- Influence of vertical and lateral heat transfer on permafrost thaw,
peatland landscape transition, and groundwater flow
- Authors: Barret L. Kurylyk; Masaki Hayashi, William L. Quinton, Jeffrey M. McKenzie, Clifford I. Voss
Abstract: Recent climate change has reduced the spatial extent and thickness of permafrost in many discontinuous permafrost regions. Rapid permafrost thaw is producing distinct landscape changes in the Taiga Plains of the Northwest Territories, Canada. As permafrost bodies underlying forested peat plateaus shrink, the landscape slowly transitions into unforested wetlands. The expansion of wetlands has enhanced the hydrologic connectivity of many watersheds via new surface and near‐surface flow paths, and increased streamflow has been observed. Furthermore, the decrease in forested peat plateaus results in a net loss of boreal forest and associated ecosystems.
This study investigates fundamental processes that contribute to permafrost thaw by comparing observed and simulated thaw development and landscape transition of a peat plateau‐wetland complex in the Northwest Territories, Canada from 1970 to 2012. Measured climate data are first used to drive surface energy balance simulations for the wetland and peat plateau. Near‐surface soil temperatures simulated in the surface energy balance model are then applied as the upper boundary condition to a three‐dimensional model of subsurface water flow and coupled energy transport with freeze‐thaw. Simulation results demonstrate that lateral heat transfer, which is not considered in many permafrost models, can influence permafrost thaw rates. Furthermore, the simulations indicate that landscape evolution arising from permafrost thaw acts as a positive feedback mechanism that increases the energy absorbed at the land surface and produces additional permafrost thaw. The modeling results also demonstrate that flow rates in local groundwater flow systems may be enhanced by the degradation of isolated permafrost bodies. This article is protected by copyright. All rights reserved.
- Deviation of permeable coarse‐grained boundary resistance from
- Abstract: Nikuradse's  rough pipe study is enormously influential in the understanding of flow resistance over a sediment bed. However, the rough boundary employed in Nikuradse's study differs from permeable sediment beds in rivers. This implies that the results derived from the rough pipe experiments may not be applicable for flows over a permeable coarse‐grained bed. The present study aimed to explore to what extent the flow resistance of a permeable coarse‐grained boundary deviates from the Nikuradse's observations. Experiments were conducted with rough pipes, which were prepared by overlaying the inner wall with one to four layers of spherical beads. The single layer roughness resembles the experimental setup reported in Nikuradse's study, while the multilayer of grains allows significant flow to pass through the porous roughness layer. In addition, the ratio of grain diameter, k, to pipe diameter, d, was chosen to be one to two orders greater than the range (0.001
- Spatiotemporal densification of river water level time series by
multimission satellite altimetry
- Authors: M. J. Tourian; A. Tarpanelli, O. Elmi, T. Qin, L. Brocca, T. Moramarco, N. Sneeuw
Abstract: Limitations of satellite radar altimetry for operational hydrology include its spatial and temporal sampling as well as measurement problems caused by local topography and heterogeneity of the reflecting surface. In this study, we develop an approach that eliminates most of these limitations to produce an approximately 3‐day‐temporal resolution water‐level time series from the original typically (sub‐)monthly datasets for the Po River in detail, and for Congo, Mississippi and Danube rivers. We follow a geodetic approach by which, after estimating and removing inter‐satellite biases, all virtual stations of several satellite altimeters are connected hydraulically and statistically to produce water‐level time series at any location along the river. We test different data‐selection strategies and validate our method against the extensive available in situ data over the Po River, resulting in an average correlation of 0.7, Root Mean Square Error of 0.8m, bias of ‐0.4m and Nash‐Sutcliffe efficiency coefficient of 0.5. We validate the transferability of our method by applying it to the Congo, Mississippi and Danube rivers, which have very different geomorphological and climatic conditions. The methodology yields correlations above 0.75 and Nash‐Sutcliffe coefficients of 0.84 (Congo), 0.34 (Mississippi), and 0.35 (Danube). This article is protected by copyright. All rights reserved.
- Effects of soil spatial variability at the hillslope and catchment scales
on characteristics of rainfall‐induced landslides
- Authors: Linfeng Fan; Peter Lehmann, Dani Or
Abstract: Spatial variations in soil properties affect key hydrological processes, yet their role in soil mechanical response to hydro‐mechanical loading is rarely considered. This study aims to fill this gap by systematically quantifying effects of spatial variations in soil type and initial water content on rapid rainfall‐induced shallow landslide predictions at the hillslope‐ and catchment‐scales. We employed a physically‐based landslide triggering model that considers mechanical interactions among soil columns governed by strength thresholds. At the hillslope scale, we found that the emergence of weak regions induced by spatial variations of soil type and initial water content resulted in early triggering of landslides with smaller volumes of released mass relative to a homogeneous slope. At the catchment scale, initial water content was linked to a topographic wetness index, whereas soil type varied deterministically with soil depth considering spatially correlated stochastic components. Results indicate that a strong spatial organization of initial water content delays landslide triggering, whereas spatially linked soil type with soil depth promoted landslide initiation. Increasing the standard deviation and correlation length of the stochastic component of soil type increases landslide volume and hastens onset of landslides. The study illustrates that for similar external boundary conditions and mean soil properties, landslide characteristics vary significantly with soil variability, hence it must be considered for improved landslide model predictions. This article is protected by copyright. All rights reserved.
- Error modeling of DEMs from topographic surveys of rivers using fuzzy
- Authors: Sara Bangen; James Hensleigh, Peter McHugh, Joseph Wheaton
Abstract: Digital elevation models (DEMs) have become common place in the earth sciences as a tool to characterize surface topography and set modelling boundary conditions. All DEMs have a degree of inherent error that is propagated to subsequent models and analyses. While previous research has shown that DEM error is spatially variable it is often represented as spatially uniform for analytical simplicity. Fuzzy inference systems (FIS) offer a tractable approach for modeling spatially variable DEM error, including flexibility in the number of inputs and calibration of outputs based on survey technique and modeling environment. We compare three FIS error models for DEMs derived from TS surveys of wadeable streams and test them at 34 sites in the Columbia River basin. The models differ in complexity regarding the number/type of inputs and degree of site‐specific parameterization. A 2‐input FIS uses inputs derived from the topographic point cloud (slope, point density). A 4‐input FIS adds interpolation error and 3D point quality. The 5‐input FIS adds bed‐surface roughness estimates. Both the 4‐ and 5‐input FIS model output were parameterized to site‐specific values. In the wetted channel we found i) the 5‐input FIS resulted in lower mean δz due to including roughness, and ii) the 4‐ and 5‐input FIS resulted in a higher standard deviation and maximum δz due to the inclusion of site‐specific bank heights. All three FIS gave plausible estimates of DEM error, with the two more complicated models offering an improvement in the ability to detect spatially localized areas of DEM uncertainty. This article is protected by copyright. All rights reserved.
- Comparison of the transport and deposition of Pseudomonas aeruginosa under
aerobic and anaerobic conditions
- Authors: Huixin Zhang; Hongbo Zeng, Ania C. Ulrich, Yang Liu
Abstract: Laboratory‐scale columns were employed to study the effect of oxygen and ionic strength on the transport of Pseudomonas aeruginosa PAO1 in porous media. In anaerobic experiments, cells were grown and transport experiments were conducted in a well‐controlled anaerobic chamber. Cell surface electrokinetic potentials were measured and surface elemental composition was analyzed using X‐ray photoelectron spectroscopy (XPS). Transport experimental results showed reduced travel distance of PAO1 with increased ionic strength under aerobic and anaerobic conditions, consistent with calculated Derjaguin‐Landau‐Verwey‐Overbeek (DLVO) theory. The deposition rates of PAO1 were significantly higher in aerobic than in anaerobic condition at higher ionic strength (10 and 100 mM), although the electrokinetic potentials were similar throughout the tested ionic strength (1, 10, and 100 mM). No difference in PAO1 deposition rate was observed at 1 mM. XPS analysis showed that variation in cell surface composition due to different growth conditions played a primary role in determining the different deposition behaviors. This article is protected by copyright. All rights reserved.
- Field assessment of noncontact stream gauging using portable surface
velocity radars (SVR)
- Abstract: The applicability of a portable, commercially available surface velocity radar (SVR) for non‐contact stream gauging was evaluated through a series of field‐scale experiments carried out in a variety of sites and deployment conditions. Comparisons with various concurrent techniques showed acceptable agreement with velocity profiles, with larger uncertainties close to the banks. In addition to discharge error sources shared with intrusive velocity‐area techniques, SVR discharge estimates are affected by flood‐induced changes in the bed profile and by the selection of a depth‐averaged to surface velocity ratio, or velocity coefficient (α). Cross‐sectional averaged velocity coefficients showed smaller fluctuations and closer agreement with theoretical values than those computed on individual verticals, especially in channels with high relative roughness. Our findings confirm that α = 0.85 is a valid default value, with a preferred site‐specific calibration to avoid underestimation of discharge in very smooth channels (relative roughness ∼ 0.001) and overestimation in very rough channels (relative roughness > 0.05). Theoretically derived and site‐calibrated values of α also give accurate SVR‐based discharge estimates (within 10%) for low and intermediate roughness flows (relative roughness 0.001 to 0.05). Moreover, discharge uncertainty does not exceed 10% even for a limited number of SVR positions along the cross‐section (particularly advantageous to gauge unsteady flood flows and very large floods), thereby extending the range of validity of rating curves. This article is protected by copyright. All rights reserved.
- A particle‐number conserving Lagrangian method for
mixing‐driven reactive transport
- Authors: Diogo Bolster; Amir Paster, David A. Benson
Abstract: The purely Lagrangian algorithm for chemical reactions introduced by Benson and Meerschaert  suffers from a low‐concentration resolution problem. We alleviate the problem by re‐defining the probabilistic collision/reaction (birth/death) stochastic process as a mass‐reduction operation. Theoretically this corresponds to replacing an on/off particle with a large number of “sub‐particles” and tracking the number fraction. The new particle reaction process maintains the original particle numbers, but adjusts each particle's mass upon reaction. Several simulations show the veracity as well as the gains in low‐concentration resolution offered by the algorithm. We also compare the results to those obtained by a traditional finite difference model with suitably defined initial condition, demonstrating that the Lagrangian models match these. This article is protected by copyright. All rights reserved.
- Evaluating geothermal and hydrogeologic controls on regional groundwater
- Authors: Erick R. Burns; Steven E. Ingebritsen, Michael Manga, Colin F. Williams
Abstract: A one‐dimensional (1‐D) analytic solution is developed for heat transport through an aquifer system where the vertical temperature profile in the aquifer is nearly uniform. The general anisotropic form of the viscous heat generation term is developed for use in groundwater flow simulations. The 1‐D solution is extended to more complex geometries by solving the equation for piece‐wise linear or uniform properties and boundary conditions. A moderately complex example, the Eastern Snake River Plain (ESRP), is analyzed to demonstrate the use of the analytic solution for identifying important physical processes. For example, it is shown that viscous heating is variably important and that heat conduction to the land surface is a primary control on the distribution of aquifer and spring temperatures. Use of published values for all aquifer and thermal properties results in a reasonable match between simulated and measured groundwater temperatures over most of the 300 km length of the ESRP, except for geothermal heat flow into the base of the aquifer within 20 km of the Yellowstone hotspot. Previous basal heat flow measurements (∼110 mW/m2) made beneath the ESRP aquifer were collected at distances of >50 km from the Yellowstone Plateau, but a higher basal heat flow of 150 mW/m2 is required to match groundwater temperatures near the Plateau. The ESRP example demonstrates how the new tool can be used during preliminary analysis of a groundwater system, allowing efficient identification of the important physical processes that must be represented during more‐complex 2‐D and 3‐D simulations of combined groundwater and heat flow. This article is protected by copyright. All rights reserved.
- Stability analysis of ecomorphodynamic equations
- Abstract: In order to shed light on the influence of riverbed vegetation on river morphodynamics, we perform a linear stability analysis on a minimal model of vegetation dynamics coupled with classical one‐ and two‐dimensional Saint‐Venant‐Exner equations of morphodynamics. Vegetation is modeled as a density field of rigid, non‐submerged cylinders and affects flow via a roughness change. Furthermore, vegetation is assumed to develop following a logistic dependence and may be uprooted by flow. First, we perform the stability analysis of the reduced one‐dimensional framework. As a result of the competitive interaction between vegetation growth and removal through uprooting, we find a domain in the parameter space where originally straight rivers are unstable towards periodic longitudinal patterns. For realistic values of the sediment transport parameter, the dominant longitudinal wavelength is determined by the parameters of the vegetation model. Bed topography is found to adjust to the spatial pattern fixed by vegetation. Subsequently, the stability analysis is repeated for the two‐dimensional framework, where the system may evolve towards alternate or multiple bars. On a fixed bed, we find instability towards alternate bars due to flow‐vegetation interaction, but no multiple bars. Both alternate and multiple bars are present on a movable, vegetated bed. Finally, we find that the addition of vegetation to a previously unvegetated riverbed favors instability towards alternate bars and thus the development of a single course rather than braiding. This article is protected by copyright. All rights reserved.
- Influence of particle size and density and channel velocity on the
deposition patterns around a circular patch of model emergent vegetation
- Authors: Ying Shi; Beihan Jiang, Heidi M. Nepf
Abstract: This laboratory study examined the influence of particle size and density, and channel velocity on the spatial deposition pattern around an emergent (extending through the entire water depth), circular patch of model vegetation located at the center of a channel. Three flow conditions and three particles of different size and density were considered. Across all particle and velocity conditions three basic deposition patterns were observed: 1) high deposition in the patch wake and low deposition in the zones adjacent to the patch; 2) high deposition in both the wake and adjacent zones; and 3) low deposition in both the wake and adjacent zones. The observed deposition pattern correlated with the ratio of channel shear velocity (u*) to critical shear velocity (u*c). Specifically, for u*/u*c 3, the deposition was high (or low, respectively) over the entire channel with little difference between the wake and adjacent regions. In contrast, for 0.7
- Cost of riparian buffer zones: A comparison of hydrologically adapted
site‐specific riparian buffers with traditional fixed widths
- Abstract: Traditional approaches aiming at protecting surface waters from the negative impacts of forestry often focus on retaining fixed width buffer zones around waterways. While this method is relatively simple to design and implement, it has been criticized for ignoring the spatial heterogeneity of biogeochemical processes and biodiversity in the riparian zone. Alternatively, a variable width buffer zone adapted to site‐specific hydrological conditions has been suggested to improve the protection of biogeochemical and ecological functions of the riparian zone. However, little is known about the monetary value of maintaining hydrologically adapted buffer zones compared to the traditionally used fixed width ones. In this study, we created a hydrologically adapted buffer zone by identifying wet areas and groundwater discharge hotspots in the riparian zone. The opportunity cost of the hydrologically adapted riparian buffer zones was then compared to that of the fixed width zones in a meso‐scale boreal catchment to determine the most economical option of designing riparian buffers. The results show that hydrologically adapted buffer zones were cheaper per hectare than the fixed width ones when comparing the total cost. This was because the hydrologically adapted buffers included more wetlands and low productive forest areas than the fixed widths. As such, the hydrologically adapted buffer zones allows more effective protection of the parts of the riparian zones that are ecologically and biogeochemically important and more sensitive to disturbances without forest landowners incurring any additional cost. This article is protected by copyright. All rights reserved.
- Probabilistic flood maps to support decision‐making: Mapping the
Value of Information
- Authors: L. Alfonso; M. M. Mukolwe, G. Di Baldasssarre
Abstract: Floods are one of the most frequent and disruptive natural hazards that affect man. Annually, significant flood damage is documented worldwide. Flood mapping is a common pre‐impact flood hazard mitigation measure, for which advanced methods and tools (such as flood inundation models) are used to estimate potential flood extent maps that are used in spatial planning. However, these tools are affected, largely to an unknown degree, by both epistemic and aleatory uncertainty. Over the past few years, advances in uncertainty analysis with respect to flood inundation modeling show that it is appropriate to adopt Probabilistic Flood Maps (PFM) to account for uncertainty. However, the following question arises; how can probabilistic flood hazard information be incorporated into spatial planning? Thus, a consistent framework to incorporate PFMs into the decision making is required. In this paper, a novel methodology based on Decision Making under Uncertainty theories, in particular Value of Information (VOI) is proposed. Specifically, the methodology entails the use of a PFM to generate a VOI map, which highlights floodplain locations where additional information is valuable with respect to available floodplain management actions and their potential consequences. The methodology is illustrated with a simplified example and also applied to a real case study in the South of France, where a VOI map is analyzed on the basis of historical land‐use change decisions over a period of 26 years. Results show that uncertain flood hazard information encapsulated in PFMs can aid decision making in floodplain planning. This article is protected by copyright. All rights reserved.
- Space‐time conditional disaggregation of precipitation at high
resolution via simulation
- Abstract: Daily rainfall data are more plentiful and reliable than pluviometer data and are the best data‐set to start data‐repair from, world‐wide. Clusters of pluviometers (a term used herein for instruments recording at sub‐daily intervals) record wet and dry periods in close synchrony and larger and smaller catches tend to be recorded in similar groups, but they have many gaps that require infilling. We present a method of disaggregating daily rainfall to sub‐daily intervals, contemporaneously infilling gaps in the pluviometers. Then the observed data, together with the infilled and disaggregated values, are interpolated over the intervening space. To achieve this disaggregation, we used a Gaussian copula based model with time‐dependent marginal distributions and censored values representing the dry periods. In addition, we generated stochastically meaningful ensembles of missing or disaggregated values, while constraining each realisation to the observed daily total where relevant. This applies to the gaps filled in the pluviometers as well as the disaggregation of the daily totals. Using the disaggregated and infilled sub‐daily ensembles, we then conditionally spatially simulated historical rainfall in the space between the gauges and pluviometers. The mean of these stochastic realizations was compared to interpolated fields using two other procedures: Rescaled Ordinary Kriging and Rescaled Nearest Neighbours, and found our method to be superior. Where there are daily data, the daily sum constrains the simulation. In the intervening space, in a chosen daily sub‐interval, there will be an ensemble of values simulated from the observations. We present the results of measurements and validation of the applications to an unusually large amount of data (not just a few convenient samples), and are confident that the methodology is sound and applicable in a variety of geographies. This article is protected by copyright. All rights reserved.
- Hydrogeophysical characterization of transport processes in fractured rock
by combining push‐pull and single‐hole ground penetrating
- Authors: Alexis Shakas; Niklas Linde, Ludovic Baron, Olivier Bochet, Olivier Bour, Tanguy Le Borgne
Abstract: The in situ characterization of transport processes in fractured media is particularly challenging due to the considerable spatial uncertainty on tracer pathways and dominant controlling processes, such as dispersion, channeling, trapping, matrix diffusion, ambient and density driven flows. We attempted to reduce this uncertainty by coupling push‐pull tracer experiments with single‐hole ground penetrating radar (GPR) time‐lapse imaging. The experiments involved different injection fractures, chaser volumes and resting times, and were performed at the fractured rock research site of Ploemeur in France (H+ network, hplus.ore.fr/en). For the GPR acquisitions we used both fixed and moving antenna setups in a borehole that was isolated with a flexible liner. During the fixed‐antenna experiment, time‐varying GPR reflections allowed us to track the spatial and temporal dynamics of the tracer during the push‐pull experiment. During the moving antenna experiments, we clearly imaged the dominant fractures in which tracer transport took place, fractures in which the tracer was trapped for longer time periods and the spatial extent of the tracer distribution (up to 8 meters) at different times. This demonstrated the existence of strongly channelized flow in the first few meters and radial flow at greater distances. By varying the resting time of a given experiment, we identified regions affected by density‐driven and ambient flow. These experiments open up new perspectives for coupled hydrogeophysical inversion aimed at understanding transport phenomena in fractured rock formations. This article is protected by copyright. All rights reserved.
- Uncertainties in vertical groundwater fluxes from 1‐D steady state
heat transport analyses caused by heterogeneity, multidimensional flow,
and climate change
- Authors: Dylan J. Irvine; Ian Cartwright, Vincent E.A. Post, Craig T. Simmons, Eddie W. Banks
Abstract: Steady state 1D analytical solutions to estimate groundwater fluxes from temperature profiles are an attractive option because they are simple to apply, with no complex boundary or initial conditions. Steady state solutions have been applied to estimate both aquifer scale fluxes as well as to estimate groundwater discharge to streams. This study explores the sources of uncertainty in flux estimates from regional scale aquifers caused by sensor precision, aquifer heterogeneity, multi‐dimensional flow and variations in surface temperature due to climate change. Synthetic temperature profiles were generated using 2D groundwater flow and heat transport models with homogeneous and heterogeneous hydraulic and thermal properties. Temperature profiles were analysed assuming temperature can be determined with a precision between 0.1°C and 0.001°C. Analysis of synthetic temperature profiles show that the Bredehoeft and Papadopulos  method can provide good estimates of the mean vertical Darcy flux over the length of the temperature profile. Reliable flux estimates were obtained when the ratio of vertical to horizontal flux was as low as 0.1, and in heterogeneous media, providing that temperature at the upper boundary was constant in time. However, temporal increases in surface temperature led to over‐estimation of fluxes. Over‐estimates increased with time since the onset of, and with the rate of surface warming. Overall, the Bredehoeft and Papadopulos  method may be more robust for the conditions with constant temperature distributions than previously thought, but that transient methods that account for surface warming should be used to determine fluxes in shallow aquifers. This article is protected by copyright. All rights reserved.
- A minimal probabilistic model for soil moisture in seasonally dry climates
- Authors: David N. Dralle; Sally E. Thompson
Abstract: In seasonally dry climates, a distinct rainy season is followed by a pronounced dry season during which rainfall often makes a negligible contribution to soil moisture. Using stochastic analytical models of soil moisture to represent the effects of this seasonal change has been hindered by the need to mathematically represent the stochastic influence of wet season climate on dry season soil water dynamics. This study presents a simple process based stochastic model for soil moisture dynamics, which explicitly models inter‐seasonal transient dynamics while accounting for carry‐over soil moisture storage between the wet and dry seasons, and allows a derivation of an analytical expression for the dry season mean first passage time below a soil moisture threshold. Such crossing times pose controls on both vegetation productivity and water stress during dry summers. The new model, along with an existing model that incorporates non‐zero dry season rainfall but not variability in the soil moisture condition at the start of the dry season, are tested against data from the Tonzi Ranch Ameriflux site. Both models predict first passage times well for high soil moisture thresholds, but the new model improves prediction at lower thresholds. The annual soil moisture probability distribution function (PDF) from the new model also compares well with observations. This article is protected by copyright. All rights reserved.
- From spatially variable streamflow to distributed hydrological models:
Analysis of key modeling decisions
- Authors: Fabrizio Fenicia; Dmitri Kavetski, Hubert H. G. Savenije, Laurent Pfister
Abstract: This paper explores the development and application of distributed hydrological models, focusing on the key decisions of how to discretize the landscape, which model structures to use in each landscape element, and how to link model parameters across multiple landscape elements. The case study considers the Attert catchment in Luxembourg – a 300 km2 mesoscale catchment with 10 nested subcatchments that exhibit clearly different streamflow dynamics. The research questions are investigated using conceptual models applied at hydrologic response unit (HRU) scales (1‐4 HRUs) on 6‐hourly time steps. Multiple model structures are hypothesized and implemented using the SUPERFLEX framework. Following calibration, space/time model transferability is tested using a split‐sample approach, with evaluation criteria including streamflow prediction error metrics and hydrological signatures. Our results suggest that: (1) models using geology‐based HRUs are more robust and capture the spatial variability of streamflow time series and signatures better than models using topography‐based HRUs; this finding supports the hypothesis that, in the Attert, geology exerts a stronger control than topography on streamflow generation, (2) streamflow dynamics of different HRUs can be represented using distinct and remarkably simple model structures, which can be interpreted in terms of the perceived dominant hydrologic processes in each geology type, and (3) the same maximum root zone storage can be used across the three dominant geological units with no loss in model transferability; this finding suggests that the partitioning of water between streamflow and evaporation in the study area is largely independent of geology and can be used to improve model parsimony. The modeling methodology introduced in this study is general and can be used to advance our broader understanding and prediction of hydrological behaviour, including the landscape characteristics that control hydrologic response, the dominant processes associated with different landscape types, and the spatial relations of catchment processes. This article is protected by copyright. All rights reserved.
- Irrigation water sources and irrigation application methods used by U.S.
plant nursery producers
- Authors: Krishna P. Paudel; Mahesh Pandit, Roger Hinson
Abstract: We examine irrigation water sources and irrigation methods used by U.S. nursery plant producers using nested multinomial fractional regression models. We use data collected from the National Nursery Survey (2009) to identify effects of different firm and sales characteristics on the fraction of water sources and irrigation methods used. We find that regions, sales of plants types, farm income, and farm age have significant roles in what water source is used. Given the fraction of alternative water sources used, results indicated that use of computer, annual sales, region, and the number of IPM practices adopted play an important role in the choice of irrigation method. Based on the findings from this study, government can provide subsidies to nursery producers in water deficit regions to adopt drip irrigation method or use recycled water or combination of both. Additionally, encouraging farmers to adopt IPM may enhance the use of drip irrigation and recycled water in nursery plant production. This article is protected by copyright. All rights reserved.
- Identification of contaminant source architectures—A statistical
inversion that emulates multiphase physics in a computationally
- Authors: J Koch; W. Nowak
Abstract: The goal of this work is to improve the inference of non‐aqueous‐phase contaminated source zone architectures (CSA) from field data. We follow the idea that a physically‐motivated model for CSA formation helps in this inference by providing relevant relationships between observables and the unknown CSA. Typical multiphase models are computationally too expensive to be applied for inverse modeling; thus, state‐of‐the‐art CSA identification techniques do not yet use physically‐based CSA formation models. To overcome this shortcoming, we apply a stochastic multiphase model with reduced computational effort that can be used to generate a large ensemble of possible CSA realizations. Further, we apply a reverse transport formulation in order to accelerate the inversion of transport‐related data such as downgradient aqueous‐phase concentrations. We combine these approaches within an inverse Bayesian methodology for joint inversion of CSA and aquifer parameters. Because we use multiphase physics to constrain and inform the inversion, (1) only physically meaningful CSAs are inferred; (2) each conditional realization is statistically meaningful; (3) we obtain physically meaningful spatial dependencies for inter‐ and extrapolation of point‐like observations between the different involved unknowns and observables, and (4) dependencies far beyond simple correlation; (5) the inversion yields meaningful uncertainty bounds. We illustrate our concept by inferring three‐dimensional probability distributions of DNAPL residence, contaminant mass discharge and of other CSA characteristics. In the inference example, we use synthetic numerical data on permeability, DNAPL saturation and downgradient aqueous‐phase concentration, and we substantiate our claims about the advantages of emulating a multiphase flow model with reduced computational requirement in the inversion. This article is protected by copyright. All rights reserved.
- The costs of coping with poor water supply in rural Kenya
- Authors: Joseph Cook; Peter Kimuyu, Dale Whittington
Abstract: As the disease burden of poor access to water and sanitation declines around the world, the non‐health benefits ‐ mainly the time burden of water collection ‐ will likely grow in importance in sector funding decisions and investment analyses. We measure the coping costs incurred by households in one area of rural Kenya. Sixty percent of the 387 households interviewed were collecting water outside the home, and household members were spending an average of two to three hours doing so per day. We value these time costs using an individual‐level value of travel time estimate based on a stated preference experiment. We compare these results to estimates obtained assuming that the value of time saved is a fraction of unskilled wage rates. Coping cost estimates also include capital costs for storage and rainwater collection, money paid either to water vendors or at sources that charge volumetrically, costs of treating diarrhea cases, and expenditures on drinking water treatment (primarily boiling in our site). Median total coping costs per month are approximately US$20 per month, higher than average household water bills in many utilities in the United States, or 12% of reported monthly cash income. We estimate that coping costs are greater than 10% of income for over half of households in our sample. They are higher among larger and wealthier households, and households whose primary source is not at home. Even households with unprotected private wells or connections to an intermittent piped network spend money on water storage containers and on treating water they recognize as unsafe. This article is protected by copyright. All rights reserved.
- On the emergence of diel solute signals in flowing waters
- Authors: Robert T. Hensley; Matthew J. Cohen
Abstract: Biota imprint their stoichiometry on relative rates of elemental cycling in the environment. Despite this coupling, producer driven diel solute variation in rivers and streams is more apparent for some solutes (e.g., dissolved oxygen ‐ DO) than others (e.g., nitrate ‐ NO3‐). We hypothesized that these differences arise from atmospheric equilibration, with signals emerging and evolving differently for gaseous and non‐gaseous solutes. Measurements of DO and NO3 in a spring‐fed river, where constant inputs isolate in‐stream processing, support this hypothesis, as do results from reactive transport modeling of river solute dynamics. Atmospheric equilibration dramatically shortens the benthic footprint over which signals integrate, facilitating emergence of diel DO signals in response to in‐stream metabolism. In contrast, upstream influences persist much further downstream for non‐gaseous solutes, confounding and potentially obscuring the diel signals from in‐stream assimilatory processing. Isolating diel NO3 signals from in‐stream processing requires a two station approach wherein metabolic impacts on solute variation are measured by difference between upstream and downstream sensors. Notably, two‐station inference improves markedly when hydraulic controls on signal propagation such as dispersion and storage are explicitly considered. We conclude that the absence of diel signals at a single station for non‐gaseous solutes such as NO3‐ cannot be interpreted as lack of autotroph demand or element coupling. As advances in sensors enable the acquisition of an increasingly rich array of solute signals, controlling for differences in the emergence and downstream evolution of gaseous versus non‐gaseous solutes will dramatically improve inferences regarding the timing and magnitude of coupled elemental processing. This article is protected by copyright. All rights reserved.
- Issue Information
- PubDate: 2016-01-19T02:59:12.840528-05:
- The fan of influence of streams and channel feedbacks to simulated land
surface water and carbon dynamics
- Authors: Chaopeng Shen; William J. Riley, Kurt R. Smithgall, John M. Melack, Kuai Fang
Abstract: Large‐scale land models assume unidirectional land‐to‐river hydrological interactions, without considering feedbacks between channels and land. Using a tested, physically‐based model with explicit multi‐way interactions between overland, channel, wetland, and groundwater flows, we assessed how the representation and properties of channels influence simulated land surface hydrologic, biogeochemical, and ecosystem dynamics. A zone near the channels where various fluxes and states are significantly influenced by the channels, referred to as the fan of influence (FoI) of channels, has been identified. We elucidated two mechanisms inducing the model‐derived FoI: the baseflow mechanism, in which incised, gaining streams lower the water table and induce more baseflow, and the relatively more efficient conveyance of the channel network compared to overland flow. We systematically varied drainage density and grid resolution to quantify the size of the FoI, which is found to span a large fraction of the watershed (25%∼50%) for hydrologic variables including depth to water table and recharge, etc. The FoI is more pronounced with low‐resolution simulations but remains noticeable in hyper‐resolution (25 m) subbasin simulations. The FoI and the channel influence on basin‐average fluxes are also similar in simulations with alternative parameter sets. We found that high order, entrenched streams cause larger FoI. In addition, removing the simulated channels has disproportionally large influence on modeled wetland areas and inundation duration, which has implications for coupled biogeochemical or ecological modeling. Our results suggest that explicit channel representation provides important feedbacks to land surface dynamics which should be considered in meso‐ or large‐scale simulations. Since grid refinement incurs prohibitive computational cost, sub‐grid channel parameterization has advantages in efficiency over grid‐based representations that do not distinguish between overland flow and channel flow. This article is protected by copyright. All rights reserved.
- Two‐phase convective CO2 dissolution in saline aquifers
- Authors: M.J. Martinez; M.A. Hesse
Abstract: Geologic carbon storage in deep saline aquifers is a promising technology for reducing anthropogenic emissions into the atmosphere. Dissolution of injected CO2 into resident brines is one of the primary trapping mechanisms generally considered necessary to provide long‐term storage security. Given that diffusion of CO2 in brine is woefully slow, convective dissolution, driven by a small increase in brine density with CO2 saturation, is considered to be the primary mechanism of dissolution trapping. Previous studies of convective dissolution have typically only considered the convective process in the single phase region below the capillary transition zone and have either ignored the overlying two‐phase region where dissolution actually takes place or replaced it with a virtual region with reduced or enhanced constant permeability. Our objective is to improve estimates of the long‐term dissolution flux of CO2 into brine by including the capillary transition zone in two‐phase model simulations. In the fully two‐phase model, there is a capillary transition zone above the brine‐saturated region over which the brine saturation decreases with increasing elevation. Our two‐phase simulations show that the dissolution flux obtained by assuming a brine‐saturated, single‐phase porous region with a closed upper boundary is recovered in the limit of vanishing entry pressure and capillary transition zone. For typical finite entry pressures and capillary transition zone, however, convection currents penetrate into the two‐phase region. This removes the mass transfer limitation of the diffusive boundary layer and enhances the convective dissolution flux of CO2 more than three times above the rate assuming single‐phase conditions. This article is protected by copyright. All rights reserved.
- Effects of alongshore morphology on groundwater flow and solute transport
in a nearshore aquifer
- Authors: Ying Zhang; Ling Li, Dirk V. Erler, Isaac Santos, David Lockington
Abstract: Variations of beach morphology in both the cross‐shore and alongshore directions, associated with tidal creeks, are common at natural coasts, as observed at a field site on the east coast of Rarotonga, Cook Islands. Field investigations and three‐dimensional (3D) numerical simulations were conducted to study the nearshore groundwater flow and solute transport in such a system. The results show that the beach morphology, combined with tides, induced a significant alongshore flow and modified local pore water circulation and salt transport in the intertidal zone substantially. The bathymetry and hydraulic head of the creek enabled further and more rapid landward intrusion of seawater along the creek than in the aquifer, which created alongshore hydraulic gradient and solute concentration gradient to drive pore water flow and salt transport in the alongshore direction within the aquifer. The effects of the creek led to the formation of a saltwater plume in groundwater at an intermediate depth between fresher water zones on a cross‐shore transect. The 3D pore water flow in the nearshore zone was also complicated by the landward hydraulic head condition, resulting in freshwater drainage across the inland section of the creek while seawater infiltrating the seaward section. These results provided new insights into the complexity, intensity and time‐scales of mixing among fresh groundwater, recirculating seawater and creek water in three dimensions. The 3D characteristics of nearshore pore water flow and solute transport have important implications for studies of submarine groundwater discharge and associated chemical input to the coastal sea, and for evaluation of the beach habitat conditions. This article is protected by copyright. All rights reserved.
- Evaluating marginal likelihood with thermodynamic integration method and
comparison with several other numerical methods
- Authors: Peigui Liu; Ahmed S. Elshall, Ming Ye, Peter Beerli, Xiankui Zeng, Dan Lu, Yuezan Tao
Abstract: Evaluating marginal likelihood is the most critical and computationally expensive task, when conducting Bayesian model averaging to quantify parametric and model uncertainties. The evaluation is commonly done by using Laplace approximations to evaluate semi‐analytical expressions of the marginal likelihood or by using Monte Carlo (MC) methods to evaluate arithmetic or harmonic mean of a joint likelihood function. This study introduces a new MC method, i.e., thermodynamic integration, which has not been attempted in environmental modeling. Instead of using samples only from prior parameter space (as in arithmetic mean evaluation) or posterior parameter space (as in harmonic mean evaluation), the thermodynamic integration method uses samples generated gradually from the prior to posterior parameter space. This is done through a path sampling that conducts Markov chain Monte Carlo simulation with different power coefficient values applied to the joint likelihood function. The thermodynamic integration method is evaluated using three analytical functions by comparing the method with two variants of the Laplace approximation method and three MC methods, including the nested sampling method that is recently introduced into environmental modeling. The thermodynamic integration method outperforms the other methods in terms of their accuracy, convergence, and consistency. The thermodynamic integration method is also applied to a synthetic case of groundwater modeling with four alternative models. The application shows that model probabilities obtained using the thermodynamic integration method improves predictive performance of Bayesian model averaging. The thermodynamic integration method is mathematically rigorous, and its MC implementation is computationally general for a wide range of environmental problems. This article is protected by copyright. All rights reserved.
- Analytical solution and computer program (FAST) to estimate fluid fluxes
from subsurface temperature profiles
- Authors: Barret L. Kurylyk; Dylan J. Irvine
Abstract: This study details the derivation and application of a new analytical solution to the one‐dimensional, transient conduction‐advection equation that is applied to trace vertical subsurface fluid fluxes. The solution employs a flexible initial condition that allows for non‐linear temperature‐depth profiles, providing a key improvement over most previous solutions. The boundary condition is comprised of any number of superimposed step changes in surface temperature, and thus it accommodates intermittent warming and cooling periods due to long term changes in climate or land cover. The solution is verified using an established numerical model of coupled groundwater flow and heat transport.
A new computer program FAST (Flexible Analytical Solution using Temperature) is also presented to facilitate the inversion of this analytical solution to estimate vertical groundwater flow. The program requires surface temperature history (which can be estimated from historic climate data), subsurface thermal properties, a present‐day temperature‐depth profile, and reasonable initial conditions. FAST is written in the Python computing language and can be run using a free graphical user interface. Herein, we demonstrate the utility of the analytical solution and FAST using measured subsurface temperature and climate data from the Sendia Plain, Japan. Results from these illustrative examples highlight the influence of the chosen initial and boundary conditions on estimated vertical flow rates. This article is protected by copyright. All rights reserved.
- Sequential approach to joint flow—Seismic inversion for improved
characterization of fractured media
- Authors: Peter K. Kang; Yingcai Zheng, Xinding Fang, Rafal Wojcik, Dennis McLaughlin, Stephen Brown, Michael C. Fehler, Daniel R. Burns, Ruben Juanes
Abstract: Seismic interpretation of subsurface structures is traditionally performed without any account of flow behavior. Here, we present a methodology for characterizing fractured geologic reservoirs by integrating flow and seismic data. The key element of the proposed approach is the identification—within the inversion—of the intimate relation between fracture compliance and fracture transmissivity, which determine the acoustic and flow responses of a fractured reservoir, respectively. Owing to the strong (but highly uncertain) dependence of fracture transmissivity on fracture compliance, the modeled flow response in a fractured reservoir is highly sensitive to the geophysical interpretation. By means of synthetic models, we show that by incorporating flow data (well pressures and tracer breakthrough curves) into the inversion workflow, we can simultaneously reduce the error in the seismic interpretation and improve predictions of the reservoir flow dynamics. While the inversion results are robust with respect to noise in the data for this synthetic example, the applicability of the methodology remains to be tested for more complex synthetic models and field cases. This article is protected by copyright. All rights reserved.
- NMR study comparing capillary trapping in Berea sandstone of air, carbon
dioxide, and supercritical carbon dioxide after imbibition of water
- Authors: Cody A. Prather; Joshua M. Bray, Joseph D. Seymour, Sarah L. Codd
Abstract: Nuclear magnetic resonance (NMR) techniques were used to study the capillary trapping mechanisms relevant to carbon sequestration. Capillary trapping is an important mechanism in the initial trapping of supercritical CO2 in the pore structures of deep underground rock formations during the sequestration process. Capillary trapping is considered the most promising trapping option for carbon sequestration. NMR techniques noninvasively monitor the drainage and imbibition of air, CO2, and supercritical CO2 with DI H2O at low capillary numbers in a Berea sandstone rock core under conditions representative of a deep underground saline aquifer. Supercritical CO2 was found to have a lower residual nonwetting (NW) phase saturation than that of air and CO2. Supercritical CO2 behaves differently than gas phase air or CO2 and leads to a reduction in capillary trapping. NMR relaxometry data suggests that the NW phase, i.e. air, CO2, or supercritical CO2, is preferentially trapped in larger pores. This is consistent with snap‐off conditions being more favorable in macroscale pores, as NW fluids minimize their contact area with the solid and hence prefer larger pores. This article is protected by copyright. All rights reserved.
- Salt water interface in a layered coastal aquifer: The only published
analytic solution is in error
- Authors: O.D.L. Strack
Abstract: We consider the approach applied by Rumer and Shiau  to interface flow in a layered coastal aquifer. The authors match the boundary conditions along the interfaces between layers of different hydraulic conductivities by changing the vertical scale of the layers, which causes violation of the governing equations. In particular, the mass balance equation is not met in each of the transformed layers. This article is protected by copyright. All rights reserved.
- Estimation of composite hydraulic resistance in ice covered alluvial
- Authors: Soheil Ghareh Aghaji Zare; Stephanie A. Moore, Colin D. Rennie, Ousmane Seidou, Habib Ahmari, Jarrod Malenchak
Abstract: Formation, propagation and recession of ice cover introduce a dynamic boundary layer to the top of rivers during northern winters. Ice cover affects water velocity magnitude and distribution, water level and consequently conveyance capacity of the river. In this research, total resistance, i.e., “composite resistance”, is studied for a four month period including stable ice cover, break up and open water stages in Lower Nelson River (LNR), northern Manitoba, Canada. Flow and ice characteristics such as water velocity and depth and ice thickness and condition were measured continuously using acoustic techniques. An Acoustic Doppler Current Profiler (ADCP) and Shallow Water Ice Profiling Sonar (SWIPS) were installed simultaneously on a bottom mount and deployed for this purpose. Total resistance to the flow and boundary roughness are estimated using measured bulk hydraulic parameters. A novel method is developed to calculate composite resistance directly from measured under‐ice velocity profiles. The results of this method are compared to the measured total resistance and to the calculated composite resistance using formulae available in literature. The new technique is demonstrated to compare favourably to measured total resistance and to outperform previously available methods. This article is protected by copyright. All rights reserved.
- Evapotranspiration of rubber (Hevea brasiliensis) cultivated at two
plantation sites in southeast Asia
- Authors: Thomas W. Giambelluca; Ryan G. Mudd, Wen Liu, Alan D. Ziegler, Nakako Kobayashi, Tomo'omi Kumagai, Yoshiyuki Miyazawa, Tiva Khan Lim, Maoyi Huang, Jefferson Fox, Song Yin, Sophea Veasna Mak, Poonpipope Kasemsap
Abstract: To investigate the effects of expanding rubber (Hevea brasiliensis) cultivation on water cycling in Mainland Southeast Asia (MSEA), evapotranspiration (ET) was measured within rubber plantations at Bueng Kan, Thailand, and Kampong Cham, Cambodia. After energy closure adjustment, mean annual rubber ET was 1,211 and 1,459 mm yr−1 at the Thailand and Cambodia sites, respectively, higher than that of other tree‐dominated land covers in the region, including tropical seasonal forest (812‐1,140 mm yr−1), and savanna (538‐1,060 mm yr−1). The mean proportion of net radiation used for ET by rubber (0.725) is similar to that of tropical rainforest (0.729) and much higher than that of tropical seasonal forest (0.595) and savanna (0.548). Plant area index (varies with leaf area changes), explains 88.2% and 73.1% of the variance in the ratio of latent energy flux (energy equivalent of ET) to potential latent energy flux (LE/LEpot) for midday rain‐free periods at the Thailand and Cambodia sites, respectively. High annual rubber ET results from high late dry season water use, associated with rapid refoliation by this brevi‐deciduous species, facilitated by tapping of deep soil water, and by very high wet season ET, a characteristic of deciduous trees. Spatially, mean annual rubber ET increases strongly with increasing net radiation (Rn) across the three available rubber plantation observation sites, unlike non‐rubber tropical ecosystems, which reduce canopy conductance at high Rn sites. High water use by rubber raises concerns about potential effects of continued expansion of tree plantations on water and food security in MSEA. This article is protected by copyright. All rights reserved.
- A lithofacies approach for modeling non‐Fickian solute transport in
a heterogeneous alluvial aquifer
- Authors: Marco Bianchi; Chunmiao Zheng
Abstract: Stochastic realizations of lithofacies assemblage based on lithological data from a relatively small number of boreholes were used to simulate solute transport at the well‐known Macrodispersion Experiment (MADE) site in Mississippi (USA). With sharp vertical contrasts and lateral connectivity explicitly accounted for in the corresponding hydraulic conductivity fields, experimental results from a large‐scale tracer experiment were adequately reproduced with a relatively simple model based on advection and local dispersion. The geologically based model of physical heterogeneity shows that one well interconnected lithofacies, with a significantly higher hydraulic conductivity and accounting for 12% of the total aquifer volume, may be responsible for the observed non‐Fickian transport behavior indicated by the asymmetric shape of the plumes and by variations of the dispersion rate in both space and time. This analysis provides a lithological basis to the hypothesis that transport at MADE site is controlled by a network of high‐conductivity sediments embedded in a less permeable matrix. It also explains the calibrated value of the ratio of mobile to total porosities used in previous modelling studies based on the dual‐domain mass transfer approach. The results of this study underscore the importance of geologically plausible conceptualizations of the subsurface for making accurate predictions of the fate and transport of contaminants in highly heterogeneous aquifers. These conceptualizations may be developed through integration of raw geological data with expert knowledge, interpretation and appropriate geostatistical methods. This article is protected by copyright. All rights reserved.
- Accurate and efficient prediction of fine‐resolution hydrologic and
carbon dynamic simulations from coarse‐resolution models
- Authors: G. S. H. Pau; C. Shen, W. J. Riley, Y. Liu
Abstract: The topography, and the biotic and abiotic parameters are typically upscaled to make watershed‐scale hydrologic‐biogeochemical models computationally tractable. However, upscaling procedure can produce biases when nonlinear interactions between different processes are not fully captured at coarse resolutions. Here we applied the Proper Orthogonal Decomposition Mapping Method (PODMM) to downscale the field solutions from a coarse (7 km) resolution grid to a fine (220 m) resolution grid. PODMM trains a reduced order model (ROM) with coarse‐ and fine‐resolution solutions, here obtained using PAWS+CLM, a quasi‐3D watershed processes model that has been validated for many temperate watersheds. Subsequent fine‐resolution solutions were approximated based only on coarse‐resolution solutions and the ROM. The approximation errors were efficiently quantified using an error estimator. By jointly estimating correlated variables and temporally varying the ROM parameters, we further reduced the approximation errors by up to 20\%. We also improved the method's robustness by constructing multiple ROMs using different set of variables, and selecting the best approximation based on the error estimator. The ROMs produced accurate downscaling of soil moisture, latent heat flux, and net primary production with O(1000) reduction in computational cost. The subgrid distributions were also nearly indistinguishable from the ones obtained using the fine‐resolution model. Compared to coarse‐resolution solutions, biases in upscaled ROM solutions were reduced by up to 80\%. This method has the potential to help address the long‐standing spatial scaling problem in hydrology and enable long‐time integration, parameter estimation, and stochastic uncertainty analysis while accurately representing the heterogeneities. This article is protected by copyright. All rights reserved.
- Hydrodynamics and sediment transport in a meandering channel with a model
axial flow hydrokinetic turbine
- Authors: Craig Hill; Jessica Kozarek, Fotis Sotiropoulos, Michele Guala
Abstract: An investigation into the interactions between a model axial flow hydrokinetic turbine (rotor diameter, dT = 0.15 m) and the complex hydrodynamics and sediment transport processes within a meandering channel was carried out in the Outdoor StreamLab research facility at the University of Minnesota St. Anthony Falls Laboratory. This field‐scale meandering stream with bulk flow and sediment discharge control provided a location for high spatio‐temporally resolved measurements of bed and water surface elevations around the model turbine. The device was installed within an asymmetric, erodible channel cross‐section under migrating bedform and fixed outer bank conditions. A comparative analysis between velocity and topographic measurements, with and without the turbine installed, highlights the local and non‐local features of the turbine‐induced scour and deposition patterns. In particular, it shows how the cross‐section geometry changes, how the bedform characteristics are altered, and how the mean flow field is distorted both upstream and downstream of the turbine. We further compare and discuss how current energy conversion deployments in meander regions would result in different interactions between the turbine operation and the local and non‐local bathymetry compared to straight channels. This article is protected by copyright. All rights reserved.
- Reconstructing input for artificial neural networks based on embedding
theory and mutual information to simulate soil porewater salinity in tidal
- Authors: Fawen Zheng; Yongshan Wan, Keunyea Song, Detong Sun, Marion Hedgepeth
Abstract: Soil porewater salinity plays an important role in the distribution of vegetation and biogeochemical processes in coastal floodplain ecosystems. In this study, artificial neural networks (ANNs) was applied to simulate the porewater salinity of a tidal floodplain in Florida. We present an approach based on embedding theory with mutual information to reconstruct ANN model input time series from one system state variable. Mutual information between system output and input was computed and the local minimum mutual information points were used to determine a time lag vector for time series embedding and reconstruction, with which the mutual information weighted average method was developed to compute the components of reconstructed time series. The optimal embedding dimension was obtained by optimizing model performance. The method was applied to simulate soil porewater salinity dynamics at 12 probe locations in the tidal floodplain influenced by saltwater intrusion using four years (2005 to 2008) data, in which adjacent river water salinity was used to reconstruct model input. The simulated electrical conductivity of the porewater showed close agreement with field observations (RMSE ≤ 0.031 S/m and R2 ≥ 0.897), suggesting the reconstructed input by the proposed approach provided adequate input information for ANN modeling. Multiple linear regression model, partial mutual information algorithm for input variable selection, k‐NN algorithm, and simple time delay embedding were also used to further verify the merit of the proposed approach. This article is protected by copyright. All rights reserved.
- Representing spatial variability of forest snow: Implementation of a new
- Authors: D. Moeser; G. Mazzotti, N. Helbig, T. Jonas
Abstract: A new interception model was integrated in a snowmelt model and for the first time the spatial variability of forest snow was effectively represented due to the inclusion of new forest structure metrics. The model was tested at 1273 field points surrounding Davos, Switzerland, that feature an extremely wide range of canopy and forest structure. The behavior of the new model was compared against a widely applied interception model. Due to the inclusion of novel forest structure parameters (mean distance to canopy and total gap area) in the new model, simulated interception mimicked the horizontal layout of canopy structure, while the standard interception model yielded fairly homogeneous interception estimations even under highly heterogeneous canopy conditions. The large variance of estimated interception between points using the new model translated into significant effects on under‐canopy snow water equivalent and snow depth. Precipitation conditions were also analyzed, and further differences between the models were related to storm intensity. In climates characterized by large storm events, the new interception model provides significantly higher interception estimations (i.e., lower under‐canopy snow) than the standard model, whereas in climates characterized by small storms events, the new model yields lower interception estimations (i.e., higher under‐canopy snow) in areas with moderately sized to large canopy gaps. This article is protected by copyright. All rights reserved.
- Use of a flux‐based field capacity criterion to identify effective
hydraulic parameters of layered soil profiles subjected to synthetic
- Authors: Paolo Nasta; Nunzio Romano
Abstract: This study explores the feasibility of identifying the effective soil hydraulic parameterization of a layered soil profile by using a conventional unsteady drainage experiment leading to field capacity. The flux‐based field capacity criterion is attained by subjecting the soil profile to a synthetic drainage process implemented numerically in the Soil‐Water‐Atmosphere‐Plant (SWAP) model. The effective hydraulic parameterization is associated to either aggregated or equivalent parameters, the former being determined by the geometrical scaling theory while the latter is obtained through the inverse modeling approach. Outcomes from both these methods depend on information that is sometimes difficult to retrieve at local scale and rather challenging or virtually impossible at larger scales. The only knowledge of topsoil hydraulic properties, for example as retrieved by a near‐surface field campaign or a data assimilation technique, is often exploited as a proxy to determine effective soil hydraulic parameterization at the largest spatial scales. Comparisons of the effective soil hydraulic characterization provided by these three methods are conducted by discussing the implications for their use and accounting for the trade‐offs between required input information and model output reliability. To better highlight the epistemic errors associated to the different effective soil hydraulic properties and to provide some more practical guidance, the layered soil profiles are then grouped by using the FAO textural classes. For the moderately heterogeneous soil profiles available, all three approaches guarantee a general good predictability of the actual field capacity values and provide adequate identification of the effective hydraulic parameters. Conversely, worse performances are encountered for the highly variable vertical heterogeneity, especially when resorting to the “topsoil only” information. In general, the best performances are guaranteed by the equivalent parameters, which might be considered a reference for comparisons with other techniques. As might be expected, the information content of the soil hydraulic properties pertaining only to the uppermost soil horizon is rather inefficient and also unable to map out the hydrologic behavior of the real vertical soil heterogeneity since the drainage process is significantly affected by profile layering in almost all cases. This article is protected by copyright. All rights reserved.
- Trading‐off tolerable risk with climate change adaptation costs in
water supply systems
- Abstract: Choosing secure water resource management plans inevitably requires trade‐offs between risks (for a variety of stakeholders), costs and other impacts. We have previously argued that water resources planning should focus upon metrics of risk of water restrictions, accompanied by extensive simulation‐ and scenario‐based exploration of uncertainty. However, the results of optimisation subject to risk constraints can be sensitive to the specification of tolerable risk, which may not be precisely or consistently defined by different stakeholders. In this paper we recast the water resources planning problem as a multi‐objective optimisation problem to identify least cost schemes that satisfy a set of criteria for tolerable risk, where tolerable risk is defined in terms of the frequency of water use restrictions of different levels of severity. Our proposed method links a very large ensemble of climate model projections to a water resource system model and a multi‐objective optimization algorithm to identify a Pareto optimal set of water resource management plans across a 25 years planning period. In a case study application to the London water supply system, we identify water resources management plans that, for a given financial cost, maximise performance with respect to one or more probabilistic criteria. This illustrates trade‐offs between financial costs of plans and risk, and between risk criteria for four different severities of water use restrictions. Graphical representation of alternative sequences of investments in the Pareto set helps to identify water management options for which there is a robust case for including them in the plan. This article is protected by copyright. All rights reserved.
- Estimation of evaporation over the Upper Blue Nile basin by combining
observations from satellites and river flow gauges
- Authors: Mariam M. Allam; Anjuli Jain Figueroa, Dennis B. McLaughlin, Elfatih A. B. Eltahir
Abstract: Reliable estimates of regional evapotranspiration are necessary to improve water resources management and planning. However, direct measurements of evaporation are expensive and difficult to obtain. Some of the difficulties are illustrated in a comparison of several satellite‐based estimates of evapotranspiration for the Upper Blue Nile (UBN) basin in Ethiopia. These estimates disagree both temporally and spatially. All the available data products underestimate evapotranspiration leading to basin‐scale mass balance errors on the order of 35 percent of the mean annual rainfall. This paper presents a methodology that combines satellite observations of rainfall, terrestrial water storage as well as river‐flow gauge measurements to estimate actual evapotranspiration over the UBN basin. The estimates derived from these inputs are constrained using a one‐layer soil water balance and routing model. Our results describe physically consistent long‐term spatial and temporal distributions of key hydrologic variables, including rainfall, evapotranspiration, and river‐flow. We estimate an annual evapotranspiration over the UBN basin of about 2.55 mm per day. Spatial and Temporal evapotranspiration trends are revealed by dividing the basin into smaller sub‐basins. The methodology described here is applicable to other basins with limited observational coverage that are facing similar future challenges of water scarcity and climate change. This article is protected by copyright. All rights reserved.
- Sediment deposition within and around a finite patch of model vegetation
over a range of channel velocity
- Authors: Chao Liu; Heidi Nepf
Abstract: The interaction between flow and vegetation creates feedbacks to deposition that vary with channel velocity. This experimental study describes how channel velocity and stem‐generated turbulence influence the deposition within and around an emergent patch of model vegetation, with a particular focus on deposition within the patch. The Reynolds number threshold for stem‐scale turbulence generation was determined using velocity spectra and flow visualization. At high channel velocity resuspension occurred in the bare regions of the channel and a non‐uniform spatial distribution of net deposition was observed around and within the patch. In contrast, at low channel velocity there was no (or limited) resuspension and a uniform distribution of net deposition was observed around and within the patch. The deposition inside the patch was enhanced, relative to a bare‐channel control, only when the following two criteria were met: (1) the absence of stem turbulence, and (2) the presence of sediment resuspension in the bare channel. Comparison to previous lab and field studies further support these criteria. This article is protected by copyright. All rights reserved.
- Enhancing multiple‐point geostatistical modeling: 2. Iterative
simulation and multiple distance function
- Authors: Pejman Tahmasebi; Muhammad Sahimi
Abstract: This series addresses a fundamental issue in multiple‐point statistical (MPS) simulation for generation of realizations of large‐scale porous media. Past methods suffer from the fact that they generate discontinuities and patchiness in the realizations that, in turn, affect their flow and transport properties. Part I of this series addressed certain aspects of this fundamental issue, and proposed two ways of improving of one such MPS method, namely, the cross correlation‐based simulation (CCSIM) method that was proposed by the authors. In the present paper a new algorithm is proposed to further improve the quality of the realizations. The method utilizes the realizations generated by the algorithm introduced in Part I, iteratively removes any possible remaining discontinuities in them, and addresses the problem with honoring hard (quantitative) data, using an error map. The map represents the differences between the patterns in the training image (TI) and the current iteration of a realization. The resulting iterative CCSIM – the iCCSIM algorithm ‐ utilizes a random path and the error map to identify the locations in the current realization in the iteration process that need further “repairing;” that is, those locations at which discontinuities may still exist. The computational time of the new iterative algorithm is considerably lower than one in which every cell of the simulation grid is visited in order to repair the discontinuities. Furthermore, several efficient distance functions are introduced by which one extracts effectively key information from the TIs. To increase the quality of the realizations and extracting the maximum amount of information from the TIs, the distance functions can be used simultaneously. The performance of the iCCSIM algorithm is studied using very complex 2D and 3D examples, including those that are process‐based. Comparison is made between the quality and accuracy of the results with those generated by the original CCSIM algorithm, which demonstrates the superior performance of the iCCSIM. This article is protected by copyright. All rights reserved.
- Enhancing multiple‐point geostatistical modeling: 1. Graph theory
and pattern adjustment
- Authors: Pejman Tahmasebi; Muhammad Sahimi
Abstract: In recent years higher‐order geostatistical methods have been used for modeling of a wide variety of large‐scale porous media, such as groundwater aquifers and oil reservoirs. Their popularity stems from their ability to account for qualitative data and the great flexibility that they offer for conditioning the models to hard (quantitative) data, which endow them with the capability for generating realistic realizations of porous formations with very complex channels, as well as features that are mainly a barrier to fluid flow. One group of such models consists of pattern‐based methods that use a set of data points for generating stochastic realizations by which the large‐scale structure and highly‐connected features are reproduced accurately. The cross correlation‐based simulation (CCSIM) algorithm, proposed previously by the authors, is a member of this group that has been shown to be capable of simulating multi‐million cell models in a matter of a few CPU seconds. The method is, however, sensitive to pattern's specifications, such as boundaries and the number of replicates. In this paper the original CCSIM algorithm is reconsidered and two significant improvements are proposed for accurately reproducing large‐scale patterns of heterogeneities in porous media. First, an effective boundary‐correction method based on the graph theory is presented by which one identifies the optimal cutting path/surface for removing the patchiness and discontinuities in the realization of a porous medium. Next, a new pattern adjustment method is proposed that automatically transfers the features in a pattern to one that seamlessly matches the surrounding patterns. The original CCSIM algorithm is then combined with the two methods and is tested using various complex two‐ and three‐dimensional examples. It should, however, be emphasized that the methods that we propose in this paper are applicable to other pattern‐based geostatistical simulation methods. This article is protected by copyright. All rights reserved.
- Estimating transmissivity from single‐well pumping tests in
- Authors: Armin Pechstein; Sabine Attinger, Ronald Krieg, Nadim K. Copty
Abstract: Although aquifers are naturally heterogeneous, the interpretation of pumping tests is commonly performed under the assumption of aquifer homogeneity. This yields interpreted hydraulic parameters averaged over a domain of uncertain extent which disguises their relation to the underlying heterogeneity. In this study, we numerically investigate the sensitivity of the transient drawdown at the pumping well, to non‐uniform distributions of transmissivity in confined aquifers. Frechet kernels and their time derivative are used to estimate two spatially averaged transmissivities, denoted the equivalent and interpreted transmissivity, Teq and Tin, respectively, for the case of single‐well pumping tests. Interrelating Teq and Tin is achieved by modeling Tin in terms of a distance‐dependent, radially heterogeneous field. In weakly heterogeneous aquifers, Teq approximates TPW, the local transmissivity at the pumped well. With increasing degree of heterogeneity, Teq deviates from TPW as pumping propagates. Tin starts at TPW, approaching the spatial geometric mean of transmissivity during late pumping times. Limits of the proposed spatial weighting functions are investigated by treating the interpreted storativity, Sest, as an indicator for ow connectivity. It is shown numerically that the spatial weights for Teq and Tin agree well to the underlying heterogeneity if Sest
- The flow of a foam in a two‐dimensional porous medium
- Abstract: Foams have been used for decades as displacing fluids for enhanced oil recovery and aquifer remediation, and more recently for remediation of the vadose zone, in which case foams carry chemical amendments. Foams are better injection fluids than aqueous solutions due to their low sensitivity to gravity and because they are less sensitive to permeability heterogeneities, thus allowing a more uniform sweep. The latter aspect results from their peculiar rheology, whose understanding motivates the present study. We investigate foam flow through a two‐dimensional porous medium consisting of circular obstacles positioned randomly in a horizontal transparent Hele‐Shaw cell. The local foam structure is recorded in situ, which provides a measure of the spatial distribution of bubble velocities and sizes at regular time intervals. The flow exhibits a rich phenomenology including preferential flow paths and local flow non‐stationarity (intermittency) despite the imposed permanent global flow rate. Moreover, the medium selects the bubble size distribution through lamella division‐triggered bubble fragmentation. Varying the mean bubble size of the injected foam, its water content, and mean velocity, we characterize those processes systematically. In particular we measure the spatial evolution of the distribution of bubble areas, and infer the efficiency of bubble fragmentation depending on the various control parameters. We furthermore show that the distributions of bubble sizes and velocities are correlated. This study sheds new light on the local rheology of foams in porous media and opens the way towards quantitative characterization of the relationship between medium geometry and foam flow properties. It also suggests that large scale models of foam flows in the subsurface should account for the correlation between bubble sizes and velocities. This article is protected by copyright. All rights reserved.
- Using time scales to characterize phytoplankton assemblages in a deep
- Abstract: A combination of field observations and 3D hydrodynamic simulations were used to identify the phytoplankton species and to estimate the various time scales of the dominant physical and biological processes in Lake Iseo, a deep subalpine lake located in northern Italy, during a stratified period (July 2010). By ordering the rate processes time scales we derive a phytoplankton patch categorization and growth interpretation that provides a general framework for the spatial distribution of phytoplankton concentration in Lake Iseo and illuminates the characteristics of their ecological niches. The results show that the diurnal surface layer was well mixed, received strong diurnal radiation, had low phosphorus concentrations and the phytoplankton biomass was sustained by the green alga Sphaerocystis schroeterii. The vertical mixing time scales were much shorter than horizontal mixing time scales causing a depth‐uniform chlorophyll a concentration. The horizontal patch scale was determined by horizontal dispersion balancing the phytoplankton growth time scale, dictating the success of the observed green algae. The strongly stratified nutrient‐rich metalimnion had mild light conditions and Diatoma elongatum and Planktothrix rubescens made up the largest proportions of the total phytoplankton biomass at the intermediate and deeper metalimnetic layers. The vertical transport time scales were much shorter than horizontal transport and vertical dispersion leading to growth niche for the observed phytoplankton. The study showed that time‐scale hierarchy mandates the essential phytoplankton attributes or traits for success in a particular section of the water column and/or water body. This article is protected by copyright. All rights reserved.
- Hierarchical Bayesian method for mapping biogeochemical hot spots using
induced polarization imaging
- Abstract: In floodplain environments, a naturally reduced zone (NRZ) is considered to be a common biogeochemical hotspot, having distinct microbial and geochemical characteristics. Although important for understanding their role in mediating floodplain biogeochemical processes, mapping the subsurface distribution of NRZs over the dimensions of a floodplain is challenging, as conventional wellbore data are typically spatially limited and the distribution of NRZs is heterogeneous. In this study, we present an innovative methodology for the probabilistic mapping of NRZs within a three‐dimensional (3D) subsurface domain using induced polarization imaging, which is a non‐invasive geophysical technique. Measurements consist of surface geophysical surveys and drilling‐recovered sediments at the U.S. Department of Energy field site near Rifle, CO (USA). Inversion of surface time‐domain induced polarization (TDIP) data yielded 3D images of the complex electrical resistivity, in terms of magnitude and phase, which are associated with mineral precipitation and other lithological properties. By extracting the TDIP data values co‐located with wellbore lithological logs, we found that the NRZs have a different distribution of resistivity and polarization from the other aquifer sediments. To estimate the spatial distribution of NRZs, we developed a Bayesian hierarchical model to integrate the geophysical and wellbore data. In addition, the resistivity images were used to estimate hydrostratigraphic interfaces under the floodplain. Validation results showed that the integration of electrical imaging and wellbore data using an Bayesian hierarchical model was capable of mapping spatially heterogeneous interfaces and NRZ distributions thereby providing a minimally invasive means to parameterize a hydro‐biogeochemical model of the floodplain. This article is protected by copyright. All rights reserved.
- Process‐based characterization of evapotranspiration sources over
the North American monsoon region
- Authors: Theodore J. Bohn; Enrique R. Vivoni
Abstract: Evapotranspiration (ET) is a poorly constrained flux in the North American monsoon (NAM) region, leading to potential errors in land‐atmosphere feedbacks. We quantified the spatio‐temporal variations of ET using the Variable Infiltration Capacity (VIC) model, modified to account for soil evaporation (Esoil), irrigated agriculture, and the variability of land surface properties derived from the Moderate Resolution Imaging Spectroradiometer during 2000‐2012. Simulated ET patterns were compared to field observations at fifty‐nine eddy covariance towers, water balance estimates in nine basins, and six available gridded ET products. The modified VIC model performed well at eddy covariance towers representing the natural and agricultural land covers in the region. Simulations revealed that major sources of ET were forested mountain areas during the summer season and irrigated croplands at peak times of growth in the winter and summer, accounting for 22% and 9% of the annual ET, respectively. Over the NAM region, Esoil was the largest component (60%) of annual ET, followed by plant transpiration (T, 32%) and evaporation of canopy interception (8%). Esoil and T displayed different relationships with P in natural land covers, with Esoil tending to peak earlier than T by up to one month, while only a weak correlation between ET and P was found in irrigated croplands. Based on the model performance, the VIC‐based estimates are the most realistic to date for this region. Furthermore, spatio‐temporal patterns reveal new information on the magnitudes, locations and timing of ET in the North American monsoon region with implications on land‐atmosphere feedbacks. This article is protected by copyright. All rights reserved.
- Distributed hydrological modeling with channel network flow of a forestry
drained peatland site
- Authors: Kersti Haahti; Lassi Warsta, Teemu Kokkonen, Bassam A. Younis, Harri Koivusalo
Abstract: Peatland drainage has been an important component of forestry management in the boreal zone and the resulting ditch networks are maintained regularly to sustain forest productivity. In Finland, this is recognized as the most detrimental forestry practice increasing diffuse loads of suspended solids. Alongside forestry management on peatlands, interest in peatland restoration has grown lately. Distributed hydrological modeling has the potential to address these matters by recognizing relevant physical mechanisms and identifying most suitable strategies for mitigating undesired outcomes. This study investigates the utility of such a modeling approach in a drained peatland forest environment. To provide a suitable tool for this purpose, we coupled channel network flow to the three‐dimensional distributed hydrological model FLUSH. The resulting model was applied to a 5.2 ha drained peatland forest catchment in Eastern Finland. The model was calibrated and validated using field measurements obtained over frost‐free periods of five months. The application showed that distributed modeling can disentangle the importance of spatial factors on local soil moisture conditions, which is significant as peatland drainage aims to control these conditions. In our application, we limited the spatial aspect to the topography and the drainage network, and found that the drainage configuration had a clear effect on the spatial soil moisture patterns but that the effect was less pronounced during the wetter summer. Future applications of distributed modeling in this field comprises investigating the impacts of other spatial factors, modeling channel erosion and solid transport to address strategies for their mitigation, and evaluating restoration schemes. This article is protected by copyright. All rights reserved.
- A Bayesian hierarchical approach to model seasonal algal variability along
an upstream to downstream river gradient
- Authors: YoonKyung Cha; Seok Soon Park, Hye Won Lee, Craig A. Stow
Abstract: Modeling to accurately predict river phytoplankton distribution and abundance is important in water quality and resource management. Nevertheless, the complex nature of eutrophication processes in highly connected river systems makes the task challenging. To model dynamics of river phytoplankton, represented by chlorophyll a (Chla) concentration, we propose a Bayesian hierarchical model that explicitly accommodates seasonality and upstream‐downstream spatial gradient in the structure. The utility of our model is demonstrated with an application to the Nakdong River (South Korea), which is a eutrophic, intensively regulated river, but functions as an irreplaceable water source for more than 13 million people. Chla is modeled with two manageable factors, river flow and total phosphorus (TP) concentration. Our model results highlight the importance of taking seasonal and spatial context into account when describing flow regimes and phosphorus delivery in rivers. A contrasting positive Chla‐flow relationship across stations vs. negative Chla‐flow slopes that arose when Chla was modeled on a station‐month basis is an illustration of Simpson's paradox, which necessitates modeling Chla‐flow relationships decomposed into seasonal and spatial components. Similar Chla‐TP slopes among stations and months suggest that, with the flow effect removed, positive TP effects on Chla are uniform regardless of the season and station in the river. Our model prediction successfully captured the shift in the spatial and monthly patterns of Chla. This article is protected by copyright. All rights reserved.
- Quantifying watershed‐scale groundwater loading and in‐stream
fate of nitrate using high‐frequency water quality data
- Authors: Matthew P. Miller; Anthony J. Tesoriero, Paul D. Capel, Brian A. Pellerin, Kenneth E. Hyer, Douglas A. Burns
Abstract: We describe a new approach that couples hydrograph separation with high‐frequency nitrate data to quantify time‐variable groundwater and runoff loading of nitrate to streams, and the net in‐stream fate of nitrate at the watershed‐scale. The approach was applied at three sites spanning gradients in watershed size and land use in the Chesapeake Bay watershed. Results indicate that 58‐73% of the annual nitrate load to the streams was groundwater‐discharged nitrate. Average annual first order nitrate loss rate constants (k) were similar to those reported in both modelling and in‐stream process‐based studies, and were greater at the small streams (0.06 and 0.22 d−1) than at the large river (0.05 d−1), but 11% of the annual loads were retained/lost in the small streams, compared with 23% in the large river. Larger streambed area to water volume ratios in small streams result in greater loss rates, but shorter residence times in small streams result in a smaller fraction of nitrate loads being removed than in larger streams. A seasonal evaluation of k values suggests that nitrate was retained/lost at varying rates during the growing season. Consistent with previous studies, streamflow and nitrate concentration were inversely related to k. This new approach for interpreting high‐frequency nitrate data and the associated findings furthers our ability to understand, predict, and mitigate nitrate impacts on streams and receiving waters by providing insights into temporal nitrate dynamics that would be difficult to obtain using traditional field‐based studies. This article is protected by copyright. All rights reserved.
- Percolating length scales from topological persistence analysis of
micro‐CT images of porous materials
- Abstract: Topological persistence is a powerful and general technique for characterising the geometry and topology of data. Its theoretical foundations are over fifteen years old and efficient computational algorithms are now available for the analysis of large digital images. We explain here how quantities derived from topological persistence relate to other measurements on porous materials such as grain and pore‐size distributions, connectivity numbers, and the critical radius of a percolating sphere. The connections between percolation and topological persistence are explored in detail using data obtained from micro‐CT images of spherical bead packings, unconsolidated sand packing, a variety of sandstones, and a limestone. We demonstrate how persistence information can be used to estimate the percolating sphere radius and to characterise the connectivity of the percolating cluster. This article is protected by copyright. All rights reserved.
- The Impact of Mucilage on Root Water Uptake – A Numerical Study
- Authors: N. Schwartz; A. Carminati, M. Javaux
Abstract: The flow of water between soil and plants follows the gradient in water potential and depends on the hydraulic properties of the soil and the root. In models for root water uptake (RWU), it is usually assumed that the hydraulic properties near the plant root (i.e. in the rhizosphere) and in the bulk soil are identical. Yet, a growing body of evidence has shown that the hydraulic properties of the rhizosphere are affected by root exudates (specifically, mucilage) and markedly differ from those of the bulk soil. In this work, we couple a 3D detailed description of RWU with a model that accounts for the rhizosphere specific properties (i.e. rhizosphere hydraulic properties and a non‐equilibrium relation between water content and matric head). We show that as the soil dries out (due to water uptake), the higher water holding capacity of the rhizosphere results in a delay of the stress onset. During rewetting, non‐equilibrium results in a slower increase of the rhizosphere water content. Furthermore, the inverse relation between water content and relaxation time implies that the drier is the rhizosphere the longer it takes to rewet. Another outcome of non‐equilibrium is the small fluctuation of the rhizosphere water content compared to the bulk soil. Overall, our numerical results are in agreement with recent experimental data and provide a tool to further examine the impact of various rhizosphere processes on RWU and water dynamics. This article is protected by copyright. All rights reserved.
- Assessing the relative effectiveness of statistical downscaling and
distribution mapping in reproducing rainfall statistics based on climate
- Authors: Andreas Langousis; Antonios Mamalakis, Roberto Deidda, Marino Marrocu
Abstract: To improve the level skill of climate models (CMs) in reproducing the statistics of daily rainfall at a basin level, two types of statistical approaches have been suggested. One is statistical correction of CM rainfall outputs based on historical series of precipitation. The other, usually referred to as statistical rainfall downscaling, is the use of stochastic models to conditionally simulate rainfall series, based on large‐scale atmospheric forcing from CMs. While promising, the latter approach attracted reduced attention in recent years, since the developed downscaling schemes involved complex weather identification procedures, while demonstrating limited success in reproducing several statistical features of rainfall. In a recent effort, Langousis and Kaleris (2014) developed a statistical framework for simulation of daily rainfall intensities conditional on upper‐air variables, which is simpler to implement and more accurately reproduces several statistical properties of actual rainfall records. Here, we study the relative performance of: a) direct statistical correction of CM rainfall outputs using non‐parametric distribution mapping, and b) the statistical downscaling scheme of Langousis and Kaleris (2014), in reproducing the historical rainfall statistics, including rainfall extremes, at a regional level. This is done for an intermediate‐sized catchment in Italy, i.e. the Flumendosa catchment, using rainfall and atmospheric data from 4 CMs of the ENSEMBLES project. The obtained results are promising, since the proposed downscaling scheme is more accurate and robust in reproducing a number of historical rainfall statistics, independent of the CM used and the characteristics of the calibration period. This is particularly the case for yearly rainfall maxima. This article is protected by copyright. All rights reserved.
- Hedging the financial risk from water scarcity for Great Lakes shipping
- Authors: Eliot S. Meyer; Gregory W. Characklis, Casey Brown, Paul Moody
Abstract: Low water levels in the Great Lakes have recently had significant financial impacts on the region's commercial shipping, which transports hundreds of millions of dollars' worth of bulk goods each year. Cargo capacity is a function of a ship's draft, the distance between water level and the ship's bottom, and lower water levels force ships to reduce cargo loads to prevent running aground in shallow harbors and locks. Financial risk transfer instruments, such as index‐based insurance contracts, may provide an adaptable method for managing these financial risks. In this work, a relationship between water levels and shipping revenues is developed and used in an actuarial analysis of the frequency and magnitude of revenue losses. This analysis is used to develop a standardized suite of binary financial contracts, which are indexed to water levels and priced according to predefined thresholds. These contracts are then combined to form hedging portfolios with different objectives for the shippers. Results suggest that binary contracts could substantially reduce the risk of financial losses during low lake level periods and at a relatively low cost of only one to three percent of total revenues, depending on coverage level. This article is protected by copyright. All rights reserved.
- Thermal conductivity in porous media: Percolation‐based
- Authors: Behzad Ghanbarian; Hugh Daigle
Abstract: Knowledge of porosity‐ and saturation‐dependent thermal conductivities is necessary to investigate heat and water transfer in natural porous media such as rocks and soils. Thermal conductivity in a porous medium is affected by the complicated relationship between the topology and geometry of the pore space and the solid matrix. However, as water content increases from completely dry to fully saturated, the effect of the liquid phase on thermal conductivity may increase substantially. Although various methods have been proposed to model the porosity and saturation dependence of thermal conductivity, most are empirical or quasi‐physical. In this study, we present a theoretical upscaling framework from percolation theory and the effective‐medium approximation, which is called percolation‐based effective‐medium approximation (P‐EMA). The proposed model predicts the thermal conductivity in porous media from endmember properties (e.g., air, solid matrix, and saturating fluid thermal conductivities), a scaling exponent, and a percolation threshold. In order to evaluate our porosity‐ and saturation‐dependent models, we compare our theory with 193 porosity‐dependent thermal conductivity measurements and 25 saturation‐dependent thermal conductivity datasets and find excellent match. We also find values for the scaling exponent different than the universal value of 2, in insulator‐conductor systems, and also different from 0.76, the exponent in conductor‐superconductor mixtures, in three dimensions. These results indicate that the thermal conductivity under fully and partially saturated conditions conforms to nonuniversal behavior. This means the value of the scaling exponent changes from medium to medium and depends not only on structural and geometrical properties of the medium but also characteristics (e.g., wetting or nonwetting) of the saturating fluid. This article is protected by copyright. All rights reserved.
- Wet channel network extraction by integrating LiDAR intensity and
- Authors: Milad Hooshyar; Seoyoung Kim, Dingbao Wang, Stephen C. Medeiros
Abstract: The temporal dynamics of stream networks are vitally important for understanding hydrologic processes including surface water and groundwater interaction and hydrograph recession. However, observations of wet channel networks are limited, especially in headwater catchments. Near infrared LiDAR data provide an opportunity to map wet channel networks owing to the fine spatial resolution and strong absorption of light energy by water surfaces. A systematic method is developed to map wet channel networks by integrating elevation and signal intensity of ground returns. The signal intensity thresholds for identifying wet pixels are extracted from frequency distributions of intensity return within the convergent topography extent using a Gaussian mixture model. Moreover, the concept of edge in digital image processing, defined based on the intensity gradient, is utilized to enhance detection of small wet channels. The developed method is applied to the Lake Tahoe area based on eight LiDAR snapshots during recession periods in five watersheds. A power‐law relationship between streamflow and wetted channel length during recession periods is derived, and the scaling exponent (L ∝ Q0.38) is within the range of reported values from fieldwork in other regions. This article is protected by copyright. All rights reserved.
- Spatiotemporal evolution of water storage changes in India from the
updated GRACE‐derived gravity records
- Authors: Dileep K. Panda; John Wahr
Abstract: Investigating changes in terrestrial water storage (TWS) is important for understanding response of the hydrological cycle to recent climate variability worldwide. This is particularly critical in India where the current economic development and food security greatly depend on its water resources. We use 129 monthly gravity solutions from NASA's Gravity Recovery and Climate Experiment (GRACE) satellites for the period of January 2003 to May 2014 to characterize spatiotemporal variations of TWS and groundwater storage (GWS). The spatiotemporal evolution of GRACE data reflects consistent patterns with that of several hydroclimatic variables, and also shows that most of the water loss has occurred in the northern parts of India. Substantial GWS depletion at the rate of 1.25 and 2.1 cm yr−1 has taken place, respectively in the Ganges Basin and Punjab state, which are known as the India's grain bowl. Of particular concern is the Ganges Basin's storage loss in drought years, primarily due to anthropogenic groundwater withdrawals that sustain rice and wheat cultivation. We estimate these losses to be approximately 41, 44 and 42 km3 in 2004, 2009 and 2012, respectively. The GWS depletions that constitute about 90% of the observed TWS loss are also influenced by a marked rise in temperatures since 2008. A high degree of correspondence between GRACE‐derived GWS and in‐situ groundwater levels from observation well validates the results. This validation increases confidence level in the application of GRACE observations in monitoring large‐scale storage changes in intensely irrigated areas in India and other regions around the world. This article is protected by copyright. All rights reserved.
- Multiobjective adaptive surrogate modeling‐based optimization for
parameter estimation of large, complex geophysical models
- Authors: Wei Gong; Qingyun Duan, Jianduo Li, Chen Wang, Zhenhua Di, Aizhong Ye, Chiyuan Miao, Yongjiu Dai
Abstract: Parameter specification is an important source of uncertainty in large, complex geophysical models. These models generally have multiple model outputs that require multi‐objective optimization algorithms. Although such algorithms have long been available, they usually require a large number of model runs and are therefore computationally expensive for large, complex dynamic models. In this paper, a multi‐objective adaptive surrogate modeling‐based optimization (MO‐ASMO) algorithm is introduced that aims to reduce computational cost while maintaining optimization effectiveness. Geophysical dynamic models usually have a prior parameterization scheme derived from the physical processes involved, and our goal is to improve all of the objectives by parameter calibration. In this study, we developed a method for directing the search processes towards the region that can improve all of the objectives simultaneously. We tested the MO‐ASMO algorithm against NSGA‐II and SUMO with 13 test functions and a land surface model ‐ the Common Land Model (CoLM). The results demonstrated the effectiveness and efficiency of MO‐ASMO. This article is protected by copyright. All rights reserved.
- Application of acoustic tomography to reconstruct the horizontal flow
velocity field in a shallow river
- Authors: Mahdi Razaz; Kiyosi Kawanisi, Arata Kaneko, Ioan Nistor
Abstract: A novel acoustic tomographic measurement system capable of resolving sound travel time in extremely shallow rivers is introduced and the results of an extensive field measurements campaign are presented and further discussed. Acoustic pulses were transmitted over a wide frequency band of 20–35 kHz between eight transducers for about a week in a meandering reach of theBāsen River, Hiroshima, Japan. The purpose of the field experiment was validating the concept of acoustic tomography in rivers for visualizing current fields. The particular novelty of the experiment resides in its unusual tomographic features: subbasin scale (100 m × 270 m) and shallowness (0.5–3.0 m) of the physical domain, frequency of the transmitted acoustic signals (central frequency of 30 kHz), and the use of small sampling intervals (105 s). Inverse techniques with no a priori statistical information were used to estimate the depth‐average current velocity components from differential travel times. Zeroth‐order Tikhonov regularization, in conjunction with L‐curve method deployed to stabilize the solution and to determine the weighting factor appearing in the inverse analysis. Concurrent direct environmental measurements were provided in the form of ADCP readings close to the right and left bank. Very good agreement found between along‐channel velocities larger than 0.2 m/s obtained from the two techniques. Inverted quantities were, however, underestimated, perhaps due to vicinity of the ADCPs to the banks and strong effect of river geometry on the readings. In general, comparing the visualized currents with direct nodal measurements illustrate the plausibility of the tomographically reconstructed flow structures.
- Improved error estimates of a discharge algorithm for remotely sensed
river measurements: Test cases on Sacramento and Garonne Rivers
- Abstract: We present an improvement to a previously‐presented algorithm that used a Bayesian Markov Chain Monte Carlo method for estimating river discharge from remotely sensed observations of river height, width, and slope. We also present an error budget for discharge calculations from the algorithm. The algorithm may be utilized by the upcoming Surface Water and Ocean Topography (SWOT) mission. We present a detailed evaluation of the method using synthetic SWOT‐like observations (i.e., SWOT and AirSWOT, an airborne version of SWOT). The algorithm is evaluated using simulated AirSWOT observations over the Sacramento and Garonne Rivers that have differing hydraulic characteristics. The algorithm is also explored using SWOT observations over the Sacramento River. SWOT and AirSWOT height, width, and slope observations are simulated by corrupting the “true” hydraulic modeling results with instrument error. Algorithm discharge root mean square error (RMSE) was 9% for the Sacramento River and 15% for the Garonne River for the AirSWOT case using expected observation error. The discharge uncertainty calculated from Manning's equation was 16.2% and 17.1%, respectively. For the SWOT scenario, the RMSE and uncertainty of the discharge estimate for the Sacramento River were 15% and 16.2%, respectively. A method based on the Kalman filter to correct errors of discharge estimates was shown to improve algorithm performance. From the error budget, the primary source of uncertainty was the a priori uncertainty of bathymetry and roughness parameters. Sensitivity to measurement errors was found to be a function of river characteristics. For example, Steeper Garonne River is less sensitive to slope errors than the flatter Sacramento River. This article is protected by copyright. All rights reserved.
- The algal lift—Buoyancy‐mediated sediment transport
- Abstract: The role of benthic algae as biostabilizers of sediments is well‐known, however, their potential to lift and transport sediments remains unclear. Under low‐flow conditions, matured algal mats may detach from the bed and may lift up sediment, thereby causing disturbance to the uppermost sediment. We tested the potential of algal mats to lift sediments in 12 indoor flumes filled with sand (0.2 – 0.8 mm), gravel (2 – 8 mm) or a sand‐gravel mixture (25/75% mass). After four weeks, the algal mats covered about 50% of the flumes area. Due to the accumulation of oxygen gas bubbles in the mats that developed from high primary production at 4.5 weeks, about half of the algal mats detached from the bed carrying entangled sediments. Both the area covered by algal mats and detached area were similar among sediment types, but the amount of sediment transported tended to be higher for sand and sand‐gravel mixture compared to gravel. Our results reveal that biologically mediated sediment transport mainly depends on the development of a dense filamentous algal matrix that traps gas bubbles, increasing the mats buoyancy. This novel mechanism of sediment transport will occur in shallow ecosystems during low‐flow periods, with the highest impact for sandy sediments. This article is protected by copyright. All rights reserved.
- Role of spatial anisotropy in design storm generation—Experiment and
- Authors: Tero J. Niemi; Joseph H. A. Guillaume, Teemu Kokkonen, Tam M. T. Hoang, Alan W. Seed
Abstract: Rainfall accumulation depths over a given area are strongly dependent on the shape of the storm together with its direction of advection. A method to produce design storms exhibiting anisotropic spatial scaling is presented by combining a state‐of‐the‐art stochastic rainfall generator STEPS with the linear generalized scale invariance (GSI) notation. The enhanced model is used to create ensembles of design storms based on an extreme storm with a distinct rainband shape observed in Melbourne, Australia. Design storms are generated both with and without accounting for anisotropy. Effect of anisotropy on precipitation characteristics is studied using the entire region covered by the radar (radar scale) and at a significantly smaller catchment scale. A rainfall‐runoff model is applied to route the rainfall through the catchment into streamflow. Accounting for anisotropy allows for a more realistic description of precipitation features at the radar scale. At the catchment scale, anisotropy increases the probability of high accumulated rainfall depths, which translates into greater flood volumes. No discernible difference was observed in streamflow characteristics after controlling for the accumulation over the catchment. This could be explained by a lower importance of anisotropy relative to other factors affecting streamflow generation, and by the difficulties in creating representative rainfall temporal properties at the catchment scale when the radar scale is used for model calibration. The proposed method provides a tool to create ensembles of design storms when the anisotropic shape of the fields is of importance. This article is protected by copyright. All rights reserved.
- Water sources and mixing in riparian wetlands revealed by tracers and
- Authors: Jason S. Lessels; Doerthe Tetzlaff, Christian Birkel, Jonathan Dick, Chris Soulsby
Abstract: Mixing of waters within riparian zones has been identified as an important influence on runoff generation and water quality. Improved understanding of the controls on the spatial and temporal variability of water sources and how they mix in riparian zones is therefore of both fundamental and applied interest. In this study, we have combined topographic indices derived from a high resolution Digital Elevation Model (DEM) with repeated spatially high resolution synoptic sampling of multiple tracers to investigate such dynamics of source water mixing. We use geostatistics to estimate concentrations of three different tracers (Deuterium, alkalinity and Dissolved Organic Carbon) across an extended riparian zone in a headwater catchment in NE Scotland, to identify spatial and temporal influences on mixing of source waters. The various biogeochemical tracers and stable isotopes helped constrain the sources of runoff and their temporal dynamics. Results show that spatial variability in all three tracers was evident in all sampling campaigns, but more pronounced in warmer dryer periods. The extent of mixing areas within the riparian area reflected strong hydro‐climatic controls and showed large degrees of expansion and contraction and was not strongly related to topographic indices. The integrated approach of using multiple tracers, geospatial statistics and topographic analysis allowed us to classify three main riparian source areas and mixing zones. This study underlines the importance of the riparian zones for mixing soil water and groundwater and introduces a novel approach how this mixing can be quantified and the effect on the downstream chemistry be assessed. This article is protected by copyright. All rights reserved.
- Methane occurrence is associated with sodium‐rich valley waters in
domestic wells overlying the Marcellus Shale in New York State
- Authors: Kayla M. Christian; Laura K. Lautz, Gregory D. Hoke, Donald I. Siegel, Zunli Lu, John Kessler
Abstract: Prior work suggests spatial parameters (e.g. landscape position, distance to nearest gas well) can be used to estimate the amount of dissolved methane in domestic drinking water wells overlying the deep Marcellus Shale. New York (NY) provides an opportunity to investigate methane occurrence prior to expansion of high volume hydraulic fracturing because unconventional gas production is currently banned in the state. We sampled domestic groundwater wells for methane in 2013 (n=137) across five counties of NY bordering Pennsylvania, and then resampled a subset of those wells in 2014 for methane concentrations and δ13C‐CH4 and δD‐CH4. The majority of waters from wells sampled (77%) had low concentrations of methane (10 mg/L). Dissolved methane concentrations did not change as a function of proximity to existing vertical gas wells, nor other parameters indicating subsurface planes of weakness (i.e. faults or lineaments). Methane levels were significantly higher in wells closer to hydrography flow lines, and most strongly correlated to Na‐HCO3 water type. The distribution of methane between Ca‐HCO3 (n=76) and Na‐HCO3 (n=23) water types significantly differed (p
- Evaluation of theoretical and empirical water vapor sorption isotherm
models for soils
- Authors: Emmanuel Arthur; Markus Tuller, Per Moldrup, Lis W. de Jonge
Abstract: The mathematical characterization of water vapor sorption isotherms of soils is crucial for modeling processes such as volatilization of pesticides and diffusive and convective water vapor transport. Although numerous physically‐based and empirical models were previously proposed to describe sorption isotherms of building materials, food, and other industrial products, knowledge about the applicability of these functions for soils is noticeably lacking. We present an evaluation of nine models for characterizing adsorption/desorption isotherms for a water activity range from 0.03 to 0.93 based on measured data of 207 soils with widely varying textures, organic carbon contents, and clay mineralogy. In addition, the potential applicability of the models for prediction of sorption isotherms from known clay content was investigated. While in general all investigated models described measured adsorption and desorption isotherms reasonably well, distinct differences were observed between physical and empirical models and due to the different degrees of freedom of the model equations. There were also considerable differences in model performance for adsorption and desorption data. While regression analysis relating model parameters and clay content and subsequent model application for prediction of measured isotherms showed promise for the majority of investigated soils, for soils with distinct kaolinitic and smectitic clay mineralogy predicted isotherms did not closely match the measurements. This article is protected by copyright. All rights reserved.
- Game theory and risk‐based leveed river system planning with
- Authors: Rui Hui; Jay R. Lund, Kaveh Madani
Abstract: Optimal risk‐based levee designs are usually developed for economic efficiency. However, in river systems containing multiple levees, the planning and maintenance of different levees are controlled by different agencies or groups. For example, along many rivers, levees on opposite riverbanks constitute a simple leveed river system with each levee designed and controlled separately. Collaborative planning of the two levees can be economically‐optimal for the whole system. Independent and self‐interested landholders on opposite riversides often are willing to separately determine their individual optimal levee planning, resulting in a less efficient leveed river system from an overall society‐wide perspective (the tragedy of commons). We apply game theory to simple leveed river system planning where landholders on each riverside independently determine their optimal risk‐based levee planning. Outcomes from non‐cooperative games are analyzed and compared with the overall economically‐optimal outcome, which minimizes net flood cost system‐wide. The system‐wide economically‐optimal solution generally transfers residual flood risk to the lower‐valued side of the river, but is often impractical without compensating for flood risk transfer to improve outcomes for all individuals involved. Such compensation can be determined and implemented with landholders' agreements on collaboration to develop the economically‐optimal planning. By examining iterative multiple‐shot non‐cooperative games with reversible and irreversible decisions, the costs of myopia for the future in making levee planning decisions show the significance of considering the externalities and evolution path of dynamic water resource problems to improve decision‐making. This article is protected by copyright. All rights reserved.
- Peak flow transition from snowmelt to rainfall dominance along the
Colorado Front Range: Historical patterns, trends, and lessons from the
2013 Colorado Front Range floods
- Authors: Stephanie K. Kampf; Michael A. Lefsky
Abstract: The Colorado Front Range has a large elevation gradient with deep seasonal snowpack in the mountains and limited snow accumulation in the foothills and plains. This study examines how water sources for annual peak flows (snowmelt, rainfall, mixed) change with the fraction of time snow persists on the ground, snow persistence (SP), and whether these sources have changed over time. Sources of peak flows for 20 gaging stations are estimated using a gridded rain and snow model forced with PRISM daily precipitation and both PRISM and TopoWx temperature. The mean snowmelt contribution to peak flow is highly correlated with SP (r2=0.86‐0.90). Watersheds with SP3100 m) are mostly snowmelt‐dominated, with mixed sources between these thresholds. Rainfall runoff peak flows are possible at all elevations, but their likelihood declines with increasing SP. Rainfall runoff from an extreme storm in September 2013 produced the highest annual peaks at many stations, including some snowmelt‐dominated watersheds. Regional Kendall trend tests indicate that the contributions of snowmelt to peak flows and total annual inputs have declined in the mixed source zone. These changes may affect hydrographs, as analyses confirm that snowmelt runoff generally produces more attenuated peaks than rainfall runoff. Discrimination of peak flow source is sensitive to input data and model structure for mixed rain and snowmelt events, and both observation and modeling research are needed to help understand potential runoff changes in these conditions. This article is protected by copyright. All rights reserved.
- Dimension reduction of decision variables for multireservoir operation: A
spectral optimization model
- Authors: Duan Chen; Arturo S. Leon, Nathan L. Gibson, Parnian Hosseini
Abstract: Optimizing the operation of a multi‐reservoir system is challenging due to the high dimension of the decision variables that lead to a large and complex search space. A spectral optimization model (SOM), which transforms the decision variables from time‐domain to frequency‐domain, is proposed to reduce the dimensionality. The SOM couples a spectral dimensionality‐reduction method called Karhunen‐Loeve (KL) expansion within the routine of Non‐dominated Sorting Genetic Algorithm (NSGA‐II). The KL expansion is used to represent the decision variables as a series of terms that are deterministic orthogonal functions with undetermined coefficients. The KL expansion can be truncated into fewer significant terms, and consequently, fewer coefficients by a predetermined number. During optimization, operators of the NSGA‐II (e.g., crossover) are conducted only on the coefficients of the KL expansion rather than the large number of decision variables, significantly reducing the search space. The SOM is applied to the short‐term operation of a ten‐reservoir system in the Columbia River of the United States. Two scenarios are considered herein, the first with 140 decision variables and the second with 3360 decision variables. The hypervolume index is used to evaluate the optimization performance in terms of convergence and diversity. The evaluation of optimization performance is conducted for both conventional optimization model (i.e., NSGA‐II without KL) and the SOM with different number of KL terms. The results show that the number of decision variables can be greatly reduced in the SOM to achieve a similar or better performance compared to the conventional optimization model. For the scenario with 140 decision variables, the optimal performance of the SOM model is found with 6 KL terms. For the scenario with 3360 decision variables, the optimal performance of the SOM model is obtained with 11 KL terms. This article is protected by copyright. All rights reserved.
- A new framework for comprehensive, robust, and efficient global
sensitivity analysis: 1. Theory
- Authors: Saman Razavi; Hoshin V. Gupta
Abstract: Computer simulation models are continually growing in complexity with increasingly more factors to be identified. Sensitivity Analysis (SA) provides an essential means for understanding the role and importance of these factors in producing model responses. However, conventional approaches to SA suffer from (1) an ambiguous characterization of sensitivity, and (2) poor computational efficiency, particularly as the problem dimension grows. Here, we present a new and general sensitivity analysis framework (called VARS), based on an analogy to ‘variogram analysis', that provides an intuitive and comprehensive characterization of sensitivity across the full spectrum of scales in the factor space. We prove, theoretically, that Morris (derivative‐based) and Sobol (variance‐based) methods and their extensions are special cases of VARS, and that their SA indices can be computed as by‐products of the VARS framework. Synthetic functions that resemble actual model response surfaces are used to illustrate the concepts, and show VARS to be as much as two orders of magnitude more computationally efficient than the state‐of‐the‐art Sobol approach. In a companion paper (Part II), we propose a practical implementation strategy, and demonstrate the effectiveness, efficiency and reliability (robustness) of the VARS approach on real‐data case studies. This article is protected by copyright. All rights reserved.
- A new framework for comprehensive, robust, and efficient global
sensitivity analysis: 2. Application
- Authors: Saman Razavi; Hoshin V. Gupta
Abstract: Based on the theoretical framework for sensitivity analysis called “Variogram Analysis of Response Surfaces” (VARS), developed in Part I, we develop and implement a practical “star‐based” sampling strategy (called STAR‐VARS), for the application of VARS to real‐world problems. We further develop a bootstrap approach to provide confidence level estimates for the VARS sensitivity measures and to evaluate the reliability of inferred factor rankings. The effectiveness, efficiency, and robustness of the new approach are demonstrated via two real‐data hydrological case studies (a 5‐parameter conceptual rainfall‐runoff model and a 45‐parameter land surface scheme hydrology model), and a comparison with the “derivative‐based” Morris and “variance‐based” Sobol approaches are provided. Our results show that STAR‐VARS provides reliable and stable assessments of “global” sensitivity across the full range of scales in the factor space, while being 1‐2 orders of magnitude more efficient than Morris or Sobol. This article is protected by copyright. All rights reserved.
- Comment on “Storage selection functions: A coherent framework for
quantifying how catchments store and release water and solutes” by
Rinaldo et al.
- Authors: Amilcare Porporato; Salvatore Calabrese
Abstract: We discuss the recent commentary by Rinaldo et al, showing how the age dependent loss functions are related to the M'Kendrick‐von Foerster equation and to the theory of Instantaneous unit hydrograph. Complications arising from the presence of nonlinearities are also discussed. This article is protected by copyright. All rights reserved.
- Reply to comment by Rinaldo et al. on “Storage selection functions:
A coherent framework for quantifying how catchments store and release
water and solutes”
- Authors: Andrea Rinaldo; Paolo Benettin, Ciaran Harman, Markus Hrachowitz, Kevin McGuire, Ype Van der Velde, Enrico Bertuzzo, Gianluca Botter
Abstract: The reply discusses why the comments by Porporato and Calabrese refer to a mathematical formulation unsuitable to jointly address transient flow and transport at catchment scales and elaborates further on the usefulness of storage selection functions. This article is protected by copyright. All rights reserved.
- Extraction of cross sections from digital elevation model for one
dimensional dam‐break wave propagation in mountain valleys
- Authors: Marco Pilotti
Abstract: Shallow Water Equations (SWE) provide a fundamental component for the quantification and mapping of hydraulic hazard. In steep mountain valleys the use of one‐dimensional SWE (also known as St. Venant Equations, SVE) is often legitimate and computationally competitive against two‐dimensional solvers. However, in the same environment the solution of SVE is hindered by the need of an accurate bathymetric reconstruction, that implies a number of cross sections which cannot be readily acquired by conventional field surveys. On the other hand, Digital Elevation Models (DEM) with resolution adequate for studies of flood propagation are available in many areas of the world. In this paper I propose to compute cross sections automatically by operating along the channel network derived from a valley's raster DEM, on the basis of algorithms that hitherto have been used for geomorphological and hydrological purposes. The extraction process can be refined by varying cross section inter‐distance and width, in order to prevent superimpositions that might occur due to the sinuosity of the thalweg and to better capture the valley's local topography. At the end of this process, the geometric functions needed by SVE solvers can be computed for each cross‐section. A software tool that implements the described algorithm is provided to the scientific community. This article is protected by copyright. All rights reserved.
- Identifying long term empirical relationships between storm
characteristics and episodic groundwater recharge
- Authors: Arik M. Tashie; Benjamin B. Mirus, Tamlin M. Pavelsky
Abstract: Shallow aquifers are an important source of water resources and provide baseflow to streams, yet actual rates of groundwater recharge are difficult to estimate. While climate change is predicted to increase the frequency and magnitude of extreme precipitation events, the resulting impact on groundwater recharge remains poorly understood. We quantify empirical relations between precipitation characteristics and episodic groundwater recharge for a wide variety of geographic and land‐use types across North Carolina. We extract storm duration, magnitude, average rate, and hourly weighted intensity from long‐term precipitation records over periods of 12 to 35 years at 10 locations. Using time‐series of water‐table fluctuations from nearby monitoring wells, we estimate relative recharge to precipitation ratios (RPR) to identify statistical trends. Increased RPR correlates with increased storm duration, whereas RPR decreases with increasing magnitude, average rate, and intensity of precipitation. Agricultural and urban areas exhibit the greatest decrease in RPR due to increasing storm magnitude, average rate, and intensity, while naturally vegetated areas exhibit a larger increase in RPR with increased storm duration. Though RPR is generally higher during the winter than the summer, this seasonal effect is magnified in the Appalachian and Piedmont regions. These statistical trends provide valuable insights into the likely consequences of climate and land‐use change for water resources in subtropical climates. If, as predicted, growing seasons lengthen and the intensity of storms increases with a warming climate, decreased recharge in Appalachia, the Piedmont, and rapidly growing urban areas of the American Southeast could further limit groundwater availability. This article is protected by copyright. All rights reserved.
- Dry season streamflow persistence in seasonal climates
- Authors: David N. Dralle; Nathaniel J. Karst, Sally E. Thompson
Abstract: Seasonally dry ecosystems exhibit periods of high water availability followed by extended intervals during which rainfall is negligible and streamflows decline. Eventually, such declining flows will fall below the minimum values required to support ecosystem functions or services. The time at which dry season flows drop below these minimum values (Q*), relative to the start of the dry season, is termed the ‘persistence time' (TQ*). The persistence time determines how long seasonal streams can support various human or ecological functions during the dry season. In this study we extended recent work in the stochastic hydrology of seasonally dry climates to develop an analytical model for the probability distribution function (PDF) of the persistence time. The proposed model accurately captures the mean of the persistence time distribution, but underestimates its variance. We demonstrate that this underestimation arises in part due to correlation between the parameters used to describe the dry season recession, but that this correlation can be removed by re‐scaling the flow variables. The mean persistence time predictions form one example of the broader class of streamflow statistics known as crossing properties, which could feasibly be combined with simple ecological models to form a basis for rapid risk assessment under different climate or management scenarios. This article is protected by copyright. All rights reserved.
- A generalized complementary relationship between actual and potential
evaporation defined by a reference surface temperature
- Authors: Milad Aminzadeh; Michael L. Roderick, Dani Or
Abstract: The definition of potential evaporation remains widely debated despite its centrality for hydrologic and climatic models. We employed an analytical pore‐scale representation of evaporation from terrestrial surfaces to define potential evaporation using a hypothetical steady‐state reference temperature that is common to both air and evaporating surface. The feedback between drying land surfaces and overlaying air properties, central in the Bouchet  complementary relationship, is implicitly incorporated in the hypothetical steady‐state where the sensible heat flux vanishes and the available energy is consumed by evaporation. Evaporation rates predicted based on the steady‐state reference temperature hypothesis were in good agreement with class A pan evaporation measurements suggesting that evaporation from pans occurs with negligible sensible heat flux. The model facilitates a new generalization of the asymmetric complementary relationship with the asymmetry parameter b analytically predicted for a wide range of meteorological conditions with initial tests yielding good agreement between measured and predicted actual evaporation. This article is protected by copyright. All rights reserved.
- Hydrologic implications of GRACE satellite data in the Colorado River
- Authors: Bridget R. Scanlon; Zizhan Zhang, Robert C. Reedy, Donald R. Pool, Himanshu Save, Di Long, Jianli Chen, David M. Wolock, Brian D. Conway, Daniel Winester
Abstract: Use of GRACE (Gravity Recovery and Climate Experiment) satellites for assessing global water resources is rapidly expanding. Here we advance application of GRACE satellites by reconstructing long‐term total water storage (TWS) changes from ground‐based monitoring and modeling data. We applied the approach to the Colorado River Basin which has experienced multiyear intense droughts at decadal intervals. Estimated TWS declined by 94 km3 during 1986–1990 and by 102 km3 during 1998–2004, similar to the TWS depletion recorded by GRACE (47 km3) during 2010–2013. Our analysis indicates that TWS depletion is dominated by reductions in surface reservoir and soil moisture storage in the upper Colorado basin with additional reductions in groundwater storage in the lower basin. Groundwater storage changes are controlled mostly by natural responses to wet and dry cycles and irrigation pumping outside of Colorado River delivery zones based on ground‐based water level and gravity data. Water storage changes are controlled primarily by variable water inputs in response to wet and dry cycles rather than increasing water use. Surface reservoir storage buffers supply variability with current reservoir storage representing ∼2.5 years of available water use. This study can be used as a template showing how to extend short‐term GRACE TWS records and using all available data on storage components of TWS to interpret GRACE data, especially within the context of droughts. This article is protected by copyright. All rights reserved.
- Representing low‐frequency variability in continuous rainfall
simulations: A Hierarchical Random Bartlett Lewis continuous rainfall
- Authors: Conrad Wasko; Alexander Pui, Ashish Sharma, Rajeshwar Mehrotra, Erwin Jeremiah
Abstract: Low frequency variability, in the form of the El Niño‐Southern Oscillation, plays a key role in shaping local weather systems. However, current continuous stochastic rainfall models do not account for this variability in their simulations. Here, a modified Random Pulse Bartlett Lewis stochastic generation model is presented for continuous rainfall simulation exhibiting low frequency variability. Termed the Hierarchical Random Bartlett Lewis Model (HRBLM), the model features a hierarchical structure to represent a range of rainfall characteristics associated with the El Niño‐Southern Oscillation with parameters conditioned to vary as functions of relevant climatic states. Long observational records of near‐continuous rainfall at various locations in Australia are used to formulate and evaluate the model. The results indicate clear benefits of using the hierarchical climate‐dependent structure proposed. In addition to accurately representing the wet spells characteristics and observed low‐frequency variability, the model replicates the interannual variability of the antecedent rainfall preceding the extremes, which is known to be of considerable importance in design flood estimation applications. This article is protected by copyright. All rights reserved.
- On the variability of the Priestley‐Taylor coefficient over water
- Abstract: Deviations in the Priestley‐Taylor (PT) coefficient αPT from its accepted 1.26 value are analyzed over large lakes, reservoirs, and wetlands where stomatal or soil controls are minimal or absent. The datasets feature wide variations in water body sizes and climatic conditions. Neither surface temperature nor sensible heat flux variations alone, which proved successful in characterizing αPT variations over some crops, explain measured deviations in αPT over water. It is shown that the relative transport efficiency of turbulent heat and water vapor is key to explaining variations in αPT over water surfaces, thereby offering a new perspective over the concept of minimal advection or entrainment introduced by PT. Methods that allow the determination of αPT based on low frequency sampling (i.e. 0.1 Hz) are then developed and tested, which are usable with standard meteorological sensors that filter some but not all turbulent fluctuations. Using approximations to the Gram determinant inequality, the relative transport efficiency is derived as a function of the correlation coefficient between temperature and water vapor concentration fluctuations (RTq). The proposed approach reasonably explains the measured deviations from the conventional αPT =1.26 value even when RTq is determined from air temperature and water vapor concentration time series that are Gaussian‐filtered and sub‐sampled to a cutoff frequency of 0.1 Hz. Because over water bodies, RTq deviations from unity are often associated with advection and/or entrainment, linkages between αPT and RTq offer both a diagnostic approach to assess their significance and a prognostic approach to correct the 1.26 value when using routine meteorological measurements of temperature and humidity. This article is protected by copyright. All rights reserved.
- Dynamic, structured heterogeneity of water isotopes inside hillslopes
- Authors: Jasper Oshun; William E. Dietrich, Todd E. Dawson, Inez Fung
Abstract: Use of the stable isotopes of water (δD, δ18O) to determine vegetative water sources, runoff paths, and residence times generally assumes that, other than shallow evaporative enrichment, the isotopic composition of precipitation is conserved as it travels through the subsurface to the stream channel. Here we follow rainfall through a thick (up to 25m) vadose zone of soil, saprolite, and weathered bedrock mostly composed of argillite, and underlying a steep (32°) forested hillslope. We discover a persistent structured heterogeneity of water isotopes inside the hillslope. Summer dry season causes evaporative enrichment of the soil, but not in the saprolite and weathered bedrock. In winter, the mobile water, generated by successive rainstorms with widely varying isotopic composition, mixes in the vadose zone, elevating soil and rock moisture content, and eventually recharging the groundwater with isotopically invariant water similar to the seasonally averaged rainfall. Yet, throughout the winter the less mobile winter soil and rock moisture remains relatively light, and water extracted from the interior of argillite lies well to the left of the local meteoric water line. This persistently light composition of soil and rock moisture and the deviation from average meteoric values suggest that subsurface fractionation, or the inheritance of paleo‐meteoric rock moisture associated with rock uplift may lead to large enduring isotopic differences between high and low mobility water. These differences suggest that the use of water isotopes as tracers must consider the possibility of subsurface isotopic evolution and the influence of exchange with more tightly held water. This article is protected by copyright. All rights reserved.
- A meteorological and snow observational data set from Snoqualmie Pass (921
m), Washington Cascades, USA
- Authors: Nicholas E. Wayand; Adam Massmann, Colin Butler, Eric Keenan, John Stimberis, Jessica D. Lundquist
Abstract: We introduce a quality controlled observational atmospheric, snow, and soil data set from Snoqualmie Pass, Washington, U.S.A., to enable testing of hydrometeorological and snow process representations within a rain‐snow transitional climate where existing observations are sparse and limited. Continuous meteorological forcing (including air temperature, total precipitation, wind speed, specific humidity, air pressure, short‐ and longwave irradiance) are provided at hourly intervals for a 24‐year historical period (water years 1989‐2012) and at half‐hourly intervals for a more‐recent period (water years 2013‐2015), separated based on the availability of observations. The majority of missing data were filled with biased‐corrected reanalysis model values (using NLDAS). Additional observations include 40‐years of snow board new snow accumulation, multiple measurements of total snow depth, and manual snow pits, while more recent years include sub‐daily surface temperature, snowpack drainage, soil moisture and temperature profiles, and eddy co‐variance derived turbulent heat flux. This data set is ideal for testing hypotheses about energy balance, soil and snow processes in the rain‐snow transition zone. This article is protected by copyright. All rights reserved.
- The relative importance of head, flux, and prior information in hydraulic
- Abstract: Using cross‐correlation analysis, we demonstrate that flux measurements at observation locations during hydraulic tomography (HT) surveys carry non‐redundant information about heterogeneity that are complementary to head measurements at the same locations. We then hypothesize that a joint interpretation of head and flux data, even when the same observation network as head has been used, can enhance the resolution of HT estimates. Subsequently, we use numerical experiments to test this hypothesis and investigate the impact of flux conditioning and prior information (such as correlation lengths, and initial mean models (i.e. uniform mean or distributed means)) on the HT estimates of a non‐stationary, layered medium. We find that the addition of flux conditioning to HT analysis improves the estimates in all of the prior models tested. While prior information on geologic structures could be useful, its influence on the estimates reduces as more non‐redundant data (i.e., flux) are used in the HT analysis. Lastly, recommendations for conducting HT surveys and analysis are presented. This article is protected by copyright. All rights reserved.
- Steady nonuniform shallow flow within emergent vegetation
- Abstract: Surface flow redistribution on flat ground from crusted bare soil to vegetated patches following intense rainfall events elevates plant available water above that provided by rainfall. The significance of this surface water redistribution to sustaining vegetation in arid and semi‐arid regions is undisputed. What is disputed is the quantity and spatial distribution of the redistributed water. In eco‐hydrological models, such non‐uniform flows are described using the Saint‐Venant equation (SVE) subject to a Manning roughness coefficient closure. To explore these assumptions in the most idealized setting, flume experiments were conducted using rigid cylinders representing rigid vegetation with varying density. Flow was induced along the streamwise x direction by adjusting the free water surface height H(x) between the upstream and downstream boundaries mimicking the non‐uniformity encountered in nature. In natural settings, such H(x) variations arise due to contrasts in infiltration capacity and ponded depths during storms. The measured H(x) values in the flume were interpreted using the SVE augmented with progressively elaborate approximations to the roughness representation. The simplest approximation employs a friction factor derived from a drag coefficient (Cd) for isolated cylinders in a locally (but not globally) uniform flow and upscaled using the rod density that was varied across experiments. Comparison between measured and modeled H(x) suggested that such a ‘naïve' approach over‐predicts H(x). Blockage was then incorporated into the SVE model calculations but resulted in underestimation of H(x). Biases in modeled H(x) suggest that Cd must be varying in x beyond what a local or bulk Reynolds number predicts. Inferred Cd(x) from the flume experiments exhibited a near‐parabolic shape most peaked in the densest canopy cases. The outcome of such Cd(x) variations are then summarized in a bulk resistance formulation that may be beneficial to modeling runon‐runoff processes on shallow slopes using SVE. This article is protected by copyright. All rights reserved.
- Recent tree die‐off has little effect on streamflow in contrast to
expected increases from historical studies
- Authors: Joel A. Biederman; Andrew J. Somor, Adrian A. Harpold, Ethan D. Gutmann, David D. Breshears, Peter A. Troch, David J. Gochis, Russell L. Scott, Arjan J.H. Meddens, Paul D. Brooks
Abstract: Recent bark beetle epidemics have caused regional‐scale tree mortality in many snowmelt‐dominated headwater catchments of western North America. Initial expectations of increased streamflow have not been supported by observations, and the basin‐scale response of annual streamflow is largely unknown. Here we quantified annual streamflow responses during the decade following tree die‐off in eight infested catchments in the Colorado River headwaters and one nearby control catchment. We employed three alternative empirical methods: (i) double‐mass comparison between impacted and control catchments, (ii) runoff ratio comparison before and after die‐off, and (iii) time‐trend analysis using climate‐driven linear models. In contrast to streamflow increases predicted by historical paired catchment studies and recent modeling, we did not detect streamflow changes in most basins following die‐off, while one basin consistently showed decreased streamflow. The three analysis methods produced generally consistent results, with time‐trend analysis showing precipitation was the strongest predictor of streamflow variability (R2 = 74–96%). Time‐trend analysis revealed post‐die‐off streamflow decreased in three catchments by 11–29%, with no change in the other five catchments. Although counter to initial expectations, these results are consistent with increased transpiration by surviving vegetation and the growing body of literature documenting increased snow sublimation and evaporation from the subcanopy following die‐off in water‐limited, snow‐dominated forests. The observations presented here challenge the widespread expectation that streamflow will increase following beetle‐induced forest die‐off and highlight the need to better understand the processes driving hydrologic response to forest disturbance. This article is protected by copyright. All rights reserved.
- Large wood transport and jam formation in a series of flume experiments
- Authors: S. L. Davidson; L.G. MacKenzie, B.C. Eaton
Abstract: Large wood has historically been removed from streams, resulting in the depletion of in‐stream wood in waterways worldwide. As wood increases morphological and hydraulic complexity, the addition of large wood is commonly employed as a means to rehabilitate in‐stream habitat. At present, however, the scientific understanding of wood mobilization and transport is incomplete. This paper presents results from a series of four flume experiments in which wood was added to a reach to investigate the piece and reach characteristics that determine wood stability and transport, as well as the time scale required for newly recruited wood to self‐organize into stable jams. Our results show that wood transitions from a randomly distributed newly recruited state, to a self‐organized, or jam‐stabilized state, over the course of a single bankfull flow event. Statistical analyses of piece mobility during this transitional period indicate that piece irregularities, especially rootwads, dictate the stability of individual wood pieces; rootwad presence or absence accounts for up to 80% of the variance explained by linear regression models for transport distance. Furthermore, small pieces containing rootwads are especially stable. Large ramped pieces provide nuclei for the formation of persistent wood jams, and the frequency of these pieces in the reach impacts the travel distance of mobile wood. This research shows that the simulation of realistic wood dynamics is possible using a simplified physical model, and also has management implications, as it suggests that randomly added wood may organize into persistent, stable jams, and characterizes the time scale for this transition. This article is protected by copyright. All rights reserved.
- Impact of switching crop type on water and solute fluxes in deep vadose
- Authors: T. Turkeltaub; D. Kurtzman, E. E. Russak, O. Dahan
Abstract: Switching crop type and consequently changing irrigation and fertilization regimes leads to alterations in deep percolation and solute concentrations of pore water. Herein, observations from the deep vadose zone and model simulations demonstrate the changes in water, chloride and nitrate fluxes under a commercial greenhouse following the change from tomato to lettuce cropping. The site, located above a phreatic aquifer, was monitored for 5 years. A vadose‐zone monitoring system was implemented under the greenhouse and provided continuous data on both temporal variations in water content and chemical composition of the pore water at multiple depths in the deep vadose zone (up to 20 m). Following crop switching, a significant reduction in chloride concentration and dramatic increase in nitrate were observed across the unsaturated zone. The changes in chemical composition of the vadose zone pore water appeared as sequential breakthroughs across the unsaturated zone, initiating at land surface and propagating down toward the water table. Today, 3 years after switching the crops, penetration of the impact exceeds 10 m depth. Variations in the isotopic composition of nitrate (18O and 15N) in water samples obtained from the entire vadose zone clearly support a fast leaching process and mobilization of solutes across the unsaturated zone following the change in crop type. Water flow and chloride transport models were calibrated to observations acquired during an enhanced infiltration experiment. Forward simulation runs were performed with the calibrated models, constrained to tomato and lettuce cultivation regimes as surface boundary conditions. Predicted chloride and nitrate concentrations were in agreement with the observed concentrations. The simulated water drainage and nitrogen leaching implied that the observed changes are an outcome of recommended agricultural management practices. This article is protected by copyright. All rights reserved.
- Practical notes on local data‐worth analysis
- Authors: Stefan Finsterle
Abstract: These notes discuss the usefulness, limitations, and potential pitfalls of using sensitivity indices as a means to evaluate data worth and to guide the formulation and solution of inverse problems. A sensitivity analysis examines changes in model output variables with respect to changes in model input parameters. It appears straightforward to use this information to select influential parameters that should be subjected to estimation by inverse modeling, and to identify the observations that contain information about these parameters and thus may be useful as calibration points. However, the results of such a sensitivity analysis does not account for parameter correlations and redundancies in observations, and may not properly separate between calibration and prediction targets if used as criteria that guide inverse modeling; they may thus yield misleading recommendations about parameter identifiability and data worth. These issues (and some remedies) are discussed using an illustrative example, in which we examine the value of data sets potentially used for the calibration of a geothermal reservoir model. These notes highlight the importance of carefully formulating the objectives of a simulation study, which controls the set‐up of the inverse problem and related data needs. This article is protected by copyright. All rights reserved.
- Control of water distribution networks with dynamic DMA topology using
strictly feasible sequential convex programming
- Authors: Robert Wright; Edo Abraham, Panos Parpas, Ivan Stoianov
Abstract: The operation of water distribution networks (WDN) with a dynamic topology is a recently pioneered approach for the advanced management of district metered areas (DMA) that integrates novel developments in hydraulic modelling, monitoring, optimization and control. A common practice for leakage management is the sectorization of WDNs into small zones, called DMAs, by permanently closing isolation valves. This facilitates water companies to identify bursts and estimate leakage levels by measuring the inlet flow for each DMA. However, by permanently closing valves, a number of problems have been created including reduced resilience to failure and suboptimal pressure management. By introducing a dynamic topology to these zones, these disadvantages can be eliminated whilst still retaining the DMA structure for leakage monitoring. In this paper, a novel optimization method based on sequential convex programming (SCP) is outlined for the control of a dynamic topology with the objective of reducing average zone pressure (AZP). A key attribute for control optimization is reliable convergence. To achieve this, the SCP method we propose guarantees that each optimization step is strictly feasible, resulting in improved convergence properties. By using a null space algorithm for hydraulic analyses, the computations required are also significantly reduced. The optimized control is actuated on a real WDN operated with a dynamic topology. This unique experimental programme incorporates a number of technologies set up with the objective of investigating pioneering developments in WDN management. Preliminary results indicate AZP reductions for a dynamic topology of up to 6.5% over optimally controlled fixed topology DMAs. This article is protected by copyright. All rights reserved.
- The compressed state Kalman filter for nonlinear state estimation:
Application to large‐scale reservoir monitoring
- Authors: Judith Yue Li; Amalia Kokkinaki, Hojat Ghorbanidehno, Eric F. Darve, Peter K. Kitanidis
Abstract: Reservoir monitoring aims to provide snapshots of reservoir conditions and their uncertainties to assist operation management and risk analysis. These snapshots may contain millions of state variables, e.g., pressures and saturations, which can be estimated by assimilating data in real time using the Kalman filter (KF). However, the KF has a computational cost that scales quadratically with the number of unknowns, m, due to the cost of computing and storing the covariance and Jacobian matrices, along with their products. The compressed state Kalman filter (CSKF) [Kitanidis, 2015] adapts the KF for solving large‐scale monitoring problems. The CSKF uses N preselected orthogonal bases to compute an accurate rank‐N approximation of the covariance that is close to the optimal spectral approximation given by SVD. The CSKF has a computational cost that scales linearly in m and uses an efficient matrix‐free approach that propagates uncertainties using N + 1 forward model evaluations, where N ≪ m. Here, we present a generalized CSKF algorithm for nonlinear state estimation problems such as CO2 monitoring. For simultaneous estimation of multiple types of state variables, the algorithm allows selecting bases that represent the variability of each state type. Through synthetic numerical experiments of CO2 monitoring, we show that the CSKF can reproduce the Kalman gain accurately even for large compression ratios (m/N). For a given computational cost, the CSKF uses a robust and flexible compression scheme that gives more reliable uncertainty estimates than the ensemble Kalman Filter, which may display loss of ensemble variability leading to suboptimal uncertainty estimates. This article is protected by copyright. All rights reserved.
- Annually‐resolved late holocene paleohydrology of the southern
Sierra Nevada and Tulare Lake, California
- Authors: Kenneth D. Adams; Robert M. Negrini, Edward R. Cook, Seshadri Rajagopal
Abstract: Here we present 2000‐year long, annually resolved records of stream flow for the Kings, Kaweah, Tule, and Kern rivers in the southwestern Sierra Nevada of California and consequent lake‐level fluctuations at Tulare Lake in the southern San Joaquin Valley. The integrated approach of using moisture‐sensitive tree ring records from the Living Blended Drought Atlas to reconstruct annual discharge and then routing this discharge to an annual Tulare Lake water balance model highlights the differences between these two types of paleoclimate records, even when subject to the same forcing factors. The reconstructed streamflow in the southern Sierra responded to yearly changes in precipitation and expressed a strong periodicity in the 2 – 8 year range over most of the reconstruction. The storage capacity of Tulare Lake caused it to fluctuate more slowly, masking the 2 ‐8 year stream flow periodicity and instead expressing a strong periodicity in the 32 – 64 year range over much of the record. Although there have been longer droughts, the 2015 water year represents the driest in the last 2015 years and the 2012‐2015 drought represents the driest four year period in the record. Under natural conditions, simulated Tulare Lake levels would now be at about 60 m, which is not as low as what occurred multiple times over the last 2000 years. This long term perspective of fluctuations in climate and water supply suggest that different drought scenarios that vary in terms of severity and duration can produce similar lake‐level responses in closed lake basins. This article is protected by copyright. All rights reserved.
- “Prophetic vision, vivid imagination”: The 1927 Mississippi
- Authors: James A. Smith; Mary Lynn Baeck
Abstract: The 1927 flood in the Lower Mississippi River was the most destructive flood in American history, inundating more than 70,000 square kilometers of land, resulting in approximately 500 fatalities and leaving more than 700,000 people homeless. Despite the prominence of the 1927 flood, details on the flood, and the storms that produced the flood, are sparse. We examine the hydrometeorology and hydroclimatolgy of the 1927 flood in the Lower Mississippi River through downscaling simulations of the storms that were responsible for catastrophic flooding and through empirical analyses of rainfall and streamflow records. We use 20th Century Reanalysis fields as boundary conditions and initial conditions for downscaling simulations using the Weather Research and Forecasting (WRF) model. We place the hydrometeorological analyses of the 1927 storms in a hydroclimatological context through analyses of the 20th Century Reanalysis fields. Analyses are designed to assess the physical processes that control the upper tail of flooding in the Lower Mississippi River. We compare the 1927 flood in the Lower Mississippi River to floods in 1937 and 2011 that represent the most extreme flooding in the Lower Mississippi River. This article is protected by copyright. All rights reserved.
- Dating base flow in streams using dissolved gases and diurnal temperature
- Authors: Ward E. Sanford; Gerolamo Casile, Karl B. Haase
Abstract: A method is presented for using dissolved CFCs or SF6 to estimate the apparent age of stream base flow by indirectly estimating the mean concentration of the tracer in the inflowing groundwater. The mean value is estimated simultaneously with the mean residence times of the gas and water in the stream by sampling the stream for one or both age tracers, along with dissolved nitrogen and argon at a single location over a period of approximately 12‐14 hours. The data are fitted to an equation representing the temporal in‐stream gas exchange as it responds to the diurnal temperature fluctuation. The efficacy of the method is demonstrated by collecting and analyzing samples at six different stream locations across parts of northern Virginia, USA. The studied streams drain watersheds with areas of between 2 and 122 km2 during periods when the diurnal stream temperature ranged between 2 and 5°C. The method has the advantage of estimating the mean groundwater residence time of discharge from the watershed to the stream without the need for the collection of groundwater infiltrating to streambeds or local groundwater sampled from shallow observation wells near the stream. This article is protected by copyright. All rights reserved.
- Accepting managed aquifer recharge of urban stormwater reuse: The role of
- Authors: Aditi Mankad; Andrea Walton
Abstract: A between‐groups experimental design examined public acceptance for managed aquifer recharge of stormwater for indirect potable and non‐potable reuse; acceptance was based on five policy‐related variables (fairness, effectiveness, trust, importance of safety assurances, and importance of communication activities). Results showed that public acceptance (N = 408) for managed aquifer recharge of stormwater was higher for non‐potable applications, as was the importance of safety assurances. Analyses of variance also showed that perceptions of fairness and effectiveness were higher for a non‐potable scheme, but not trust. A three‐step hierarchical regression (Step 1: age, gender, education, income; Step 2: type of use; Step 3: fairness, effectiveness, trust, safety assurance, communication activities) demonstrated that type of stormwater use and the policy related factors accounted for 73% of the variance in acceptance of stormwater (R2 = .74, adjusted R2 = .74, F (10, 397) = 113.919, p
- Hydration and diffusion processes shape microbial community organization
and function in model soil aggregates
- Authors: Ali Ebrahimi; Dani Or
Abstract: The constantly changing soil hydration status affects gas and nutrient diffusion through soil pores and thus the functioning of soil microbial communities. The conditions within soil aggregates are of particular interest due to limitations to oxygen diffusion into their core, and the presence of organic carbon often acting as binding agent. We developed a model for microbial life in simulated soil aggregates comprising of 3‐D angular pore networks (APNM) that mimic soil hydraulic and transport properties. Within these APNM, we introduced individual motile (flagellated) microbial cells with different physiological traits that grow, disperse, and respond to local nutrients and oxygen concentrations. The model quantifies the dynamics and spatial extent of anoxic regions that vary with hydration conditions, and their role in shaping microbial community size and activity and the spatial (self) segregation of anaerobes and aerobes. Internal carbon source and opposing diffusion directions of oxygen and carbon within an aggregate were essential to emergence of stable coexistence of aerobic and anaerobic communities (anaerobes become extinct when carbon sources are external). The model illustrates a range of hydration conditions that promote or suppress denitrification or decomposition of organic matter and thus affect soil GHG emissions. Model predictions of CO2 and N2O production rates were in good agreement with limited experimental data. These limited tests support the dynamic modeling approach whereby microbial community size, composition and spatial arrangement emerge from internal interactions within soil aggregates. The upscaling of the results to a population of aggregates of different sizes embedded in a soil profile is underway. This article is protected by copyright. All rights reserved.
- Upwelling of deep water during thermal stratification onset – A
major mechanism of vertical transport in small temperate lakes in
- Abstract: Using airborne thermal infrared imaging and horizontally‐resolved in situ temperature monitoring at the lake surface we estimated strength and duration of regular wind‐driven upwelling of dense deep water to the lake surface in two small (in terms of Rossby radius) temperate lakes during the initial phase of summer thermal stratification. The onset and duration of the upwelling events correlated well with the balance between stratification (in terms of Schmidt Stability) and wind forcing, as expressed by Lake and Wedderburn Numbers: The period of regular upwelling appearances lasted 7‐15 days, identified by Schmidt stabilities around 30 J m−2 and Lake Numbers between 0 and 1, and resulted in persistent temperature gradients of up to 2°C across the lake surface. Our results suggest that spring upwelling should inevitably take place in all freshwater temperate lakes with mean temperatures crossing the maximum density value of freshwater on annual cycle, whereas duration and intensity of the upwelling would vary depending on lake morphometry and weather conditions. Our results suggest major contribution of upwelling in nutrient supply to the upper waters, oxygenation of the deep water column, and air‐lake gas exchange, in particular, the release of the sediment‐produced methane into the atmosphere. This article is protected by copyright. All rights reserved.
- The formation of viscous limited saturation zones behind rapid drainage
fronts in porous media
- Authors: Frouke Hoogland; Peter Lehmann, Dani Or
Abstract: Drainage characteristics of porous media are shaped by an interplay between gravitational, capillary and viscous forces that result in complex phase invasion patterns and dynamics. We propose a mechanistic model for viscous separation of temporary phase detention behind rapidly moving drainage fronts. The viscous‐limited region forming behind the front tip (tip of furthest penetrated air finger) drains at a slower rate with a characteristic time scale τ dictated by hydraulic decoupling expressed by the hydraulic properties of the medium. The region where saturation becomes viscously detained (temporarily entrapped) is determined by a critical water content θcrit that defines a viscous length LV behind the front tip. Theory developed to predict the critical water content θcrit and the characteristic secondary timescale τ was in good agreement with measured drainage characteristics using neutron radiography and direct imaging. The observed critical water content θcrit increased with higher drainage rates as predicted by theory with consideration of a percolation threshold. The observed slow drainage timescale τ as a function of mean drainage rate depended on the critical water content θcrit and the resulting counteracting effects of increased detained liquid volume and increased conductivity of the viscous limited region. The concept of drainage zonation illustrates how increasing flow rates enhances the extent of viscous limitations behind the main drainage front. The new insights could be useful for management of immiscible fluid displacement, quantification of averaging effects in experimental measurements (dynamic effects on pc‐S relationship), and explain some of the underpinnings of the field capacity phenomenon. This article is protected by copyright. All rights reserved.
- A pseudogenetic model of coarse braided‐river deposits
- Authors: Guillaume Pirot; Julien Straubhaar, Philippe Renard
Abstract: A new method is proposed to produce three‐dimensional facies models of braided‐river aquifers based on analog data. The algorithm consists of two steps. The first step involves building the main geological units. The production of the principal inner structures of the aquifer is achieved by stacking Multiple‐Point‐Statistics simulations of successive topographies, thus mimicking the major successive flooding events responsible for the erosion and deposition of sediments. The second step of the algorithm consists of generating fine scale heterogeneity within the main geological units. These smaller‐scale structures are generated by mimicking the trough‐filling process occurring in braided rivers; the imitation of the physical processes relies on the local topography and on a local approximation of the flow. This produces realistic cross‐stratified sediments, comparable to what can be observed in outcrops. The three main input parameters of the algorithm offer control over the proportions, the continuity and the dimensions of the deposits. Calibration of these parameters does not require invasive field measurements and can rely partly on analog data. This article is protected by copyright. All rights reserved.
- Independent component analysis of local‐scale temporal variability
in sediment‐water interface temperature
- Authors: M. A. Middleton; P.H. Whitfield, D.M. Allen
Abstract: Temperature recorded at the sediment‐water interface has been identified as a valuable tracer for understanding groundwater‐surface water interactions. However, factors contributing to the variability in temperatures can be difficult to distinguish. In this study, the temporal variability in daily temperatures at the sediment‐water interface is evaluated for a 40‐m reach of a coastal stream using Independent Component Analysis (ICA). ICA separation is used to identify three independent temperature components within the reach for each of four summer periods (2008‐2011). Extracted temperature signals correlate with stream discharge, estimated streambed temperature, and groundwater level, but the strength of the correlations varies from summer to summer. Overall, variations in the temperature signals have clearer separation in summers with lower stream discharge and greater stream temperature ranges. Surface heating from solar radiation is the dominant factor influencing the sediment‐water interface temperature in most years, but there is evidence that thermal exchanges are taking place other than at the air‐water interface. These exchanges take place at the sediment‐water interface, and the correlation with groundwater levels indicates these heat exchanges are associated with groundwater inflow. This study demonstrates that ICA can be used effectively to aid in identifying component signals in environmental applications of small spatial scale. This article is protected by copyright. All rights reserved.
- Beyond annual streamflow reconstructions for the Upper Colorado River
Basin: A paleo‐water‐balance approach
- Authors: Subhrendu Gangopadhyay; Gregory J. McCabe, Connie Woodhouse
Abstract: In this paper, we present a methodology to use annual tree‐ring chronologies and a monthly water balance model to generate annual reconstructions of water balance variables (e.g. potential evapotranspiration (PET), actual evapotranspiration (AET), snow water equivalent (SWE), soil moisture storage (SMS), and runoff (R)). The method involves resampling monthly temperature and precipitation from the instrumental record directed by variability indicated by the paleoclimate record. The generated time series of monthly temperature and precipitation are subsequently used as inputs to a monthly water balance model. The methodology is applied to the Upper Colorado River basin and results indicate that the methodology reliably simulates water‐year runoff, maximum snow water equivalent, and seasonal soil moisture storage for the instrumental period. As a final application, the methodology is used to produce time series of PET, AET, SWE, SMS, and R for the 1404 through 1905 period for the Upper Colorado River basin. This article is protected by copyright. All rights reserved.
- Contribution of alluvial groundwater to the outflow of mountainous
- Abstract: Alluvial aquifers in mountainous regions cover typically a limited area. Their contribution to catchment storage and outflow is rarely isolated; alluvial groundwater discharge under gauging‐stations is generally assumed negligible; and hydrological models tend to lump alluvial storage with other units. The role of alluvial aquifers remains therefore unclear: can they contribute significantly to outflow when they cover a few percent of catchment area? Should they be considered a dynamic storage unit or merely a transmission zone? We address these issues based on the continuous monitoring of groundwater discharge, river discharge (one year), and aquifer storage (six months) in the 6 km2 alluvial system of a 194 km2 catchment. River and groundwater outflow were measured jointly through “coupled gauging stations”. The contribution of alluvial groundwater to outflow was highest at the outlet of a sub‐catchment (52 km2), where subsurface discharge amounted to 15% of mean annual outflow, and 85% of outflow during the last week of a drought. In this period, alluvial‐aquifer depletion supported 75% of the sub‐catchment outflow and 35% of catchment outflow – thus 3% of the entire catchment supported a third of the outflow. Storage fluctuations occurred predominantly in the aquifer's upstream part, where heads varied over 6 m. Not only does this section act as a significant water source, but storage recovers also rapidly at the onset of precipitation. Storage dynamics were best conceptualized along the valley‐axis, rather than across the more conventional riparian‐channel transect. Overall the contribution of alluvial aquifers to catchment outflow deserves more attention. This article is protected by copyright. All rights reserved.
- Solar radiation as a global driver of hillslope asymmetry: Insights from
an ecogeomorphic landscape evolution model
- Authors: Omer Yetemen; Erkan Istanbulluoglu, Alison R. Duvall
Abstract: Observations at the field, catchment, and continental scales across a range of arid and semiarid climates and latitudes reveal aspect‐controlled patterns in soil properties, vegetation types, ecohydrologic fluxes, and hillslope morphology. Although the global distribution of solar radiation on earth's surface and its implications on vegetation dynamics are well documented, we know little about how variation of solar radiation across latitudes influence landscape evolution and resulting geomorphic difference. Here, we used a landscape evolution model that couples the continuity equations for water, sediment, and aboveground vegetation biomass at each model element in order to explore the controls of latitude and mean annual precipitation (MAP) on the development of hillslope asymmetry (HA). In our model, asymmetric hillslopes emerged from the competition between soil creep and vegetation‐modulated fluvial transport, driven by spatial distribution of solar radiation. Latitude was a primary driver of HA because of its effects on the global distribution of solar radiation. In the Northern Hemisphere, north‐facing slopes (NFS), which support more vegetation cover and have lower transport efficiency, get steeper toward the North Pole while south‐facing slopes (SFS) get gentler. In the Southern Hemisphere, the patterns are reversed and SFS get steeper towards the South Pole. For any given latitude, MAP is found to have minor control on HA. Our results underscore the potential influence of solar radiation as a global control on the development of asymmetric hillslopes in fluvial landscapes. This article is protected by copyright. All rights reserved.
- Rapid bacteriophage MS2 transport in an oxic sandy aquifer in cold
climate: Field experiments and modeling
- Abstract: Virus removal during rapid transport in an unconfined, low‐temperature (6°C) sand and gravel aquifer was investigated at a riverbank field site, 25 km south of Trondheim in central Norway. The data from bacteriophage MS2 inactivation and transport experiments were applied in a two‐site kinetic transport model using HYDRUS‐1D, to evaluate the mechanisms of virus removal and whether these mechanisms were sufficient to protect the groundwater supplies. The results demonstrated that inactivation was negligible to the overall removal, and that irreversible MS2 attachment to aquifer grains, coated with iron precipitates, played a dominant role in the removal of MS2; 4.1 log units of MS2 were removed by attachment during 38 m travel distance and less than 2 days residence time. Although the total removal was high, pathways capable of allowing virus migration at rapid velocities were present in the aquifer. The risk of rapid transport of viable viruses should be recognized, particularly for water supplies without permanent disinfection. This article is protected by copyright. All rights reserved.
- Characterising flow behavior for gas injection: Relative permeability of
CO2‐brine and N2‐water in heterogeneous rocks
- Authors: C. A. Reynolds; S. Krevor
Abstract: We provide a comprehensive experimental study of steady state, drainage relative permeability curves with CO2‐brine and N2‐deionised water, on a single Bentheimer sandstone core with a simple two‐layer heterogeneity. We demonstrate that, if measured in the viscous limit, relative permeability is invariant with changing reservoir conditions, and is consistent with the continuum scale multiphase flow theory for water wet systems. Furthermore, we show that under capillary limited conditions, the CO2‐brine system is very sensitive to heterogeneity in capillary pressure, and by performing core floods under capillary limited conditions, we produce effective relative permeability curves that are flow rate and fluid parameter dependent. We suggest that the major uncertainty in past observations of CO2‐brine relative permeability curves is due to the interaction of CO2 flow with pore space heterogeneity under capillary limited conditions and is not due to the effects of changing reservoir conditions. We show that the appropriate conditions for measuring intrinsic or effective relative permeability curves can be selected simply by scaling the driving force for flow by a quantification of capillary heterogeneity. Measuring one or two effective curves on a core with capillary heterogeneity that is representative of the reservoir will be sufficient for reservoir simulation This article is protected by copyright. All rights reserved.
- Advective transport in heterogeneous aquifers: Are proxy models
- Authors: A. Fiori; A. Zarlenga, H. Gotovac, I. Jankovic, E. Volpi, V. Cvetkovic, G. Dagan
Abstract: We examine the prediction capability of two approximate models (Multi Rate Mass Transfer, MRMT, and Continuous Time Random Walk, CTRW) of non‐Fickian transport, by comparison with accurate 2D and 3D numerical simulations. Both non‐local in time approaches circumvent the need to solve the flow and transport equations by using proxy models to advection, providing the breakthrough curves (BTC) at control planes at any x, depending on a vector of 5 unknown parameters. Although underlain by different mechanisms, the two models have an identical structure in the Laplace Transform domain and have the Markovian property of independent transitions. We show that also the numerical BTCs enjoy the Markovian property. Following the procedure recommended in the literature, along a practitioner perspective, we first calibrate the parameters values by a best fit with the numerical BTC at a control plane at x1, close to the injection plane, and subsequently use it for prediction at further control planes for a few values of σY2≤8. Due to a similar structure and Markovian property, the two methods perform equally well in matching the numerical BTC. The identified parameters are generally not unique, making their identification somewhat arbitrary. The inverse Gaussian model and the recently developed Multi Indicator model (MIM), which does not require any fitting as it relates the BTC to the permeability structure, are also discussed. The application of the proxy models for prediction requires carrying out transport field tests of large plumes for a long duration. This article is protected by copyright. All rights reserved.
- Are all runoff processes the same? Numerical experiments comparing a
Darcy‐Richards solver to an overland flow‐based approach for
subsurface storm runoff simulation
- Authors: A. A. Ameli; J.R. Craig, J.J. McDonnell
Abstract: Hillslope runoff theory is based largely on the differentiation between infiltration excess overland flow, saturation excess overland flow and subsurface stormflow. Here we explore to what extent a 2‐D friction‐based overland flow model is useful for predicting hillslope‐scale subsurface stormflow, posited here as phenomenologically the same as infiltration excess at depth. We compare our results to a 3‐D variably saturated Darcy‐Richards subsurface solver for individual rainfall runoff events. We use field data from the well‐studied Panola Mountain Experimental hillslope in Georgia USA. Our results show that the two models are largely indistinguishable in terms of their ability to simulate the hillslope hydrograph magnitude and timing for a range of slopes and rainfall depths. Furthermore, we find that the descriptive ability of the overland flow model is comparable to the variably saturated subsurface flow model in terms of its ability to represent the spatial distribution of subsurface stormflow and infiltration across the soil‐bedrock interface. More importantly, these results imply that the physics of infiltration excess subsurface storm flow at the soil‐bedrock interface is similar to infiltration excess overland flow at the soil surface, in terms of detention storage, loss along the lower boundary and threshold‐like activation at the larger hillslope scale. Given the phenomenological similarity of overland flow and subsurface stormflow and the fact that overland flow model predictions are considerably faster to run (particularly as slope and rainfall depth increase), these findings imply that new forms of hillslope‐scale subsurface storm runoff predictions may be possible with the knowledge of bedrock permeability and limited soil information. Finally, this work suggests that the role of soil mantle vis‐à‐vis subsurface stormflow is mainly as a filter that delays the development of patches of saturation along the bedrock surface. Our model results show that simple realizations of soil based on a few soil depth measurements can possibly be enough to characterize this filtering effect. This article is protected by copyright. All rights reserved.
- Persistence of legacy soil‐P and elevated background water‐P
concentrations in Water Conservation Area 2A, a northern Everglades
- Authors: John M. Juston; Robert H. Kadlec, William F. DeBusk, Mike J. Jerauld, Thomas A. DeBusk
Abstract: Upstream source control and Stormwater Treatment Areas (STAs) have reduced phosphorus (P) loads to Water Conservation Area 2A (WCA‐2A), a northern Everglades wetland, by three‐quarters since year 2000. Nevertheless, large storages of P remain in enriched peat soils and it is unclear how legacy stores will impact spatial and temporal scales of recovery. We re‐measured soil P enrichment along a well‐studied eutrophication gradient in WCA‐2A and applied a profile modeling approach with uncertainty analysis to assess changes in longitudinal soil P gradients 13‐years after load reductions. We then analyzed existing internal water P data, using a novel data screening approach, for evidence of lowest possible water P concentrations independent from inflows. We interpret such water P limits as evidence of the strength of internal loading at a location. Results indicate that soil P enrichment persists in the ∼7.5 km long “impacted” zone, with no significant evidence of net advancement or recession, while a large pool of labile P in the flocculent layer consolidated and diminished. There is indeed evidence, both spatial and temporal, that this extensive zone of enriched soil P continues to elevate lowest achievable water P concentrations. The corresponding gradient of elevated water P limits is both receding and diminishing since load reductions, thus providing further evidence toward recovery. However, results also suggest that these “transitory P limits” due to internal loading are likely to persist for decades above water quality targets. These results advance our understanding of recovery in impacted wetlands and are relevant to Everglades restoration. This article is protected by copyright. All rights reserved.
- Analysis of groundwater flow in mountainous, headwater catchments with
- Authors: Sarah G. Evans; Shemin Ge, Sihai Liang
Abstract: Headwater catchments have a direct impact on the water resources of downstream lowland regions as they supply freshwater in the form of surface runoff and discharging groundwater. Often, these mountainous catchments contain expansive permafrost that may alter the natural topographically‐controlled groundwater flow system. As permafrost could degrade with climate change, it is imperative to understand the effect of permafrost on groundwater flow in headwater catchments. This study characterizes groundwater flow in mountainous headwater catchments and evaluates the effect of permafrost in the context of climate change on groundwater movement using a three‐dimensional, finite element, hydrogeologic model. The model is applied to a representative headwater catchment on the Qinghai‐Tibet Plateau, China. Results from the model simulations indicate that groundwater contributes significantly to streams in the form of baseflow and the majority of groundwater flow is from the shallow aquifer above the permafrost, disrupting the typical topographically‐controlled flow pattern observed in most permafrost‐free headwater catchments. Under a warming scenario where mean annual surface temperature is increased by 2 °C, reducing the areal extent of permafrost in the catchment, groundwater contribution to streamflow may increase three‐fold. These findings suggest that, in headwater catchments, permafrost has a large influence on groundwater flow and stream discharge. Increased annual air temperatures may increase groundwater discharge to streams, which has implications for ecosystem health and the long‐term availability of water resources to downstream regions. This article is protected by copyright. All rights reserved.
- Combining regional estimation and historical floods: A multivariate
semiparametric peaks‐over‐threshold model with censored data
- Authors: Anne Sabourin; Benjamin Renard
Abstract: The estimation of extreme flood quantiles is challenging due to the relative scarcity of extreme data compared to typical target return periods. Several approaches have been developed over the years to face this challenge, including regional estimation and the use of historical flood data. This paper investigates the combination of both approaches using a multivariate peaks‐over‐threshold model, that allows estimating altogether the intersite dependence structure and the marginal distributions at each site. The joint distribution of extremes at several sites is constructed using a semi‐parametric Dirichlet Mixture model. The existence of partially missing and censored observations (historical data) is accounted for within a data augmentation scheme. This model is applied to a case study involving four catchments in Southern France, for which historical data are available since 1604. The comparison of marginal estimates from four versions of the model (with or without regionalizing the shape parameter; using or ignoring historical floods) highlights significant differences in terms of return level estimates. Moreover, the availability of historical data on several nearby catchments allows investigating the asymptotic dependence properties of extreme floods. Catchments display a a significant amount of asymptotic dependence, calling for adapted multivariate statistical models. This article is protected by copyright. All rights reserved.
- The atmospheric transport of iodine‐129 from Fukushima to British
Columbia, Canada, and its deposition and transport into groundwater
- Authors: Matt N. Herod; Martin Suchy, R. Jack Cornett, W.E. Kieser, Ian D. Clark, Gwyn Graham
Abstract: The Fukushima‐Daiichi nuclear accident (FDNA) released iodine‐129 (15.7 million year half‐life) and other fission product radionuclides into the environment in the spring and summer of 2011. 129I is recognized as a useful tracer for the short‐lived radiohazard 131I, which has a mobile geochemical behaviour with potential to contaminate water resources. To trace 129I released by the FDNA reaching Canada, pre‐ and post‐accident rain samples collected in Vancouver, on Saturna Island and from the National Atmospheric Deposition Program in Washington State were measured. Groundwater from the Abbotsford‐Sumas Aquifer was sampled to determine the fate of 129I that infiltrates below the root zone. Modelling of vadose zone transport was performed to constrain the travel time and retardation of 129I. The mean pre‐accident 129I concentration in rain was 31 x 106 atoms/L (n=4). Immediately following the FDNA, 129I values increased to 211 x 106 atoms/L and quickly returned to near‐background levels. However, pulses of elevated 129I continued for several months. The increases in 129I concentrations from both Vancouver and Saturna Island were synchronized, and occurred directly after the initial release from the FDNA. The 129I in shallow (3H/3He age
- Reconstruction of missing daily streamflow data using dynamic regression
- Abstract: River discharge is one of the most important quantities in hydrology. It provides fundamental records for water resources management and climate change monitoring. Even very short data‐gaps in this information can cause extremely different analysis outputs. Therefore, reconstructing missing data of incomplete datasets is an important step regarding the performance of the environmental models, engineering and research applications, thus it presents a great challenge. The objective of this paper is to introduce an effective technique for reconstructing missing daily discharge data when one has access to only daily streamflow data. The proposed procedure uses a combination of regression and autoregressive integrated moving average models (ARIMA) called dynamic regression model. This model uses the linear relationship between neighbor and correlated stations and then adjusts the residual term by fitting an ARIMA structure. Application of the model to eight daily streamflow data for the Durance river watershed showed that the model yields reliable estimates for the missing data in the time series. Simulation studies were also conducted to evaluate the performance of the procedure. This article is protected by copyright. All rights reserved.
- Derivation of operation rules for reservoirs in parallel with joint water
- Authors: Xiang Zeng; Tiesong Hu, Lihua Xiong, Zhixian Cao, Chongyu Xu
Abstract: The purpose of this paper is to derive the general optimality conditions of the commonly used operating policies for reservoirs in parallel with joint water demand, which are defined in terms of system‐wide release rules and individual reservoir storage balancing functions. Following that, a new set of release rules for individual reservoirs are proposed in analytical forms by considering the optimality conditions for the balance of total water delivery utility and carryover storage value of individual reservoirs. Theoretical analysis indicates that the commonly used operating policies are a special case of the newly derived rules. The derived release rules are then applied to simulating the operation of a parallel reservoir system in northeastern China. Compared to the performance of the commonly used policies, some advantages of the proposed operation rules are illustrated. Most notably, less water shortage occurrence and higher water supply reliability are obtained from the proposed operation rules. This article is protected by copyright. All rights reserved.
- The role of dynamic surface water‐groundwater exchange on streambed
denitrification in a first‐order, low‐relief agricultural
- Authors: Mina Rahimi; Hedeff I. Essaid, John T. Wilson
Abstract: The role of temporally‐varying surface water ‐ groundwater (SW‐GW) exchange on nitrate removal by streambed denitrification was examined along a reach of Leary Weber Ditch (LWD), Indiana, a small, first‐order, low‐relief agricultural watershed within the Upper Mississippi River basin, using data collected in 2004 and 2005. Stream stage, GW heads (H) and temperatures (T) were continuously monitored in streambed piezometers and stream bank wells for two transects across LWD accompanied by synoptic measurements of stream stage, H, T and nitrate (NO3) concentrations along the reach. The H and T data were used to develop and calibrate vertical two‐dimensional, models of streambed water flow and heat transport across and along the axis of the stream. Model‐estimated SW‐GW exchange varied seasonally and in response to high streamflow events due to dynamic interactions between SW stage and GW H. Comparison of 2004 and 2005 conditions showed that small changes in precipitation amount and intensity, evapotranspiration, and/or nearby GW levels within a low‐relief watershed can readily impact SW‐GW interactions. The calibrated LWD flow models and observed stream and streambed NO3 concentrations were used to predict temporal variations in streambed NO3 removal in response to dynamic SW‐GW exchange. NO3 removal rates underwent slow seasonal changes, but also underwent rapid changes in response to high flow events. These findings suggest that increased temporal variability of SW‐GW exchange in low‐order, low relief watersheds may be a factor contributing their more efficient removal of NO3. This article is protected by copyright. All rights reserved.
- Macroweather precipitation variability up to global and centennial scales
- Authors: M. I. P. de Lima; S. Lovejoy
Abstract: We investigate precipitation variability in the “macroweather” regime – the intermediate regime between the familiar weather and climate regimes – which is associated to time scales from about 10 days to 30–100 years. Macroweather precipitation is characterized by negative fluctuation exponents. This implies – contrary to the weather regime – that fluctuations tend to cancel each other out, they diminish with time scale, this is important for seasonal, annual and decadal forecasts. Aiming at a wide scale range space‐time statistical description of macroweather precipitation, we study the scaling of three centennial, global scale precipitation products (one gauge based, one reanalysis based, one satellite based) and systematically compare them over wide ranges of time and space scales. Although these products have very similar temporal statistics, at 5° resolution, they only agree with each other after being averaged over scales of several years. In space, there is less agreement on the statistics but since the data have low resolutions (mostly 5°×5°), the disagreement is only over a small overall range of scales: the monthly data agree fairly well at scales 20°–30° and larger. Moreover, we quantify the outer scale limit of the temporal scaling (20‐40 years, depending on the product, on the spatial scale, pixel or global). Overall, results show that precipitation can be modelled with space‐time scaling processes. The improved understanding of the space‐time macroweather precipitation variability and the limitations of precipitation products provided by this work opens new perspectives to the stochastic modelling and forecasting of macroweather fields. This article is protected by copyright. All rights reserved.
- Accelerating advances in continental domain hydrologic modeling
- Abstract: In the past, hydrologic modeling of surface water resources has mainly focused on simulating the hydrologic cycle at local to regional catchment modeling domains. There now exists a level of maturity amongst the catchment, global water security, and land surface modeling communities such that these communities are converging towards continental domain hydrologic models. This commentary, written from a catchment hydrology community perspective, provides a review of progress in each community towards this achievement, identifies common challenges the communities face, and details immediate and specific areas in which these communities can mutually benefit one another from the convergence of their research perspectives. Those include: (1) creating new incentives and infrastructure to report and share model inputs, outputs, and parameters in data services and open access, machine‐independent formats for model replication or re‐analysis; (2) ensuring that hydrologic models have: sufficient complexity to represent the dominant physical processes and adequate representation of anthropogenic impacts on the terrestrial water cycle, a process‐based approach to model parameter estimation, and appropriate parameterizations to represent large‐scale fluxes and scaling behavior; (3) maintaining a balance between model complexity and data availability as well as uncertainties and (4) quantifying and communicating significant advancements towards these modeling goals. This article is protected by copyright. All rights reserved.
- Hydrological response to changing climate conditions: Spatial streamflow
variability in the boreal region
- Authors: C. Teutschbein; T. Grabs, R. H. Karlsen, H. Laudon, K. Bishop
Abstract: In this paper we combined a multi‐model ensemble based on 15 regional climate models with a multi‐catchment approach to explore the hydrologic sensitivity of 14 neighboring and rather similar catchments to changing climate conditions. Current (1982‐2010) and future (2062‐2090) streamflow was simulated with the HBV model. A diagnostic approach was used, which considered major behavioral catchment functions by using hydrologically relevant signatures related to overall water balance, flow duration curves and hydrograph attributes. Projected increases in temperature and precipitation resulted in increased total available streamflow, with lower spring and summer flows, but substantially higher winter streamflow. Furthermore, significant changes in flow durations with lower chances of both high and low flows can be expected in boreal Sweden in the future. This overall trend in projected streamflow pattern changes was comparable among the analyzed catchments but the magnitude of change differed considerably. This suggests that catchments belonging to the same region can show distinctly different degrees of hydrological responses to the same external climate change signal. We reason that differences in spatially distributed physical catchment properties within catchments are not only of great importance for current streamflow behavior, but also play a major role in the sensitivity of catchments to changing climate conditions. This article is protected by copyright. All rights reserved.
- Robust optimization of well location to enhance hysteretical trapping of
CO2: Assessment of various uncertainty quantification methods and
utilization of mixed response surface surrogates
- Authors: Masoud Babaei; Indranil Pan, Ali Alkhatib
Abstract: The paper aims to solve a robust optimization problem (optimization in presence of uncertainty) for finding the optimal locations of a number of CO2 injection wells for geological sequestration of carbon dioxide in a saline aquifer. The parametric uncertainties are the interfacial tension between CO2 and aquifer brine, the Land's trapping coefficient and the boundary aquifer's absolute. The spatial uncertainties are due to the channelized permeability field which exhibits a binary channel‐non‐channel system. The objective function of the optimization is the amount of residually trapped CO2 due to the hysteresis of the relative permeability curves. A risk‐averse value derived from the cumulative density function of the distribution of the amount of trapped gas is chosen as the objective function value. In order to ensure that the uncertainties are effectively taken into account, Monte Carlo simulation and Polynomial Chaos Expansion (PCE)‐based methods are used and compared with each other. For different cases of parametric and spatial uncertainties, the most accurate uncertainty quantification (UQ) method is chosen to be integrated within the optimization algorithm. While for parametric uncertainty cases of up to two uncertain variables, PCE‐based methods computationally outperform Monte Carlo simulations, it is shown that for the multimodal distributions of the function of trapped gas occurring for the spatial uncertainty case, Monte Carlo simulations are more reliable than PCE‐based UQ methods. For the discrete (integer) optimization problem, various mixed response surface surrogate models are tested and the robust optimization resulted in optimal CO2 injection well locations. This article is protected by copyright. All rights reserved.
- Depth, ice thickness, and ice‐out timing cause divergent hydrologic
responses among Arctic lakes
- Authors: Christopher D. Arp; Benjamin M. Jones, Anna K. Liljedahl, Kenneth M. Hinkel, Jeffery A. Welker
Pages: 9379 - 9401
Abstract: Lakes are prevalent in the Arctic and thus play a key role in regional hydrology. Since many Arctic lakes are shallow and ice grows thick (historically 2 m or greater), seasonal ice commonly freezes to the lake bed (bedfast ice) by winter's end. Bedfast ice fundamentally alters lake energy balance and melt‐out processes compared to deeper lakes that exceed the maximum ice thickness (floating ice) and maintain perennial liquid water below floating ice. Our analysis of lakes in northern Alaska indicated that ice‐out of bedfast ice lakes occurred on average 17 days earlier (22 June) than ice‐out on adjacent floating ice lakes (9 July). Earlier ice‐free conditions in bedfast ice lakes caused higher open‐water evaporation, 28% on average, relative to floating ice lakes and this divergence increased in lakes closer to the coast and in cooler summers. Water isotopes (18O and 2H) indicated similar differences in evaporation between these lake types. Our analysis suggests that ice regimes created by the combination of lake depth relative to ice thickness and associated ice‐out timing currently cause a strong hydrologic divergence among Arctic lakes. Thus, understanding the distribution and dynamics of lakes by ice regime is essential for predicting regional hydrology. An observed regime shift in lakes to floating ice conditions due to thinner ice growth may initially offset lake drying because of lower evaporative loss from this lake type. This potential negative feedback caused by winter processes occurs in spite of an overall projected increase in evapotranspiration as the Arctic climate warms.