forces among them. These aggregates had even less tendency to attach to various interfaces, due to an increase in the corresponding energy barrier. For the colloid retention at fluid‐fluid‐solid contact lines, we found that the role of DLVO interactions was less important than capillary forces.
During transient events, we found that attached colloids become remobilized. The colloids deposited on the solid‐fluid interface were detached by the moving fluid‐fluid‐solid contact lines. But, this happened only when the liquid containing colloids was being displaced by the other liquid. We simulated breakthrough curves using a model based on a coupled system of equations for two‐phase flow, advection‐dispersion of colloids, adsorption to and desorption from fluid‐fluid interfaces and fluid‐solid interfaces. Very good agreements were obtained among measured breakthrough curves, visualization results, and numerical modeling.
- Assimilation of point SWE data into a distributed snow cover model
comparing two contrasting methods
- Authors: Jan Magnusson; David Gustafsson, Fabia Hüsler, Tobias Jonas
Pages: n/a - n/a
Abstract: In alpine and high latitude regions water resource decision making often requires large scale estimates of snow amounts and melt rates. Such estimates are available through distributed snow models which in some situations can be improved by assimilation of remote sensing observations. However, in regions with frequent cloud cover, complex topography, or large snow amounts satellite observations may feature information of limited quality. In this study we examine whether assimilation of snow water equivalent (SWE) data from ground observations can improve model simulations in a region largely lacking reliable remote sensing observations. We combine the model output with the point data using three‐dimensional sequential data assimilation methods, the ensemble Kalman filter and statistical interpolation. The filter performance was assessed by comparing the simulation results against observed SWE and snow covered fraction. We find that a method which assimilates fluxes (snowfall and melt rates computed from SWE) showed higher model performance than a control simulation not utilizing the filter algorithms. However, an alternative approach for updating the model results using the SWE data directly did not show a significantly higher performance than the control simulation. The results show that three‐dimensional data assimilation methods can be useful for transferring information from point snow observations to the distributed snow model.
- Multiobjective optimization of cluster‐scale urban water systems
investigating alternative water sources and level of decentralization
- Authors: J. P. Newman; G. C. Dandy, H. R. Maier
Pages: n/a - n/a
Abstract: In many regions, conventional water supplies are unable to meet projected consumer demand. Consequently, interest has arisen in integrated urban water systems, which involve the reclamation or harvesting of alternative, localised water sources. However, this makes the planning and design of water infrastructure more difficult, as multiple objectives need to be considered, water sources need to be selected from a number of alternatives and end‐uses of these sources need to be specified. In addition, the scale at which each treatment, collection and distribution network should operate needs to be investigated. In order to deal with this complexity, a framework for planning and designing water infrastructure taking into account integrated urban water management principles is presented in this paper and applied to a rural greenfield development. Various options for water supply, and the scale at which they operate were investigated in order to determine the life‐cycle trade‐offs between water savings, cost and GHG emissions as calculated from models calibrated using Australian data. The decision space includes the choice of water sources, storage tanks, treatment facilities and pipes for water conveyance. For each water system analysed, infrastructure components were sized using multiobjective genetic algorithms. The results indicate that local water sources are competitive in terms of cost and GHG emissions, and can reduce demand on the potable system by as much as 54%. Economies of scale in treatment dominated the diseconomies of scale in collection and distribution of water. Therefore, water systems that connect large clusters of households tend to be more cost efficient and have lower GHG emissions. In addition, water systems that recycle wastewater tended to perform better than systems that captured roof‐runoff. Through these results, the framework was shown to be effective at identifying near optimal trade‐offs between competing objectives, thereby enabling informed decisions to be made when planning water systems for greenfield developments.
- Bayesian inference of a lake water quality model by emulating its
- Authors: A. Dietzel; P. Reichert
Pages: n/a - n/a
Abstract: We use a Gaussian stochastic process emulator to interpolate the posterior probability density of a computationally demanding application of the biogeochemical‐ecological lake model BELAMO to accelerate statistical inference of deterministic model and error model parameters. The deterministic model consists of a mechanistic description of key processes influencing the mass balance of nutrients, dissolved oxygen, organic particles, and phyto‐ and zooplankton in the lake. This model is complemented by a Gaussian stochastic process to describe the remaining model bias and by Normal, independent observation errors. A small sub‐sample of the Markov chain representing the posterior of the model parameters is propagated through the full model to get model predictions and uncertainty estimates. We expect this approximation to be more accurate at only slightly higher computational costs compared to using a Normal approximation to the posterior probability density and linear error propagation to the results as we did in an earlier paper. The performance of the two techniques is compared for a didactical example as well as for the lake model. As expected, for the didactical example, the use of the emulator led to posterior marginals of the model parameters that are closer to those calculated by Markov chain simulation using the full model than those based on the Normal approximation. For the lake model, the new technique proved applicable without an excessive increase in computational requirements, but we faced challenges in the choice of the design data set for emulator calibration. As the posterior is a scalar function of the parameters, the suggested technique is an alternative to the emulation of a potentially more complex, structured output of the simulation model that allows for the use of a less case‐specific emulator. This is at the cost that still the full model has to be used for prediction (which can be done with a smaller, approximately independent subsample of the Markov chain).
- A dynamic watershed model for determining the effects of transient storage
on nitrogen export to rivers
- Authors: Dingjiang Chen; Minpeng Hu, Randy A. Dahlgren
Pages: n/a - n/a
Abstract: Legacy anthropogenic nitrogen (N) has been suggested as a major cause for increasing riverine N exports despite significant declines in anthropogenic N inputs in many regions. However, little quantitative knowledge exists concerning the contribution of the legacy N pool to riverine N export. This study developed a dynamic watershed N delivery model to address the role of transient storage of anthropogenic N inputs on riverine N flux. Employing simple mass balance and equivalent substitution rules, the model expresses the transient storage of legacy N mass with a term that combines the previous one year's riverine total N (TN) flux, relevant explanatory variables, and unknown parameters, enabling us to inversely calibrate the model parameters from measurable variables using Bayesian statistics. The model efficacy was demonstrated through application to the Yong'an River watershed in eastern China based on a 31‐year record (1980–2010) of riverine TN fluxes. The model can quantify annual transient storage of legacy N and its resulting contribution to annual riverine N flux. The model also allows partitioning of the complete long‐term mass balance for the fate (e.g., transient storage, riverine export, and loss/retention by denitrification, biomass uptake and wood product export) of annual anthropogenic N inputs. To further improve the model, various N input‐output processes can be specified and long‐term measurements of N fates are required to further verify the model results. This study demonstrates the need to consider transient storage effects as an improvement to current watershed models and for developing and assessing N pollution control measures.
- With or against the tide: The influence of bed form asymmetry on the
formation of macroturbulence and suspended sediment patterns
- Authors: E. Kwoll; M. Becker, C. Winter
Pages: n/a - n/a
Abstract: This study examines tide‐dependent variations in the formation and dynamics of suspended sediment patterns coupled to mean flow and turbulence above asymmetric bedforms. In the Danish Knudedyb inlet, very large primary bedforms remain ebb‐oriented during a tidal cycle while smaller superimposed bedforms reverse direction with each tidal phase. Hydro‐acoustic in‐situ observations reveal pronounced differences in suspended sediment transport patterns between tidal phases caused by the relative orientation of primary bedforms and the mean tidal flow and flow unsteadiness during a single tidal phase. When flow and primary bedform orientation are aligned, water‐depth‐scale macroturbulence develops in the bedform lee‐sides in the presence of flow separation. Macroturbulent flow structures occur at high flow stages and are coupled to increased amounts of sediment in suspension. When flow and bedform orientation are opposed no evidence of flow separation associated with primary bedforms is found. Sediment‐laden macroturbulence at high flow velocities is of a smaller scale and attributed to the superimposed secondary bedforms. The flow structures are advected along the primary bedform stoss‐side (temporary hydraulic lee‐side). The steep primary bedform lee‐side (temporary hydraulic stoss‐side) however, limits transport capabilities beyond the scale of primary bedforms.
- A novel infrastructure modularity index for the segmentation of water
- Authors: O. Giustolisi; L. Ridolfi
Pages: n/a - n/a
Abstract: The search for suitable segmentations is a challenging and urgent issue for the analysis, planning and management of complex water distribution networks (WDNs). In fact, complex and large size hydraulic systems require the division into modules in order to simplify the analysis and the management tasks. In the complex network theory, modularity index has been proposed as a measure of the strength of the network division into modules and its maximization is used in order to identify community of nodes (i.e., modules) which are characterized by strong inter‐connections. Nevertheless, modularity index needs to be revised considering the specificity of the hydraulic systems as infrastructure systems. To this aim, the classic modularity index has been recently modified and tailored for WDNs. Nevertheless, the WDN‐oriented modularity is affected by the resolution limit stemming from classic modularity index. Such a limit hampers the identification/design of small modules and this is a major drawback for technical tasks requiring a detailed resolution of the network segmentation. In order to get over this problem, we propose a novel infrastructure modularity index that is not affected by the resolution limits of the classic one. The rationale and good features of the proposed index are theoretically demonstrated and discussed using two real hydraulic networks.
- Spatiotemporal variation of long‐term drought propensity through
- Authors: Kironmala Chanda; Rajib Maity, Ashish Sharma, Rajeshwar Mehrotra
Pages: n/a - n/a
Abstract: This paper characterizes the long‐term, spatio‐temporal variation of drought propensity through a newly proposed, namely Drought Management Index (DMI) and explores its predictability in order to assess the future drought propensity and adapt drought management policies for a location. The DMI was developed using the Reliability‐Resilience‐Vulnerability (RRV) rationale commonly used in water resources systems analysis, under the assumption that depletion of soil moisture across a vertical soil column is equivalent to the operation of a water supply reservoir, and that drought should be managed not simply using a measure of system reliability, but should also take into account the readiness of the system to bounce back from drought to a normal state. Considering India as a test bed, five year long monthly gridded (0.5° Lat x 0.5° Lon) soil moisture data is used to compute the RRV at each grid location falling within the study domain. The Permanent Wilting Point (PWP) is used as the threshold, indicative of transition into water stress. The association between resilience and vulnerability is then characterized through their joint probability distribution ascertained using Plackett copula models for four broad soil types across India. The joint cumulative distribution functions (CDF) of resilience and vulnerability forms the basis for estimating the DMI as a five‐yearly time series at each grid location assessed. The status of DMI over the past 50 years indicate that drought propensity is consistently low towards northern and north eastern parts of India, but higher in the western part of peninsular India. Based on the observed past behaviour of DMI series on a climatological time scale, a DMI prediction model comprising of deterministic and stochastic components is developed. The predictability of DMI for a lead time of 5 years is found to vary across India, with a Pearson correlation coefficient between observed and predicted DMI above 0.6 over most of the study area, indicating a reasonably good potential for drought management in the medium term water resources planning horizon.
- An adaptive ant colony optimization framework for scheduling environmental
flow management alternatives under varied environmental water availability
- Authors: J. M. Szemis; H.R. Maier, G.C. Dandy
Pages: n/a - n/a
Abstract: Human water use is increasing and, as such, water for the environment is limited and needs to be managed efficiently. One method for achieving this is the scheduling of environmental flow management alternatives (EFMAs) (e.g. releases, wetland regulators), with these schedules generally developed over a number of years. However, the availability of environmental water changes annually as a result of natural variability (e.g. drought, wet years). To incorporate this variation and schedule EFMAs in a operational setting, a previously formulated multi‐objective optimization approach for EFMA schedule development used for long‐term planning has been modified and incorporated into an adaptive framework. As part of this approach, optimal schedules are updated at regular intervals during the planning horizon based on environmental water allocation forecasts, which are obtained using artificial neural networks. In addition, the changes between current and updated schedules can be minimized to reduce any disruptions to long‐term planning. The utility of the approach is assessed by applying it to an 89km section of the River Murray in South Australia. Results indicate that the approach is beneficial under a range of hydrological conditions and an improved ecological response is obtained in a operational setting compared with previous long‐term approaches. Also, it successfully produces trade‐offs between the number of disruptions to schedules and the ecological response, with results suggesting that ecological response increases with minimal alterations required to existing schedules. Overall, the results indicate that the information obtained using the proposed approach potentially aides managers in the efficient management of environmental water.
- Effects of rainfall spatial variability and intermittency on shallow
landslide triggering patterns at a catchment scale
- Authors: J. von Ruette; P. Lehmann, D. Or
Pages: n/a - n/a
Abstract: The occurrence of shallow landslides is often associated with intense and prolonged rainfall events, where infiltrating water reduces soil strength and may lead to abrupt mass release. Despite general understanding of the role of rainfall water in slope stability, the prediction of rainfall–induced landslides remains a challenge due to natural heterogeneity that affect hydrologic loading patterns and the largely unobservable internal progressive failures. An often overlooked and potentially important factor is the role of rainfall variability in space and time on landslide triggering that is often obscured by coarse information (e.g. hourly radar–data at spatial resolution of a few kilometers). To quantify potential effects of rainfall variability on failure dynamics, spatial patterns, landslide numbers and volumes, we employed a physically–based ‘Catchment–scale Hydromechanical Landslide Triggering' (CHLT) model [von Ruette et al., 2013] for a study area where a summer storm in 2002 triggered 51 shallow landslides. In numerical experiments based on the CHLT model we applied the measured rainfall amount of 53 mm in different artificial spatio–temporal rainfall patterns, resulting in between 30 and 100 landslides and total released soil volumes between 3,000 and 60,000 m3 for the various scenarios. Results indicate that low intensity rainfall below soil's infiltration capacity resulted in the largest mechanical perturbation. This study illustrates how small scale rainfall variability that is often overlooked by present operational rainfall data may play a key role in shaping landslide patterns.
- Estimating porosity and solid dielectric permittivity in the Miami
limestone using high‐frequency ground penetrating radar (GPR)
measurements at the laboratory scale
- Authors: Gregory J. Mount; Xavier Comas
Pages: n/a - n/a
Abstract: Subsurface water flow in South Florida is largely controlled by the heterogeneous nature of the karst limestone in the Biscayne aquifer and its upper formation, the Miami Limestone. These heterogeneities are amplified by dissolution structures that induce changes in the aquifer’s material and physical properties (i.e. porosity and dielectric permittivity) and create preferential flow paths. Understanding such patterns are critical for the development of realistic groundwater flow models, particularly in the Everglades, where restoration of hydrological conditions is intended. In this work we used non‐invasive ground penetrating radar (GPR) to estimate the spatial variability in porosity and the dielectric permittivity of the solid phase of the limestone at centimeter scale resolution to evaluate the potential for field‐based GPR studies. A laboratory setup that included high frequency GPR measurements under completely unsaturated and saturated conditions was used to estimate changes in electromagnetic wave velocity through Miami Limestone samples. The Complex Refractive Index Model was used to derive estimates of porosity and dielectric permittivity of the solid phase of the limestone. Porosity estimates of the samples ranged between 45.2 and 66.0 % and showed good correspondence with estimates of porosity using analytical and digital image techniques. Solid dielectric permittivity values ranged between 7.0 and 13.0. This study shows the ability of GPR to image the spatial variability of porosity and dielectric permittivity in the Miami Limestone and shows potential for expanding these results to larger scales and other karst aquifers.
- Beyond optimality: Multistakeholder robustness trade‐offs for
regional water portfolio planning under deep uncertainty
- Authors: Jonathan D. Herman; Harrison B. Zeff, Patrick M. Reed, Gregory W. Characklis
Pages: n/a - n/a
Abstract: While optimality is a foundational mathematical concept in water resources planning and management, “optimal” solutions may be vulnerable to failure if deeply uncertain future conditions deviate from those assumed during optimization. These vulnerabilities may produce severely asymmetric impacts across a region, making it vital to evaluate the robustness of management strategies as well as their impacts for regional stakeholders. In this study, we contribute a multi‐stakeholder many‐objective robust decision making (MORDM) framework that blends many‐objective search and uncertainty analysis tools to discover key tradeoffs between water supply alternatives and their robustness to deep uncertainties (e.g., population pressures, climate change, financial risks, etc.). The proposed framework is demonstrated for four interconnected water utilities representing major stakeholders in the “Research Triangle” region of North Carolina, USA. The utilities supply well over one million customers and have the ability to collectively manage drought via transfer agreements and shared infrastructure. We show that water portfolios for this region that compose optimal tradeoffs (i.e., Pareto‐approximate solutions) under expected future conditions may suffer significantly degraded performance with only modest changes in deeply uncertain hydrologic and economic factors. We then use the Patient Rule Induction Method (PRIM) to identify which uncertain factors drive the individual and collective vulnerabilities for the four cooperating utilities. Our framework identifies key stakeholder dependencies and robustness tradeoffs associated with cooperative regional planning, which are critical to understanding the tensions between individual versus regional water supply goals. Cooperative demand management was found to be the key factor controlling the robustness of regional water supply planning, dominating other hydroclimatic and economic uncertainties through the 2025 planning horizon. Results suggest that a modest reduction in the projected rate of demand growth (from approximately 3% per year to 2.4%) will substantially improve the utilities’ robustness to future uncertainty and reduce the potential for regional tensions. The proposed multi‐stakeholder MORDM framework offers critical insights into the risks and challenges posed by rising water demands and hydrological uncertainties, providing a planning template for regions now forced to confront rapidly evolving water scarcity risks.
- Quantitative characterization of stream turbidity‐discharge behavior
using event loop shape modeling and power law parameter decorrelation
- Authors: Amanda L. Mather; Richard L. Johnson
Pages: n/a - n/a
Abstract: Turbidity behavior in streams is a complex and dynamic function of both source material supply and event‐driven transport. While the primary controls on turbidity behavior across time and space are still not fully understood, recent increases in the availability of high temporal resolution, co‐located stream turbidity and discharge data provide an opportunity for more‐detailed analysis. Here we examine methods to quantitatively characterize event responses by modeling the shape of turbidity‐discharge hysteresis loops. A total of 1559 events from 20 gages in the Mid‐Atlantic region of the U.S. were modeled using both previously‐reported and new models combining elements of existing models. The results suggest that a more general power law based model, utilizing both a discharge rate of change term and a “supply” term, allows characterization of a wide range of simple and complex events. Additionally, this study explores a decorrelation approach to address the strong correlation frequently observed between the power law model coefficient (a) and exponent (b), with the goal of exposing the underlying behavior of each parameter individually. An examination of seasonal parameter behavior suggests that this approach may facilitate greater physically‐based interpretation of the power law coefficient. The power law parameter decorrelation strategy and the loop models examined here provide a step towards the larger goal of understanding the physical controls on turbidity‐discharge hysteretic behavior.
- Mathematical analysis of the Saint‐Venant‐Hirano model for
- Authors: G. Stecca; A. Siviglia, A. Blom
Pages: n/a - n/a
Abstract: Sediments of different size are transported in rivers under the action of flow. The first and still most popular sediment continuity model able to deal with mixed sediment is the so called active layer model proposed by Hirano [1971, 1972]. In this paper we consider the one‐dimensional hydro‐morpohodynamic model given by the Saint‐Venant equations for free‐surface flow coupled with the active layer model. We perform a mathematical analysis of this model, extending the previous analysis by Ribberink , including full unsteadiness and grainsize‐selectivity of the transported load by explicitly considering multiple sediment fractions. The presence of multiple fractions gives rise to distinct waves traveling in the downstream direction, for which we provide an analytical approximation of propagation velocity under any Froude regime. We finally investigate the role of different waves in advecting morphodynamic changes through the domain. To this aim, we implement an analytical linearized solver to analyze the propagation of small‐amplitude perturbations of the bed elevation and grainsize distribution of the active layer as described by the system of governing equations. We find that initial gradients in the grainsize distribution of the active layer are able to trigger significant bed variations, which propagate in the downstream direction at faster pace than the “bed” wave arising from the uniform‐sediment Saint‐Venant‐Exner model. We also verify that multiple “sorting” waves carry multiple associated bed perturbations, traveling at different speeds.
- Climate change, water rights, and water supply: The case of irrigated
agriculture in Idaho
- Authors: Wenchao Xu; Scott E. Lowe, Richard M. Adams
Pages: n/a - n/a
Abstract: We conduct a hedonic analysis to estimate the response of agricultural land use to water supply information under the Prior Appropriation Doctrine by using Idaho as a case study. Our analysis includes long‐term weather trends and water supply conditions as well as seasonal water supply forecasts. A farm‐level panel data set, which accounts for the priority effects of water rights and controls for diversified crop mixes and rotation practices, is used. Our results indicate that farmers respond to long‐term surface and ground water conditions as well as to the seasonal water supply variation. Climate change‐induced variations in weather and water supply conditions could lead to substantial damages to irrigated agriculture. We project substantial losses (up to 32%) of the average crop revenue for major agricultural areas under future climate scenarios in Idaho. Finally, farmers demonstrate significantly varied responses given their water rights priorities, which implies that the distributional impact of climate change is sensitive to institutions such as the Prior Appropriation Doctrine.
- Robust changes and sources of uncertainty in the projected hydrological
regimes of Swiss catchments
- Authors: Nans Addor; Ole Rössler, Nina Köplin, Matthias Huss, Rolf Weingartner, Jan Seibert
Pages: 7541 - 7562
Abstract: Projections of discharge are key for future water resources management. These projections are subject to uncertainties, which are difficult to handle in the decision process on adaptation strategies. Uncertainties arise from different sources such as the emission scenarios, the climate models and their postprocessing, the hydrological models, and the natural variability. Here we present a detailed and quantitative uncertainty assessment, based on recent climate scenarios for Switzerland (CH2011 data set) and covering catchments representative for midlatitude alpine areas. This study relies on a particularly wide range of discharge projections resulting from the factorial combination of 3 emission scenarios, 10–20 regional climate models, 2 postprocessing methods, and 3 hydrological models of different complexity. This enabled us to decompose the uncertainty in the ensemble of projections using analyses of variance (ANOVA). We applied the same modeling setup to six catchments to assess the influence of catchment characteristics on the projected streamflow, and focused on changes in the annual discharge cycle. The uncertainties captured by our setup originate mainly from the climate models and natural climate variability, but the choice of emission scenario plays a large role by the end of the 21st century. The contribution of the hydrological models to the projection uncertainty varied strongly with catchment elevation. The discharge changes were compared to the estimated natural decadal variability, which revealed that a climate change signal emerges even under the lowest emission scenario (RCP2.6) by the end of the century. Limiting emissions to RCP2.6 levels would nevertheless reduce the largest regime changes by the end of the century by approximately a factor of two, in comparison to impacts projected for the high emission scenario SRES A2. We finally show that robust regime changes emerge despite the projection uncertainty. These changes are significant and are consistent across a wide range of scenarios and catchments. We propose their identification as a way to aid decision making under uncertainty.
- USDA‐ARS Riesel Watersheds, Riesel, Texas, USA: Water quality
- Authors: R. Daren Harmel; Richard L. Haney, Douglas R. Smith, Michael White, Kevin W. King
Pages: 8374 - 8382
Abstract: The 75 year legacy database including discharge, sediment loss, land management, and meteorological data for the USDA‐ARS Riesel Watersheds, Riesel, TX, USA has been available on the web for more than a decade (www.ars.usda.gov/spa/hydro‐data) and used in numerous studies and publications; however, only recently have these data been added to the Sustaining the Earth's Watersheds, Agricultural Research Data System (STEWARDS) database (www.nrrig.mwa.ars.usda.gov/stewards/stewards.html). In addition, water quality data including dissolved inorganic N and P compounds measured from more than 1000 storm runoff events, 1300 base flow sampling events (lateral subsurface return flow or seepage flow), and 157 precipitation events through 2012 were added. The objectives of this manuscript are to present relevant background information on these data, summarize the data collection and analysis methodology, present the measured data along with cursory analyses, and convey the commitment of the USDA‐ARS Riesel Watersheds to long‐term data accessibility and database enhancement for water quality data and research.