- Microbubble transport in water‐saturated porous media
- Authors: Y. Ma; X.‐Z Kong, A. Scheuermann, SA. Galindo‐Torres, D. Bringemeier, L. Li
Abstract: Laboratory experiments were conducted to investigate flow of discrete microbubbles through a water‐saturated porous medium. During the experiments, bubbles, released from a diffuser, moved upward through a quasi‐2D flume filled with transparent water‐based gelbeads and formed a distinct plume that could be well registered by a calibrated camera. Outflowing bubbles were collected on the top of the flume using volumetric burettes for flux measurements. We quantified the scaling behaviors between the gas (bubble) release rates and various characteristic parameters of the bubble plume, including plume tip velocity, plume width and breakthrough time of the plume front. The experiments also revealed circulations of ambient pore water induced by the bubble flow. Based on a simple momentum exchange model, we showed that the relationship between the mean pore water velocity and gas release rate is consistent with the scaling solution for the bubble plume. These findings have important implications for studies of natural gas emission and air sparging, as well as fundamental research on bubble transport in porous media. This article is protected by copyright. All rights reserved.
- A spatiotemporal precipitation generator based on a censored latent
- Authors: Anastassia Baxevani; Jan Lennatsson
Abstract: A daily stochastic spatio‐temporal precipitation generator that yields precipitation realisations that are quantitatively consistent, is described. The methodology relies on a latent Gaussian field that drives both the occurrence and intensity of the precipitation process. For the precipitation intensity the marginal distributions, which are space and time dependent, are described by a composite model of a gamma‐distribution for observations below some threshold with a generalized Pareto distribution modeling the excesses above the threshold. Model parameters are estimated from data and extrapolated to locations and times with no direct observations using linear regression of position covariates. One advantage of such a model is that stochastic generator parameters are readily available at any location and time of the year inside the stationarity regions. The methodology is illustrated for a network of 12 locations in Sweden. Performance of the model is judged through its ability to accurately reproduce a series of spatial dependence measures and weather indices. This article is protected by copyright. All rights reserved.
- On the optimal design of experiments for conceptual and predictive
discrimination of hydrologic system models
- Authors: C.P. Kikuchi; T.P.A. Ferre, J.A. Vrugt
Abstract: Experimental design and data collection constitute two main steps of the iterative research cycle (aka the scientific method). To help evaluate competing hypotheses, it is critical to ensure that the experimental design is appropriate and maximizes information retrieval from the system of interest. Scientific hypothesis testing is implemented by comparing plausible model structures (conceptual discrimination) and sets of predictions (predictive discrimination). This research presents a new Discrimination‐Inference (DI) methodology to identify prospective datasets highly suitable for either conceptual or predictive discrimination. The DI methodology uses preposterior estimation techniques to evaluate the expected change in the conceptual or predictive probabilities, as measured by the Kullback‐Leibler divergence. We present two case studies with increasing complexity to illustrate implementation of the DI for maximizing information withdrawal from a system of interest. The case studies show that highly informative datasets for conceptual discrimination are in general those for which between‐model (conceptual) uncertainty is large relative to the within‐model (parameter) uncertainty, and the redundancy between individual measurements in the set is minimized. The optimal dataset differs if predictive, rather than conceptual, discrimination is the experimental design objective. Our results show that DI analyses highlight measurements that can be used to address critical uncertainties related to the prediction of interest. Finally, we find that the optimal dataset for predictive discrimination is sensitive to the predictive grouping definition in ways that are not immediately apparent from inspection of the model structure and parameter values. This article is protected by copyright. All rights reserved.
- Landscape reorganization under changing climatic forcing: Results from an
- Authors: Arvind Singh; Liam Reinhardt, Efi Foufoula‐Georgiou
Abstract: Understanding how landscapes respond to climate dynamics in terms of macro‐scale (average topographic features) and micro‐scale (landform re‐organization) is of interest both for deciphering past climates from today's landscapes and for predicting future landscapes in view of recent climatic trends. Although several studies have addressed macro‐scale response, only a few have focused on quantifying smaller‐scale basin re‐organization. To that goal, a series of controlled laboratory experiments were conducted where a self‐organized complete drainage network emerged under constant precipitation and uplift dynamics. Once steady state was achieved, the landscape was subjected to a five‐fold increase in precipitation (transient state). Throughout the evolution, high resolution spatio‐temporal topographic data in the form of digital elevation models were collected. The steady state landscape was shown to possess three distinct geomorphic regimes (unchannelized hillslopes, debris‐dominated channels, and fluvially‐dominated channels). During transient state, landscape re‐organization was observed to be driven by hillslopes via accelerated erosion, ridge lowering, channel widening, and reduction of basin relief as opposed to channel base‐level reduction. Quantitative metrics on which these conclusions were based included slope‐area curve, correlation analysis of spatial and temporal elevation increments, and wavelet spectral analysis of the evolving landscapes. Our results highlight that landscape re‐organization in response to increased precipitation seems to follow “an arrow of scale”: major elevation change initiates at the hillslope scale driving erosional regime change at intermediate scales and further cascading to geomorphic changes at the channel scale as time evolves. This article is protected by copyright. All rights reserved.
- Detection and attribution of urbanization effect on flood extremes using
nonstationary flood frequency models
- Authors: I. Prosdocimi; T. R. Kjeldsen, J. D. Miller
Abstract: This study investigates whether long‐term changes in observed series of high flows can be attributed to changes in land‐use via non‐stationary flood frequency analyses. A point process characterization of threshold exceedances is used, which allows for direct inclusion of covariates in the model; as well as a non‐stationary model for block maxima series. In particular, changes in annual, winter and summer block maxima and peaks over threshold extracted from gauged instantaneous flows records in two hydrologically similar catchments located in close proximity to one another in northern England are investigated. The study catchment is characterized by large increases in urbanization levels in recent decades, while the paired control catchment has remained undeveloped during the study period (1970‐2010). To avoid the potential confounding effect of natural variability, a covariate which summarize key climatological properties is included in the flood frequency model. A significant effect of the increasing urbanization levels on high flows is detected, in particular in the summer season. Point process models appear to be superior to block maxima models in their ability to detect the effect of the increase in urbanization levels on high flows. This article is protected by copyright. All rights reserved.
- Issue Information
- PubDate: 2015-05-19T07:06:16.020095-05:
- Probabilistic inference of multi‐Gaussian fields from indirect
hydrological data using circulant embedding and dimensionality reduction
- Authors: Eric Laloy; Niklas Linde, Diederik Jacques, Jasper A. Vrugt
Abstract: We present a Bayesian inversion method for the joint inference of high‐dimensional multi‐Gaussian hydraulic conductivity fields and associated geostatistical parameters from indirect hydrological data. We combine Gaussian process generation via circulant embedding to decouple the variogram from grid cell specific values, with dimensionality reduction by interpolation to enable Markov chain Monte Carlo (MCMC) simulation. Using the Matérn variogram model, this formulation allows inferring the conductivity values simultaneously with the field smoothness (also called Matérn shape parameter) and other geostatistical parameters such as the mean, sill, integral scales and anisotropy direction(s) and ratio(s). The proposed dimensionality reduction method systematically honors the underlying variogram and is demonstrated to achieve better performance than the Karhunen‐Loéve expansion. We illustrate our inversion approach using synthetic (error corrupted) data from a tracer experiment in a fairly heterogeneous 10,000‐dimensional 2D conductivity field. A 40‐times reduction of the size of the parameter space did not prevent the posterior simulations to appropriately fit the measurement data and the posterior parameter distributions to include the true geostatistical parameter values. Overall, the posterior field realizations covered a wide range of geostatistical models, questioning the common practice of assuming a fixed variogram prior to inference of the hydraulic conductivity values. Our method is shown to be more efficient than sequential Gibbs sampling (SGS) for the considered case study, particularly when implemented on a distributed computing cluster. It is also found to outperform the method of anchored distributions (MAD) for the same computational budget. This article is protected by copyright. All rights reserved.
- The emergence of topographic steady state in a perpetually dynamic
self‐organized critical landscape
- Authors: Liam Reinhardt; Michael A. Ellis
Abstract: We conducted a series of four physical modelling experiments of mountain growth at differing rates of uplift and three distinct climates ranging from relatively wet to relatively dry. The spatial and temporal pattern of landscape behavior is characterized by ∼f−1 scaling in sediment discharge and power law scaling in the magnitude and frequency of ridge movement in all four experiments. We find that internally generated SOC (self‐organized critical) processes generate dynamically stable catchment geometries after ∼1 relief depths of erosion: these regularly spaced catchments have an average outlet spacing ratio of 2.16, well within the range of values reported in field studies. Once formed, large catchment bounding ridges oscillate about a critically balanced mean location, with occasional large scale changes in catchment size. Ridge movement appears to be driven by the competition for discharge as landslides push ridges back and forth. These dynamics lead to the emergence of a complex two‐fold scaling in catchment dynamics that is fully established by 1.8 relief depths of erosion; at this stage a clear threshold has emerged separating two distinct scaling regimes, where large ridge mobility is insensitive to relief and small ridge mobility is relief dependent. Overall, we demonstrate that the development of dynamically‐stable large scale landforms is related to the emergence of a complex‐system hierarchy in topographic dynamics. Once formed these landscapes do not evolve; statistical properties such as average topography and discharge become stationary whilst topography remains highly dynamic at smaller length‐scales. This article is protected by copyright. All rights reserved.
- Water conservation and hydrological transitions in cities in the United
- Authors: George M. Hornberger; David J. Hess, Jonathan Gilligan
Abstract: Cities across the world have had to diversify and expand their water‐supply systems in response to demand growth, groundwater depletion and pollution, and instability and inadequacy of regional surface freshwater sources. In the U.S., these problems plague not only the arid Western cities but increasingly cities in the Eastern portions of the country. Although cities continue to seek out new sources of water via Promethean projects of long‐distance supply systems, desalinization plants, and the recharge of aquifers with surface water, they also pursue water conservation because of its low cost and other benefits. We examine water conservation as a complex sociotechnical system comprising interactions of political, sociodemographic, economic, and hydroclimatological factors. We provide quantitative data on the factors that affect more and less advanced transitions in water conservation regimes, and we show that water stress and other hydrological data can only partially predict the transition. We also provide qualitative case studies to identify institutional and political barriers to more advanced water conservation regimes. This interdisciplinary, mixed‐methods approach typifies the need for knowledge that informs hydrologists about how their research may or may not be adopted by decision‐makers. This article is protected by copyright. All rights reserved.
- An entropy‐based measure of hydrologic complexity and its
- Authors: Aldrich Castillo; Fabio Castelli, Dara Entekhabi
Abstract: Basin response and hydrologic fluxes are functions of hydrologic states, most notably of soil moisture. However, characterization of hillslope‐scale soil moisture is challenging since it is both spatially heterogeneous and dynamic. This paper introduces an entropy‐based and discretization‐invariant dimensionless index of hydrologic complexity H that measures the distance of a given distribution of soil moisture from a Dirac delta (most organization) and a uniform distribution (widest distribution). Applying the distributed hydrologic model MOBIDIC to seven test basins with areas ranging 10° – 103 km2 and representing semiarid and temperate climates, H is shown to capture distributional characteristics of soil moisture fields. It can also track the temporal evolution of the distributional features. Furthermore, this paper explores how basin attributes affect the characteristic H, and how H can be used to explain inter‐basin variability in hydrologic response. Relationships are found only by grouping basins with the same climate or size. For the semiarid basins, H scales with catchment area, topographic wetness, infiltration ratio and baseflow index; while H is inversely related to relief ratio. This article is protected by copyright. All rights reserved.
- Effects of salinity variations on pore water flow in salt marshes
- Authors: Chengji Shen; Guangqiu Jin, Pei Xin, Jun Kong, Ling Li
Abstract: Spatial and temporal salinity variations in surface water and pore water commonly exist in salt marshes under the combined influence of tidal inundation, precipitation, evapotranspiration and inland freshwater input. Laboratory experiments and numerical simulations were conducted to investigate how density gradients associated with salinity variations affect pore‐water flow in the salt marsh system. The results showed that upward salinity (density) gradients could lead to flow instability and the formation of salt fingers. These fingers, varying in size with the distance from the creek, modified significantly the pore‐water flow field, especially in the marsh interior. While the flow instability enhanced local salt transport and mixing considerably, the net effect was small, causing only a slight increase in the overall mass exchange across the marsh surface. In contrast, downward salinity gradients exerted less influence on the pore‐water flow in the marsh soil and slightly weakened the surface water and groundwater exchange across the marsh surface. Numerical simulations revealed similar density effects on pore‐water flow at the field scale under realistic conditions. These findings have important implications for studies of marsh soil conditions concerning plant growth as well as nutrient exchange between the marsh and coastal marine system. This article is protected by copyright. All rights reserved.
- What time scales are important for monitoring tidally influenced submarine
groundwater discharge? Insights from a salt marsh
- Authors: Alicia M. Wilson; Tyler Evans, Willard Moore, Charles A. Schutte, Samantha B. Joye
Abstract: Submarine groundwater discharge (SGD) varies significantly across time scales ranging from hours to years, but studies that allow quantitative comparisons between different time scales are few. Most of these studies have focused on beach settings, where the combined variations in fresh and saline SGD can be difficult to interpret. We calculated variations in saline SGD based on a 1‐year record of hydraulic head in a salt marsh, where we could isolate variations in saline, tidally‐driven SGD. Observed SGD varied by an order of magnitude over the course of the year. Groundwater discharge was proportional to tidal amplitude and varied by at least a factor of two between spring and neap tides. Monthly average SGD was inversely proportional to average sea level; it increased by nearly a factor of two as sea level declined by ∼50 cm from late summer to late winter. This variation was far larger than that predicted by analytic models, owing to the flat topography and inundation of the marsh platform. The effect of short‐term (days) variations in sea level associated with wind events and storms was small in comparison. SGD is probably proportional to tidal amplitude in nearly all coastal settings, including beaches. Seasonal variations in sea level may not affect the volume of SGD as significantly in coastal settings where the slope of the intertidal zone is relatively constant, but such variations have the potential to strongly affect the composition of SGD. This article is protected by copyright. All rights reserved.
- Mechanisms driving the seasonality of catchment‐scale nitrate
export: Evidence for riparian ecohydrologic controls
- Authors: Jonathan M. Duncan; Lawrence E. Band, Peter M. Groffman, Emily S. Bernhardt
Abstract: Considerable variability in the seasonal patterns of streamwater nitrate (NO3‐) has been observed in forested watersheds throughout the world. While many forested headwater catchments exhibit winter and early spring peaks in NO3‐ concentrations, several watersheds have peak concentrations during the summer months. Pond Branch, a headwater catchment in Maryland monitored for over 10 years, exhibits recurrent and broad summer peaks in both NO3‐ concentrations and watershed export. Higher NO3‐ export from June to September is particularly surprising, given that these summer months typically have the year's lowest discharge. A key challenge is identifying the source(s) of NO3‐ and the mechanism(s) by which it is transported to the watershed outlet during the summer. In this study, we assessed multiple hypotheses (not mutually exclusive) that could account for the seasonal trend including proximal controls of groundwater‐surface water interactions, in‐stream processes, and riparian groundwater‐N cycling interactions, as well as two distal controls: geochemical weathering and senescence of riparian vegetation. A combination of long‐term weekly and limited duration high‐frequency sensor data reveal the importance of riparian ecohydrologic processes during baseflow. In this watershed, patterns of seasonal streamwater NO3‐ concentrations and fluxes depend fundamentally on interactions between groundwater dynamics and nitrogen (N) cycling in the riparian zone. Groundwater tables control nitrification‐denitrification dynamics as well as hydrologic transport. Our results suggest that in many watersheds, a more sophisticated exploration of NO3‐ production and NO3‐ transport mechanisms is required to identify critical points in the landscape and over time that disproportionately drive patterns of watershed NO3‐ export. This article is protected by copyright. All rights reserved.
- Probabilistic human health risk assessment of degradation‐related
chemical mixtures in heterogeneous aquifers: Risk statistics, hot spots,
and preferential channels
- Authors: Christopher V. Henri; Daniel Fernàndez‐Garcia, Felipe P. J. de Barros
Abstract: The increasing presence of toxic chemicals released in the subsurface has led to a rapid growth of social concerns and the need to develop and employ models that can predict the impact of groundwater contamination on human health risk under uncertainty. Monitored natural attenuation is a common remediation action in many contamination cases. However, natural attenuation can lead to the production of daughter species of distinct toxicity that may pose challenges in pollution management strategies. The actual threat that these contaminants pose to human health depends on the interplay between the complex structure of the geological media and the toxicity of each pollutant byproduct. This work addresses human health risk for chemical mixtures resulting from the sequential degradation of a contaminant (such as a chlorinated solvent) under uncertainty through high resolution three‐dimensional numerical simulations. We systematically investigate the interaction between aquifer heterogeneity, flow connectivity, contaminant injection model and chemical toxicity in the probabilistic characterization of health risk. We illustrate how chemical‐specific travel times control the regime of the expected risk and its corresponding uncertainties. Results indicate conditions where preferential flow paths can favor the reduction of the overall risk of the chemical mixture. The overall human risk response to aquifer connectivity is shown to be non‐trivial for multi‐species transport. This non‐triviality is a result of the interaction between aquifer heterogeneity and chemical toxicity. To quantify the joint effect of connectivity and toxicity in health risk, we propose a toxicity‐based Damköhler number. Furthermore, we provide a statistical characterization in terms of low‐order moments and the probability density function of the individual and total risks. This article is protected by copyright. All rights reserved.
- The emergence of hydrogeophysics for improved understanding of subsurface
processes over multiple scales
- Authors: Andrew Binley; Susan S. Hubbard, Johan A. Huisman, André Revil, David A. Robinson, Kamini Singha, Lee D. Slater
Abstract: Geophysics provides a multi‐dimensional suite of investigative methods that are transforming our ability to see into the very fabric of the subsurface environment, and monitor the dynamics of its fluids and the biogeochemical reactions that occur within it. Here, we document how geophysical methods have emerged as valuable tools for investigating shallow subsurface processes over the past two decades and offer a vision for future developments relevant to hydrology and also ecosystem science. The field of “hydrogeophysics” arose in the late 1990s, prompted, in part, by the wealth of studies on stochastic subsurface hydrology that argued for better field‐based investigative techniques. These new hydrogeophysical approaches benefited from the emergence of practical and robust data inversion techniques, in many cases with a view to quantify shallow subsurface heterogeneity and the associated dynamics of subsurface fluids. Furthermore, the need for quantitative characterization stimulated a wealth of new investigations into petrophysical relationships that link hydrologically relevant properties to measurable geophysical parameters. Development of time‐lapse approaches provided a new suite of tools for hydrological investigation, enhanced further with the realization that some geophysical properties may be sensitive to biogeochemical transformations in the subsurface environment, thus opening up the new field of “biogeophysics”. Early hydrogeophysical studies often concentrated on relatively small ‘plot‐scale' experiments. More recently, however, the translation to larger‐scale characterization has been the focus of a number of studies. Geophysical technologies continue to develop, driven, in part, by the increasing need to understand and quantify key processes controlling sustainable water resources and ecosystem services. This article is protected by copyright. All rights reserved.
- Exploring scale‐dependent ecohydrological responses in a large
endorheic river basin through integrated surface water‐groundwater
- Authors: Yong Tian; Yi Zheng, Chunmiao Zheng, Honglang Xiao, Wenjie Fan, Songbing Zou, Bin Wu, Yingying Yao, Aijing Zhang, Jie Liu
Abstract: Ecohydrological processes in a water‐limited environment are sensitive to both climate conditions and human activities, but the response mechanisms have rarely been explored for large endorheic river basins via an integrated modeling approach. This study established an integrated surface water‐groundwater model for the Heihe River Basin (HRB), China's second largest endorheic river basin, using GSFLOW as the modeling platform. Evapotranspiration (ET) and Leaf Area Index (LAI) data independently derived from remote sensing products were compared and correlated, respectively, with the modeling results. Scale‐dependent interrelationships among ecological, hydrological and human‐impact (i.e., diversion and pumping) variables were revealed through multiple regression analyses. Major study findings include: (1) the independent ET and LAI data enabled the modeler to crosscheck the modeling results from a unique angle not possible with conventional groundwater and streamflow observations; (2) controlling factors for the temporal variability of ET and LAI exhibit notable scale‐dependence, reflecting distinctive climate and human impacts on different land covers; and (3) there exists an intricate linkage between the hydrological regimes in the lower HRB and the middle HRB, essentially equivalent to a tradeoff between the ecosystem health of the lower HRB and the sustainable development of the middle HRB. Overall, the integrated modeling assisted by the independent ET and LAI data has provided a coherent understanding on the regional water cycle, and led to new insights on tackling the existing water conflicts in HRB. This article is protected by copyright. All rights reserved.
- Model‐based analysis of the influence of catchment properties on
hydrologic partitioning across five mountain headwater subcatchments
- Authors: Christa Kelleher; Thorsten Wagener, Brian McGlynn
Abstract: Ungauged headwater basins are an abundant part of the river network, but dominant influences on headwater hydrologic response remain difficult to predict. To address this gap, we investigated the ability of a physically‐based watershed model (the Distributed Hydrology‐Soil‐Vegetation Model) to represent controls on metrics of hydrologic partitioning across five adjacent headwater sub‐catchments. The five study sub‐catchments, located in Tenderfoot Creek Experimental Forest in central Montana, have similar climate but variable topography and vegetation distribution. This facilitated a comparative hydrology approach to interpret how parameters that influence partitioning, detected via global sensitivity analysis, differ across catchments. Model parameters were constrained a priori using existing regional information and expert knowledge. Influential parameters were compared to perceptions of catchment functioning and its variability across sub‐catchments. Despite between‐catchment differences in topography and vegetation, hydrologic partitioning across all metrics and all sub‐catchments was sensitive to a similar subset of snow, vegetation, and soil parameters. Results also highlighted one sub‐catchment with low certainty in parameter sensitivity, indicating that the model poorly represented some complexities in this sub‐catchment likely because an important process is missing or poorly characterized in the mechanistic model. For use in other basins, this method can assess parameter sensitivities as a function of the specific ungauged system to which it is applied. Overall, this approach can be employed to identify dominant modeled controls on catchment response and their agreement with system understanding. This article is protected by copyright. All rights reserved.
- Validation of hydraulic tomography in an unconfined aquifer: A controlled
- Authors: Zhanfeng Zhao; Walter A. Illman, Tian.‐ Chyi, J. Yeh, Steven J. Berg, Deqiang Mao
Abstract: In this study, we demonstrate the effectiveness of hydraulic tomography (HT) that considers variably saturated flow processes in mapping the heterogeneity of both the saturated and unsaturated zones in a laboratory unconfined aquifer. The successive linear estimator (SLE) developed by Mao et al., (2013c) for interpreting HT in unconfined aquifers is utilized to obtain tomograms of hydraulic conductivity (K), specific storage (Ss), and the unsaturated zone parameters [pore size parameter (α) and saturated water content (θs)] for the Gardner‐Russo's model. The estimated tomograms are first evaluated by visually comparing them with stratigraphy visible in the sandbox. Results reveal that the HT analysis is able to accurately capture the location and extent of heterogeneity including high and low K layers within the saturated and unsaturated zones, as well as reasonable distribution patterns of α and θs for the Gardner‐Russo's model. We then validate the estimated tomograms through predictions of drawdown responses of pumping tests not used during the inverse modeling effort. The strong agreement between simulated and observed drawdown curves obtained by pressure transducers and tensiometers demonstrates the robust performance of HT that considers variably saturated flow processes in unconfined aquifers and the unsaturated zone above it. In addition, compared to the case using the homogeneous assumption, HT results, as expected, yield significantly better predictions of drawdowns in both the saturated and unsaturated zones. This comparison further substantiates the unbiased and minimal variance of HT analysis with the SLE algorithm. This article is protected by copyright. All rights reserved.
- Measurement and simulation of subsurface tracer migration to tile drains
in low‐permeability, macroporous soil
- Authors: Joshua M. Bishop; Michael V. Callaghan, Edwin E. Cey, Larry R. Bentley
Abstract: Multi‐year monitoring and simulation of a conservative tracer was used in this study to investigate preferential flow and macropore‐matrix interactions in low permeability, macroporous soil. 2,6‐Difluorobenzoic acid (DFBA) tracer was applied to a 20 × 20m drip irrigated test plot situated over two tile drains. Tracer movement over the 2009 and 2010 field seasons was monitored using tile drain effluent, suction lysimeters, monitoring wells, and soil cores. Despite similar volumes of water application to the plot in each season, 10 times more water and 14 times more DFBA were captured by the drains in 2010 due to wetter regional hydrologic conditions. The importance of preferential flow along macropores was shown by rapid DFBA breakthrough to the tile (
- Temporal dynamics of catchment transit times from stable isotope data
- Authors: Julian Klaus; Kwok P. Chun, Kevin J. McGuire, Jeffrey J. McDonnell
Abstract: Time variant catchment transit time distributions are fundamental descriptors of catchment function but yet not fully understood, characterized, and modeled. Here we present a new approach for use with standard runoff and tracer datasets that is based on tracking of tracer and age information and time‐variant catchment mixing. Our new approach is able to deal with non‐stationarity of flow paths and catchment mixing, and an irregular shape of the transit time distribution. The approach extracts information on catchment mixing from the stable isotope time series instead of prior assumptions of mixing or the shape of transit time distribution. We first demonstrate proof of concept of the approach with artificial data; the Nash‐Sutcliffe efficiencies in tracer and instantaneous transit times were >0.9. The model provides very accurate estimates of time variant transit times when the boundary conditions and fluxes are fully known. We then tested the model with real rainfall‐runoff flow and isotope tracer time series from the HJ Andrews Watershed 10 (WS10) in Oregon. Model efficiencies were 0.37 for the 18O modeling for a 2‐year time series; the efficiencies increased to 0.86 for the second year underlying the need of long time tracer time series with a long overlap of tracer input and output. The approach was able to determine time variant transit time of WS10 with field data and showed how it follows the storage dynamics and related changes in flow paths where wet periods with high flows resulted in clearly shorter transit times compared to dry low flow periods. This article is protected by copyright. All rights reserved.
- Geomechanics of subsurface water withdrawal and injection
- Authors: Giuseppe Gambolati; Pietro Teatini
Abstract: Land subsidence and uplift, ground ruptures, and induced seismicity are the principal geomechanic effects of groundwater withdrawal and injection. The major environmental consequence of groundwater pumping is anthropogenic land subsidence. The first observation concerning land settlement linked to subsurface processes was made in 1926 by the American geologists Pratt and Johnson, who wrote that “the cause of subsidence is to be found in the extensive extraction of fluid from beneath the affected area”. Since then, impressive progress has been made in terms of: a) recognizing the basic hydrologic and geomechanic principles underlying the occurrence; b) measuring aquifer compaction and ground displacements, both vertical and horizontal; c) modelling and predicting the past and future event; d) mitigating environmental impact through aquifer recharge and/or surface water injection. The first milestone in the theory of pumped aquifer consolidation was reached in 1923 by Terzaghi, who introduced the principle of “effective intergranular stress”. In the early 70s the emerging computer technology facilitated development of the first mathematical model of the subsidence of Venice, made by Gambolati and Freeze. Since then the comprehension, measuring, and simulation of the occurrence have improved dramatically. More challenging today are the issues of ground ruptures and induced/triggered seismicity, which call for a shift from the classical continuum approach to discontinuous mechanics. Although well‐known for decades, anthropogenic land subsidence is still threatening large urban centres and deltaic areas worldwide, such as Bangkok, Jakarta, and Mexico City, at rates in the order of 10 cm/yr. This article is protected by copyright. All rights reserved.
- The spatial and temporal evolution of contributing areas
- Authors: Fabian Nippgen; Brian L. McGlynn, Ryan E. Emanuel
Abstract: Predicting runoff source areas and how they change through time is a challenge in hydrology. Topographically induced lateral water redistribution and water removal through evapotranspiration lead to spatially and temporally variable patterns of watershed water storage. These dynamic storage patterns combined with threshold mediation of saturated subsurface throughflow lead to runoff source areas that are dynamic through time. To investigate these processes and their manifestation in watershed runoff, we developed and applied a parsimonious but spatially distributed model (WECOH ‐ Watershed ECOHydrology). Evapotranspiration was measured via an eddy‐covariance tower located within the catchment and disaggregated as a function of vegetation structure. This modeling approach reproduced the stream hydrograph well and was internally consistent with observed watershed runoff patterns and behavior. We further examined the spatial patterns of water storage and their evolution through time by building on past research focused on landscape hydrologic connectivity. The percentage of landscape area connected to the stream network ranged from less than 1% during the fall and winter baseflow period to 71% during snowmelt. Over the course of the two‐year study period 90% of the watershed areas were connected to the stream network for at least one day, leaving 10% of area that never became connected. Runoff source areas during the event shifted from riparian dominated runoff to areas at greater distances from the stream network when hillslopes became connected. Our modeling approach elucidates and enables quantification and prediction of watershed active areas and those active areas connected to the stream network through time. This article is protected by copyright. All rights reserved.
- Optimizing hydrological consistency by incorporating hydrological
signatures into model calibration objectives
- Authors: Mahyar Shafii; Bryan A. Tolson
Abstract: The simulated outcome of a calibrated hydrologic model should be hydrologically consistent with the measured response data. Hydrologic modelers typically calibrate models to optimize residual‐based goodness‐of‐fit measures, e.g., the Nash‐Sutcliffe Efficiency measure, and then evaluate the obtained results with respect to hydrological signatures, e.g., the flow duration curve indices. The literature indicates that the consideration of a large number of hydrologic signatures has not been addressed in a full multi‐objective optimization context. This research develops a model calibration methodology to achieve hydrological consistency using goodness‐of‐fit measures, many hydrological signatures, as well as a level of acceptability for each signature. The proposed framework relies on a scoring method that transforms any hydrological signature to a calibration objective. These scores are used to develop the hydrological consistency metric, which is maximized to obtain hydrologically consistent parameter sets during calibration. This consistency metric is implemented in different signature‐based calibration formulations that adapt the sampling according to hydrologic signature values. These formulations are compared with the traditional formulations found in the literature for seven case studies. The results reveal that Pareto dominance‐based multi‐objective optimization yields the highest level of consistency among all formulations. Furthermore, it is found that the choice of optimization algorithms does not affect the findings of this research. This article is protected by copyright. All rights reserved.
- Isotope‐based Fluvial Organic Carbon (ISOFLOC) Model: Model
Formulation, Sensitivity and Evaluation
- Authors: William I. Ford; James F. Fox
Abstract: Watershed‐scale carbon budgets remain poorly understood, in part due to inadequate simulation tools to assess in‐stream carbon fate and transport. A new numerical model termed ISOtope‐based FLuvial Organic Carbon (ISOFLOC) is formulated to simulate the fluvial organic carbon budget in watersheds where hydrologic, sediment transport, and biogeochemical processes are coupled to control benthic and transported carbon composition and flux. One ISOFLOC innovation is the formulation of new stable carbon isotope model subroutines that include isotope fractionation processes in order to estimate carbon isotope source, fate, and transport. A second innovation is the coupling of transfers between carbon pools, including algal particulate organic carbon, fine particulate and dissolved organic carbon, and particulate and dissolved inorganic carbon, to simulate the carbon cycle in a comprehensive manner beyond that of existing watershed water quality models. ISOFLOC was tested and verified in a low‐gradient, agriculturally‐impacted stream. Results of a global sensitivity analysis suggested the isotope response variable had unique sensitivity to the coupled interaction between fluvial shear resistance of algal biomass and the concentration of dissolved inorganic carbon. Model calibration and validation suggested good agreement at event, seasonal, and annual timescales. Multi‐objective uncertainty analysis suggested inclusion of the carbon stable isotope routine reduced uncertainty by 80% for algal particulate organic carbon flux estimates. This article is protected by copyright. All rights reserved.
- Identifying multiple timescale rainfall controls on Mojave Desert
ecohydrology using an integrated data and modeling approach for Larrea
- Authors: Gene‐Hua Crystal Ng; David R. Bedford, David M. Miller
Abstract: The perennial shrub Larrea tridentata is widely successful in North American warm deserts but is also susceptible to climatic perturbations. Understanding its response to rainfall variability requires consideration of multiple timescales. We examine intra‐annual to multi‐year relationships using model simulations of soil moisture and vegetation growth over 50 years in the Mojave National Preserve in southeastern California (USA). Ecohydrological model parameters are conditioned on field and remote sensing data using an ensemble Kalman filter. Although no specific periodicities were detected in the rainfall record, simulated leaf‐area‐index exhibits multi‐year dynamics that are driven by multi‐year (∼3‐years) rains, but with up to a 1‐year delay in peak response. Within a multi‐year period, Larrea tridentata is more sensitive to winter rains than summer. In the most active part of the root zone (above ∼80 cm), >1‐year average soil moisture drives vegetation growth, but monthly average soil moisture is controlled by root uptake. Moisture inputs reach the lower part of the root zone (below ∼80 cm) infrequently, but once there they can persist over a year to help sustain plant growth. Parameter estimates highlight efficient plant physiological properties facilitating persistent growth and high soil hydraulic conductivity allowing deep soil moisture stores. We show that soil moisture as an ecological indicator is complicated by bidirectional interactions with vegetation that depend on timescale and depth. Under changing climate, Larrea tridentata will likely be relatively resilient to shorter‐term moisture variability but will exhibit higher sensitivity to shifts in seasonal to multi‐year moisture inputs. This article is protected by copyright. All rights reserved.
- Changes in dissolved organic carbon and total dissolved nitrogen fluxes
across subtropical forest ecosystems at different successional stages
- Authors: Junhua Yan; Kun Li, Wantong Wang, Deqiang Zhang, Guoyi Zhou
Abstract: Lateral transports of carbon and nitrogen are important processes linking terrestrial ecosystems and aquatic systems. Most previous studies made in temperate forests found that fluxes of carbon and nitrogen by runoff water varied in different forests, but few studies have been made in subtropical forests. This study was to investigate dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) fluxes at the catchment scale along a subtropical forest succession gradient from pine forest (pioneer) to coniferous and broadleaved mixed forest (transitional) to broadleaved forest (mature). Our results showed that DOC concentration significantly decreased (p
- Analyzing the effects of excess rainfall properties on the scaling
structure of peak discharges: Insights from a mesoscale river basin
- Authors: Tibebu B. Ayalew; Witold F. Krajewski, Ricardo Mantilla
Abstract: Key theoretical and empirical results from the past two decades have established that peak discharges resulting from a single rainfall‐runoff event in a nested watershed exhibit a power‐law, or scaling, relation to drainage area and that the parameters of the power‐law relation, henceforth referred to as the flood scaling exponent and intercept, change from event to event. To date, only two studies have been conducted using empirical data, both using data from the 21 km2 Goodwin Creek Experimental Watershed that is located in Mississippi, in an effort to uncover the physical processes that control the event‐to‐event variability of the flood scaling parameters. Our study expands the analysis to the mesoscale Iowa River basin (A=32,400 km2), which is located in eastern Iowa, and provides additional insights into the physical processes that control the flood scaling parameters. Using 51 rainfall‐runoff events that we identified over the 12 year period since 2002, we show how the duration and depth of excess rainfall, which is the portion of rainfall that contributes to direct runoff, control the flood scaling exponent and intercept. Moreover, using a diagnostic simulation study that is guided by evidence found in empirical data, we show that the temporal structure of excess rainfall has a significant effect on the scaling structure of peak discharges. These insights will contribute towards ongoing efforts to provide a framework for flood prediction in ungauged basins (PUB). This article is protected by copyright. All rights reserved.
- Observations and modeling of hillslope throughflow temperatures in a
coastal forested catchment
- Authors: J. A. Leach; R. D. Moore
Abstract: A growing body of research on stream thermal regimes has highlighted the importance of heat advection associated with surface water and groundwater interactions, such as hyporheic exchange, groundwater discharge, and hillslope throughflow inputs. Existing catchment models that predict stream temperature use a variety of approaches to estimate throughflow temperatures, but none has been evaluated against field measurements of throughflow temperature. In this study, throughflow temperatures were monitored over two winters at fifty locations adjacent to a headwater stream (11 ha catchment area) located in the rain‐on‐snow zone of the Pacific Northwest. Existing approaches to estimating throughflow temperature under‐ or over‐predicted throughflow temperatures by up to 5 ∘C, or were unable to represent the influence of transient snow cover. Therefore, a conceptual‐parametric model that is computationally efficient was developed that simulates hillslope hydrology and throughflow temperatures. The model structure includes an upslope reservoir that drains into a downslope reservoir that, in turn, drains into the stream. Vertical and lateral energy and water fluxes are simulated using simplified process representations. The model successfully predicts throughflow temperatures and highlights the dominant role of throughflow advection and the influence of snow cover on stream thermal regimes during high flow periods and rain‐on‐snow events. This article is protected by copyright. All rights reserved.
- Comment on “Effects of tidal fluctuations on mixing and spreading in
coastal aquifers: Homogeneous case” by M. Pool, V. E. A. Post, and
C. T. Simmons
- Authors: Behzad Ataie‐Ashtiani
- Reply to comment by M. Pool, V. E. A. Post, and C. T. Simmons on
“Effects of tidal fluctuations on mixing and spreading in coastal
aquifers: Homogeneous case”
- Authors: María Pool; Vincent E.A. Post, Craig T. Simmons
- Flood response for the watersheds of the Fernow Experimental Forest in the
- Authors: Naomi S. Bates; James A. Smith, Gabriele Villarini
Abstract: We examine flood response of high‐gradient, forested central Appalachian watersheds through analyses of rainfall, streamflow and piezometer observations from the Fernow Experimental Forest near Parsons, West Virginia. Analyses focus on hydrologic processes that control the “upper tail” of flood distributions. The largest flood peaks in the Fernow are an order of magnitude smaller than record floods in the central Appalachian region (for basins of comparable drainage area). We examine flood distributions in the Fernow using extreme value distributions (Generalized Extreme Value and Generalized Pareto distributions) and compare them to other watersheds in the central Appalachians. To examine the role of antecedent soil moisture on flood response, we installed a network of 415 crest‐stage piezometers on two headwater watersheds (0.30 and 0.14 km2) of the Fernow. Observations show pronounced heterogeneity of subsurface saturation even within the unchannelized swales of headwater watersheds. Shallow perched water tables over large portions of a watershed occur infrequently in forested central Appalachian basins, but may play an important role in extreme flood response. Fernow watersheds include “treated” and control watersheds with stream gaging records extending back to 1951. We examine nonstationarites in flood frequency in the Fernow and show that forest management practices have had relatively minor impacts on flood frequency. This article is protected by copyright. All rights reserved.
- Seeing the landscape for the trees: Metrics to guide riparian shade
management in river catchments
- Authors: Matthew F. Johnson; Robert L. Wilby
Abstract: Rising water temperature (Tw) due to anthropogenic climate change may have serious consequences for river ecosystems. Conservation and/or expansion of riparian shade could counter warming and buy time for ecosystems to adapt. However, sensitivity of river reaches to direct solar radiation is highly heterogeneous in space and time, so benefits of shading are also expected to be site specific. We use a network of high‐resolution temperature measurements from two upland rivers in the UK, in conjunction with topographic shade modelling, to assess the relative significance of landscape and riparian shade to the thermal behaviour of river reaches. Trees occupy 7% of the study catchments (comparable with the UK national average) yet shade covers 52% of the area and is concentrated along river corridors. Riparian shade is most beneficial for managing Tw at distances 5 to 20 km downstream from the source of the rivers where discharge is modest, flow is dominated by near‐surface hydrological pathways, there is a wide floodplain with little landscape shade, and where cumulative solar exposure times are sufficient to affect Tw. For the rivers studied, we find that approximately 0.5 km of complete shade is necessary to off‐set Tw by 1°C during July (the month with peak Tw) at a headwater site; whereas 1.1 km of shade is required 25 km downstream. Further research is needed to assess the integrated effect of future changes in air temperature, sunshine duration, direct solar radiation and downward diffuse radiation on Tw to help tree planting schemes achieve intended outcomes. This article is protected by copyright. All rights reserved.
- Estimation of the bed shear stress in vegetated and bare channels with
- Authors: Judy Q. Yang; Francois Kerger, Heidi M. Nepf
Abstract: The shear stress at the bed of a channel influences important benthic processes such as sediment transport. Several methods exist to estimate the bed shear stress in bare channels without vegetation, but most of these are not appropriate for vegetated channels due to the impact of vegetation on the velocity profile and turbulence production. This study proposes a new model to estimate the bed shear stress in both vegetated and bare channels with smooth beds. The model, which is supported by measurements, indicates that for both bare and vegetated channels with smooth beds, within a viscous sub‐layer at the bed, the viscous stress decreases linearly with increasing distance from the bed, resulting in a parabolic velocity profile at the bed. For bare channels, the model describes the velocity profile in the overlap region of the Law of the Wall. For emergent canopies of sufficient density (frontal area per unit canopy volume a≥4.3m‐1), the thickness of the linear‐stress layer is set by the stem diameter, leading to a simple estimate for bed shear stress. This article is protected by copyright. All rights reserved.
- A revised model for microbially induced calcite precipitation ‐
improvements and new insights based on recent experiments
- Authors: Johannes Hommel; Ellen Lauchnor, Adrienne Phillips, Robin Gerlach, Alfred B. Cunningham, Rainer Helmig, Anozie Ebigbo, Holger Class
Abstract: The model for microbially induced calcite precipitation (MICP) published by Ebigbo et al. (2012) has been improved based on new insights obtained from experiments and model calibration. The challenge in constructing a predictive model for permeability reduction in the underground with MICP is the quantification of the complex interaction between flow, transport, biofilm growth, and reaction kinetics. New data from Lauchnor et al. (2015) on whole‐cell ureolysis kinetics from batch experiments was incorporated into the model, which has allowed for a more precise quantification of the relevant parameters as well as a simplification of the reaction kinetics in the equations of the model. Further, the model has been calibrated objectively by inverse modeling using quasi‐1D column experiments and a radial flow experiment. From the post‐processing of the inverse modeling, a comprehensive sensitivity analysis has been performed with focus on the model input parameters that were fitted in the course of the model calibration. It reveals that calcite precipitation and concentrations of and Ca2+ are particularly sensitive to parameters associated with the ureolysis rate and the attachment behavior of biomass. Based on the determined sensitivities and the ranges of values for the estimated parameters in the inversion, it is possible to identify focal areas where further research can have a high impact towards improving the understanding and engineering of MICP. This article is protected by copyright. All rights reserved.
- Digital catchment observatories: A platform for engagement and knowledge
exchange between catchment scientists, policy makers, and local
- Authors: E. B. Mackay; M. E. Wilkinson, C.J.A. Macleod, K. Beven, B.J. Percy, M.G. Macklin, P.F. Quinn, M. Stutter, P.M. Haygarth
Abstract: Increasing pressures on the hydrological cycle from our changing planet have led to calls for a refocus of research in the sciences of hydrology and water resources. Opportunities for new and innovative research into these areas are being facilitated by advances in the use of cyberinfrastructure, such as the development of digital catchment observatories. This is enabling research into hydrological issues such as flooding to be approached differently. The ability to combine different sources of data, knowledge and modelling capabilities from different groups such as scientists, policy makers and the general public has the potential to provide novel insights into the way individual catchments respond at different temporal and spatial scales. While the potential benefits of the digital catchment observatory are large, this new way of carrying out research into hydrological sciences is likely to prove challenging on many levels. Along with the obvious technical and infrastructural challenges to this work, an important area for consideration is how to enable a digital observatory to work for a range of potential end‐users, paving the ways for new areas of research through developing a platform effective for engagement and knowledge exchange. Using examples from the recent local‐scale hydrological exemplar in the Environmental Virtual Observatory pilot project (http://www.evo‐uk.org), this commentary considers a number of issues around the communication between and engagement of different users, the use of local knowledge and uncertainty with cloud‐based models and the potential for decision support and directions for future research. This article is protected by copyright. All rights reserved.
- Stormflow generation: A metaanalysis of field evidence from small,
- Authors: Frauke K. Barthold; Ross A. Woods
Abstract: Combinations of runoff characteristics are commonly used to represent distinct conceptual models of stormflow generation. In this study, three runoff characteristics: hydrograph response, time source of runoff water, and flow path, are used to classify catchments. Published data from the scientific literature are used to provide evidence from small, forested catchments. Each catchment was assigned to one of the eight conceptual models, depending on the combination of quick/slow response, old/new water, and overland/subsurface flow. A standard procedure was developed to objectively diagnose the predominant conceptual model of stormflow generation for each catchment and assess its temporal and spatial support.
The literature survey yielded 42 catchments, of which 30 catchments provide a complete set of qualitative runoff characteristics resulting in one of the 8 conceptual models. The majority of these catchments classify as subsurface flow path dominated. No catchments were found for conceptual models representing combinations of quick response – new water – subsurface flow (SSF), slow – new – SSF, slow – old – overland flow (OF) nor new – slow – OF. Of the 30 qualitatively classified catchments, 24 provide a complete set of quantitative measures. In summary, the field support is strong for 19 subsurface‐dominated catchments and is weak for 5 surface flow path dominated catchments (6 catchments had insufficient quantitative data). Two alternative explanations exist that may explain the imbalance of field support between the two flow path classes: (1) the selection of research catchments in past field studies was mainly to explain quick hydrograph response in subsurface dominated catchments; (2) catchments with prevailing subsurface flow paths are more common in nature. We conclude that the selection of research catchments needs to cover a wider variety of environmental conditions which should lead to a broader, and more widely applicable, spectrum of resulting conceptual models and process mechanisms. This is a prerequisite in studies where catchment organization and similarity approaches are used to develop catchment classification systems in order to regionalize stormflow. This article is protected by copyright. All rights reserved.
- Declining rainfall and regional variability changes in Jordan
- Authors: Kazi Rahman; Steven M Gorelick, P James Dennedy‐Frank, Jim Yoon, Bala Rajaratnam
Abstract: Jordan, with limited rainfall, has per capita water availability of 135 m3/y making it one of the water‐poorest countries in the world. We analyzed the most comprehensive modern rainfall data set to date, consisting of 44 years of daily measurements from 58 stations primarily in the western, populated and agricultural portion of Jordan over the period 1970‐2013 to assess temporal trends, variability, and spatial patterns. From 1995‐2013, 13 of 19 years showed rainfall less than the mean, which has a probability
- A new general 1‐D vadose zone flow solution method
- Authors: Fred L. Ogden; Wencong Lai, Robert C. Steinke, Jianting Zhu, Cary A. Talbot, John L. Wilson
Abstract: We have developed an alternative to the one‐dimensional partial differential equation (PDE) attributed to Richards  that describes unsaturated porous media flow in homogeneous soil layers. Our solution is a set of three ordinary differential equation (ODEs) derived from unsaturated flux and mass conservation principles. We used a hodograph transformation, the Method of Lines, and a finite water‐content discretization to produce ODEs that accurately simulate infiltration, falling slugs, and groundwater table dynamic effects on vadose zone fluxes. This formulation, which we refer to as "finite water‐content" simulates sharp fronts, and is guaranteed to conserve mass using a finite‐volume solution. Our ODE solution method is explicitly integrable, does not require iterations and therefore has no convergence limits and is computationally efficient. The method accepts boundary fluxes including arbitrary precipitation, bare soil evaporation and evapotranspiration. The method can simulate heterogeneous soils using layers. Results are presented in terms of fluxes and water content profiles. Comparing our method against analytical solutions, laboratory data, and the Hydrus‐1D solver, we find that predictive performance of our finite water‐content ODE method is comparable to or in some cases exceeds that of the solution of Richards' equation, with or without a shallow water table. The presented ODE method is transformative in that it offers accuracy comparable to the Richards  PDE numerical solution, without the numerical complexity, in a form that is robust, continuous, and suitable for use in large watershed and land‐atmosphere simulation models, including regional‐scale models of coupled climate and hydrology. This article is protected by copyright. All rights reserved.
- On the assessment of reliability in probabilistic hydrometeorological
- Authors: Caleb M. DeChant; Hamid Moradkhani
Abstract: Probabilistic forecasts are commonly used to communicate uncertainty in the occurrence of hydro‐meteorological events. Although probabilistic forecasting is common, conventional methods for assessing the reliability of these forecasts are approximate. Among the most common methods for assessing reliability, the decomposed Brier Score and Reliability Diagram treat an observed string of events as samples from multiple Binomial distributions, but this is an approximation of the forecast reliability, leading to unnecessary loss of information. This article suggests testing the hypothesis of reliability via the Poisson‐Binomial distribution, which is a generalized solution to the Binomial distribution, providing a more accurate model of the probabilistic event forecast verification setting. Further, a two‐stage approach to reliability assessment is suggested to identify errors in the forecast related to both bias and overly/insufficiently sharp forecasts. Such a methodology is shown to more effectively distinguish between reliable and unreliable forecasts, leading to more robust probabilistic forecast verification. This article is protected by copyright. All rights reserved.
- A general analytical solution for steady flow in heterogeneous porous
- Authors: J. R. Craig
Abstract: A novel analytical solution approach for problems of steady flow in two‐dimensional heterogeneous porous media is presented, where the hydraulic conductivity may be be represented as an arbitrary polynomial in space. The solution approach uses Wirtinger calculus and the Bers‐Vekua theory of elliptical functions. The final form of the solution comprises an arbitrary complex polynomial solution to the Laplace equation and additional non‐holomorphic terms which are determined directly from the coefficients of this polynomial. The arbitrary polynomial coefficients may be chosen to satisfy general flow conditions along system boundaries. The approach is also extended to singular flow, such as that induced by pumping wells. The solution is demonstrated to be effectively exact for a number of test cases; the problems are solved to machine precision. This article is protected by copyright. All rights reserved.
- Debates—Perspectives on sociohydrology: Capturing feedbacks between
physical and social processes
- Authors: Giuliano Di Baldassarre; Alberto Viglione, Gemma Carr, Linda Kuil, Kun Yan, Luigia Brandimarte, Günter Blöschl
Abstract: In flood risk assessment, there remains a lack of analytical frameworks capturing the dynamics emerging from two‐way feedbacks between physical and social processes, such as adaptation and levee effect. The former, “adaptation effect”, relates to the observation that the occurrence of more frequent flooding is often associated with decreasing vulnerability. The latter, “levee effect”, relates to the observation that the non‐occurrence of frequent flooding (possibly caused by flood protection structures, e.g. levees) is often associated to increasing vulnerability. As current analytical frameworks do not capture these dynamics, projections of future flood risk are not realistic.
In this paper, we develop a new approach whereby the mutual interactions and continuous feedbacks between floods and societies are explicitly accounted for. Moreover, we show an application of this approach by using a socio‐hydrological model to simulate the behavior of two main prototypes of societies: green societies, which cope with flooding by resettling out of flood‐prone areas; and technological societies, which deal with flooding also by building levees or dikes. This application shows that the proposed approach is able to capture and explain the aforementioned dynamics (i.e. adaptation and levee effect) and therefore contribute to a better understanding of changes in flood risk, within an iterative process of theory development and empirical research. This article is protected by copyright. All rights reserved.
- Debates—Perspectives on sociohydrology: Introduction
- Authors: Alberto Montanari
- Debates—Perspectives on sociohydrology: Changing water systems and
the “tyranny of small problems”—Sociohydrology
- Authors: Murugesu Sivapalan
Abstract: We are well and truly in the Anthropocene. Humans can no longer be considered as mere external drivers or boundary conditions in the hydrologic systems we study. The interactions and feedbacks between human actions and water cycle dynamics on the planet, combined with the evolution of human norms/values in relation to water, are throwing up a range of emergent “big problems”. Understanding and offering sustainable solutions to these “big problems” require a broadening of hydrologic science to embrace the perspectives of both social and natural scientists. The new science of socio‐hydrology was introduced with this in mind, yet faces major challenges due to the wide gulf that separates the knowledge foundations and methodologies of natural and social sciences. Yet, the benefits of working together are enormous, including through adoption of natural science methods for social science problems, and vice versa. Bringing together the perspectives of both social and natural scientists dealing with water is good for hydrologic science, having the salutary effect of revitalizing it as use‐inspired basic science. It is good for management too, in that the broader, holistic perspectives provided by socio‐hydrology can help recognize potential “big” problems that may otherwise be unforeseen and, equally, identify potential “alternative” solutions to otherwise intractable problems. This article is protected by copyright. All rights reserved.
- Comment on “Debates—Perspectives on sociohydrology: Simulating
hydrologic‐human interactions” by M. Sivapalan
- Authors: Daniel P. Loucks
Abstract: Humans and their social institutions have a strong impact on the design and operation of our water resource systems, whether in response to flooding, drought, or just normally occurring events that include allocating water to different water users. Humans are a part of most water resource systems that modelers are asked to study. If modelers were able to predict human behavior under various hydrologic scenarios, and how that behavior affects the performance of our water resource systems, we would be better able to manage them and perhaps derive additional benefits from them. Di Baldassarre, et al. (2013a, 2013b, 2015) and Sivapalan et al. (2012), have suggested a modeling approach that couples the hydrologic and social components of water resource systems in order to better understand such interactions between these components. Their dynamic modeling approach coupling the social and hydrologic components of an urban flood model motivates the discussion in this paper on how their modeling approach might be extended to estimate possible human responses resulting from their perceptions of the effectiveness of a range of flood management and mitigation measures. But even with such extensions this writer is somewhat pessimistic as to the eventual ability of any mathematical models to predict even the probabilities of possible human or social actions without the direct participation of stakeholders. It is such stakeholders whose behavior and decisions will impact how we design and operate our water resource systems and will influence how well these systems will meet various economic and social objectives. This article is protected by copyright. All rights reserved.
- Debates—Perspectives on sociohydrology: Sociohydrologic
modeling—Tradeoffs, hypothesis testing, and validation
- Authors: Tara J. Troy; Mitchell Pavao‐Zuckerman, Tom P. Evans
Abstract: Socio‐hydrology focuses on studying the dynamics and co‐evolution of coupled human and water systems. Recently, several new socio‐hydrologic models have been published that explore these dynamics, and these models offer unique opportunities to better understand these coupled systems and to understand how water problems evolve similarly in different regions. These models also offer challenges, as decisions need to be made by the modeler on trade‐offs between generality, precision, and realism. In addition, traditional hydrologic model validation techniques, such as evaluating simulated streamflow, are insufficient, and new techniques must be developed. As socio‐hydrology progresses, these models offer a robust, invaluable tool to test hypotheses about the relationships between aspects of the coupled human‐water systems. They will allow us to explore multiple working hypotheses to greatly expand insights and understanding of coupled socio‐hydrologic systems. This article is protected by copyright. All rights reserved.
- Hydrology: The interdisciplinary science of water
- Authors: Richard M. Vogel; Upmanu Lall, Ximing Cai, Balaji Rajagopalan, Peter Weiskel, Richard P. Hooper, Nicholas C. Matalas
Abstract: We live in a world where biophysical and social processes are tightly coupled. Hydrologic systems change in response to a variety of natural and human forces such as climate variability and change, water use and water infrastructure and land cover change. In turn, changes in hydrologic systems impact socio‐economic, ecological and climate systems at a number of scales, leading to a co‐evolution of these interlinked systems. The Harvard Water Program, Hydro‐sociology, Integrated Water Resources Management, Eco‐Hydrology, Hydromorphology and Socio‐Hydrology were all introduced to provide different, interdisciplinary perspectives on water problems to address the dynamics of human interaction with the hydrosphere and the evolution Earth's hydrologic systems during the Anthropocene epoch. Each of them addresses scientific, social and engineering challenges related to how humans influence water systems and vice versa. There are now numerous examples in the literature of how holistic approaches can provide a structure and vision of the future of hydrology. We review selected examples, which taken together, describe the type of theoretical and applied integrated hydrologic analyses and associated curricular content required to address the societal issue of water‐resources sustainability. We describe a modern interdisciplinary science of hydrology needed to develop an in‐depth understanding of the dynamics of the connectedness between human and natural systems and to determine effective solutions to resolve the complex water problems that the world faces today. Nearly every theoretical hydrologic model introduced previously is in need of revision to accommodate how climate, land, vegetation and socio‐economic factors interact, change and evolve over time. This article is protected by copyright. All rights reserved.
- Debates perspectives on sociohydrology: Modeling flood risk as a public
- Authors: Patricia Gober; Howard S. Wheater
Abstract: Socio‐hydrology views human activities as endogenous to water system dynamics; it is the interaction between human and biophysical processes that threatens the viability of current water systems through positive feedbacks and unintended consequences. Di Baldassarre et al.  implement socio‐hydrology as a flood risk problem using the concept of social memory as a vehicle to link human perceptions to flood damage. Their mathematical model has heuristic value in comparing potential flood damages in green versus technological societies. It can also support communities in exploring the potential consequences of policy decisions and evaluating critical policy tradeoffs, for example, between flood protection and economic development. The concept of social memory does not, however, adequately capture the social processes whereby public perceptions are translated into policy action, including the pivotal role played by the media in intensifying or attenuating perceived flood risk, the success of policy entrepreneurs in keeping flood hazard on the public agenda during short windows of opportunity for policy action, and different societal approaches to managing flood risk that derive from cultural values and economic interests. We endorse the value of seeking to capture these dynamics in a simplified conceptual framework, but favor a broader conceptualization of socio‐hydrology that includes a knowledge exchange component, including the way modeling insights and scientific results are communicated to floodplain managers. The social processes used to disseminate the products of socio‐hydrological research are as important as the research results themselves in determining whether modeling is used for real‐world decision making. This article is protected by copyright. All rights reserved.
- Should hydraulic tomography data be interpreted using geostatistical
inverse modeling? A laboratory sandbox investigation
- Authors: Walter A. Illman; Steven J. Berg, Zhanfeng Zhao
Abstract: The robust performance of hydraulic tomography (HT) based on geostatistics has been demonstrated through numerous synthetic, laboratory and field studies. While geostatistical inverse methods offer many advantages, one key disadvantage is its highly parameterized nature, which renders it computationally intensive for large‐scale problems. Another issue is that geostatistics‐based HT may produce overly smooth images of subsurface heterogeneity when there are few monitoring interval data. Therefore, some may question the utility of the geostatistical inversion approach in certain situations and seek alternative approaches. To investigate these issues, we simultaneously calibrated different groundwater models with varying subsurface conceptualizations and parameter resolutions using a laboratory sandbox aquifer. The compared models included: 1) isotropic and anisotropic effective parameter models; 2) a heterogeneous model that faithfully represents the geological features; and 3) a heterogeneous model based on geostatistical inverse modeling. The performance of these models was assessed by quantitatively examining the results from model calibration and validation. Calibration data consisted of steady state drawdown data from eight pumping tests and validation data consisted of data from 16 separate pumping tests not used in the calibration effort. Results revealed that the geostatistical inversion approach performed the best among the approaches compared, although the geological model that faithfully represented stratigraphy came a close second. In addition, when the number of pumping tests available for inverse modeling was small, then the geological modeling approach yielded more robust validation results. This may suggest that the better knowledge of stratigraphy obtained via geophysics or other means may contribute to improved results for HT. This article is protected by copyright. All rights reserved.
- Groundwater in the Earth's critical zone—Relevance to
large‐scale patterns and processes
- Authors: Ying Fan
Abstract: Although we have an intuitive understanding of the behavior and functions of groundwater in the Earth's critical zone at the scales of a column (atmosphere‐plant‐soil‐bedrock), along a topo‐sequence (ridge to valley), and across a small catchment (
- Uncertainty in modeled and observed climate change impacts on American
- Authors: Jonathan M. Winter; Pat J.‐F. Yeh, Xiaojing Fu, Elfatih A.B. Eltahir
Abstract: An important potential consequence of climate change is the modification of the water cycle in agricultural areas, such as the American Midwest. Soil moisture is the integrand of the water cycle, reflecting dynamics of precipitation, evapotranspiration, and runoff in space and time, and a key determinant of yield. Here we present projected changes in the hydrologic cycle over a representative area of the American Midwest from regional climate model experiments that sample a range of model configurations. While significant summer soil moisture drying is predicted in some ensemble members others predict soil moisture wetting, with the sign of soil moisture response strongly influenced by our choice of boundary conditions. To resolve the contradictory predictions of soil moisture across ensemble members, we assess an extensive and unique observational dataset of the water budget in Illinois. No statistically significant monotonic trends are found in observed soil moisture, precipitation, streamflow, groundwater level, or 2‐m air temperature over a recent 26‐year period (soil moisture 25 years). Based on this analysis of model simulations and observations, we conclude that the sign of climate change impacts on the regional hydrology of the American Midwest remains uncertain. This article is protected by copyright. All rights reserved.
- Delta channel networks: 2. Metrics of topologic and dynamic complexity for
delta comparison, physical inference, and vulnerability assessment
- Authors: Alejandro Tejedor; Anthony Longjas, Ilya Zaliapin, Efi Foufoula‐Georgiou
Abstract: Deltas are landforms that deliver water, sediment and nutrient fluxes from upstream rivers to the deltaic surface and eventually to oceans or inland water bodies via multiple pathways. Despite their importance, quantitative frameworks for their analysis lack behind those available for tributary networks. In a companion paper [Tejedor et al., 2015], we conceptualized delta channel networks as directed graphs and used spectral graph theory to design a quantitative framework for exploring delta connectivity and flux dynamics. Here we use this framework to introduce a suite of graph‐theoretic and entropy‐based metrics, to quantify two components of a delta's complexity: (1) Topologic, imposed by the network connectivity and (2) Dynamic, dictated by the flux partitioning and distribution. The metrics are aimed to facilitate comparing, contrasting, and establishing connections between deltaic structure, process, and form. We illustrate the proposed analysis using seven deltas in diverse morphodynamic environments and of various degrees of channel complexity. We project deltas into a topo‐dynamic space whose coordinates are given by topologic and dynamic delta complexity metrics, and show that this space provides a basis for delta comparison and physical insight into their dynamic behavior. We also show that the examined metrics relate to the intuitive notion of vulnerability, measured by the impact of upstream flux changes to the shoreline flux, and reveal that complexity and vulnerability are inversely related. Finally, we use a spatially explicit metric, akin to a delta width function, to classify shapes of different delta types. This article is protected by copyright. All rights reserved.
- Delta channel networks: A graph‐theoretic approach for studying
connectivity and steady state transport on deltaic surfaces
- Authors: Alejandro Tejedor; Anthony Longjas, Ilya Zaliapin, Efi Foufoula‐Georgiou
Abstract: River deltas are intricate landscapes with complex channel networks that self‐organize to deliver water, sediment, and nutrients from the apex to the delta top and eventually to the coastal zone. The natural balance of material and energy fluxes, which maintains a stable hydrologic, geomorphologic, and ecological state of a river delta, is often disrupted by external perturbations causing topological and dynamical changes in the delta structure and function. A formal quantitative framework for studying delta channel network connectivity and transport dynamics and their response to change is lacking. Here we present such a framework based on spectral graph theory and demonstrate its value in computing delta's steady state fluxes and identifying upstream (contributing) and downstream (nourishment) areas and fluxes from any point in the network. We use this framework to construct vulnerability maps that quantify the relative change of sediment and water delivery to the shoreline outlets in response to possible perturbations in hundreds of upstream links. The framework is applied to the Wax Lake delta in the Louisiana coast of the US and the Niger delta in West Africa. In a companion paper, we present a comprehensive suite of metrics that quantify topologic and dynamic complexity of delta channel networks and, via application to seven deltas in diverse environments, demonstrate their potential to reveal delta morphodynamics and relate to notions of vulnerability and robustness. This article is protected by copyright. All rights reserved.
- Partitioning of evapotranspiration using high frequency water vapor
isotopic measurement over a rice paddy field
- Authors: Zhongwang Wei; Kei Yoshimura, Atsushi Okazaki, Wonsik Kim, Zhongfang Liu, Masaharu Yokoi
Abstract: Partitioning ecosystem evapotranspiration (ET) into soil evaporation (E) and transpiration (T) is crucial for understanding hydrological processes. In this study, by using high‐frequency isotope measurements and continuous surface water measurements, we investigated the isotope ratios in soil‐vegetation‐atmosphere transfer and the physical mechanisms involved over a paddy field for a full growing season. The isotopic signals of δET, δT, and δE were determined by the Keeling plot method, surface water isotopic measurements, and the Craig‐Gordon model, respectively. The fraction of transpiration in evapotranspiration (FT) ranged from 0.2 to 1, with an almost continuous increase in the early growing season and a relatively constant value close to 1 later in the year. The result was supported by FT derived from simulated T and eddy correlation measured ET. The seasonal change in the transpiration fraction could be described quite well as a function of the LAI (FT=0.67LAI0.25, R2 = 0.80), implying that transpiration plays a dominant role in the soil‐vegetation‐atmosphere continuum during the growing season. The two end‐member uncertainty analysis suggested that further improvement in the estimation of δT and δET is necessary for partitioning evapotranspiration using the isotopic method. In the estimation of δET, the assumptions underlying Keeling plot method were rarely met and the uncertainty was quite large. A high frequency of precise isotopic measurements in surface water was also necessary for δT estimation. Furthermore, special care must be taken concerning the kinetic fractionation parameter in the Craig and Gordon Equation for δE estimation under low‐LAI conditions. The results demonstrated the robustness of using isotope measurements for partitioning evapotranspiration. This article is protected by copyright. All rights reserved.
- On the effect of connectivity on solute transport in spatially
heterogeneous combined unsaturated‐saturated flow systems
- Authors: David Russo
Abstract: Detailed numerical analyses of flow and transport were used to investigate the effect of spatially connected features on transport in three‐dimensional (3‐D), spatially heterogeneous, combined vadose zone‐groundwater flow systems. Formations with spatially connected fine‐ and coarse‐textured features (F‐ and C‐formations, respectively), representing the10th and the 90th percentiles of the distributions of the formation's hydraulic parameters, respectively, were considered here. Results of the analyses suggest that in steady‐state flow, when the unsaturated zone of the combined flow domains is relatively wet, as compared with a Multivariate‐Gaussian (MG) formation, spatially connected features may reduce the solute first arrival time, particularly in the C‐formation, and may enhance the spreading of the solute breakthrough, particularly in the F‐formation. The effect of the spatially connected features on the hydrological response, however, decreases as the unsaturated zone becomes drier. The latter result stems from the decrease in the fraction of the water‐filled, pore‐space occupied by the connected structures, with decreasing water content. The latter finding also explains the result that the response of more realistic, combined flow systems, whose unsaturated zone is associated with relatively low, intermittent water contents, is essentially independent of the spatially connected features of the formations, regardless of their soil texture. This article is protected by copyright. All rights reserved.
- On the probabilistic structure of water age
- Authors: Amilcare Porporato; Salvatore Calabrese
Abstract: The age distribution of water in hydrologic systems has received renewed interest recently, especially in relation to watershed response to rainfall inputs. The purpose of this contribution is first to draw attention to existing theories of age distributions in population dynamics, fluid mechanics and stochastic groundwater, and in particular to the McKendrick‐von Foerster equation and its generalizations and solutions. A second and more important goal is to clarify that, when hydrologic fluxes are modeled by means of time‐varying stochastic processes, the age distributions must themselves be treated as random functions. Once their probabilistic structure is obtained, it can be used to characterize the variability of age distributions in real systems and thus help quantify the inherent uncertainty in the field determination of water age. We illustrate these concepts with reference to a stochastic storage model, which has been used as a minimalist model of soil moisture and streamflow dynamics. This article is protected by copyright. All rights reserved.
- Reactive transport modeling of geochemical controls on secondary water
quality impacts at a crude oil spill site near Bemidji, MN
- Authors: Gene‐Hua Crystal Ng; Barbara A. Bekins, Isabelle M. Cozzarelli, Mary Jo Baedecker, Philip C. Bennett, Richard T. Amos, William N. Herkelrath
Abstract: Anaerobic biodegradation of organic amendments and contaminants in aquifers can trigger secondary water quality impacts that impair groundwater resources. Reactive transport models help elucidate how diverse geochemical reactions control the spatiotemporal evolution of these impacts. Using extensive monitoring data from a crude oil spill site near Bemidji, Minnesota (USA), we implemented a comprehensive model that simulates secondary plumes of depleted dissolved O2 and elevated concentrations of Mn2+, Fe2+, CH4, and Ca2+ over a two‐dimensional cross‐section for 30 years following the spill. The model produces observed changes by representing multiple oil constituents and coupled carbonate and hydroxide chemistry. The model includes reactions with carbonates and Fe and Mn mineral phases, outgassing of CH4 and CO2 gas phases, and sorption of Fe, Mn, and H+. Model results demonstrate that most of the carbon loss from the oil (70%) occurs through direct outgassing from the oil source zone, greatly limiting the amount of CH4 cycled downgradient. The vast majority of reduced Fe is strongly attenuated on sediments, with most (91%) in the sorbed form in the model. Ferrous carbonates constitute a small fraction of the reduced Fe in simulations, but may be important for furthering the reduction of ferric oxides. The combined effect of concomitant redox reactions, sorption, and dissolved CO 2 inputs from source‐zone degradation, successfully reproduced observed pH. The model demonstrates that secondary water quality impacts may depend strongly on organic carbon properties, and impacts may decrease due to sorption and direct outgassing from the source zone. This article is protected by copyright. All rights reserved.
- Balancing water scarcity and quality for sustainable irrigated agriculture
- Authors: Shmuel Assouline; David Russo, Avner Silber, Dani Or
Abstract: The challenge of meeting the projected doubling of global demand for food by 2050 is monumental. It is further exacerbated by the limited prospects for land expansion and rapidly dwindling water resources. A promising strategy for increasing crop yields per unit land requires the expansion of irrigated agriculture and the harnessing of water sources previously considered “marginal” (saline, treated effluent, and desalinated water). Such an expansion, however, must carefully consider potential long‐term risks on soil hydro‐ecological functioning. The study provides critical analyses of use of marginal water and management approaches to map out potential risks. Long‐term application of treated effluent (TE) for irrigation has shown adverse impacts on soil transport properties, and introduces certain health risks due to the persistent exposure of soil biota to anthropogenic compounds (e.g., promoting antibiotic resistance). The availability of desalinated water (DS) for irrigation expands management options, and improves yields while reducing irrigation amounts and salt loading into the soil. Quantitative models are used to delineate trends associated with long‐term use of TE and DS considering agricultural, hydrological and environmental aspects. The primary challenges to the sustainability of agro‐ecosystems lies with the hazards of saline and sodic conditions, and the unintended consequences on soil hydro‐ecological functioning. Multidisciplinary approaches that combine new scientific knowhow with legislative, economic and societal tools are required to ensure safe and sustainable use of water resources of different qualities. The new scientific knowhow should provide quantitative models for integrating key biophysical processes with ecological interactions at appropriate spatial and temporal scales. This article is protected by copyright. All rights reserved.
- Assessment of reservoir system variable forecasts
- Authors: Martin Kistenmacher; Aris P. Georgakakos
Abstract: Forecast ensembles are a convenient means to model water resources uncertainties and to inform planning and management processes. For multi‐purpose reservoir systems, forecast types include (i) forecasts of upcoming inflows, and (ii) forecasts of system variables and outputs such as reservoir levels, releases, flood damage risks, hydropower production, water supply withdrawals, water quality conditions, navigation opportunities, and environmental flows, among others. Forecasts of system variables and outputs are conditional on forecasted inflows as well as on specific management policies, and can provide useful information for decision making processes.
Unlike inflow forecasts (in ensemble or other forms), which have been the subject of many previous studies, reservoir system variable and output forecasts are not formally assessed in water resources management theory or practice. This article addresses this gap and develops methods to rectify potential reservoir system forecast inconsistencies and improve the quality of management‐relevant information provided to stakeholders and managers. The overarching conclusion is that system variable and output forecast consistency is critical for robust reservoir management and needs to be routinely assessed for any management model used to inform planning and management processes. The above are demonstrated through an application from the Sacramento‐American‐San Joaquin reservoir system in northern California. This article is protected by copyright. All rights reserved.
- Rainfall hotspots over the southern tropical Andes: Spatial distribution,
rainfall intensity and relations with large‐scale atmospheric
- Authors: Jhan Carlo Espinoza; Steven Paul Chavez, Josyane Ronchail, Clémentine Junquas, Ken Takahashi, Waldo Lavado
Abstract: The Andes/Amazon transition is among the rainiest regions of the world and the interactions between large‐scale circulation and the topography that determine its complex rainfall distribution remain poorly known. This work provides an in‐depth analysis of the spatial distribution, variability and intensity of rainfall in the southern Andes/Amazon transition, at seasonal and intraseasonal time scales. The analysis is based on comprehensive daily rainfall datasets from meteorological stations in Peru and Bolivia. We compare our results with high‐resolution rainfall TRMM‐PR 2A25 estimations. Hotspot regions are identified at low elevations in the Andean foothills (400‐700 masl) and in windward conditions at Quincemil and Chipiriri, where more than 4,000 mm rainfall per year are recorded. Orographic effects and exposure to easterly winds produce a strong annual rainfall gradient between the lowlands and the Andes that can reach 190 mm/km. Although TRMM‐PR reproduces the spatial distribution satisfactorily, it underestimates rainfall by 35% in the hotspot regions. In the Peruvian hotspot, exceptional rainfall occurs during the austral dry season (around 1,000 mm in June‐July‐August; JJA), but not in the Bolivian hotspot. The direction of the low‐level winds over the Andean foothills partly explains this difference in the seasonal rainfall cycle. At intraseasonal scales in JJA, we found that, during northerly wind regimes, positive rainfall anomalies predominate over the lowland and the eastern flank of the Andes, whereas less rain falls at higher altitudes. On the other hand, during southerly regimes, rainfall anomalies are negative in the hotspot regions. The influence of cross‐equatorial winds is particularly clear below 2000 masl. This article is protected by copyright. All rights reserved.
- Controls on the diurnal streamflow cycles in two subbasins of an alpine
- Authors: Raphael Mutzner; Steven V. Weijs, Paolo Tarolli, Marc Calaf, Holly J. Oldroyd, Marc B. Parlange
Abstract: In high altitude alpine catchments, diurnal streamflow cycles are typically dominated by snowmelt or ice melt. Evapotranspiration‐induced diurnal streamflow cycles are less observed in these catchments but might happen simultaneously. During a field campaign in the summer 2012 in an alpine catchment in the Swiss Alps (Val Ferret catchment, 20.4 km2, glaciarized area: 2%), we observed a transition in the early season from a snowmelt to an evapotranspiration‐induced diurnal streamflow cycle in one of two monitored sub‐basins. The two different cycles were of comparable amplitudes and the transition happened within a time span of several days. In the second monitored sub‐basin, we observed an ice melt‐dominated diurnal cycle during the entire season due to the presence of a small glacier. Comparisons between ice melt and evapotranspiration cycles showed that the two processes were happening at the same times of day but with a different sign and a different shape. The amplitude of the ice melt cycle decreased exponentially during the season and was larger than the amplitude of the evapotranspiration cycle which was relatively constant during the season. Our study suggests that an evapotranspiration‐dominated diurnal streamflow cycle could damp the ice melt‐dominated diurnal streamflow cycle. The two types of diurnal streamflow cycles were separated using a method based on the identification of the active riparian area and measurement of evapotranspiration. This article is protected by copyright. All rights reserved.
- Multimodel framework for characterization of transport in porous media
- Authors: Valentina Ciriello; Yaniv Edery, Alberto Guadagnini, Brian Berkowitz
Abstract: We consider modeling approaches to characterize solute transport in porous media, integrating them into a unique theoretical and experimental framework for model evaluation and data interpretation. To date, development of (conservative and reactive chemical) transport models and formulation of model calibration methods grounded on sensitivity‐based collection of measurements have been pursued in parallel. Key questions that remain include: For a given set of measurements, which conceptual picture of the transport processes, as embodied in a mathematical model or models, is most appropriate? What are the most valuable space‐time locations for solute concentration measurements, depending on the model selected? How is model parameter uncertainty propagated to model output, and how does this propagation affect model calibration? We address these questions by merging parallel streams of research – model formulation, reduction, calibration, sensitivity analysis, and discrimination – offering our view on an emerging framework that guides (i) selection of an appropriate number and location of time‐dependent concentration measurements given a transport model; and (ii) assessment (through discrimination criteria) of the relative benefit of applying any particular model from a set of several models. Our strategy is to employ metrics to quantify the relative contribution of each uncertain model parameter to the variability of the model output. We evaluate these metrics through construction of a surrogate (or "meta") transport model that has the additional benefit of enabling sensitivity analysis and model calibration at a highly reduced computational cost. We demonstrate the applicability of this framework, focusing on transport of reactive chemicals in laboratory‐scale porous media. This article is protected by copyright. All rights reserved.
- Does improved SSTA prediction ensure better seasonal rainfall
- Authors: Mohammad Zaved Kaiser Khan; Ashish Sharma, Rajeshwar Mehrotra, Andrew Schepen, Q. J. Wang
Abstract: Seasonal rainfall forecasts in Australia are issued based on concurrent sea surface temperature anomalies (SSTAs) using a Bayesian model averaging (BMA) approach. The SSTA fields are derived from the Predictive Ocean‐Atmosphere Model for Australia (POAMA) initialized in the preceding season. This study investigates the merits of the rainfall forecasted using POAMA SSTAs in contrast to that forecasted using a multi‐model combination of SSTAs derived using five existing models. In addition, seasonal rainfall forecasts derived from multi‐model and POAMA SSTA fields are subsequently combined to obtain a single weighted forecast over Australia. These three forecasts are compared against “idealized” forecasts where observed SSTAs are used instead of those predicted.The results indicate that while seasonal rainfall forecasts derived using multi‐model based SSTA indices offer improvements in selected seasons over a majority of grid cells in comparison to the case where a single SSTA model is used in two seasons, these improvements are not as significant as the improvements in the SSTA field that drive the rainfall forecasting model. The forecasts derived from the combination of multi‐model and POAMA SSTA indices forecasts are found to offer greater improvements over the multi‐model or the POAMA forecasts for a majority of grid cells in all seasons. It is also observed that these combined forecasts are touching the upper limits of forecastability, which are reached when observed SSTAs are used to forecast the rainfall. This suggests further improvements in rainfall forecasting are only possible through the use of an improved forecasting algorithm, and not the driver (SSTA) information used in the current study. This article is protected by copyright. All rights reserved.
- A multi‐objective short‐term optimal operation model for a
cascade system of reservoirs considering the impact on long‐term
- Authors: Bin Xu; Ping‐An Zhong, Zachary Stanko, Yunfa Zhao, William W.‐G. Yeh
Abstract: This paper examines the impact of short‐term operation on long‐term energy production. We propose a multiobjective optimization model for the short‐term, daily operation of a system of cascade reservoirs. The two objectives considered in the daily model are: 1) minimizing the total amount of water released, and 2) maximizing the stored energy in the system. Optimizing short‐term operation without considering its impact on long‐term energy production does not guarantee maximum energy production in the system. Therefore, a major goal of this paper is to identify desirable short‐term operation strategies that, at the same time, optimize long‐term energy production. First, we solve the daily model for one month (30 days) using a non‐dominated genetic algorithm (NSGAII). We then use the non‐dominated solutions obtained by NSGAII to assess the impact on long‐term energy production using a monthly model. We use historical monthly inflows to characterize the inflow variability. We apply the proposed methodology to the Qingjiang cascade system of reservoirs in China. The results show: 1) in average hydrology scenarios, the solution maximizing stored energy produces the most overall long‐term energy production; 2) in moderately wet hydrology scenarios, the solution minimizing water released outperforms the maximizing stored energy solution; and 3) when extremely wet hydrology scenarios are expected, a compromise solution is the best strategy. This article is protected by copyright. All rights reserved.
- Assessment of flow regime alterations over a spectrum of temporal scales
using wavelet‐based approaches
- Authors: Fu‐Chun Wu; Ching‐Fu Chang, Jenq‐Tzong Shiau
Abstract: The full range of natural flow regime is essential for sustaining the riverine ecosystems and biodiversity, yet there are still limited tools available for assessment of flow regime alterations over a spectrum of temporal scales. Wavelet analysis has proven useful for detecting hydrologic alterations at multiple scales via the wavelet power spectrum (WPS) series. The existing approach based on the global WPS (GWPS) ratio tends to be dominated by the rare high‐power flows so that alterations of the more frequent low‐power flows are often underrepresented. We devise a new approach based on individual deviations between WPS (DWPS) that are root‐mean‐squared to yield the global DWPS (GDWPS). We test these two approaches on the three reaches of the Feitsui Reservoir system (Taiwan) that are subjected to different classes of anthropogenic interventions. The GDWPS reveal unique features that are not detected with the GWPS ratios. We also segregate the effects of individual sub‐flow components on the overall flow regime alterations using the sub‐flow GDWPS. The results show that the daily hydropeaking waves below the reservoir not only intensified the flow oscillations at daily scale but most significantly eliminated subweekly flow variability. Alterations of flow regime were most severe below the diversion weir, where the residual hydropeaking resulted in a maximum impact at daily scale while the post‐diversion null flows led to large hydrologic alterations over submonthly scales. The smallest impacts below the confluence reveal that the hydrologic alterations at scales longer than 2 days were substantially mitigated with the joining of the unregulated tributary flows, whereas the daily‐scale hydrologic alteration was retained because of the hydropeaking inherited from the reservoir releases. The proposed DWPS approach unravels for the first time the details of flow regime alterations at these intermediate scales that are overridden by the low‐frequency high‐power flows when the long‐term averaged GWPS are used. This article is protected by copyright. All rights reserved.
- Chute cutoff as a morphological response to stream reconstruction: The
possible role of backwater
- Authors: J.P.C. Eekhout; A.J.F. Hoitink
Abstract: Stream restoration efforts often aim at creating new unconstrained meandering channels without weirs and bank revetments. In reconstructed streams, the initial morphological response of the new streams is often rapid, until a dynamic equilibrium is reached. Here we report on a chute cutoff that occurred within 3 months after realization of a stream restoration project, caused by a plug bar that formed in response to a backwater effect. The temporal evolution of the morphology of both the new and the old channel was monitored over a period of nearly 8 months, including pre‐cutoff conditions. The observations can be separated into three stages. Stage 1 is the initial period leading to cutoff vulnerability, stage 2 is the actual cutoff, and stage 3 is the morphological adjustment in response to the cutoff. In stage 1, a plug bar was deposited in one of the channel bends. Hydrodynamic model results show the location of the plug bar coincides with a region where bed shear stress decreased in downstream direction due to backwater. Longitudinal channel bed profiles show that the channel slope decreased soon after channel reconstruction. Hence, sediment from upstream was available to form the plug bar. After the plug bar was deposited, an embayment formed in the floodplain at a location where the former channel was located (stage 2). The former channel was filled with sediment prior to channel construction. It is likely that the sediment at this location was less consolidated, and therefore, prone to erosion. The chute channel continued to incise and widen into the floodplain and, after 6 months, acted as the main channel, conveying the discharge during the majority of time (stage 3). The cutoff channel gradually continued to fill with sediment, from the moment the plug bar formed until the chute channel incised into the floodplain. Sedimentary successions of the deposited material show upward fining, which is in agreement with observations of chute cutoffs in rivers. Although the artificial setting limits the degree in which the observed processes can be projected on natural rivers, the observations prompt to investigate the role of backwater effects in natural chute initiation. This article is protected by copyright. All rights reserved.
- Abiotic and biotic controls of soil moisture spatio‐temporal
variability and the occurrence of hysteresis
- Authors: Simone Fatichi; Gabriel G. Katul, Valeriy Y. Ivanov, Christoforos Pappas, Athanasios Paschalis, Ada Consolo, Jongho Kim, Paolo Burlando
Abstract: An expression that separates biotic and abiotic controls on the temporal dynamics of the soil moisture spatial coefficient of variation Cv(θ) was explored via numerical simulations using a mechanistic ecohydrological model, Tethys‐Chloris. Continuous soil moisture spatio‐temporal dynamics at an exemplary hillslope domain were computed for six case studies characterized by different climate and vegetation cover and for three configurations of soil properties. It was shown that abiotic controls largely exceed their biotic counterparts in wet climates. Biotic controls on Cv(θ) were found to be more pronounced in Mediterranean climates. The relation between Cv(θ) and spatial mean soil moisture θ¯ was found to be unique in wet locations, regardless of the soil properties. For the case of homogeneous soil texture, hysteretic cycles between Cv(θ) and θ¯ were observed in all Mediterranean climate locations considered here and to a lesser extent in a deciduous temperate forest. Heterogeneity in soil properties increased Cv(θ) to values commensurate with field observations and weakened signatures of hysteresis at all of the studied locations. This finding highlights the role of site‐specific heterogeneities in hiding or even eliminating the signature of climatic and biotic controls on Cv(θ), thereby offering a new perspective on causes of confounding results reported across field experiments. This article is protected by copyright. All rights reserved.
- A framework to identify Pareto‐efficient subdaily environmental flow
constraints on hydropower reservoirs using a grid‐wide power
- Authors: Marcelo A. Olivares; Jannik Haas, Rodrigo Palma‐Behnke, Carlos Benavides
Abstract: Hydrologic alteration due to hydropeaking reservoir operations is a main concern worldwide. Subdaily environmental flow constraints (ECs) on operations can be promising alternatives for mitigating negative impacts. However, those constraints reduce the flexibility of hydropower plants, potentially with higher costs for the power system. To study the economic and environmental efficiency of ECs, this work proposes a novel framework comprising four steps: i) assessment of the current subdaily hydrologic alteration; ii) formulation and implementation of a short‐term, grid‐wide hydrothermal coordination model; iii) design of ECs in the form of maximum ramping rates (MRRs) and minimum flows (MIFs) for selected hydropower reservoirs; and iv) identification of Pareto‐efficient solutions in terms of grid‐wide costs and the Richard‐Baker flashiness index for subdaily hydrologic alteration (SDHA). The framework was applied to Chile's main power grid, assessing 25 EC cases, involving five MIFs and five MRRs. Each case was run for a dry, normal and wet water year type. Three Pareto‐efficient ECs are found, with remarkably small cost increase below 2% and a SDHA improvement between 28% and 90%. While the case involving the highest MIF worsens the flashiness of another basin, the other two have no negative effect on other basins and can be recommended for implementation. This article is protected by copyright. All rights reserved.
- Modeling the Release of E. coli D21g with Transients in Water Content
- Authors: Scott A. Bradford; Yusong Wang, Saeed Torkzaban, Jiri Šimůnek
Abstract: Transients in water content are well known to mobilize colloids that are retained in the vadose zone. However, there is no consensus on the proper model formulation to simulate colloid release during drainage and imbibition. We present a model that relates colloid release to changes in the air‐water interfacial area (Aaw) with transients in water content. Colloid release from the solid‐water interface (SWI) is modeled in two steps. First, a fraction of the colloids on the SWI partitions to the mobile aqueous phase and air‐water interface (AWI) when the Aaw increases during drainage. Second, colloids that are retained on the AWI or at the air‐water‐solid triple line are released during imbibition as the AWI is destroyed. The developed model was used to describe the release of Escherichia coli D21g during cycles of drainage and imbibition under various saturation conditions. Simulations provided a reasonable description of experimental D21g release results. Only two model parameters were optimized to the D21g release data: (i) the cell fraction that was released from the SWI (fr); and (ii) the cell fraction that partitioned from the SWI to the AWI (fawi). Numerical simulations indicated that cell release was proportional to fr and the initial amount of retention on the SWI and AWI. Drainage to a lower water content enhanced cell release, especially during subsequent imbibition, because more bacteria on the SWI were partitioned to the AWI and/or aqueous phase. Imbibition to a larger water content produced greater colloid release because of higher flow rates, and more destruction of the AWI (smaller Aaw). Variation in the value of fawi was found to have a pronounced influence on the amount of cell release in both drainage and imbibition due to changes in the partitioning of cells from the SWI to the aqueous phase and the AWI. This article is protected by copyright. All rights reserved.
- Hyporheic zone hydrologic science: a historical account of its emergence
and a prospectus
- Authors: M. Bayani Cardenas
Abstract: The hyporheic zone, defined by shallow subsurface pathways through river beds and banks beginning and ending at the river, is an integral and unique component of fluvial systems. It hosts myriad hydrologically‐controlled processes that are potentially coupled in complex ways. Understanding these processes and the connections between them is critical since these processes are not only important locally but integrate to impact increasingly larger scale biogeochemical functioning of the river corridor up to the river network scale. Thus, the hyporheic zone continues to be a growing research focus for many hydrologists for more than half the history of Water Resources Research. This manuscript partly summarizes the historical development of hyporheic zone hydrologic science as gleaned from papers published in Water Resources Research, from the birth of the concept of the hyporheic zone as a hydrologic black box (sometimes referred to as transient storage zone), to its adolescent years of being torn between occasionally competing research perspectives of interrogating the hyporheic zone from a surface or subsurface view, to its mature emergence as an interdisciplinary research field that employs the wide array of state‐of‐the‐art tools available to the modern hydrologist. The field is vibrant and moving in the right direction of addressing critical fundamental and applied questions with no clear end in sight in its growth. There are exciting opportunities for scientists that are able to tightly link the allied fields of geology, geomorphology, hydrology, geochemistry and ecology to tackle the many open problems in hyporheic zone science. This article is protected by copyright. All rights reserved.
- A methodology for velocity field measurement in multiphase
high‐pressure flow of CO2 and water in micromodels
- Authors: Farzan Kazemifar; Gianluca Blois, Dimitrios C. Kyritsis, Kenneth T. Christensen
Abstract: This paper presents a novel methodology for capturing instantaneous, temporally and spatially resolved velocity fields in an immiscible multi‐phase flow of liquid/supercritical CO2 and water through a porous micromodel. Of interest is quantifying pore‐scale flow processes relevant to geological CO2 sequestration and enhanced oil recovery, for example, at thermodynamic conditions relevant to geological reservoirs. A previously‐developed two‐color microscopic particle image velocimetry approach based upon proxy fluids [Blois et al. 2015] is adapted to a high‐pressure apparatus, facilitating flow quantification of water interacting with supercritical CO2. This technique simultaneously resolves (in space and time) the aqueous phase velocity field as well as the dynamics of the menisci. The method and the experimental apparatus are detailed, and results are presented to demonstrate its unique capabilities for studying pore‐scale dynamics of CO2‐water interactions. Simultaneous identification of the boundary between the two fluid phases and quantification of the instantaneous velocity field in the aqueous phase provides a step‐change in capability for investigating multi‐phase flow physics at the pore scale at reservoir‐relevant conditions. This article is protected by copyright. All rights reserved.
- Long‐range seasonal streamflow forecasting over the Iberian
Peninsula using large‐scale atmospheric and oceanic information
- Authors: J.M. Hidalgo‐Muñoz; S.R. Gámiz‐Fortis, Y. Castro‐Díez, D. Argüeso, M.J. Esteban‐Parra
Abstract: Identifying the relationship between large‐scale climate signals and seasonal streamflow may provide a valuable tool for long‐range seasonal forecasting in regions under water stress, such as the Iberian Peninsula (IP). The skill of the main teleconnection indices as predictors of seasonal streamflow in the IP was evaluated. The streamflow database used was composed of 382 stations, covering the period 1975‐2008. Predictions were made using a leave‐one‐out cross‐validation approach based on multiple linear regression, combining Variance Inflation Factor and Stepwise Backward selection to avoid multicollinearity and select the best subset of predictors. Predictions were made for four forecasting scenarios, from one to four seasons in advance. The correlation coefficient (RHO), Root Mean Square Error Skill Score (RMSESS) and the Gerrity Skill Score (GSS) were used to evaluate the forecasting skill.
For autumn streamflow, good forecasting skill (RHO>0.5, RMSESS>20%, GSS>0.4) was found for a third of the stations located in the Mediterranean Andalusian Basin, the North Atlantic Oscillation of the previous winter being the main predictor. Also, fair forecasting skill (RHO>0.44, RMSESS>10%, GSS>0.2) was found in stations in the northwestern IP (16 of these located in the Douro and Tagus Basins) with two seasons in advance. For winter streamflow, fair forecasting skill was found for one season in advance in 168 stations, with the Snow Advance Index as the main predictor. Finally, forecasting was poorer for spring streamflow than for autumn and winter, since only 16 stations showed fair forecasting skill in with one season in advance, particularly in north‐western of IP. This article is protected by copyright. All rights reserved.
- Coupled and uncoupled hydrogeophysical inversions using ensemble Kalman
filter assimilation of ERT‐monitored tracer test data
- Authors: Matteo Camporese; Giorgio Cassiani, Rita Deiana, Paolo Salandin, Andrew Binley
Abstract: Recent advances in geophysical methods have been increasingly exploited as inverse modeling tools in groundwater hydrology. In particular, several attempts to constrain the hydrogeophysical inverse problem to reduce inversion errors have been made using time‐lapse geophysical measurements through both coupled and uncoupled (also known as sequential) inversion approaches. Despite the appeal and popularity of coupled inversion approaches, their superiority over uncoupled methods has not been proven conclusively; the goal of this work is to provide an objective comparison between the two approaches within a specific inversion modeling framework based on the ensemble Kalman filter (EnKF). Using EnKF and a model of Lagrangian transport, we compare the performance of a fully coupled and uncoupled inversion method for the reconstruction of heterogeneous saturated hydraulic conductivity fields through the assimilation of ERT‐monitored tracer test data. The two inversion approaches are tested in a number of different scenarios, including isotropic and anisotropic synthetic aquifers, where we change the geostatistical parameters used to generate the prior ensemble of hydraulic conductivity fields. Our results show that the coupled approach outperforms the uncoupled when the prior statistics are close to the ones used to generate the true field. Otherwise, the coupled approach is heavily affected by “filter inbreeding” (an undesired effect of variance underestimation typical of EnKF), while the uncoupled approach is more robust, being able to correct biased prior information thanks to its capability of capturing the solute travel times even in presence of inversion artifacts such as the violation of mass balance. Furthermore, the coupled approach is more computationally intensive than the uncoupled, due to the much larger number of forward runs required by the electrical model. Overall, we conclude that the relative merit of the coupled versus the uncoupled approach cannot be assumed a priori and should be assessed case by case. This article is protected by copyright. All rights reserved.
- Drag force parameters of rigid and flexible vegetal elements
- Authors: John A. Chapman; Bruce N. Wilson, John S. Gulliver
Abstract: This paper compares parameters that characterize vegetation flexibility effects on flow resistance and drag. Drag forces have been measured in a flume for simple cylindrical obstructions of the same shape and size but with different flexibility under several flow conditions. This data set is used to fit drag parameters and to relate their value to flexibility through the Cauchy Number. A formulation is presented where the drag coefficient is evaluated as a function of a new calibration velocity parameter which is related to the elastic modulus of the obstruction. While the use of a Vogel exponent and reference velocity provides a similar response, the reference velocity when used is somewhat nebulous and appears to have a critical impact on the parameter and the drag force calculated. The proposed formulation for drag reduction is more consistently estimated for the range of flexibilities in this study. This article is protected by copyright. All rights reserved.
- Hydroeconomic optimization of integrated water management and transfers
under stochastic surface water supply
- Authors: Tingju Zhu; Guilherme Fernandes Marques, Jay R. Lund
Abstract: Efficient re‐allocation and conjunctive operation of existing water supplies is gaining importance as demands grow, competitions among users intensify, and new supplies become more costly. This paper analyzes the roles and benefits of conjunctive use of surface water and groundwater and market‐based water transfers in an integrated regional water system where agricultural and urban water users coordinate supply and demand management based on supply reliability and economic values of water. Agricultural users optimize land and water use for annual and perennial crops to maximize farm income, while urban users choose short‐term and long‐term water conservation actions to maintain reliability and minimize costs. The temporal order of these decisions is represented in a two‐stage optimization that maximizes the net expected benefits of crop production, urban conservation and water management including conjunctive use and water transfers. Long‐term decisions are in the first stage and short‐term decisions are in a second stage based on probabilities of water availability events. Analytical and numerical analyses are made. Results show that conjunctive use and water transfers can substantially stabilize farmer's income and reduce system costs by reducing expensive urban water conservation or construction. Water transfers can equalize marginal values of water across users, while conjunctive use minimizes water marginal value differences in time. Model results are useful for exploring the integration of different water demands and supplies through water transfers, conjunctive use, and conservation, providing valuable insights for improving system management. This article is protected by copyright. All rights reserved.
- Modeling chloride transport using travel time distributions at Plynlimon,
- Authors: Paolo Benettin; James Kirchner, Andrea Rinaldo, Gianluca Botter
Abstract: Here we present a theoretical interpretation of high‐frequency, high‐quality tracer time series from the Hafren catchment at Plynlimon in mid‐Wales. We make use of the formulation of transport by travel time distributions to model chloride transport originating from atmospheric deposition and compute catchment‐scale travel time distributions. The relevance of the approach lies in the explanatory power of the chosen tools, particularly to highlight hydrologic processes otherwise clouded by the integrated nature of the measured outflux signal. The analysis reveals the key role of residual storages that are poorly visible in the hydrological response, but are shown to strongly affect water quality dynamics. A significant accuracy in reproducing data is shown by our calibrated model. A detailed representation of catchment‐scale travel time distributions has been derived, including the time evolution of the overall dispersion processes (which can be expressed in terms of time‐varying storage sampling functions). Mean computed travel times span a broad range of values (from 80 to 800 days) depending on the catchment state. Results also suggest that, in the average, discharge waters are younger than storage water. The model proves able to capture high‐frequency fluctuations in the measured chloride concentrations, which are broadly explained by the sharp transition between groundwaters and faster flows originating from topsoil layers. This article is protected by copyright. All rights reserved.
- Assimilation of stream discharge for flood forecasting: Updating a
semidistributed model with an integrated data assimilation scheme
- Authors: Yuan Li; Dongryeol Ryu, Andrew W. Western, Q. J. Wang
Abstract: Real‐time discharge observations can be assimilated into flood models to improve forecast accuracy; however, the presence of time lags in the routing process and a lack of methods to quantitatively represent different sources of uncertainties challenge the implementation of data assimilation techniques for operational flood forecasting. To address these issues, an integrated error parameter estimation and lag‐aware data assimilation (IEELA) scheme was recently developed for a lumped model. The scheme combines an ensemble‐based maximum a posteriori (MAP) error estimation approach with a lag‐aware ensemble Kalman smoother (EnKS).
In this study, the IEELA scheme is extended to a semi‐distributed model to provide for more general application in flood forecasting by including spatial and temporal correlations in model uncertainties between sub‐catchments. The result reveals that using a semi‐distributed model leads to more accurate forecasts than a lumped model in an open‐loop scenario. The IEELA scheme improves the forecast accuracy significantly in both lumped and semi‐distributed models, and the superiority of the semi‐distributed model remains in the data assimilation scenario. However, the improvements resulting from IEELA are confined to the outlet of the catchment where the discharge observations are assimilated. Forecasts at “ungauged” internal locations are not improved, and in some instances, even become less accurate. This article is protected by copyright. All rights reserved.
- A thermodynamic interpretation of Budyko and L'vovich formulations of
annual water balance: Proportionality hypothesis and maximum entropy
- Authors: Dingbao Wang; Jianshi Zhao, Yin Tang, Murugesu Sivapalan
Abstract: The paper forms part of the search for a thermodynamic explanation for the empirical Budyko Curve, addressing a long‐standing research question in hydrology. Here, this issue is pursued by invoking the Proportionality Hypothesis underpinning the Soil Conservation Service (SCS) curve number method widely used for estimating direct runoff at the event scale. In this case, the Proportionality Hypothesis posits that the ratio of continuing abstraction to its potential value is equal to the ratio of direct runoff to its potential value. Recently, the validity of the Proportionality Hypothesis has been extended to the partitioning of precipitation into runoff and evaporation at the annual time scale as well. In this case, the Proportionality Hypothesis dictates that the ratio of continuing evaporation to its potential value is equal to the ratio of runoff to its potential value. The Budyko Curve could then be seen as the straightforward outcome of the application of the Proportionality Hypothesis to estimate mean annual water balance. In this paper, we go further and demonstrate that the Proportionality Hypothesis itself can be seen as a result of the application of the thermodynamic principle of Maximum Entropy Production (MEP). In this way, we demonstrate a possible thermodynamic basis for the Proportionality Hypothesis, and consequently for the Budyko Curve. As a further extension, the L'vovich formulation for the two‐stage partitioning of annual precipitation is also demonstrated to be a result of MEP: one for the competition between soil wetting and fast flow during the first stage; another for the competition between evaporation and base flow during the second stage. This article is protected by copyright. All rights reserved.
- Comparison of three dual‐source remote sensing evapotranspiration
models during the MUSOEXE‐12 campaign: Revisit of model physics
- Authors: Yuting Yang; Di Long, Huade Guan, Wei Liang, Craig Simmons, Okke Batelaan
Abstract: Various remote sensing‐based terrestrial evapotranspiration (ET) models have been developed during the past four decades. These models vary in conceptual and mathematical representations of the physics, consequently leading to different performances. Examination of uncertainties associated with limitations in model physics will be useful for model selection and improvement. Here, three dual‐source remote sensing ET models (i.e. the Hybrid dual‐source scheme and Trapezoid framework‐based ET Model (HTEM), the Two‐Source Energy Balance (TSEB) model and the MOD16 ET algorithm) using ASTER images were compared during the MUSOEXE‐12 campaign in the Heihe River Basin in Northwest China, aiming to better understand the differences in model physics that potentially lead to differences in model performance. Model results were firstly compared against observations from a dense network of eddy covariance towers and isotope‐based evaporation (E) and transpiration (T) partitioning. Results show that HTEM outperformed the other two models in simulating ET and its partitioning, whereas MOD16 performed worst (i.e. ET root‐mean‐square errors are 42.3 W/m2 (HTEM), 49.8 W/m2 (TSEB), and 95.3 W/m2 (HTEM)). On to model limitations, HTEM tends to underestimate ET under high advection due mostly to the underestimation of temperatures for the wet edge in its trapezoidal space. For TSEB, large uncertainties occur in determining the initial Priestley‐Taylor coefficient and the iteration procedure for ET partitioning, leading to overestimation/underestimation of T/E in most cases, particularly over sparse vegetation. Primary use of meteorological data for MOD16 does not effectively capture the soil moisture restriction on ET, and therefore results in unreasonable spatial ET patterns. This article is protected by copyright. All rights reserved.
- Mathematical equivalence between time‐dependent single‐rate
and multirate mass transfer models
- Authors: D. Fernàndez‐Garcia; X. Sanchez‐Vila
Abstract: The often observed tailing of tracer breakthrough curves is caused by a multitude of mass transfer processes taking place over multiple scales. Yet, in some cases it is convenient to fit a transport model with a single‐rate mass transfer coefficient that lumps all the non‐Fickian observed behavior. Since mass transfer processes take place at all characteristic times, the single‐rate mass transfer coefficient derived from measurements in the laboratory or in the field vary with time, ω(t). The literature review and tracer experiments compiled by Haggerty et al.  from a number of sites worldwide suggest that the characteristic mass transfer time, which is proportional to ω(t)−1, scales as a power law of the advective and experiment duration. This paper studies the mathematical equivalence between the Multi‐Rate Mass Transfer Model (MRMT) and a time‐dependent single‐rate mass transfer model (t‐SRMT). In doing this, we provide new insights into the previously observed scale‐dependence of mass transfer coefficients. The memory function, g(t), which is the most salient feature of the MRMT model, determines the influence of the past values of concentrations on its present state. We found that the t‐SRMT model can also be expressed by means of a memory function φ(t,τ). In this case though the memory function is non‐stationary, meaning that in general it cannot be written as φ(t‐τ). Nevertheless, the full behavior of the concentrations using a single time‐dependent rate ω(t) is approximately analogous to that of the MRMT model provided that the equality ω(t) = ‐d In g(t)/dt holds and the field capacity is properly chosen. This relationship suggests that when the memory function is a power law, g(t) ∼ t1‐k, the equivalent mass transfer coefficient scales as ω(t) ∼ t−1, nicely fitting without calibration the estimated mass transfer coefficients compiled by Haggerty et al. . This article is protected by copyright. All rights reserved.
- Using the level set method to study the effects of heterogeneity and
anisotropy on hyporheic exchange
- Authors: Cheng Chen; Lingzao Zeng
Abstract: The level set method was used to simulate the interface movement when a conservative solute migrated from stream water to subsurface water, and study the effects of streambed heterogeneity and anisotropy on solute penetration. The level set method is a numerical technique for tracking moving interfaces based on the idea that the interface is a level set curve of a higher‐dimensional function. Numerical simulations were compared to experiments conducted in a recirculating flume. Streambed heterogeneity led to water exchange between multiple bedforms, while in homogeneous streambeds the water exchange was restricted within a single bedform. A thin layer of homogeneous sediments at the top of the heterogeneous streambeds significantly increased the interfacial water influx, resulting in faster solute penetration and stream concentration decrease. Streambed heterogeneity generated horizontal preferential flow paths in the upper part of the bed while decreasing pore water velocities deeper in the bed, which hindered vertical penetration and consequently led to slower stream concentration decrease. Decreasing vertical permeability or increasing horizontal permeability led to slower vertical penetration and stream concentration decrease. Decreasing vertical permeability had a much more significant impact on solute penetration than increasing horizontal permeability, because mass transfer in hyporheic exchange is greatly dominated by vertical advection which depends primarily on the vertical permeability. This study was the first to apply the level set method in the study of hyporheic exchange. The theoretical and numerical methods have important applications in subsurface flow and transport processes. This article is protected by copyright. All rights reserved.
- What does it take to flood the Pampas? Lessons from a decade of strong
- Authors: S. Kuppel; J. Houspanossian, M. D. Nosetto, E. G. Jobbágy
Abstract: While most landscapes respond to extreme rainfalls with increased surface water outflows, very flat and poorly drained ones have little capacity to do this and their most common responses include (i) increased water storage leading to rising water tables and floods, (ii) increased evaporative water losses and, after reaching high levels of storage, (iii) increased liquid water outflows. The relative importance of these pathways was explored in the extensive plains of the Argentine Pampas, where two significant flood episodes (denoted FE1 and FE2) occurred in 2000‐2003 and 2012‐2013. In two of the most flood‐prone areas (Western and Lower Pampa, 60 000 km2 each), surface water cover reached 31 and 19% during FE1 in each subregion, while FE2 covered up to 22 and 10%, respectively. From the spatiotemporal heterogeneity of the flood events, we distinguished slow floods lasting several years when the water table is brought to the surface following sustained precipitation excesses in groundwater‐connected systems (Western Pampa), and "fast" floods triggered by surface water accumulation over the course of weeks to months, typical of poor surface‐groundwater connectivity (Lower Pampa) or when exceptionally‐strong rainfalls overwhelm infiltration capacity. Because of these different hydrological responses, precipitation and evapotranspiration were strongly linked in the Lower Pampa only, while the connection between water fluxes and storage was limited to the Western Pampa. In both regions, evapotranspirative losses were strongly linked to flooded conditions as a regulatory feedback, while liquid water outflows remained negligible. This article is protected by copyright. All rights reserved.
- Canopy influence on snow depth distribution in a pine stand determined
from terrestrial laser data
- Authors: J. Revuelto; J.I. López‐Moreno, C. Azorin‐Molina, S.M. Vicente‐Serrano
Abstract: In this study we analyzed the effects of the forest canopy and trunks of a pine stand in the central Spanish Pyrenees on the snow depth (SD) distribution. Using LiDAR technology with a terrestrial laser scanner (TLS), high‐resolution data on the SD distribution were acquired during the 2011–12 and 2012–13 snow seasons, which were two years having very contrasting climatic and snow accumulation conditions. Average SD evolution in open and canopy areas was characterized. Principal component analysis was applied to identify days having similar spatial patterns of SD distribution. There was a clear contrast in the temporal variability of the snowpack in different areas of the forest stand, corresponding generally to beneath the canopy, and in open sites. The canopy and openings showed markedly different accumulation and melting, with higher snow accumulation found in openings. Differences ranged from 14 to 80% reduction (average 49%) in the SD beneath the canopy relative to open sites. The difference in SD between open and canopy areas increased throughout the snow season. The surveyed days were classified in terms of SD distribution, and included days associated with: high SD, low SD, intense melting conditions and periods when the SD distribution was driven by wind conditions. The SD increased with distance from the trunks to a distance of 3.5–4.5 m, coinciding with the average size of the crown of individual trees. This article is protected by copyright. All rights reserved.
- Three‐dimensional versus two‐dimensional bed
form‐induced hyporheic exchange
- Authors: Xiaobing Chen; M. Bayani Cardenas, Li Chen
Abstract: The hyporheic zone is often a critical component of river systems. Hyporheic exchange is generally forced by variation in riverbed topography such as due to bedforms. Most previous research on bedform‐driven hyporheic flow has focused on two‐dimensional (2D) dunes and ripples, while little has been done on their three‐dimensional (3D) counterparts. Here we compared hyporheic exchange and associated metrics for a previously studied pair of corresponding 2D and 3D bedforms. To accomplish this, a series of multiphysics computational fluid dynamics models were conducted both in 2D and 3D with similar open channel Reynolds numbers (Re). Results show that the pressure gradient along the sediment‐water interface is highly sensitive to the spatial structure of bedforms, which consequently determines hyporheic flow dynamics. Hyporheic flux is a function of Re for both 2D and 3D dunes via a power law; however, the equivalent 3D dunes have a higher flux since the 3D form induces more drag. The hyporheic zone depths and volumes are only slightly different with the 3D case having a larger volume. The mean fluid residence times for both cases are related to Re by an inverse power law relationship, with the 3D dune having smaller residence times at moderate to high Re. The effects of increasing flux on residence time in 3D dunes is partly modulated by a slightly increasing hyporheic volume. Our results suggest that a 2D idealization is a reasonable approximation for the more complex 3D situation if local details are unimportant but that development of predictive models for mean fluxes and residence times, which are critical for biogeochemical processes, based on 2D models may be insufficient. This article is protected by copyright. All rights reserved.
- Two‐phase flow properties of a sandstone rock for the CO2/water
system: Core‐flooding experiments, and focus on impacts of
- Authors: JC. Manceau; J. Ma, R. Li, P. Audigane, P. X. Jiang, R. Xu, J. Tremosa, C. Lerouge
Abstract: The two‐phase flow characterization (CO2/water) of a Triassic sandstone core from the Paris Basin, France, is reported in this paper. Absolute properties (porosity and water permeability), capillary pressure, relative permeability with hysteresis between drainage and imbibition, and residual trapping capacities have been assessed at 9 MPa pore pressure and 28 °C (CO2 in liquid state) using a single core‐flooding apparatus associated with magnetic resonance imaging. Different methodologies have been followed to obtain a data‐set of flow properties to be up‐scaled and used in large scale CO2 geological storage evolution modeling tools. The measurements are consistent with the properties of well‐sorted water‐wet porous systems. As the mineralogical investigations showed a non‐negligible proportion of carbonates in the core, the experimental protocol was designed to observe potential impacts on flow properties of mineralogical changes. The magnetic resonance scanning and mineralogical observations indicate mineral dissolution during the experimental campaign, and the core‐flooding results show an increase in porosity and water absolute permeability. The changes in two‐phase flow properties appear coherent with the pore structure modifications induced by the carbonates dissolution but the changes in relative permeability could also be explained by a potential increase of the water‐wet character of the core. Further investigations on the impacts of mineral changes are required with other reactive formation rocks, especially carbonate‐rich ones, because the implications can be significant both for the validity of laboratory measurements and for the outcomes of in situ operations modeling. This article is protected by copyright. All rights reserved.
- Sediment transport and shear stress partitioning in a vegetated flow
- Authors: C. Le Bouteiller; J. G. Venditti
Abstract: Vegetation is a common feature in natural coastal and riverine water ways, interacting with both the water flow and sediment transport. However, the physical processes governing these interactions are still poorly understood, which makes it difficult to predict sediment transport and morphodynamics in a vegetated environment. We performed a simple experiment to study how sediment transport responds to the presence of flexible, single‐blade vegetation and how this response is influenced by the vegetation density. We found that the skin friction and sediment transport are reduced in a plant patch, and that this effect is larger for denser vegetation. We then evaluated several methods to calculate the skin friction in a vegetated flow, which is the key to sediment transport prediction. Among these, the inversion of bedload transport formulas and the Einstein and Banks (1950) methods appeared to produce the most reasonable values of the skin friction. Finally, we suggest using the parameter α, which is the ratio of the skin friction computed by these methods to the total bed shear stress, to make more realistic sediment transport predictions in morphodynamic models. This article is protected by copyright. All rights reserved.
- Water‐quality trading: Can we get the prices of pollution right?
- Authors: Yoshifumi Konishi; Jay S. Coggins, Bin Wang
Abstract: Water‐quality trading requires inducing permit prices that account properly for spatially explicit damage relationships. We compare recent work by Hung and Shaw  and Farrow et al.  for river systems exhibiting branching and nonlinear damages. The Hung‐Shaw scheme is robust to nonlinear damages, but not to hot spots occurring at the confluence of two branches. The Farrow et al. scheme is robust to branching, but not to nonlinear damages. We also compare the two schemes to each other. Neither dominates from a welfare perspective, but the comparison appears to tilt in favor of the Farrow et al. scheme. This article is protected by copyright. All rights reserved.
- Biodegradation of subsurface oil in a tidally influenced sand beach:
Impact of hydraulics and interaction with pore water chemistry
- Authors: Xiaolong Geng; Michel C. Boufadel, Kenneth Lee, Stewart Abrams, Makram Suidan
Abstract: The aerobic biodegradation of oil in tidally influenced beaches was investigated numerically in this work using realistic beach and tide conditions. A numerical model BIOMARUN, coupling a multiple‐Monod kinetic model BIOB to a density‐dependent variably saturated groundwater flow model 2‐D MARUN, was used to simulate the biodegradation of low solubility hydrocarbon and transport processes of associated solute species (i.e., oxygen and nitrogen) in a tidally influenced beach environment. It was found that different limiting factors affect different portions of the beach. In the upper intertidal zone, where the inland incoming nutrient concentration was large (1.2 mg‐N/L), oil biodegradation occurred deeper in the beach (i.e., 0.3 m below the surface). In the mid‐intertidal zone, a reversal was noted where the biodegradation was fast at shallow locations (i.e., 0.1 m below the surface), and it was due to the decrease of oxygen with depth due to consumption, which made oxygen the limiting factor for biodegradation. Oxygen concentration in the mid‐intertidal zone exhibited two peaks as a function of time. One peak was associated with the high tide, when dissolved oxygen laden seawater filled the beach and a second oxygen peak was observed during low tides, and it was due to pore oxygen replenishment from the atmosphere. The effect of the capillary fringe (CF) height was investigated, and it was found that there is an optimal CF for the maximum biodegradation of oil in the beach. Too large a CF (i.e., very fine material) would attenuate oxygen replenishment (either from seawater or the atmosphere), while too small a CF (i.e., very coarse material) would reduce the interaction between microorganisms and oil in the upper intertidal zone due to rapid reduction in the soil moisture at low tide. This article is protected by copyright. All rights reserved.
- Point rainfall statistics for ecohydrological analyses derived from
satellite‐integrated rainfall measurements
- Authors: Manuel del Jesus; Andrea Rinaldo, Ignacio Rodríguez‐Iturbe
Abstract: Satellite rainfall measurements, nowadays commonly available, provide valuable information about the spatial structure of rainfall. In areas with low‐density rain gage networks, or where these networks are non‐existent, satellite rainfall measurements can also provide useful estimates to be used as virtual rain gages. However, satellite and rain gage measurements are statistically different in nature and cannot be directly compared to one another. In the present paper we develop a methodology to downscale satellite rainfall measurements to generate rain‐gage‐equivalent statistics. We apply the methodology to four locations along a strong rainfall gradient in the Kalahari transect, southern Africa, to validate the methodology. We show that the method allows the estimation of point rainfall statistics where only satellite measurements exist. Point rainfall statistics are key descriptors for ecohydrologic studies linking the response of vegetation to rainfall dynamics. This article is protected by copyright. All rights reserved.
- Evaluation of measurement sensitivity and design improvement for time
domain reflectometry penetrometers
- Authors: Tony Liang‐tong Zhan; Qing‐yi Mu, Yun‐min Chen, Han Ke
Abstract: The time domain reflectometry (TDR) penetrometer, which can measure both the apparent dielectric permittivity and the bulk electrical conductivity of soils, is an important tool for the site investigation of contaminated land. This paper presents a theoretical method for evaluating the measurement sensitivity and an improved design of the TDR penetrometer. The sensitivity evaluation method is based on a spatial weighting analysis of the electromagnetic field using a seepage analysis software. This method is used to quantify the measurement sensitivity for the three types of TDR penetrometers reported in literature as well as guide the design improvement of the TDR penetrometer. The improved design includes the use of semicircle‐shaped conductors and the optimization of the conductor diameter. The measurement sensitivity to the targeted medium for the improved TDR penetrometer is evaluated to be greater than those of the three types of TDR penetrometers reported in literature. The performance of the improved TDR penetrometer was demonstrated by conducting an experimental calibration of the probe and penetration tests in a chamber containing a silty soil column. The experimental results demonstrate that the measurements from the improved TDR penetrometer are able to capture the variation in the water content profiles as well as the leachate contaminated soil. This article is protected by copyright. All rights reserved.
- An active heat tracer experiment to determine groundwater velocities using
fiber‐optic cables installed with direct push equipment
- Authors: Mark Bakker; Ruben Caljé, Frans Schaars, Kees‐Jan van der Made, Sander de Haas
Abstract: A new approach is developed to insert fiber optic cables vertically into the ground with direct push equipment. Groundwater temperatures may be measured along the cables with high spatial and temporal resolution using a Distributed Temperature Sensing system. The cables may be inserted up to depths of tens of meters in unconsolidated sedimentary aquifers. The main advantages of the method are that the cables are in direct contact with the aquifer material, the disturbance of the aquifer is minor, and no borehole is needed. This cost‐effective approach may be applied to both passive and active heat tracer experiments. An active heat tracer experiment was conducted to estimate horizontal groundwater velocities in a managed aquifer recharge system in the Netherlands. Six fiber optic cables and a separate heating cable were inserted with a one‐meter spacing at the surface. The heating cable was turned on for 4 days and temperatures were measured during both heating and cooling of the aquifer. Temperature measurements at the heating cable alone were used to estimate the magnitude of the groundwater velocity and the thermal conductivity of the solids. The direction of the velocity and heat capacity of the solids were estimated by including temperature measurements at the other fiber optic cables in the analysis. The latter analysis suffered from the fact that the cables were not inserted exactly vertical. The three‐dimensional position of the fiber optic cables must be measured for future active heat tracer experiments. This article is protected by copyright. All rights reserved.
- Calibrating remotely sensed river bathymetry in the absence of field
measurements: Flow resistance equation‐based imaging of river depths
- Authors: Carl J. Legleiter
Abstract: Remote sensing could enable high‐resolution mapping of long river segments, but realizing this potential will require new methods for inferring channel bathymetry from passive optical image data without using field measurements for calibration. As an alternative to regression‐based approaches, this study introduces a novel framework for Flow REsistance Equation‐Based Imaging of River Depths (FREEBIRD). This technique allows for depth retrieval in the absence of field data by linking a linear relation between an image‐derived quantity X and depth d to basic equations of open channel flow: continuity and flow resistance. One FREEBIRD algorithm takes as input an estimate of the channel aspect (width/depth) ratio A and a series of cross‐sections extracted from the image and returns the coefficients of the X vs. d relation. A second algorithm calibrates this relation so as to match a known discharge Q. As an initial test of FREEBIRD, these procedures were applied to panchromatic satellite imagery and publicly available aerial photography of a clear‐flowing gravel‐bed river. Accuracy assessment based on independent field surveys indicated that depth retrieval performance was comparable to that achieved by direct, field‐based calibration methods. Sensitivity analyses suggested that FREEBIRD output was not heavily influenced by misspecification of A or Q, or by selection of other input parameters. By eliminating the need for simultaneous field data collection, these methods create new possibilities for large‐scale river monitoring and analysis of channel change, subject to the important caveat that the underlying relationship between X and d must be reasonably strong. This article is protected by copyright. All rights reserved.
- How well do CMIP5 climate simulations replicate historical trends and
patterns of meteorological droughts?
- Authors: Nasrin Nasrollahi; Amir AghaKouchak, Linyin Cheng, Lisa Damberg, Thomas J. Phillips, Chiyuan Miao, Kuolin Hsu, Soroosh Sorooshian
Abstract: Assessing the uncertainties and understanding the deficiencies of climate models is fundamental to developing adaptation strategies. The objective of this study is to understand how well Coupled Model Intercomparison‐Phase 5 (CMIP5) climate model simulations replicate ground‐based observations of continental drought areas and their trends. The CMIP5 multi‐model ensemble encompasses the Climatic Research Unit (CRU) ground‐based observations of area under drought at all time‐steps. However, most model members overestimate the areas under extreme drought, particularly in the Southern Hemisphere (SH). Furthermore, the results show that the time series of observations and CMIP5 simulations of areas under drought exhibit more variability in the SH than in the Northern Hemisphere (NH). The trend analysis of areas under drought reveals that the observational data exhibit a significant positive trend at the significance level of 0.05 over all land areas. The observed trend is reproduced by about three‐fourths of the CMIP5 models when considering total land areas in drought. While models are generally consistent with observations at a global (or hemispheric) scale, most models do not agree with observed regional drying and wetting trends. Over many regions, at most 40% of the CMIP5 models are in agreement with the trends of CRU observations. The drying/wetting trend calculated using the 3 months Standardized Precipitation Index (SPI) values show better agreement with the corresponding CRU values than with the observed annual mean precipitation rates. Pixel scale evaluation of CMIP5 models indicates that no single model demonstrates an overall superior performance relative to the other models. This article is protected by copyright. All rights reserved.
- Bayesian model averaging to explore the worth of data for soil‐plant
model selection and prediction
- Authors: Thomas Wöhling; Anneli Schöniger, Sebastian Gayler, Wolfgang Nowak
Abstract: A Bayesian Model Averaging (BMA) framework is presented to evaluate the worth of different observation types and experimental design options for 1) more confidence in model selection and 2) for increased predictive reliability. These two modeling tasks are handled separately, because model selection aims at identifying the most appropriate model with respect to a given calibration data set, while predictive reliability aims at reducing uncertainty in model predictions through constraining the plausible range of both models and model parameters. For that purpose, we pursue an optimal design of measurement framework that is based on BMA and that considers uncertainty in parameters, measurements, and model structures. We apply this framework to select between four crop models (the vegetation components of CERES, SUCROS, GECROS and SPASS), which are coupled to identical routines for simulating soil carbon and nitrogen turnover, soil heat and nitrogen transport, and soil water movement. An ensemble of parameter realizations was generated for each model using Monte‐Carlo simulation. We assess each model's plausibility by determining its posterior weight, which signifies the probability to have generated a given experimental data set. Several BMA analyses were conducted for different data packages with measurements of soil moisture, evapotranspiration (ETa), and leaf area index (LAI). The posterior weights resulting from the different BMA runs were compared to the weight distribution of a reference run with all data types to investigate the utility of different data packages and monitoring design options in identifying the most appropriate model in the ensemble. We found that different (combinations of) data types support different models and none of the four crop models outperforms all others under all data scenarios. The best model discrimination was observed for those data where the competing models disagree the most. The data worth for reducing prediction uncertainty depends on the prediction to be made. LAI data have the highest utility for predicting ETa, while soil moisture data is better for predicting soil water drainage. Our study illustrates, that BMA provides an objective framework for data worth analysis with respect to both model discrimination and model calibration for a wide range of applications. This article is protected by copyright. All rights reserved.
- Global change and the groundwater management challenge
- Authors: Steven M. Gorelick; Chunmiao Zheng
Abstract: With rivers in critical regions already exploited to capacity throughout the world and groundwater overdraft as well as large‐scale contamination occurring in many areas, we have entered an era in which multiple simultaneous stresses will drive water management. Increasingly, groundwater resources are taking a more prominent role in providing freshwater supplies. We discuss the competing fresh groundwater needs for human consumption, food production, energy, and the environment, as well as physical hazards, and conflicts due to transboundary overexploitation. During the past 50 years, groundwater management modeling has focused on combining simulation with optimization methods to inspect important problems ranging from contaminant remediation to agricultural irrigation management. The compound challenges now faced by water planners require a new generation of aquifer management models that address the broad impacts of global change on aquifer storage and depletion trajectory management, land subsidence, groundwater‐dependent ecosystems, seawater intrusion, anthropogenic and geogenic contamination, supply vulnerability, and long‐term sustainability. The scope of research efforts is only beginning to address complex interactions using multi‐agent system models that are not readily formulated as optimization problems and that consider a suite of human behavioral responses. This article is protected by copyright. All rights reserved.
- New ν‐type relative permeability curves for two‐phase
flows through subsurface fractures
- Authors: Noriaki Watanabe; Keisuke Sakurai, Takuya Ishibashi, Yutaka Ohsaki, Tetsuya Tamagawa, Masahiko Yagi, Noriyoshi Tsuchiya
Abstract: Appropriate relative permeability curves for two‐phase flows through subsurface fractures remain unclear. We have conducted decane‐water and nitrogen‐water two‐phase flow experiments and simulations on real variable‐aperture fractures in rocks under confining stress. Experiments have been conducted on fractures for different combinations of rock type (granite or limestone), wettability (contact angle of water: 0° or 90°), and intrinsic fracture permeability (10−11 m2 or 10−10 m2) using different combinations of shear displacement (0 mm or 1 mm) and effective confining stress (1 MPa or 40 MPa). It has been demonstrated that non‐wetting phase relative permeability depends on capillary pressure, except at either a higher contact angle or higher intrinsic permeability (i.e., bigger aperture), where no influence of capillarity is expected from the Young‐Laplace equation. In the absence of an influence of capillarity, relations between wetting and non‐wetting phase relative permeabilities agree with that of the X‐type relative permeability curves. In order to determine the relative permeability curves under the influence of capillarity, the experimental results have been analyzed by two‐phase flow simulations of the aperture distributions of the fractures. It has been revealed that non‐wetting phase relative permeability becomes zero, even at a small wetting phase saturation of approximately 0.3, while wetting phase relative permeability exhibits Corey‐type behavior, resulting in ν‐shaped relative permeability curves. Similar curves have been reported in the literature, but have not been demonstrated for real fractures. It has been revealed that the new ν‐type and traditional X‐type relative permeability curves are appropriate for describing two‐phase flows through subsurface fractures. This article is protected by copyright. All rights reserved.
- Morphodynamics: Rivers beyond steady state
- Authors: M. Church; R. I. Ferguson
Abstract: The morphology of an alluvial river channel affects the movement of water and sediment along it, but in the longer run is shaped by those processes. This interplay has mostly been investigated empirically within the paradigm of Newtonian mechanics. In rivers this has created an emphasis on equilibrium configurations with simple morphology and uniform steady flow. But transient adjustment, whether between equilibrium states or indefinitely, is to be expected in a world in which hydrology, sediment supply, and base level are not fixed. More fundamentally, water flows and all the phenomena that accompany them are inherently unsteady and flows in natural channels are characteristically non‐uniform. The morphodynamics of alluvial river channels is the striking consequence. In this paper we develop the essential connection between the episodic nature of bed material transport and the production of river morphology, emphasizing the fundamental problems of sediment transport, the role of bar evolution in determining channel form, the role of riparian vegetation, and the wide range of timescales for change. As the key integrative exercise, we emphasize the importance of physics‐based modeling of morphodynamics. We note consequences that can be of benefit to society if properly understood. These include the possibility to better be able to model how varying flows drive morphodynamic change, to understand the influence of the sediments themselves on morphodynamics, and to recognize the inherent necessity for rivers that transport bed material to deform laterally. We acknowledge pioneering contributions in WRR and elsewhere that have introduced some of these themes. This article is protected by copyright. All rights reserved.
- Diagnosis of insidious data disasters
- Authors: Jessica D. Lundquist; Nicholas E. Wayand, Adam Massmann, Martyn P. Clark, Fred Lott, Nicoleta C. Cristea
Abstract: Everyone taking field observations has a story of data collection gone wrong, and in most cases, the errors in the data are immediately obvious. A more challenging problem occurs when the errors are insidious, i.e., not readily detectable, and the error‐laden data appear useful for model testing and development. We present two case studies, one related to the water balance in the snow‐fed Tuolumne River, Sierra Nevada, California, combined with modeling using the Distributed Hydrology Soil Vegetation Model (DHSVM); and one related to the energy balance at Snoqualmie Pass, Washington, combined with modeling using the Structure for Unifying Multiple Modeling Alternatives (SUMMA). In the Tuolumne, modeled streamflow in one year was more than twice as large as observed; at Snoqualmie, modeled nighttime surface temperatures were biased by about +10 °C. Both appeared to be modeling failures, until detective work uncovered observational errors. We conclude with a discussion of what these cases teach us about science in an age of specialized research, when one person collects data, a separate person conducts model simulations, and a computer is charged with data quality‐assurance. This article is protected by copyright. All rights reserved.
- Metapopulation capacity of evolving fluvial landscapes
- Authors: Enrico Bertuzzo; Ignacio Rodriguez‐Iturbe, Andrea Rinaldo
Abstract: The form of fluvial landscapes is known to attain stationary network configurations that settle in dynamically accessible minima of total energy dissipation by landscape‐forming discharges. Recent studies have highlighted the role of the dendritic structure of river networks in controlling population dynamics of the species they host and large scale biodiversity patterns. Here, we systematically investigate the relation between energy dissipation, the physical driver for the evolution of river networks, and the ecological dynamics of their embedded biota. To that end we use the concept of metapopulation capacity, a measure to link landscape structures with the population dynamics they host. Technically, metapopulation capacity is the leading eigenvalue λM of an appropriate ‘landscape' matrix subsuming whether a given species is predicted to persist in the long run. λM can conveniently be used to rank different landscapes in terms of their capacity to support viable metapopulations. We study how λM changes in response to the evolving network configurations of spanning trees. Such sequence of configurations is theoretically known to relate network selection to general landscape evolution equations through imperfect searches for dynamically accessible states frustrated by the vagaries of Nature. Results show that the process shaping the metric and the topological properties of river networks, prescribed by physical constraints, leads to a progressive increase in the corresponding metapopulation capacity and therefore on the landscape capacity to support metapopulations – with implications on biodiversity in fluvial ecosystems. This article is protected by copyright. All rights reserved.
- Transport of fluorobenzoate tracers in a vegetated hydrologic control
volume: 1. Experimental results
- Authors: Pierre Queloz; Enrico Bertuzzo, Luca Carraro, Gianluca Botter, Franco Miglietta, P.S.C. Rao, Andrea Rinaldo
Abstract: This paper reports about the experimental evidence collected on the transport of five fluorobenzoate tracers injected under controlled conditions in a vegetated hydrologic volume, a large lysimeter (fitted with load cells, sampling ports and an underground chamber) where two willows prompting large evapotranspiration fluxes had been grown. The relevance of the study lies in the direct and indirect measures of the ways in which hydrologic fluxes, in this case evapotranspiration from the upper surface and discharge from the bottom drainage, sample water and solutes in storage at different times under variable hydrologic forcings. Methods involve the accurate control of hydrologic inputs and outputs and a large number of suitable chemical analyses of water samples in discharge waters. Mass‐extraction from biomass has also been performed ex‐post. The results of the two‐year long experiment established that our initial premises on the tracers' behavior, known to be sorption‐free under saturated conditions which we verified in column leaching tests, were unsuitable as large differences in mass recovery appeared. Issues on reactivity thus arose and were addressed in the paper, in this case attributed to microbial degradation and solute plant uptake. Our results suggest previously unknown features of fluorobenzoate compounds as hydrologic tracers, potentially interesting for catchment studies owing to their suitability for distinguishable multiple injections, and an outlook on direct experimental closures of mass balance in hydrologic transport volumes involving fluxes that are likely to sample differently stored water and solutes. This article is protected by copyright. All rights reserved.
- Transport of fluorobenzoate tracers in a vegetated hydrologic control
volume: 2. Theoretical inferences and modeling
- Authors: Pierre Queloz; Luca Carraro, Paolo Benettin, Gianluca Botter, Andrea Rinaldo, Enrico Bertuzzo
Abstract: A theoretical analysis of transport in a controlled hydrologic volume, inclusive of two willow trees and forced by erratic water inputs, is carried out contrasting the experimental data described in a companion paper. The data refer to the hydrologic transport in a large lysimeter of different fluorobenzoic acids seen as tracers. Export of solute is modeled through a recently developed framework which accounts for non‐stationary travel time distributions where we parametrize how output fluxes (namely, discharge and evapotranspiration) sample the available water ages in storage. The relevance of this work lies in the study of hydrologic drivers of the non‐stationary character of residence and travel time distributions, whose definition and computation shape this theoretical transport study. Our results show that a large fraction of the different behaviors exhibited by the tracers may be charged to the variability of the hydrologic forcings experienced after the injection. Moreover, the results highlight the crucial, and often overlooked, role of evapotranspiration and plant uptake in determining the transport of water and solutes. This application also suggests that the ways evapotranspiration selects water with different ages in storage can be inferred through model calibration contrasting only tracer concentrations in the discharge. A view on upscaled transport volumes like hillslopes or catchments is maintained throughout the paper. This article is protected by copyright. All rights reserved.
- Planform evolution of two anabranching structures in the Upper Peruvian
- Authors: C. E. Frias; J. D. Abad, A. Mendoza, J. Paredes, C. Ortals, H. Montoro
Abstract: We present a study to relate the sinuosity of the main channel and its effect on the dynamics of the secondary channels of anabranching structures. For this purpose, two locations of the Peruvian Amazon River were selected: 1) a site with a medium to high‐sinuosity main channel (MS site: Muyuy, Peru) and 2) a site with a low‐sinuosity main channel (LS site: at the triple boundary between Brazil, Colombia and Peru). The main channels for both the MS and LS anabranching structures have freedom to migrate in the lateral direction, while at least one of their secondary channels' is adjacent to the geological valley. For MS and LS sites, temporal analysis of planform evolution was carried out using 30 years of satellite imagery from which metrics such as width, sinuosity and annual maximum migration rates of main and secondary channels were calculated. Additionally, detailed hydrodynamic and bed morphology field measurements were carried out, and a two‐dimensional shallow water numerical model was developed. For a medium‐ to high‐sinuosity main channel anabranching structure, the secondary channels present a dominant mechanism for reworking the floodplain, while for the low‐sinuosity anabranching structure, the main channel planform dynamics is dominant. Flow velocities along the main and secondary channels for low‐, transition‐, and high‐flow discharges describe that for MS (LS) site, the velocities are much higher along the secondary (main) channels. This article is protected by copyright. All rights reserved.
- Linking bed morphology changes of two sediment mixtures to sediment
transport predictions in unsteady flows
- Authors: Kevin A. Waters; Joanna Crowe Curran
Abstract: Flume experiments were conducted to measure bed morphology adjustments in sand/gravel and sand/silt sediment mixtures during repeated hydrographs and to link these changes to sediment transport patterns over multiple time scales. Sediment composition and hydrograph flow magnitude greatly influenced channel morphology, which impacted sediment yield, hysteresis, and transport predictions. Bed load yields were larger and more variable for the sand/silt mixture, as gravel in the sand/gravel sediment inhibited grain entrainment, limited bedform growth, and acted to stabilize the bed. Hysteresis patterns varied due to bedform and surface structure adjustments, as well as the stabilizing effect of antecedent low flows. Using half the data set, a dimensionless fractional transport equation was derived based on excess shear stress. Dimensionless reference shear stresses were estimated in two ways: as bulk values from all transport measurements and by applying a separate limb approach in which values were estimated for each limb of each hydrograph. For the other half of the data set, transport predictions with the separate limb approach were more accurate than those from six existing transport equations and the fractional relationship applied with bulk reference shear stresses. Thus, hydrograph limb‐dependent dimensionless reference shear stress links changing bed morphology and sediment transport, providing a parameter to improve transport predictions during individual flood events and in unsteady flow regimes. This approach represents a framework with which to develop site‐specific transport relationships for varying flow regimes, particularly in cases where detailed bed morphology measurements are not feasible and heterogeneous sediment complicates bed structure over time. This article is protected by copyright. All rights reserved.
- Validation of finite water‐content vadose zone dynamics method using
column experiments with a moving water table and applied surface flux
- Authors: Fred L. Ogden; Wencong Lai, Robert C. Steinke, Jianting Zhu
Abstract: Data from laboratory experiments on a 143 cm tall and 14.5 cm diameter column, packed with Wedron sand with varied constant upper boundary fluxes and water table velocities for both falling and rising water tables are used to validate a finite water‐content vadose zone simulation methodology. The one‐dimensional finite water‐content Talbot and Ogden  (T‐O) infiltration and redistribution method was improved to simulate groundwater table dynamic effects, and compared against the numerical solution of the Richards' equation using Hydrus‐1D. Both numerical solutions agreed satisfactorily with time‐series measurements of water content. Results showed similar performance for both methods, with the T‐O method on average having higher Nash‐Sutcliffe efficiencies and smaller absolute biases. Hydrus‐1D was more accurate in predicting de‐ponding times in the case of a falling water table, while Hydrus‐1D and the T‐O method had similar errors in predicted ponding times in the case of a rising water table in 6 of 9 tests. The improved T‐O method was able to predict general features of vadose zone moisture dynamics with moving water table and surface infiltration using an explicit, mass‐conservative formulation. The advantage of an explicit formulation is that it is numerically simple, using forward Euler solution methodology, and is guaranteed to converge and to conserve mass. These properties make the improved T‐O method presented in this paper a robust and computationally efficient alternative to the numerical solution of Richards' equation in hydrological modeling applications involving groundwater table dynamic effects on vadose zone soil moistures. This article is protected by copyright. All rights reserved.
- Potential accumulation of contaminated sediments in a reservoir of a
high‐Andean watershed: Morphodynamic connections with geochemical
- Authors: María Teresa Contreras; Daniel Müllendorff, Pablo Pastén, Gonzalo E. Pizarro, Chris Paola, Cristián Escauriaza
Abstract: Rapid changes due to anthropic interventions in high‐altitude environments, such as the Altiplano region in South America, require new approaches to understand the connections between physical and geochemical processes. Alterations of the water quality linked to the river morphology can affect the ecosystems and human development in the long‐term. The future construction of a reservoir in the Lluta river, located in northern Chile, will change the spatial distribution of arsenic‐rich sediments, which can have significant effects on the lower parts of the watershed. In this investigation we develop a coupled numerical model to predict and evaluate the interactions between morphodynamic changes in the Lluta reservoir [based on the work of Kostic and Parker, 2003a,b], and conditions that can potentially desorb arsenic from the sediments. Assuming that contaminants are mobilized under anaerobic conditions, we calculate the oxygen concentration within the sediments to study the interactions of the delta progradation with the potential arsenic release. This work provides a framework for future studies aimed to analyze the complex connections between morphodynamics and water quality, when contaminant‐rich sediments accumulate in a reservoir. The tool can also help to design effective risk management and remediation strategies in these extreme environments. This article is protected by copyright. All rights reserved.
- What do we mean by sensitivity analysis? The need for comprehensive
characterization of ‘Global’ sensitivity in Earth and
Environmental Systems Models
- Authors: Saman Razavi; Hoshin V. Gupta
Abstract: Sensitivity analysis is an essential paradigm in Earth and Environmental Systems modelling. However, the term ‘sensitivity' has a clear definition, based in partial derivatives, only when specified locally around a particular point (e.g., optimal solution) in the problem space. Accordingly, no unique definition exists for ‘global sensitivity' across the problem space, when considering one or more model responses to different factors such as model parameters or forcings.
A variety of approaches have been proposed for global sensitivity analysis, based on different philosophies and theories, and each of these formally characterizes a different ‘intuitive' understanding of sensitivity. These approaches focus on different properties of the model response at a fundamental level and may therefore lead to different (even conflicting) conclusions about the underlying sensitivities. Here, we revisit the theoretical basis for sensitivity analysis, summarize and critically evaluate existing approaches in the literature, and demonstrate their flaws and shortcomings through conceptual examples. We also demonstrate the difficulty involved in interpreting ‘global' interaction effects, which may undermine the value of exiting interpretive approaches. With this background, we identify several important properties of response surfaces that are associated with the understanding and interpretation of sensitivities in the context of Earth and Environmental System models. Finally, we highlight the need for a new, comprehensive framework for sensitivity analysis that effectively characterizes all of the important sensitivity‐related properties of model response surfaces. This article is protected by copyright. All rights reserved.
- Spatial and temporal characteristics of rainfall in Africa: Summary
statistics for temporal downscaling
- Authors: Armel T. Kaptué; Niall P. Hanan, Lara Prihodko, Jorge A. Ramirez
Abstract: An understanding of rainfall characteristics at multiple spatiotemporal scales is of great importance for hydrological, biogeochemical and land surface modeling studies. In the present study, patterns of rainfall are analyzed over the African continent based on 3‐hourly 0.25° Tropical Rainfall Measuring Mission (TRMM) estimates between 1998 and 2012 to produce monthly statistical summaries. The selected rain event properties are multi‐year means of precipitation total amount (mm), event frequency (number), rate (mm/hr) and duration (hr) calculated independently for each calendar month. Analysis of 3‐hourly and daily events in the 1998‐2012 period suggests that rainfall amount can be summarized using gamma probability density functions. Assuming stationarity, gamma probability density functions of the total depth of 3‐hourly and daily events are estimated and then used for temporal down‐scaling of monthly rainfall estimates (past or future). As a result, we generate 3‐hourly and daily rainfall estimates that are pixel‐wise statistically indistinguishable from the observations while preserving monthly totals. Example scripts are provided that can be used to access monthly statistics and implement down‐scaling using archival (or projected) monthly rainfall estimates. These statistics could also be utilized for the assessment of rainfall from atmospheric models. This article is protected by copyright. All rights reserved.
- Front spreading with nonlinear sorption for oscillating flow
- Authors: D.G. (Gijsbert) Cirkel; S.E.A.T.M. (Sjoerd) van der Zee, J.C.L. (Hans) Meeussen
Abstract: In this paper we consider dispersive and chromatographic mixing at an interface, under alternating flow conditions. In case of a nonreactive or linearly sorbing solute, mixing is in complete analogy with classical dispersion theory. For nonlinear exchange however, oscillating convective flow leads to an alternation of sharpening (Traveling Wave TW) and spreading (Rarefaction Wave RW). As the limiting TW form is not necessarily accomplished at the end of the TW half cycle, the oscillating fronts show gradual continuous spreading that converges to a zero‐convection nonlinear pure diffusion spreading, which is mathematically of quite different nature. This behavior is maintained in case the total (background) concentration differs at both sides of the initial exchange front. This article is protected by copyright. All rights reserved.
- Effects of lateral nitrate flux and instream processes on dissolved
inorganic nitrogen export in a forested catchment: A model sensitivity
- Authors: Laurence Lin; Jackson R. Webster, Taehee Hwang, Lawrence E. Band
Abstract: The importance of terrestrial and aquatic ecosystems in controlling nitrogen dynamics in streams is a key interest of ecologists studying dissolved inorganic nitrogen (DIN) export from watersheds. In this study, we coupled a stream model with a terrestrial ecohydrological model and conducted a global sensitivity analysis to evaluate the relative importance of both ecosystems to nitrogen export. We constructed two scenarios (“normal” and high nitrate loads) to explore conditions under which terrestrial (lateral nitrate flux) or aquatic ecosystems (instream nutrient processes) may be more important in controlling DIN export. In a forest catchment, although the forest ecosystem controls the nitrogen load to streams, sensitivity results suggested that most nitrogen output from the terrestrial ecosystem was taken up by instream microbial immobilization associated with benthic detritus and retained in detritus. Later the immobilized nitrogen was re‐mineralized as DIN. Therefore, the intra‐annual pattern of DIN concentration in the stream was low in fall and became high in spring. Not only was instream microbial immobilization saturated with the high nitrogen load scenario, but also the net effect of immobilization and mineralization on DIN export was minimized because nitrogen cycling between organic and inorganic forms was accelerated. Overall, our linked terrestrial‐aquatic model simulations demonstrated that stream process could significantly affect the amount and timing of watershed nitrogen export when nitrogen export from the terrestrial system is low. However, when nitrogen export from the terrestrial system is high, the effect of stream processes in minimal. This article is protected by copyright. All rights reserved.
- Evapotranspiration based on Equilibrated Relative Humidity (ETRHEQ):
Evaluation over the Continental United States
- Authors: Angela J. Rigden; Guido D. Salvucci
Abstract: A novel method of estimating evapotranspiration (ET), referred to as the ETRHEQ method, is further developed, validated, and applied across the Unites States of America from 1961‐2010. The ETRHEQ method estimates the surface conductance to water vapor transport, which is the key rate‐limiting parameter of typical ET models, by choosing the surface conductance that minimizes the vertical variance of the calculated relative humidity profile averaged over the day. The ETRHEQ method, which was previously tested at five AmeriFlux sites, is modified for use at common weather stations and further validated at twenty AmeriFlux sites that span a wide range of climates and limiting factors. Averaged across all sites, the daily latent heat flux RMSE is ∼26 W·m−2 (or 15%). The method is applied across the United States at 305 weather stations and spatially interpolated using ANUSPLIN software. Gridded annual mean ETRHEQ ET estimates are compared with four datasets, including water balance‐derived ET, machine learning ET estimates based on FLUXNET data, North American Land Data Assimilation System project phase 2 ET, and a benchmark product that integrates 14 global ET datasets, with RMSEs ranging from 8.7 cm·yr−1 to 12.5 cm·yr−1. The ETRHEQ method relies only on data measured at weather stations, an estimate of vegetation height derived from land cover maps, and an estimate of soil thermal inertia. These data requirements allow it to have greater spatial coverage than direct measurements, greater historical coverage than satellite methods, significantly less parameter specification than most land surface models, and no requirement for calibration. This article is protected by copyright. All rights reserved.
- Numerical simulation of the environmental impact of hydraulic fracturing
of tight/shale gas reservoirs on near‐surface groundwater:
Background, base cases, shallow reservoirs, short‐term gas, and
- Authors: Matthew T. Reagan; George J. Moridis, Noel D. Keen, Jeffrey N. Johnson
Abstract: Hydrocarbon production from unconventional resources and the use of reservoir stimulation techniques, such as hydraulic fracturing, has grown explosively over the last decade. However, concerns have arisen that reservoir stimulation creates significant environmental threats through the creation of permeable pathways connecting the stimulated reservoir with shallower fresh‐water aquifers, thus resulting in the contamination of potable groundwater by escaping hydrocarbons or other reservoir fluids. This study investigates, by numerical simulation, gas and water transport between a shallow tight‐gas reservoir and a shallower overlying fresh‐water aquifer following hydraulic fracturing operations, if such a connecting pathway has been created. We focus on two general failure scenarios: 1) communication between the reservoir and aquifer via a connecting fracture or fault and 2) communication via a deteriorated, preexisting nearby well. We conclude that the key factors driving short‐term transport of gas include high permeability for the connecting pathway and the overall volume of the connecting feature. Production from the reservoir is likely to mitigate release through reduction of available free gas and lowering of reservoir pressure, and not producing may increase the potential for release. We also find that hydrostatic tight‐gas reservoirs are unlikely to act as a continuing source of migrating gas, as gas contained within the newly formed hydraulic fracture is the primary source for potential contamination. Such incidents of gas escape are likely to be limited in duration and scope for hydrostatic reservoirs. Reliable field and laboratory data must be acquired to constrain the factors and determine the likelihood of these outcomes. This article is protected by copyright. All rights reserved.
- Toward hyperresolution land surface modeling: The effects of
fine‐scale topography and soil texture on CLM4.0 simulations over
the southwestern U.S
- Authors: R. S. Singh; J.T. Reager, N.L. Miller, J.S. Famiglietti
Abstract: Increasing computational efficiency and the need for improved accuracy are currently driving the development of “hyper‐resolution” land surface models that can be implemented at continental scales with resolutions of 1km or finer. Here, we report research incorporating fine‐scale grid resolutions into the NCAR Community Land Model (CLM v4.0) for simulations at 1‐km, 25‐km, and 100‐km resolution using 1‐km soil and topographic information. Multi‐year model runs were performed over the southwestern United States, including the entire state of California and the Colorado River basin. The results show changes in the total amount of CLM‐modeled water storage, and changes in the spatial and temporal distributions of water in snow and soil reservoirs, as well as changes in surface fluxes and the energy balance. To inform future model progress and continued development needs and weaknesses, we compare simulation outputs to station and gridded observations of model fields. Although the higher grid‐resolution model is not driven by high‐resolution forcing, grid resolution changes alone yield significant improvement (reduction in error) between model outputs and observations, where the RMSE decreases by more than 35%, 36%, 34% and 12% for soil moisture, terrestrial water storage anomaly, sensible heat and snow water equivalent, respectively. As an additional exercise, we performed a 100m‐resolution simulation over a spatial sub‐domain. Those results indicate that parameters such as drainage, runoff, and infiltration are significantly impacted when hillslope scales of ∼100 meters or finer are considered, and we show the ways in which limitations of the current model physics, including no lateral flow between grid cells, may affect model simulation accuracy. This article is protected by copyright. All rights reserved.
- A unified approach for process‐based hydrologic modeling: 2. Model
implementation and case studies
- Authors: Martyn P. Clark; Bart Nijssen, Jessica D. Lundquist, Dmitri Kavetski, David E. Rupp, Ross A. Woods, Jim E. Freer, Ethan D. Gutmann, Andrew W. Wood, David J. Gochis, Roy M. Rasmussen, David G. Tarboton, Vinod Mahat, Gerald N. Flerchinger, Danny G. Marks
Abstract: This work advances a unified approach to process‐based hydrologic modeling, which we term the Structure for Unifying Multiple Modeling Alternatives (SUMMA). The modeling framework, introduced in the companion paper, uses a general set of conservation equations with flexibility in the choice of process parameterizations (closure relationships) and spatial architecture. This second paper specifies the model equations and their spatial approximations, describes the hydrologic and biophysical process parameterizations currently supported within the framework, and illustrates how the framework can be used in conjunction with multivariate observations to identify model improvements and future research and data needs. The case studies illustrate the use of SUMMA to select among competing modeling approaches based on both observed data and theoretical considerations. Specific examples of preferable modeling approaches include the use of physiological methods to estimate stomatal resistance, careful specification of the shape of the within‐ and below‐canopy wind profile, explicitly accounting for dust concentrations within the snowpack, and explicitly representing distributed lateral flow processes. Results also demonstrate that changes in parameter values can make as much or more difference to the model predictions than changes in the process representation. This emphasizes that improvements in model fidelity require a sagacious choice of both process parameterizations and model parameters. In conclusion, we envisage that SUMMA can facilitate ongoing model development efforts, the diagnosis and correction of model structural errors, and improved characterization of model uncertainty. This article is protected by copyright. All rights reserved.
- Impact of viscous fingering and permeability heterogeneity on fluid mixing
in porous media
- Authors: Christos Nicolaides; Birendra Jha, Luis Cueto‐Felgueroso, Ruben Juanes
Abstract: Fluid mixing plays a fundamental role in many natural and engineered processes, including groundwater flows in porous media, enhanced oil recovery, and microfluidic lab‐on‐a‐chip systems. Recent developments have explored the effect of viscosity contrast on mixing, suggesting that the unstable displacement of fluids with different viscosities, or viscous fingering, provides a powerful mechanism to increase fluid–fluid interfacial area and enhance mixing. However, existing studies have not incorporated the effect of medium heterogeneity on the mixing rate. Here, we characterize the evolution of mixing between two fluids of different viscosity in heterogeneous porous media. We focus on a practical scenario of divergent–convergent flow in a quarter five‐spot geometry prototypical of well‐driven groundwater flows. We study by means of numerical simulations the impact of permeability heterogeneity and viscosity contrast on the breakthrough curves and mixing efficiency, and we rationalize the nontrivial mixing behavior that emerges from the competition between the creation of fluid‐fluid interfacial area and channeling. This article is protected by copyright. All rights reserved.
- Estimation of soil salinity in a drip irrigation system by using joint
inversion of multicoil electromagnetic induction measurements
- Authors: Khan Zaib Jadoon; Davood Moghadas, Aurangzeb Jadoon, Thomas M. Missimer, Samir K. Al‐Mashharawi, Matthew F. McCabe
Abstract: Low frequency electromagnetic induction (EMI) is becoming a useful tool for soil characterization due to its fast measurement capability and sensitivity to soil moisture and salinity. In this research, a new EMI system (the CMD mini‐Explorer) is used for sub‐surface characterization of soil salinity in a drip irrigation system via a joint inversion approach of multi‐configuration EMI measurements. EMI measurements were conducted across a farm where Acacia trees are irrigated with brackish water. In‐situ measurements of vertical bulk electrical conductivity (σb) were recorded in different pits along one of the transects to calibrate the EMI measurements and to compare with the modeled electrical conductivity (σ) obtained by the joint inversion of multi‐configuration EMI measurements. Estimates of σ were then converted into the universal standard of soil salinity measurement (i.e. electrical conductivity of a saturated soil paste extract – ECe). Soil apparent electrical conductivity (ECa) was repeatedly measured with the CMD mini‐Explorer to investigate the temperature stability of the new system at a fixed location, where the ambient air temperature increased from 26 °C to 46 °C. Results indicate that the new EMI system is very stable in high temperature environments, especially above 40 °C, where most other approaches give unstable measurements. In addition, the distribution pattern of soil salinity is well estimated quantitatively by the joint inversion of multi‐component EMI measurements. The approach of joint inversion of EMI measurements allows for the quantitative mapping of the soil salinity distribution pattern and can be utilized for the management of soil salinity. This article is protected by copyright. All rights reserved.
- Soil hydrology: Recent methodological advances, challenges, and
- Authors: H. Vereecken; J.A. Huisman, H.J. Hendricks Franssen, N. Brüggemann, H.R. Bogena, S. Kollet, M. Javaux, J van der Kruk, J. Vanderborght
Abstract: Technological and methodological progress is essential to improve our understanding of fundamental processes in natural and engineering sciences. In this paper, we will address the potential of new technological and methodological advancements in soil hydrology to move forward our understanding of soil water related processes across a broad range of scales. We will focus on advancements made in quantifying root water uptake processes, subsurface lateral flow, and deep drainage at the field and catchment scale, respectively. We will elaborate on the value of establishing a science‐driven network of hydrological observatories to test fundamental hypotheses, to study organizational principles of soil hydrologic processes at catchment scale, and to provide data for the development and validation of models. Finally, we discuss recent developments in data assimilation methods, which provide new opportunities to better integrate observations and models and to improve predictions of the short‐term evolution of hydrological processes. This article is protected by copyright. All rights reserved.
- An exact analytical solution for steady seepage from a Perched Aquifer to
a low‐permeable sublayer: Kirkham‐Brock's legacy revisited
- Authors: A.R. Kacimov; Yu.V. Obnosov
Abstract: An analytical solution is obtained for steady 2‐D potential seepage flow from a non‐clogged and non‐lined soil channel into a highly‐permeable porous layer, with phreatic surfaces tapering towards a horizontal interface with a subjacent low‐permeable formation. Along this boundary, a vertical component of the Darcian velocity vector equals the formation saturated hydraulic conductivity. The image of the physical flow domain in the hodograph plane is a circular polygon, a triangle or digon in a limiting case of a” phreatic jet” impinging on the low‐permeable substratum. The polygon is mapped onto an auxiliary half plane, where the complex physical coordinate and complex potential are reconstructed by the Polubarinova‐Kochina method, i.e. by solution of a Riemann BVP. The seepage flow rate from the channel, free surfaces and a saturated (water‐logged) area are found for different thicknesses of the top layer, channel widths and conductivity ratios of the two strata. In particular, the earlier results of Brock, Kirkham and Youngs, which are based on a numerical solution, Dupuit‐Forchheimer (DF) approximation and approximate potential model, are confirmed in the full 2‐D models. Sufficiently far from the channel the phreatic surface and interface make a wedge. For a sufficiently deep substratum, three zones are analytically distinguished: an almost vertical 1‐D descending flow, an almost wedge‐configured 1‐D flow and an essentially 2‐D zone in between, where neither a standard infiltration theory nor DF analysis are valid. This article is protected by copyright. All rights reserved.
- Evaluating snow models with varying process representations for
- Authors: Jan Magnusson; Nander Wever, Richard Essery, Nora Helbig, Adam Winstral, Tobias Jonas
Abstract: Much effort has been invested in developing snow models over several decades, resulting in a wide variety of empirical and physically based snow models. For the most part, these models are built on similar principles. The greatest differences are found in how each model parameterizes individual processes (e.g. surface albedo and snow compaction). Parameterization choices naturally span a wide range of complexities. In this study, we evaluate the performance of different snow model parameterizations for hydrological applications using an existing multi‐model energy‐balance framework and data from two well‐instrumented alpine sites with seasonal snow cover. We also include two temperature‐index snow models and an intensive, physically based multi‐layer snow model in our analyses. Our results show that snow mass observations provide useful information for evaluating the ability of a model to predict snowpack runoff, whereas snow depth data alone are not. For snow mass and runoff, the energy‐balance models appear transferable between our two study sites, a behaviour which is not observed for snow surface temperature predictions due to site‐specificity of turbulent heat transfer formulations. Errors in the input and validation data, rather than model formulation, seem to be the greatest factor affecting model performance. The three model types provide similar ability to reproduce daily observed snowpack runoff when appropriate model structures are chosen. Model complexity was not a determinant for predicting daily snowpack mass and runoff reliably. Our study shows the usefulness of the multi‐model framework for identifying appropriate models under given constraints such as data availability, properties of interest and computational cost. This article is protected by copyright. All rights reserved.
- Prolonged river water pollution due to variable‐density flow and
solute transport in the riverbed
- Authors: Guangqiu Jin; Hongwu Tang, Ling Li, D. A. Barry
Pages: 1898 - 1915
Abstract: A laboratory experiment and numerical modeling were used to examine effects of density gradients on hyporheic flow and solute transport under the condition of a solute pulse input to a river with regular bed forms. Relatively low‐density gradients due to an initial salt pulse concentration of 1.55 kg m−3 applied in the experiment were found to modulate significantly the pore‐water flow and solute transport in the riverbed. Such density gradients increased downward flow and solute transport in the riverbed by factors up to 1.6. This resulted in a 12.2% increase in the total salt transfer from the water column to the riverbed over the salt pulse period. As the solute pulse passed, the effect of the density gradients reversed, slowing down the release of the solute back to the river water by a factor of 3.7. Numerical modeling indicated that these density effects intensified as salt concentrations in the water column increased. Simulations further showed that the density gradients might even lead to unstable flow and result in solute fingers in the bed of large bed forms. The slow release of solute from the bed back to the river led to a long tail of solute concentration in the river water. These findings have implications for assessment of impact of pollution events on river systems, in particular, long‐term effects on both the river water and riverbed due to the hyporheic exchange.
- Variational Lagrangian data assimilation in open channel networks
- Authors: Qingfang Wu; Andrew Tinka, Kevin Weekly, Jonathan Beard, Alexandre M. Bayen
Pages: 1916 - 1938
Abstract: This article presents a data assimilation method in a tidal system, where data from both Lagrangian drifters and Eulerian flow sensors were fused to estimate water velocity. The system is modeled by first‐order, hyperbolic partial differential equations subject to periodic forcing. The estimation problem can then be formulated as the minimization of the difference between the observed variables and model outputs, and eventually provide the velocity and water stage of the hydrodynamic system. The governing equations are linearized and discretized using an implicit discretization scheme, resulting in linear equality constraints in the optimization program. Thus, the flow estimation can be formed as an optimization problem and efficiently solved. The effectiveness of the proposed method was substantiated by a large‐scale field experiment in the Sacramento‐San Joaquin River Delta in California. A fleet of 100 sensors developed at the University of California, Berkeley, were deployed in Walnut Grove, CA, to collect a set of Lagrangian data, a time series of positions as the sensors moved through the water. Measurements were also taken from Eulerian sensors in the region, provided by the United States Geological Survey. It is shown that the proposed method can effectively integrate Lagrangian and Eulerian measurement data, resulting in a suited estimation of the flow variables within the hydraulic system.
- The value of multiple data set calibration versus model complexity for
improving the performance of hydrological models in mountain catchments
- Authors: David Finger; Marc Vis, Matthias Huss, Jan Seibert
Pages: 1939 - 1958
Abstract: The assessment of snow, glacier, and rainfall runoff contribution to discharge in mountain streams is of major importance for an adequate water resource management. Such contributions can be estimated via hydrological models, provided that the modeling adequately accounts for snow and glacier melt, as well as rainfall runoff. We present a multiple data set calibration approach to estimate runoff composition using hydrological models with three levels of complexity. For this purpose, the code of the conceptual runoff model HBV‐light was enhanced to allow calibration and validation of simulations against glacier mass balances, satellite‐derived snow cover area and measured discharge. Three levels of complexity of the model were applied to glacierized catchments in Switzerland, ranging from 39 to 103 km2. The results indicate that all three observational data sets are reproduced adequately by the model, allowing an accurate estimation of the runoff composition in the three mountain streams. However, calibration against only runoff leads to unrealistic snow and glacier melt rates. Based on these results, we recommend using all three observational data sets in order to constrain model parameters and compute snow, glacier, and rain contributions. Finally, based on the comparison of model performance of different complexities, we postulate that the availability and use of different data sets to calibrate hydrological models might be more important than model complexity to achieve realistic estimations of runoff composition.
- Seasonal hydrologic responses to climate change in the Pacific Northwest
- Authors: Julie A. Vano; Bart Nijssen, Dennis P. Lettenmaier
Pages: 1959 - 1976
Abstract: Increased temperatures and changes in precipitation will result in fundamental changes in the seasonal distribution of streamflow in the Pacific Northwest and will have serious implications for water resources management. To better understand local impacts of regional climate change, we conducted model experiments to determine hydrologic sensitivities of annual, seasonal, and monthly runoff to imposed annual and seasonal changes in precipitation and temperature. We used the Variable Infiltration Capacity (VIC) land‐surface hydrology model applied at 1/16° latitude‐longitude spatial resolution over the Pacific Northwest (PNW), a scale sufficient to support analyses at the hydrologic unit code eight (HUC‐8) basin level. These experiments resolve the spatial character of the sensitivity of future water supply to precipitation and temperature changes by identifying the seasons and locations where climate change will have the biggest impact on runoff. The PNW exhibited a diversity of responses, where transitional (intermediate elevation) watersheds experience the greatest seasonal shifts in runoff in response to cool season warming. We also developed a methodology that uses these hydrologic sensitivities as basin‐specific transfer functions to estimate future changes in long‐term mean monthly hydrographs directly from climate model output of precipitation and temperature. When principles of linearity and superposition apply, these transfer functions can provide feasible first‐order estimates of the likely nature of future seasonal streamflow change without performing downscaling and detailed model simulations.
- Influence of surfactants on unsaturated water flow and solute transport
- Authors: Ahmet Karagunduz; Michael H. Young, Kurt D. Pennell
Pages: 1977 - 1988
Abstract: Surfactants can reduce soil water retention by changing the surface tension of water and the contact angle between the liquid and solid phases. As a result, water flow and solute transport in unsaturated soil may be altered in the presence of surfactants. In this study, the effects of a representative nonionic surfactant, Triton X‐100, on coupled water flow and nonreactive solute transport during unsaturated flow conditions were evaluated. Batch reactor experiments were conducted to measure the surfactant sorption characteristics, while unsaturated transport experiments were performed in columns packed with 40–270 mesh Ottawa sand at five initial water contents. Following the introduction of surfactant solution, the rate of water percolation through the sand increased; however, this period of rapid water drainage was followed by decreased water percolation due to the reduction in soil water content and the corresponding decrease in unsaturated hydraulic conductivity behind the surfactant front. The observed changes in water percolation occurred sequentially, and resulted in faster nonreactive solute transport than was observed in the absence of surfactant. A one‐dimensional mathematical model accurately described coupled water flow, surfactant, and solute transport under most experimental conditions. Differences between model predictions and experimental data were observed in the column study performed at the lowest water content (0.115 cm3/cm3), which was attributed to surfactant adsorption at the air‐water interface. These findings demonstrate the potential influence of surfactants additives on unsaturated water flow and solute transport in soils, and demonstrate a methodology to couple these processes in a predictive modeling tool.
- Occurrence of seawater intrusion overshoot
- Authors: Leanne K. Morgan; Mark Bakker, Adrian D. Werner
Pages: 1989 - 1999
Abstract: A number of numerical modeling studies of transient sea level rise (SLR) and seawater intrusion (SI) in flux‐controlled aquifer systems have reported an overshoot phenomenon, whereby the freshwater‐saltwater interface temporarily extends further inland than the eventual steady state position. Recently, physical sand‐tank modeling has shown overshoot to be a physical process. In this paper, we have carried out numerical modeling of SLR‐SI to demonstrate that overshoot can occur at the field scale within unconfined aquifers. This result is contrary to previous conclusions drawn from a restricted number of cases. In addition, we show that SI overshoot is plausible under scenarios of gradual sea level rise that are consistent with conditions predicted by the Intergovernmental Panel for Climate Change. Overshoot was found to be largest in flux‐controlled unconfined aquifers characterized by low freshwater flux, high specific yield, and large inland extent. These conditions result in longer timeframes for the aquifer to reach new steady state conditions following SLR, and the extended period prior to reequilibration of the groundwater flow field produces more extensive overshoot.
- Contextual and sociopsychological factors in predicting habitual cleaning
of water storage containers in rural Benin
- Authors: Andrea Stocker; Hans‐Joachim Mosler
Pages: 2000 - 2008
Abstract: Recontamination of drinking water occurring between water collection at the source and the point of consumption is a current problem in developing countries. The household drinking water storage container is one source of contamination and should therefore be cleaned regularly. First, the present study investigated contextual factors that stimulate or inhibit the development of habitual cleaning of drinking water storage containers with soap and water. Second, based on the Risk, Attitudes, Norms, Abilities, and Self‐regulation (RANAS) Model of behavior, the study aimed to determine which sociopsychological factors should be influenced by an intervention to promote habitual cleaning. In a cross‐sectional study, 905 households in rural Benin were interviewed by structured face‐to‐face interviews. A forced‐entry regression analysis was used to determine potential contextual factors related to habitual cleaning. Subsequently, a hierarchical regression was conducted with the only relevant contextual factor entered in the first step (R2 = 6.7%) and the sociopsychological factors added in the second step (R2 = 62.5%). Results showed that households using a clay container for drinking water storage had a significantly weaker habit of cleaning their water storage containers with soap and water than did households using other types of containers (β = −0.10). The most important sociopsychological predictors of habitual cleaning were commitment (β = 0.35), forgetting (β = −0.22), and self‐efficacy (β = 0.14). The combined investigation of contextual and sociopsychological factors proved beneficial in terms of developing intervention strategies. Possible interventions based on these findings are recommended.
- The impact of reservoir conditions on the residual trapping of carbon
dioxide in Berea sandstone
- Authors: Ben Niu; Ali Al‐Menhali, Samuel C. Krevor
Pages: 2009 - 2029
Abstract: The storage of carbon dioxide in deep brine‐filled permeable rocks is an important tool for CO2 emissions mitigation on industrial scales. Residual trapping of CO2 through capillary forces within the pore space of the reservoir is one of the most significant mechanisms for storage security and is also a factor determining the ultimate extent of CO2 migration within the reservoir. In this study we have evaluated the impact of reservoir conditions of pressure, temperature, and brine salinity on the residual trapping characteristic curve of a fired Berea sandstone rock. The observations demonstrate that the initial‐residual characteristic trapping curve is invariant across a wide range of pressure, temperature, and brine salinities and is also the same for CO2‐brine systems as a N2‐water system. The observations were made using a reservoir condition core‐flooding laboratory that included high‐precision pumps, temperature control, the ability to recirculate fluids for weeks at a time, and an X‐ray CT scanner. Experimental conditions covered pressures of 5–20 MPa, temperatures of 25–50°C, and 0–5 mol/kg NaCl brine salinity. A novel coreflooding approach was developed, making use of the capillary end effect to create a large range in initial CO2 saturation (0.15–0.6) in a single coreflood. Upon subsequent flooding with CO2‐equilibriated brine, the observation of residual saturation corresponded to the wide range of initial saturations before flooding resulting in a rapid construction of the initial‐residual curve. For each condition we report the initial‐residual curve and the resulting parameterization of the Land hysteresis models.
- An empirical vegetation correction for soil water content quantification
using cosmic ray probes
- Authors: R. Baatz; H. R. Bogena, H.‐J. Hendricks Franssen, J. A. Huisman, C. Montzka, H. Vereecken
Pages: 2030 - 2046
Abstract: Cosmic ray probes are an emerging technology to continuously monitor soil water content at a scale significant to land surface processes. However, the application of this method is hampered by its susceptibility to the presence of aboveground biomass. Here we present a simple empirical framework to account for moderation of fast neutrons by aboveground biomass in the calibration. The method extends the N0‐calibration function and was developed using an extensive data set from a network of 10 cosmic ray probes located in the Rur catchment, Germany. The results suggest a 0.9% reduction in fast neutron intensity per 1 kg of dry aboveground biomass per m2 or per 2 kg of biomass water equivalent per m2. We successfully tested the novel vegetation correction using temporary cosmic ray probe measurements along a strong gradient in biomass due to deforestation, and using the COSMIC, and the hmf method as independent soil water content retrieval algorithms. The extended N0‐calibration function was able to explain 95% of the overall variability in fast neutron intensity.
- Blended near‐optimal alternative generation, visualization, and
interaction for water resources decision making
- Authors: David E. Rosenberg
Pages: 2047 - 2063
Abstract: State‐of‐the‐art systems analysis techniques focus on efficiently finding optimal solutions. Yet an optimal solution is optimal only for the modeled issues and managers often seek near‐optimal alternatives that address unmodeled objectives, preferences, limits, uncertainties, and other issues. Early on, Modeling to Generate Alternatives (MGA) formalized near‐optimal as performance within a tolerable deviation from the optimal objective function value and identified a few maximally different alternatives that addressed some unmodeled issues. This paper presents new stratified, Monte‐Carlo Markov Chain sampling and parallel coordinate plotting tools that generate and communicate the structure and extent of the near‐optimal region to an optimization problem. Interactive plot controls allow users to explore region features of most interest. Controls also streamline the process to elicit unmodeled issues and update the model formulation in response to elicited issues. Use for an example, single‐objective, linear water quality management problem at Echo Reservoir, Utah, identifies numerous and flexible practices to reduce the phosphorus load to the reservoir and maintain close‐to‐optimal performance. Flexibility is upheld by further interactive alternative generation, transforming the formulation into a multiobjective problem, and relaxing the tolerance parameter to expand the near‐optimal region. Compared to MGA, the new blended tools generate more numerous alternatives faster, more fully show the near‐optimal region, and help elicit a larger set of unmodeled issues.
- Modification of the Local Cubic Law of fracture flow for weak inertia,
tortuosity, and roughness
- Authors: Lichun Wang; M. Bayani Cardenas, Donald T. Slottke, Richard A. Ketcham, John M. Sharp
Pages: 2064 - 2080
Abstract: The classical Local Cubic Law (LCL) generally overestimates flow through real fractures. We thus developed and tested a modified LCL (MLCL) which takes into account local tortuosity and roughness, and works across a low range of local Reynolds Numbers. The MLCL is based on (1) modifying the aperture field by orienting it with the flow direction and (2) correcting for local roughness changes associated with local flow expansion/contraction. In order to test the MLCL, we compared it with direct numerical simulations with the Navier‐Stokes equations using real and synthetic three‐dimensional rough‐walled fractures, previous corrected forms of the LCL, and experimental flow tests. The MLCL performed well and the effective errors (δ) in volumetric flow rate range from −3.4% to 13.4% with an arithmetic mean of δ (< δ >) equal to 3.7%. The MLCL is more accurate than previous modifications of the LCL. We also investigated the error associated with applying the Cubic Law (CL) while utilizing modified aperture field. The δ from the CL ranges from −14.2% to 11.2%, with a slightly higher < δ > = 6.1% than the MLCL. The CL with the modified aperture field considering local tortuosity and roughness may also be sufficient for predicting the hydraulic properties of rough fractures.
- Canopy edge flow: A momentum balance analysis
- Authors: Sharon Moltchanov; Yardena Bohbot‐Raviv, Tomer Duman, Uri Shavit
Pages: 2081 - 2095
Abstract: Canopy flow models are often dedicated to ideal, infinite, homogenous systems. However, real canopy systems have physical boundaries, where the flow enters and leaves patches of vegetation, generating a complex pressure field and velocity variations. Here we focus our study on the canopy entry region by examining the terms involved in the double (space and time) averaged momentum equations and their relative contribution to the total momentum balance. The estimation of each term is made possible by particle image velocimetry (PIV) measurements in a model canopy constructed of randomly distributed thin glass plates. The instantaneous velocity fields were used to calculate the mean velocities, pressure, drag, Reynolds stresses, and dispersive stresses. It was found that within the entry region, the pressure gradient, the drag forces, and dispersive stresses are the three most significant terms that affect the balance in the streamwise momentum equation. In the vertical direction, the dispersive stresses are also significant and their contribution to the total momentum cannot be ignored. The study shows that dispersive stresses are initially formed around canopy edges; at both the entry region and the canopy top boundary. They start as a sink term, extracting momentum from the flow, and then become a source term that contributes momentum to the flow until they eventually decay at some short penetration distance into the canopy. These results reveal a new understanding on the evolution of momentum within the entry region, necessary in any closure modeling of flow in real canopies.
- Estimating hydraulic conductivity of fractured rocks from
high‐pressure packer tests with an Izbash's law‐based
- Authors: Yi‐Feng Chen; Shao‐Hua Hu, Ran Hu, Chuang‐Bing Zhou
Pages: 2096 - 2118
Abstract: High‐pressure packer test (HPPT) is an enhanced constant head packer test for characterizing the permeability of fractured rocks under high‐pressure groundwater flow conditions. The interpretation of the HPPT data, however, remains difficult due to the transition of flow conditions in the conducting structures and the hydraulic fracturing‐induced permeability enhancement in the tested rocks. In this study, a number of HPPTs were performed in the sedimentary and intrusive rocks located at 450 m depth in central Hainan Island. The obtained Q‐P curves were divided into a laminar flow phase (I), a non‐Darcy flow phase (II), and a hydraulic fracturing phase (III). The critical Reynolds number for the deviation of flow from linearity into phase II was 25−66. The flow of phase III occurred in sparsely to moderately fractured rocks, and was absent at the test intervals of perfect or poor intactness. The threshold fluid pressure between phases II and III was correlated with RQD and the confining stress. An Izbash's law‐based analytical model was employed to calculate the hydraulic conductivity of the tested rocks in different flow conditions. It was demonstrated that the estimated hydraulic conductivity values in phases I and II are basically the same, and are weakly dependent on the injection fluid pressure, but it becomes strongly pressure dependent as a result of hydraulic fracturing in phase III. The hydraulic conductivity at different test intervals of a borehole is remarkably enhanced at highly fractured zone or contact zone, but within a rock unit of weak heterogeneity, it decreases with the increase of depth.
- A direct simulation demonstrating the role of spacial heterogeneity in
determining anomalous diffusive transport
- Authors: Vaughan R. Voller
Pages: 2119 - 2127
Abstract: Typically the spreading length‐scale in a diffusion process increases in time as
ℓ∼tn,n=12. In the presence of spatial heterogeneities, however, so‐called anomalous diffusion can occur, here the time exponent
n≠12. The objective of this paper is to present a numerical simulation that directly demonstrates the link between spacial heterogeneity and anomalous diffusion. The simulation is of the infiltration of a fluid into a horizontal unit area Hele‐Shaw cell in which the permeability at specified locations, laid out as a Sierpinski fractal carpet pattern, can differ from the nominal value K = 1. When there is no permeability contrast, the fluid infiltration has a diffusion‐like behavior, i.e., the filled plan‐form area increases in time as
F=Atn,n=12. When there is a permeability contrast, however, although the evolution of infiltration still follows a power law, anomalous behavior is observed; subdiffusive (
n 1. These anomalous behaviors persist, even when the permeability contrast is only imposed over the largest sized fractal pattern element. But, if the pattern over which the permeability contrast is imposed has a subdomain length scale (obtained by filling in the largest sized element in the Sierpinski carpet with the smaller sized elements), normal diffusion
n=12 behavior is recovered. In the special case that the imposed permeability in the pattern elements is
K≪1, an approximate model, directly relating the coefficient and exponent in the infiltration power law to the porosity and fractal dimension of the carpet pattern, is derived and validated.
- On the formation of multiple local peaks in breakthrough curves
- Authors: Erica R. Siirila‐Woodburn; Xavier Sanchez‐Vila, Daniel Fernàndez‐Garcia
Pages: 2128 - 2152
Abstract: The analysis of breakthrough curves (BTCs) is of interest in hydrogeology as a way to parameterize and explain processes related to anomalous transport. Classical BTCs assume the presence of a single peak in the curve, where the location and size of the peak and the slope of the receding limb has been of particular interest. As more information is incorporated into BTCs (for example, with high‐frequency data collection, supercomputing efforts), it is likely that classical definitions of BTC shapes will no longer be adequate descriptors for contaminant transport problems. We contend that individual BTCs may display multiple local peaks depending on the hydrogeologic conditions and the solute travel distance. In such cases, classical definitions should be reconsidered. In this work, the presence of local peaks in BTCs is quantified from high‐resolution numerical simulations in synthetic fields with a particle tracking technique and a kernel density estimator to avoid either overly jagged or smoothed curves that could mask the results. Individual BTCs from three‐dimensional heterogeneous hydraulic conductivity fields with varying combinations of statistical anisotropy, heterogeneity models, and local dispersivity are assessed as a function of travel distance. The number of local peaks, their corresponding slopes, and a transport connectivity index are shown to strongly depend on statistical anisotropy and travel distance. Results show that the choice of heterogeneity model also affects the frequency of local peaks, but the slope is less sensitive to model selection. We also discuss how solute shearing and rerouting can be determined from local peak quantification.
- Optimizing water resources management in large river basins with
integrated surface water‐groundwater modeling: A
- Authors: Bin Wu; Yi Zheng, Xin Wu, Yong Tian, Feng Han, Jie Liu, Chunmiao Zheng
Pages: 2153 - 2173
Abstract: Integrated surface water‐groundwater modeling can provide a comprehensive and coherent understanding on basin‐scale water cycle, but its high computational cost has impeded its application in real‐world management. This study developed a new surrogate‐based approach, SOIM (Surrogate‐based Optimization for Integrated surface water‐groundwater Modeling), to incorporate the integrated modeling into water management optimization. Its applicability and advantages were evaluated and validated through an optimization research on the conjunctive use of surface water (SW) and groundwater (GW) for irrigation in a semiarid region in northwest China. GSFLOW, an integrated SW‐GW model developed by USGS, was employed. The study results show that, due to the strong and complicated SW‐GW interactions, basin‐scale water saving could be achieved by spatially optimizing the ratios of groundwater use in different irrigation districts. The water‐saving potential essentially stems from the reduction of nonbeneficial evapotranspiration from the aqueduct system and shallow groundwater, and its magnitude largely depends on both water management schemes and hydrological conditions. Important implications for water resources management in general include: first, environmental flow regulation needs to take into account interannual variation of hydrological conditions, as well as spatial complexity of SW‐GW interactions; and second, to resolve water use conflicts between upper stream and lower stream, a system approach is highly desired to reflect ecological, economic, and social concerns in water management decisions. Overall, this study highlights that surrogate‐based approaches like SOIM represent a promising solution to filling the gap between complex environmental modeling and real‐world management decision‐making.
- Developing effective messages about potable recycled water: The importance
of message structure and content
- Authors: J. Price; K. S. Fielding, J. Gardner, Z. Leviston, M. Green
Pages: 2174 - 2187
Abstract: Community opposition is a barrier to potable recycled water schemes. Effective communication strategies about such schemes are needed. Drawing on social psychological literature, two experimental studies are presented, which explore messages that improve public perceptions of potable recycled water. The Elaboration‐Likelihood Model of information processing and attitude change is tested and supported. Study 1 (N = 415) premeasured support for recycled water, and trust in government information at Time 1. Messages varied in complexity and sidedness were presented at Time 2 (3 weeks later), and support and trust were remeasured. Support increased after receiving information, provided that participants received complex rather than simple information. Trust in government was also higher after receiving information. There was tentative evidence of this in response to two‐sided messages rather than one‐sided messages. Initial attitudes to recycled water moderated responses to information. Those initially neutral or ambivalent responded differently to simple and one‐sided messages, compared to participants with positive or negative attitudes. Study 2 (N = 957) tested the effectiveness of information about the low relative risks, and/or benefits of potable recycled water, compared to control groups. Messages about the low risks resulted in higher support when the issue of recycled water was relevant. Messages about benefits resulted in higher perceived issue relevance, but did not translate into greater support. The results highlight the importance of understanding people's motivation to process information, and need to tailor communication to match attitudes and stage of recycled water schemes' development.
- Spin‐up behavior and effects of initial conditions for an integrated
- Authors: Alimatou Seck; Claire Welty, Reed M. Maxwell
Pages: 2188 - 2210
Abstract: Initial conditions have been shown to have a strong effect on outputs of surface water models, but their impact on integrated hydrologic models is not well documented. We investigated the effects of initial conditions on an integrated hydrologic model of a 5632 km2 domain in the northeastern U.S. Simulations were run for the year 1980 using four initial conditions spanning a range of average depth to water table, including 1 m (“wet”), 3m, 5m, and 7 m (“dry”) below land surface. Model outputs showed significant effects of initial conditions on basin‐averaged variables such as subsurface storage, surface storage, and surface runoff, with the greatest impact observed on surface storage and runoff. Effects of initial conditions were related to meteorological conditions, with precipitation reducing the effects of initial conditions on surface storage and runoff. Additionally, feedbacks between soil moisture and land‐energy fluxes affected the impacts of initial conditions: higher temperatures magnified the differences in storage, recharge, and discharge among the four initial‐condition scenarios. Ten year recursive runs were conducted for the wet and dry scenarios. Spin‐up times varied by model components and were considerably smaller for land‐surface states and fluxes. Spin‐up for dry initial conditions was slower than for wet initial conditions, indicating longer system memory for dry initial conditions. These variations in persistence of initial conditions should be taken into consideration when designing model initialization approaches. More broadly, this behavior is indicative of increased persistence of the effects of dry years as opposed to wet years in hydrologic systems.
- Lowland fluvial phosphorus altered by dams
- Authors: Jianjun Zhou; Man Zhang, Binliang Lin, Pingyu Lu
Pages: 2211 - 2226
Abstract: Dams affect ecosystems, but their physical link to the variations in fluvial fluxes and downstream ecological consequences are inadequately understood. After estimating the current effects of the Three Gorges project and other reservoirs upstream on the Yangtze River on the fluvial phosphorus (P) in the middle and lower Yangtze River, we further investigated the long‐term effects of dams on the fluvial regimes of P and P‐enriched sediment (PES). Simultaneously measured P distributions with sediment size (PDSS) from the Three Gorges Reservoir (TGR) proved that the areal density of particulate P (PP) bound on graded sediment can be measured using the surface area concentration of the total sediment. A PDSS relationship is obtained and the selective transport and long‐term sedimentation of P are simulated using a nonuniform suspended sediment model, which incorporates the PDSS formula. The computations revealed that a reservoir would significantly lower the downstream availability of P in the dry season and promote high pulses of P in summer when the reservoir is flushed as sedimentation accumulates. As a result, the P buffering and replenishing mechanism in the pristine ecosystem from upstream supplies and local re‐suspension are permanently eliminated when a regulating reservoir is built upstream. This change is irreversible if reservoir regulation continues. Changes could potentially aggravate the existing P‐limitation, decrease the water's ability to adjust nutrient/pollutant fluctuations, accumulate a greater surplus of carbon and nitrogen, and even exacerbate blooms in favorable conditions.
- Inverse sequential simulation: A new approach for the characterization of
hydraulic conductivities demonstrated on a non‐Gaussian field
- Authors: Teng Xu; J. Jaime Gómez‐Hernández
Pages: 2227 - 2242
Abstract: Inverse sequential simulation (iSS) is a new inverse modeling approach for the characterization of hydraulic conductivity fields based on sequential simulation. It is described and demonstrated in a synthetic aquifer with non‐Gaussian spatial features, and compared against the normal‐score ensemble Kalman filter (NS‐EnKF). The new approach uses the sequential simulation paradigm to generate realizations borrowing from the ensemble Kalman filter the idea of using the experimental nonstationary cross‐covariance between conductivities and piezometric heads computed on an ensemble of realizations. The resulting approach is fully capable of retrieving the non‐Gaussian patterns of the reference field after conditioning on the piezometric heads with results comparable of those obtained by the NS‐EnKF.
- Hydraulic controls of in‐stream gravel bar hyporheic exchange and
- Authors: Nico Trauth; Christian Schmidt, Michael Vieweg, Sascha E. Oswald, Jan H. Fleckenstein
Pages: 2243 - 2263
Abstract: Hyporheic exchange transports solutes into the subsurface where they can undergo biogeochemical transformations, affecting fluvial water quality and ecology. A three‐dimensional numerical model of a natural in‐stream gravel bar (20 m × 6 m) is presented. Multiple steady state streamflow is simulated with a computational fluid dynamics code that is sequentially coupled to a reactive transport groundwater model via the hydraulic head distribution at the streambed. Ambient groundwater flow is considered by scenarios of neutral, gaining, and losing conditions. The transformation of oxygen, nitrate, and dissolved organic carbon by aerobic respiration and denitrification in the hyporheic zone are modeled, as is the denitrification of groundwater‐borne nitrate when mixed with stream‐sourced carbon. In contrast to fully submerged structures, hyporheic exchange flux decreases with increasing stream discharge, due to decreasing hydraulic head gradients across the partially submerged structure. Hyporheic residence time distributions are skewed in the log‐space with medians of up to 8 h and shift to symmetric distributions with increasing level of submergence. Solute turnover is mainly controlled by residence times and the extent of the hyporheic exchange flow, which defines the potential reaction area. Although streamflow is the primary driver of hyporheic exchange, its impact on hyporheic exchange flux, residence times, and solute turnover is small, as these quantities exponentially decrease under losing and gaining conditions. Hence, highest reaction potential exists under neutral conditions, when the capacity for denitrification in the partially submerged structure can be orders of magnitude higher than in fully submerged structures.
- Hydrological connectivity in river deltas: The first‐order
importance of channel‐island exchange
- Authors: Matthew Hiatt; Paola Passalacqua
Pages: 2264 - 2282
Abstract: Deltaic systems are composed of distributary channels and interdistributary islands. While previous work has focused either on the channels or on the islands, here we study the hydrological exchange between channels and islands and point at its important role in delta morphology and ecology. We focus our analysis on Wax Lake Delta in coastal Louisiana (USA) and characterize the surface water component of hydrological connectivity through measurements of water discharge and hydraulic tracer propagation. We find that deltaic islands are zones of significant water flux as 23–54% of the incoming distributary channel flux enters the islands. A calculation of the travel times through a channel‐island complex shows travel times through the islands to be at least 3 times their channel counterparts. A dye release experiment also indicates that travel times in islands are much longer that those within channels as dye remained in the island for the 3.8 day duration of the experiment. Additionally, islands are more sensitive than channels to environmental forces such as tides, which cause flow reversal and thus can increase travel times through the islands. Our work defines the “hydrological network” of a river delta to include not only the distributary channel network but also the interdistributary islands, quantifies the implications of channel‐island hydrological connectivity to travel times through the system, and discusses the relevance of our findings to channel mouth dynamics at the delta front and the potential for denitrification in coastal systems.
- The influence of water table depth and the free atmospheric state on
convective rainfall predisposition
- Authors: Sara Bonetti; Gabriele Manoli, Jean‐Christophe Domec, Mario Putti, Marco Marani, Gabriel G. Katul
Pages: 2283 - 2297
Abstract: A mechanistic model for the soil‐plant system is coupled to a conventional slab representation of the atmospheric boundary layer (ABL) to explore the role of groundwater table (WT) variations and free atmospheric (FA) states on convective rainfall predisposition (CRP) at a Loblolly pine plantation site situated in the lower coastal plain of North Carolina. Predisposition is quantified using the crossing between modeled lifting condensation level (LCL) and convectively grown ABL depth. The LCL‐ABL depth crossing is necessary for air saturation but not sufficient for cloud formation and subsequent convective rainfall occurrence. However, such crossing forms the main template for which all subsequent dynamical processes regulating the formation (or suppression) of convective rainfall operate on. If the feedback between surface fluxes and FA conditions is neglected, a reduction in latent heat flux associated with reduced WT levels is shown to enhance the ABL‐LCL crossing probability. When the soil‐plant system is fully coupled with ABL dynamics thereby allowing feedback with ABL temperature and humidity, FA states remain the leading control on CRP. However, vegetation water stress plays a role in controlling ABL‐LCL crossing when the humidity supply by the FA is within an intermediate range of values. When FA humidity supply is low, cloud formation is suppressed independent of surface latent heat flux. Similarly, when FA moisture supply is high, cloud formation can occur independent of surface latent heat flux. In an intermediate regime of FA moisture supply, the surface latent heat flux controlled by soil water availability can supplement (or suppress) the necessary water vapor leading to reduced LCL and subsequent ABL‐LCL crossing. It is shown that this intermediate state corresponds to FA values around the mode in observed humidity lapse rates γw (between −2.5 × 10−6 and −1.5 × 10−6 kg kg−1m−1), suggesting that vegetation water uptake may be controlling CRP at the study site.
- Estimating bankfull discharge and depth in ungauged estuaries
- Authors: Jacqueline Isabella Anak Gisen; Hubert H.G. Savenije
Pages: 2298 - 2316
Abstract: It is difficult to measure river discharge accurately in an estuary, and particularly, in the region where the tidal flow dominates over the river discharge. River discharge is important for the morphology and hydrodynamics of estuaries as it influences the salt intrusion process, tidal dynamics, freshwater supply (water resources management), and the occurrence of floods. Here we try to derive river regime characteristics from the seaward end: the estuary. It is found that there are empirical relationships that link the geometry of an estuary to its river regime, which can be used to estimate river discharge characteristics with the least of data available. The aims of this study are: (1) to derive empirical relations between geometrical characteristics of estuaries and the bankfull discharge; (2) to explore a physical explanation for this relation; and (3) to estimate the bankfull discharge in estuaries. The physical connection between an estuary and its river regime is found by combining estuary shape analysis, tidal dynamic analysis, and Lacey's hydraulic geometry theory. The relationships found between the estuary depth, width, and bankfull river discharge have been tested in 23 estuaries around the world (including seven recently surveyed estuaries). From the analysis, it shows that the depth of an estuary is a function of the bankfull flood discharge to the power of 1/3, which is in agreement with Lacey's formula. This finding not only provides a method to estimate estuary depth, it also allows estimating flood discharge characteristics from readily available estuary shape indicators.
- High‐resolution experiments on chemical oxidation of DNAPL in
- Authors: Masoud Arshadi; Harihar Rajaram, Russell L. Detwiler, Trevor Jones
Pages: 2317 - 2335
Abstract: Chemical oxidation of dense nonaqueous‐phase liquids (DNAPLs) by permanganate has emerged as an effective remediation strategy in fractured rock. We present high‐resolution experimental investigations in transparent analog variable‐aperture fractures to improve understanding of chemical oxidation of residual entrapped trichloroethylene (TCE) in fractures. Four experiments were performed with different permanganate concentrations, flow rates, and initial TCE phase geometry. The initial aperture field and evolving entrapped‐phase geometry were quantified for each experiment. The integrated mass transfer rate from the TCE phase for all experiments exhibited three time regimes: an early‐time regime with slower mass transfer rates limited by low specific interfacial area; an intermediate‐time regime with higher mass transfer rates resulting from breakup of large TCE blobs, which greatly increases specific interfacial area; and a late‐time regime with low mass transfer rates due to the deposition of MnO2 precipitates. In two experiments, mass balance analyses suggested that TCE mass removal rates exceeded the maximum upper bound mass removal rates derived by assuming that oxidation and dissolution are the only mechanisms for TCE mass removal. We propose incomplete oxidation by permanganate and TCE solubility enhancement by intermediate reaction products as potential mechanisms to explain this behavior. We also speculate that some intermediate reaction products with surfactant‐like properties may play a role in lowering the TCE‐water interfacial tension, thus causing breakup of large TCE blobs. Our quantitative experimental measurements will be useful in the context of developing accurate computational models for chemical oxidation of TCE in fractures.
- A hydrometeorological approach for probabilistic simulation of monthly
soil moisture under bare and crop land conditions
- Authors: Sarit Kumar Das; Rajib Maity
Pages: 2336 - 2355
Abstract: This study focuses on the probabilistic estimation of monthly soil moisture variation by considering (a) the influence of hydrometeorological forcing to model the temporal variation and (b) the information of Hydrological Soil Groups (HSGs) and Agro‐Climatic Zones (ACZs) to capture the spatial variation. The innovative contributions of this study are: (i) development of a Combined Hydro‐Meteorological (CHM) index to extract the information of different influencing hydrometeorological variables, (ii) consideration of soil‐hydrologic characteristics (through HSGs) and climate regime‐based zoning for agriculture (through ACZs), and (iii) quantification of uncertainty range of the estimated soil moisture. Usage of Supervised Principal Component Analysis (SPCA) in the development of the CHM index helps to eliminate the “curse of dimensionality,” typically arises in the multivariate analysis. The usage of SPCA also ensures the maximum possible association between the developed CHM index and soil moisture variation. The association between these variables is modeled through their joint distribution which is obtained by using the theory of copula. The proposed approach is also spatially transferable, since the information on HSGs and ACZs is considered. The “leave‐one‐out” cross‐validation (LOO‐CV) approach is adopted for stations belong to a particular HSG to examine the spatial transferability. The simulated soil moisture values are also compared with a few existing soil moisture data sets, derived from different Land Surface Models (LSMs) or retrieved from different satellite‐based missions. The potential of the proposed approach is found to be promising and even applicable to crop land also, though with a lesser degree of efficiency as compared to bare land conditions.
- Resolution analysis of tomographic slug test head data:
Two‐dimensional radial case
- Authors: Daniel Paradis; Erwan Gloaguen, René Lefebvre, Bernard Giroux
Pages: 2356 - 2376
Abstract: Hydraulic tomography inverse problems, which are solved to estimate aquifer hydraulic properties between wells, are known to be ill‐conditioned and a priori information is often added to regularize numerical inversion of head data. Because both head data and a priori information have effects on the inversed solution, assessing the meaningful information contained in head data alone is required to ensure comprehensive interpretation of inverse solutions, whether they are regularized or not. This study thus aims to assess the amount of information contained in tomographic slug tests head data to resolve heterogeneity in Kh, Kv/Kh, and Ss. Therefore, a resolution analysis based on truncated singular value decomposition of the sensitivity matrix with a noise level representative of field measurements is applied using synthetic data reflecting a known littoral aquifer. As an approximation of the hydraulic behavior of a real aquifer system, synthetic tomographic experiments and associated sensitivity matrices are generated using a radial flow model accounting for wellbore storage to simulate slug tests in a plane encompassing a stressed well and an observation well. Although fine‐scale resolution of heterogeneities is limited by the diffusive nature of the groundwater flow equations, inversion of tomographic slug tests head data holds the potential to uniquely resolve coarse‐scale heterogeneity in Kh, Kv/Kh, and Ss, as inscribed in the resolution matrix. This implies that tomographic head data can provide key information on aquifer heterogeneity and anisotropy, but that fine‐scale information must be supplied by a priori information to obtain finer details.
- Spatiotemporal decomposition of solute dispersion in watersheds
- Authors: Joakim Riml; Anders Wörman
Pages: 2377 - 2392
Abstract: Information about the effect of different dispersion mechanisms on the solute response in watersheds is crucial for understanding the temporal dynamics of many water quality problems. However, to quantify these processes from stream water quality time series may be difficult because the governing mechanisms responsible for the concentration fluctuations span a wide range of temporal and spatial scales. In an attempt to address the quantification problem, we propose a novel methodology that includes a spectral decomposition of the watershed solute response using a distributed solute transport model for the network of transport pathways in surface and subsurface water. Closed form solutions of the transport problem in both the Laplace and Fourier domains are used to derive formal expressions of (i) the central temporal moments of a solute pulse response and (ii) the power spectral response of a solute concentration time series. By evaluating high‐frequency hydrochemical data from the Upper Hafren Watershed, Wales, we linked the watershed dispersion mechanisms to the damping of the concentration fluctuations in different frequency intervals reflecting various environments responsible for the damping. The evaluation of the frequency‐dependent model parameters indicate that the contribution of the different environments to the concentration fluctuations at the watershed effluent varies with period. For the longest periods (predominantly groundwater transport pathways) we found that the frequency typical transport time of chloride was 100 times longer and that sodium had a 2.5 times greater retardation factor compared with the shortest periods (predominantly shallow groundwater and surface water transport pathways).
- A new frequency domain analytical solution of a cascade of diffusive
channels for flood routing
- Authors: Luigi Cimorelli; Luca Cozzolino, Renata Della Morte, Domenico Pianese, Vijay P. Singh
Pages: 2393 - 2411
Abstract: Simplified flood propagation models are often employed in practical applications for hydraulic and hydrologic analyses. In this paper, we present a new numerical method for the solution of the Linear Parabolic Approximation (LPA) of the De Saint Venant equations (DSVEs), accounting for the space variation of model parameters and the imposition of appropriate downstream boundary conditions. The new model is based on the analytical solution of a cascade of linear diffusive channels in the Laplace Transform domain. The time domain solutions are obtained using a Fourier series approximation of the Laplace Inversion formula. The new Inverse Laplace Transform Diffusive Flood Routing model (ILTDFR) can be used as a building block for the construction of real‐time flood forecasting models or in optimization models, because it is unconditionally stable and allows fast and fairly precise computation.
- Natural gas price uncertainty and the cost‐effectiveness of hedging
against low hydropower revenues caused by drought
- Authors: Jordan D. Kern; Gregory W. Characklis, Benjamin T. Foster
Pages: 2412 - 2427
Abstract: Prolonged periods of low reservoir inflows (droughts) significantly reduce a hydropower producer's ability to generate both electricity and revenues. Given the capital intensive nature of the electric power industry, this can impact hydropower producers’ ability to pay down outstanding debt, leading to credit rating downgrades, higher interests rates on new debt, and ultimately, greater infrastructure costs. One potential tool for reducing the financial exposure of hydropower producers to drought is hydrologic index insurance, in particular, contracts structured to payout when streamflows drop below a specified level. An ongoing challenge in developing this type of insurance, however, is minimizing contracts’ “basis risk,” that is, the degree to which contract payouts deviate in timing and/or amount from actual damages experienced by policyholders. In this paper, we show that consideration of year‐to‐year changes in the value of hydropower (i.e., the cost of replacing it with an alternative energy source during droughts) is critical to reducing contract basis risk. In particular, we find that volatility in the price of natural gas, a key driver of peak electricity prices, can significantly degrade the performance of index insurance unless contracts are designed to explicitly consider natural gas prices when determining payouts. Results show that a combined index whose value is derived from both seasonal streamflows and the spot price of natural gas yields contracts that exhibit both lower basis risk and greater effectiveness in terms of reducing financial exposure.
- Global sensitivity analysis of the radiative transfer model
- Authors: Maheshwari Neelam; Binayak P. Mohanty
Pages: 2428 - 2443
Abstract: With the recently launched Soil Moisture Active Passive (SMAP) mission, it is very important to have a complete understanding of the radiative transfer model for better soil moisture retrievals and to direct future research and field campaigns in areas of necessity. Because natural systems show great variability and complexity with respect to soil, land cover, topography, precipitation, there exist large uncertainties and heterogeneities in model input factors. In this paper, we explore the possibility of using global sensitivity analysis (GSA) technique to study the influence of heterogeneity and uncertainties in model inputs on zero order radiative transfer (ZRT) model and to quantify interactions between parameters. GSA technique is based on decomposition of variance and can handle nonlinear and nonmonotonic functions. We direct our analyses toward growing agricultural fields of corn and soybean in two different regions, Iowa, USA (SMEX02) and Winnipeg, Canada (SMAPVEX12). We noticed that, there exists a spatio‐temporal variation in parameter interactions under different soil moisture and vegetation conditions. Radiative Transfer Model (RTM) behaves more non‐linearly in SMEX02 and linearly in SMAPVEX12, with average parameter interactions of 14% in SMEX02 and 5% in SMAPVEX12. Also, parameter interactions increased with vegetation water content (VWC) and roughness conditions. Interestingly, soil moisture shows an exponentially decreasing sensitivity function whereas parameters such as root mean square height (RMS height) and vegetation water content show increasing sensitivity with 0.05 v/v increase in soil moisture range. Overall, considering the SMAPVEX12 fields to be water rich environment (due to higher observed SM) and SMEX02 fields to be energy rich environment (due to lower SM and wide ranges of TSURF), our results indicate that first order as well as interactions between the parameters change with water and energy rich environments.
- The influence of multiyear drought on the annual rainfall‐runoff
relationship: An Australian perspective
- Authors: Margarita Saft; Andrew W. Western, Lu Zhang, Murray C. Peel, Nick J. Potter
Pages: 2444 - 2463
Abstract: Most current long‐term (decadal and longer) hydrological predictions implicitly assume that hydrological processes are stationary even under changing climate. However, in practice, we suspect that changing climatic conditions may affect runoff generation processes and cause changes in the rainfall‐runoff relationship. In this article, we investigate whether temporary but prolonged (i.e., of the order of a decade) shifts in rainfall result in changes in rainfall‐runoff relationships at the catchment scale. Annual rainfall and runoff records from south‐eastern Australia are used to examine whether interdecadal climate variability induces changes in hydrological behavior. We test statistically whether annual rainfall‐runoff relationships are significantly different during extended dry periods, compared with the historical norm. The results demonstrate that protracted drought led to a significant shift in the rainfall‐runoff relationship in ∼44% of the catchment‐dry periods studied. The shift led to less annual runoff for a given annual rainfall, compared with the historical relationship. We explore linkages between cases where statistically significant changes occurred and potential explanatory factors, including catchment properties and characteristics of the dry period (e.g., length, precipitation anomalies). We find that long‐term drought is more likely to affect transformation of rainfall to runoff in drier, flatter, and less forested catchments. Understanding changes in the rainfall‐runoff relationship is important for accurate streamflow projections and to help develop adaptation strategies to deal with multiyear droughts.
- Prediction in ungauged estuaries: An integrated theory
- Authors: Hubert H. G. Savenije
Pages: 2464 - 2476
Abstract: Many estuaries in the world are ungauged. The International Association of Hydrological Sciences completed its science decade on Prediction in Ungauged Basins (PUB) in 2012 (Hrachowitz et al., ). Prediction on the basis of limited data is a challenge in hydrology, but not less so in estuaries, where data on fundamental processes are often lacking. In this paper, relatively simple, but science‐based, methods are presented that allow researchers, engineers, and water managers to obtain first‐order estimates of essential process parameters in estuaries, such as the estuary depth, the tidal amplitude, the tidal excursion, the phase lag, and the salt water intrusion, on the basis of readily obtainable information, such as topographical maps and tidal tables. These apparently simple relationships are assumed to result from the capacity of freely erodible water bodies to adjust themselves to external drivers and to dissipate the free energy from these drivers as efficiently as possible. Thus, it is assumed that these systems operate close to their thermodynamic limit, resulting in predictable patterns that can be described by relatively simple equations. Although still much has to be done to develop an overall physics‐based theory, this does not prevent us from making use of the empirical “laws” that we observe in alluvial estuaries.
- The use of discharge perturbations to understand in situ vegetation
resistance in wetlands
- Authors: A. M. Wasantha Lal; M. Zaki Moustafa, Walter M. Wilcox
Pages: 2477 - 2497
Abstract: The ability to better quantify resistance to water flow exerted by vegetation is receiving increased attention due to ongoing worldwide efforts to restore natural vegetation communities in the wetlands and use of vegetation for environmental benefits in streams and wetlands. In south Florida, vegetation resistance affects discharge through shallow wetlands of the Everglades and projects under way in the system to restore remaining natural systems. A more detailed knowledge of the flow dynamics in these wetlands is required to improve modeling of these systems that supports restoration and management efforts. The goal of this investigation is to understand the flow dynamics and the vegetation resistance within a 3 km by 7 km area in the Everglades referred to as STA‐3/4 Cell 3A. Methods are developed to demonstrate the use of analytical solutions of partial differential equations (PDEs) and inverse methods to obtain bulk and spatially varying resistance parameters. To achieve this goal, a field test was conducted using sinusoidal discharge disturbances capable of creating water waves in the storm water treatment area (STAs). The discharges, wave speeds, and the wave attenuation rates from the test are used to develop graphical and empirical functions expressing discharge in terms of water depth and energy slope. The empirical functions developed are power law type, and different functions are developed for different depths. The results show that the Manning's equation is not applicable for wetlands with thick emergent vegetation, as well as the difficulty of applying a single power law‐type expression for vegetation resistance over a wide range of depths and energy slopes without errors. This is partly due to the existence of multiple flow regimes and different power exponents over depth and energy slopes in these regimes. Results show that the flow regime at low depths is similar to porous media flow, and the flow regime at higher depths is more turbulent.
- A unified approach for process‐based hydrologic modeling: 1.
- Authors: Martyn P. Clark; Bart Nijssen, Jessica D. Lundquist, Dmitri Kavetski, David E. Rupp, Ross A. Woods, Jim E. Freer, Ethan D. Gutmann, Andrew W. Wood, Levi D. Brekke, Jeffrey R. Arnold, David J. Gochis, Roy M. Rasmussen
Pages: 2498 - 2514
Abstract: This work advances a unified approach to process‐based hydrologic modeling to enable controlled and systematic evaluation of multiple model representations (hypotheses) of hydrologic processes and scaling behavior. Our approach, which we term the Structure for Unifying Multiple Modeling Alternatives (SUMMA), formulates a general set of conservation equations, providing the flexibility to experiment with different spatial representations, different flux parameterizations, different model parameter values, and different time stepping schemes. In this paper, we introduce the general approach used in SUMMA, detailing the spatial organization and model simplifications, and how different representations of multiple physical processes can be combined within a single modeling framework. We discuss how SUMMA can be used to systematically pursue the method of multiple working hypotheses in hydrology. In particular, we discuss how SUMMA can help tackle major hydrologic modeling challenges, including defining the appropriate complexity of a model, selecting among competing flux parameterizations, representing spatial variability across a hierarchy of scales, identifying potential improvements in computational efficiency and numerical accuracy as part of the numerical solver, and improving understanding of the various sources of model uncertainty.
- Global analysis of approaches for deriving total water storage changes
from GRACE satellites
- Authors: Di Long; Laurent Longuevergne, Bridget R. Scanlon
Pages: 2574 - 2594
Abstract: Increasing interest in use of GRACE satellites and a variety of new products to monitor changes in total water storage (TWS) underscores the need to assess the reliability of output from different products. The objective of this study was to assess skills and uncertainties of different approaches for processing GRACE data to restore signal losses caused by spatial filtering based on analysis of 1° × 1° grid‐scale data and in 60 river basins globally. Results indicate that scaling factors from six LSMs, including GLDAS‐1 four models (Noah2.7, Mosaic, VIC, and CLM 2.0), CLM 4.0, and WGHM, are similar over most of humid, subhumid, and high‐latitude regions but can differ by up to 100% over arid and semiarid basins and areas with intensive irrigation. Temporal variability in scaling factors is generally minor at the basin scale except in arid and semiarid regions, but can be appreciable at the 1° × 1° grid scale. Large differences in TWS anomalies from three processing approaches (scaling factor, additive, and multiplicative corrections) were found in arid and semiarid regions, areas with intensive irrigation, and relatively small basins (e.g., ≤200,000 km2). Furthermore, TWS anomaly products from gridded data with CLM4.0 scaling factors and the additive correction approach more closely agree with WGHM output than the multiplicative correction approach. This comprehensive evaluation of GRACE processing approaches should provide valuable guidance on applicability of different processing approaches with different climate settings and varying levels of irrigation.
- CO2 dissolution in the presence of background flow of deep saline aquifers
- Authors: Hamid Emami‐Meybodi; Hassan Hassanzadeh, Jonathan Ennis‐King
Pages: 2595 - 2615
Abstract: We study the effect of background flow on the dissolution and transport of carbon dioxide (CO2) during geological storage in saline aquifers, and include the processes of diffusion, advection, and free convection. We develop a semianalytical model that captures the evolution of the dissolution in the absence of free convection. Using the semianalytical solution, we determine scaling relations for the steady rate of dissolution that follow either
Jst∼Pe R or
Jst∼Pe depending on the value of Pe/R, where R represents the ratio of the extent of CO2 plume to the aquifer thickness and Pe is the Péclet number. Using direct numerical simulations, we provide detailed behavior of the convective mixing during the dissolution. We establish the criteria for forced and mixed (combined free and forced) convection in aquifers that is governed by the background flow. Accordingly, we provide the scaling relations
Jst∼Pe R and
Jst∼Ra R representing the forced and free convection asymptotes, respectively, where Ra is a Rayleigh number based on aquifer thickness. The results reveal that the background velocity can delay the onset of free convection and can alter the subsequent mixing. This phenomenon is more profound in the systems subject to strong background flows wherein horizontal component of the velocity field generated by background flow hinders the establishments of vertical component of the velocity field. Finally, by applying the proposed relations to several potential storage sites, we demonstrate the screening process in finding aquifers where the background flow exerts an important influence on the dissolution.