- High resolution aquifer characterization using crosshole GPR full-waveform
tomography: Comparison with direct-push and tracer test data
- Authors: Nils Gueting; Thomas Vienken, Anja Klotzsche, Jan van der Kruk, Jan Vanderborght, Jef Caers, Harry Vereecken, Andreas Englert
Abstract: Limited knowledge about the spatial distribution of aquifer properties typically constrains our ability to predict subsurface flow and transport. Here, we investigate the value of using high resolution full-waveform inversion of cross-borehole ground penetrating radar (GPR) data for aquifer characterization. By stitching together GPR tomograms from multiple adjacent crosshole planes, we are able to image, with a decimeter scale resolution, the dielectric permittivity and electrical conductivity of an alluvial aquifer along cross-sections of 50 m length and 10 m depth. A logistic regression model is employed to predict the spatial distribution of lithological facies on the basis of the GPR results. Vertical profiles of porosity and hydraulic conductivity from direct-push, flowmeter and grain size data suggest that the GPR predicted facies classification is meaningful with regard to porosity and hydraulic conductivity, even though the distributions of individual facies show some overlap and the absolute hydraulic conductivities from the different methods (direct-push, flowmeter, grain size) differ up to approximately one order of magnitude. Comparison of the GPR predicted facies architecture with tracer test data suggests that the plume splitting observed in a tracer experiment was caused by a hydraulically low-conductive sand layer with a thickness of only a few decimeters. Because this sand layer is identified by GPR full-waveform inversion but not by conventional GPR ray-based inversion we conclude that the improvement in spatial resolution due to full-waveform inversion is crucial to detect small-scale aquifer structures that are highly relevant for solute transport. This article is protected by copyright. All rights reserved.
- Intra-wellbore kinematic and frictional losses in a horizontal well in a
bounded confined aquifer
- Authors: Quanrong Wang; Hongbin Zhan
Abstract: Horizontal drilling has become an appealing technology for water resource exploration or aquifer remediation in recent decades, due to decreasing operational cost and many technical advantages over vertical wells. However, many previous studies on flow into horizontal wells were based on the Uniform Flux Boundary Condition (UFBC), which does not reflect the physical processes of flow inside the well accurately. In this study, we investigated transient flow into a horizontal well in an anisotropic confined aquifer laterally bounded by two constant-head boundaries. Three types of boundary conditions were employed to treat the horizontal well, including UFBC, Uniform Head Boundary Condition (UHBC), and Mixed-Type Boundary Condition (MTBC). The MTBC model considered both kinematic and frictional effects inside the horizontal well, in which the kinematic effect referred to the accelerational and fluid-inflow effects. A new solution of UFBC was derived by superimposing the point sink/source solutions along the axis of a horizontal well with a uniform flux distribution. New solutions of UHBC and MTBC were obtained by a hybrid analytical-numerical method, and an iterative method was proposed to determine the well discretization required for achieving sufficiently accurate results. This study showed that the differences among the UFBC, UHBC, and MTBC solutions were obvious near the well screen, decreased with distance from the well, and became negligible near the constant-head boundary. The relationship between the flowrate and the drawdown was non-linear for the MTBC solution, while it was linear for the UFBC and UHBC solutions. This article is protected by copyright. All rights reserved.
- Capillary pressure across a pore throat in the presence of surfactants
- Authors: Junbong Jang; Zhonghao Sun, J. Carlos Santamarina
Abstract: Capillarity controls the distribution and transport of multi-phase and immiscible fluids in soils and fractured rocks; therefore, capillarity affects the migration of non-aqueous contaminants and remediation strategies for both LNAPLs and DNAPLs, constrains gas and oil recovery, and regulates CO2 injection and geological storage. Surfactants alter interfacial tension and modify the invasion of pores by immiscible fluids. Experiments are conducted to explore the propagation of fluid interfaces along cylindrical capillary tubes and across pore constrictions in the presence of surfactants. Measured pressure signatures reflect the interaction between surface tension, contact angle and the pore geometry. Various instabilities occur as the interface traverses the pore constriction, consequently, measured pressure signatures differ from theoretical trends predicted from geometry, lower capillary pressures are generated in advancing wetting fronts, and jumps are prone to under-sampling. Contact angle and instabilities are responsible for pronounced differences between pressure signatures recorded during advancing and receding tests. Pressure signatures gathered with surfactant solutions suggest changes in interfacial tension at the constriction; the transient surface tension is significantly lower than the value measured in quasi-static conditions. Interface stiffening is observed during receding fronts for solutions near the critical micelle concentration. Wetting liquids tend to form plugs at pore constrictions after the invasion of a non-wetting fluid; plugs split the non-wetting fluid into isolated globules and add resistance against fluid flow. This article is protected by copyright. All rights reserved.
- An improved multilevel Monte Carlo method for estimating probability
distribution functions in stochastic oil reservoir simulations
- Authors: Dan Lu; Guannan Zhang, Clayton Webster, Charlotte Barbier
Abstract: In this work we develop an improved multilevel Monte Carlo (MLMC) method for estimating cumulative distribution functions (CDFs) of a quantity of interest (QoI), coming from numerical approximation of large-scale stochastic subsurface simulations. Compared with Monte Carlo (MC) methods, that require a significantly large number of high-fidelity model executions to achieve a prescribed accuracy when computing statistical expectations, MLMC methods were originally proposed to significantly reduce the computational cost with the use of multi-fidelity approximations. The improved performance of the MLMC methods depends strongly on the decay of the variance of the integrand as the level increases. However, the main challenge in estimating CDFs is that the integrand is a discontinuous indicator function whose variance decays slowly. To address this difficult task, we approximate the integrand using a smoothing function that accelerates the decay of the variance. In addition, we design a a-posteriori optimization strategy to calibrate the smoothing function, so as to balance the computational gain and the approximation error. The combined proposed techniques are integrated into a very general and practical algorithm that can be applied to a wide range of subsurface problems for high-dimensional uncertainty quantification, such as a fine-grid oil reservoir model considered in this effort. The numerical results reveal that with the use of the calibrated smoothing function, the improved MLMC technique significantly reduces the computational complexity compared to the standard MC approach. Finally, we discuss several factors that affect the performance of the MLMC method and provide guidance for effective and efficient usage in practice. This article is protected by copyright. All rights reserved.
- Debates—Stochastic subsurface hydrology from theory to practice: A
- Authors: Graham E. Fogg; Yong Zhang
Abstract: A geologic perspective on stochastic subsurface hydrology offers insights on representativeness of prominent field experiments and their general relevance to other hydrogeologic settings. Although the gains in understanding afforded by some 30 years of research in stochastic hydrogeology have been important and even essential, adoption of the technologies and insights by practitioners has been limited, due in part to a lack of geologic context in both the field and theoretical studies. In general, unintentional, biased sampling of hydraulic conductivity (K) using mainly hydrologic, well-based methods has resulted in the tacit assumption by many in the community that the subsurface is much less heterogeneous than in reality. Origins of the bias range from perspectives that are limited by scale and the separation of disciplines (geology, soils, aquifer hydrology, groundwater hydraulics, etc.). Consequences include a misfit between stochastic hydrogeology research results and the needs of, for example, practitioners who are dealing with local plume site cleanup that is often severely hampered by very low velocities in the very aquitard facies that are commonly overlooked or missing from low-variance stochastic models or theories. We suggest that answers to many of the problems exposed by stochastic hydrogeology research can be found through greater geologic integration into the analyses, including the recognition of not only the nearly ubiquitously high variances of K, but also the strong tendency for the good connectivity of the high K facies when spatially persistent geologic unconformities are absent. We further suggest that, although such integration may appear to make the contaminant transport problem more complex, expensive and intractable, it may in fact lead to greater simplification and more reliable, less expensive site characterizations and models. This article is protected by copyright. All rights reserved.
- Bed load transport in a very steep mountain stream (Riedbach,
Switzerland): Measurement and prediction
- Authors: Johannes M. Schneider; Dieter Rickenmann, Jens M. Turowski, Bastian Schmid, James W. Kirchner
Abstract: Compared to lower-gradient channels, steep mountain streams typically have rougher beds and shallower flow depths, making macro-scale flow resistance (due to, for example, immobile boulders and irregular bedforms) more important as controls on sediment transport. The marked differences in hydraulics, flow resistance, and grain mobility between steep and lower-gradient streams raise the question of whether the same equations can predict bed load transport rates across wide ranges of channel gradients. We studied a steep, glacier-fed mountain stream (Riedbach, Ct. Valais, Switzerland) that provides a natural experiment for exploring how stream gradients affect bed load transport rates. The streambed gradient increases over a 1-km stream reach by roughly one order of magnitude (from 3% to 38%), while flow discharge and width remain approximately constant. Sediment transport rates were determined in the 3% reach using Bunte bed load traps and in the 38% reach using the Swiss plate geophone system. Despite a ten-fold increase in bed gradient, bed load transport rates did not increase substantially. Observed transport rates for these two very different bed gradients could be predicted reasonably well by using a flow resistance partitioning approach to account for increasing bed roughness (D84 changes from 0.17 m to 0.91 m) within a fractional bed load transport equation. This suggests that sediment transport behavior across this large range of steep slopes agrees with patterns established in previous studies for both lower-gradient and steep reaches, and confirms the applicability of the flow resistance and bed load transport equations at very steep slopes. This article is protected by copyright. All rights reserved.
- Three-dimensional flow structure and bed morphology in large elongate
meander loops with different outer bank roughness characteristics
- Authors: Kory M. Konsoer; Bruce L. Rhoads, James L. Best, Eddy J. Langendoen, Jorge D. Abad, Dan R. Parsons, Marcelo H. Garcia
Abstract: Few studies have examined the three-dimensional flow structure and bed morphology within elongate loops of large meandering channels. The present study focuses on the spatial patterns of three-dimensional flow structure and bed morphology within two elongate meander loops and examines how differences in outer bank roughness influence near-bank flow characteristics. Three-dimensional velocities were measured during two different events – a near-bankfull flow and an overbank event. Detailed data on channel bathymetry and bedform geometry were obtained during a near-bankfull event. Flow structure within the loops is characterized by strong topographic steering by the point bar, by the development of helical motion associated with flow curvature, and by acceleration of flow where bedrock is exposed along the outer bank. Near-bank velocities during the overbank event are less than those for the near-bankfull flow, highlighting the strong influence of the point bar on redistribution of mass and momentum of the flow at sub-bankfull stages. Multiple outer bank pools are evident within the elongate meander loop with low outer bank roughness, but are not present in the loop with high outer bank roughness, which may reflect the influence of abundant large woody debris on near-bank velocity characteristics. The positions of pools within both loops can be linked to spatial variations in planform curvature. The findings indicate that flow structure and bed morphology in these large elongate loops is similar to that in small elongate loops, but differs somewhat from flow structure and bed morphology reported for experimental elongate loops. This article is protected by copyright. All rights reserved.
- Linking hydromorphology with invertebrate ecology in diverse morphological
units of a large river-floodplain system
- Authors: Martín C.M. Blettler; Mario L. Amsler, Eliana G. Eberle, Ricardo Szupiany, Francisco G. Latosinski, Elie Abrial, Paul J. Oberholster, Luis A. Espinola, Aldo Paira, Ailen Poza, Alberto Rodrigues Capítulo
Abstract: Interdisciplinary research in the fields of ecohydrology and ecogeomorphology is becoming increasingly important as a way to understand how biological and physical processes interact with each other in river systems. The objectives of the current study were 1) to determine changes in invertebrate community due to hydrological stages, 2) to link local physical features (flow configuration, sediment composition and morphological feature) with the ecological structure between and within dissimilar morphological units (meander and confluence), and 3) to determine the existence and the origin of bed hydro-geomorphic patches, determining their ecological structure. Results were discussed in the frame of prevailing ecological models and concepts.The study site extends over a floodplain area of the large Paraná River (Argentina), including minor and major secondary channels as well as the main channel.Overall results suggested that hydrodynamics was the driving force determining distribution patterns of benthic assemblages in the floodplain. However, while the invertebrates living in minor secondary channels seem to benefit from flooding, this hydrological phase had the opposite effect on organisms from the main and major secondary channels. We also found a clear linkage between physical features and invertebrate ecology, which caused a dissimilar fauna structure between and within the meander and the confluence. Furthermore, several sandy-patches were recorded in the confluence. These patches were colonized by the particular benthic assemblage recorded in the main channel, supported the view of rivers as patchy discontinua, under uncertain ecological equilibrium. This article is protected by copyright. All rights reserved.
- Implementation and evaluation of a monthly water balance model over the
U.S. on an 800 m grid
- Authors: S.W. Hostetler; J.R. Alder
Abstract: We simulate the 1950-2010 water balance for the conterminous U.S. (CONUS) with a monthly water balance model (MWBM) using the 800-m Parameter-elevation Regression on Independent Slopes Model (PRISM) data set as model input. We employed observed snow and streamflow data sets to guide modification of the snow and potential evapotranspiration components in the default model and to evaluate model performance. Based on various metrics and sensitivity tests, the modified model yields reasonably good simulations of seasonal snowpack in the West (range of bias of ±50 mm at 68% of 713 SNOTEL sites), the gradients and magnitudes of actual evapotranspiration, and runoff (median correlation of 0.83 and median Nash-Sutcliff efficiency of 0.6 between simulated and observed annual time series at 1427 USGS gage sites). The model generally performs well along the Pacific Coast, the high elevations of the Basin and Range and over the Midwest and East, but not as well over the dry areas of the Southwest and upper Plains regions due, in part, to the apportioning of direct versus delayed runoff. Sensitivity testing and application of the MWBM to simulate the future water balance at four National Parks when driven by 30 climate models from the Climate Model Intercomparison Program Phase 5 (CMIP5) demonstrate that the model is useful for evaluating first-order, climate driven hydrologic change on monthly and annual time scales. This article is protected by copyright. All rights reserved.
- Debates—Stochastic subsurface hydrology from theory to practice: The
relevance of stochastic subsurface hydrology to practical problems of
contaminant transport and remediation. What is characterization and
stochastic theory good for?
- Authors: A. Fiori; V. Cvetkovic, G. Dagan, S. Attinger, A. Bellin, P. Dietrich, A. Zech, G. Teutsch
Abstract: The emergence of stochastic subsurface hydrology stemmed from the realization that spatial variability of aquifer properties (primarily permeability K) has a profound impact on solute transport. Heterogeneity is characterized by much larger scale than the pore scale and the seemingly erratic variation of K and the uncertainty of its distribution called for its modeling as a random space function, which renders the fluid Darcian velocity and the concentration random as well. This article is protected by copyright. All rights reserved.
- Regional evaporation estimates in the eastern monsoon region of China:
Assessment of a nonlinear formulation of the complementary principle
- Authors: Xiaomang Liu; Changming Liu, Wilfried Brutsaert
Abstract: The performance of a nonlinear formulation of the complementary principle for evaporation estimation was investigated in 241 catchments with different climate conditions in the eastern monsoon region of China. Evaporation (Ea) calculated by the water balance equation was used as the reference. Ea estimated by the calibrated nonlinear formulation was generally in good agreement with the water balance results, especially in relatively dry catchments. The single parameter in the nonlinear formulation, namely αe as a weak analog of the alpha parameter of Priestley and Taylor , tended to exhibit larger values in warmer and humid near-coastal areas, but smaller values in colder, drier environments inland, with a significant dependency on the aridity index (AI). The nonlinear formulation combined with the equation relating the one parameter and AI provides a promising method to estimate regional Ea with standard and routinely measured meteorological data. This article is protected by copyright. All rights reserved.
- Distributed temperature sensing as a down-hole tool in hydrogeology
- Authors: V.F. Bense; T. Read, O. Bour, T. Le Borgne, T. Coleman, S. Krause, A. Chalari, M. Mondanos, F. Ciocca, J.S. Selker
Abstract: Distributed Temperature Sensing (DTS) technology enables down-hole temperature monitoring to study hydrogeological processes at unprecedentedly high frequency and spatial resolution. DTS has been widely applied in passive mode in site investigations of groundwater flow, in-well flow, and subsurface thermal property estimation. However, recent years have seen the further development of the use of DTS in an active mode (A-DTS) for which heat sources are deployed. A suite of recent studies using A-DTS down-hole in hydrogeological investigations illustrate the wide range of different approaches and creativity in designing methodologies. The purpose of this review is to outline and discuss the various applications and limitations of DTS in down-hole investigations for hydrogeological conditions and aquifer geological properties. To this end, we first review examples where passive DTS has been used to study hydrogeology via down-hole applications. Secondly, we discuss and categorize current A-DTS borehole methods into three types. These are thermal advection tests, hybrid cable flow logging, and heat pulse tests. We explore the various options with regards to cable installation, heating approach, duration, and spatial extent in order to improve their applicability in a range of settings. These determine the extent to which each method is sensitive to thermal properties, vertical in well flow, or natural gradient flow. Our review confirms that the application of DTS has significant advantages over discrete point temperature measurements, particularly in deep wells, and highlights the potential for further method developments in conjunction with other emerging fiber optic based sensors such as Distributed Acoustic Sensing. This article is protected by copyright. All rights reserved.
- Debates—Stochastic subsurface hydrology from theory to practice:
- Authors: Harihar Rajaram
Abstract: This paper introduces the papers in the “Debates – Stochastic Subsurface Hydrology from Theory to Practice” series. Beginning in the 1970s, the field of stochastic subsurface hydrology has been an active field of research, with over 3500 journal publications, of which over 850 have appeared in Water Resources Research. We are fortunate to have insightful contributions from four groups of distinguished authors who discuss the reasons why the advanced research framework established in stochastic subsurface hydrology has not impacted the practice of groundwater flow and transport modeling and design significantly. There is reasonable consensus that a community effort aimed at developing “toolboxes” for applications of stochastic methods will make them more accessible and encourage practical applications. This article is protected by copyright. All rights reserved.
- Debates—Stochastic subsurface hydrology from theory to practice: Does
stochastic subsurface hydrology help solving practical problems of
- Authors: Olaf A. Cirpka; Albert J. Valocchi
Abstract: While stochastic subsurface hydrology has been tremendously successful in understanding how the spatial variability of hydraulic conductivity affects conservative solute transport in idealized settings, it has gained little impact in practice. This is the case because typical assumptions needed for the derivation of analytical expressions are too restrictive for practical applications and often geologically implausible, small-scale variation of hydraulic conductivity is by far not the only cause of uncertainty when considering the fate and remediation of pollutants, and the research community has not developed enough methods that can directly be used by practitioners. To overcome these shortcomings we propose putting more emphasis on providing easy-to-use tools to generate realistic realizations of subsurface properties that are conditioned on all data measured at a site, extending the focus from hydraulic conductivity only to all parameters and processes relevant for reactive transport, making use of self-organizing principles of reactive transport to conceptually simplify the problem, and addressing conceptual uncertainty by stochastic methods. This article is protected by copyright. All rights reserved.
- Debates—Stochastic subsurface hydrology from theory to practice: Why
stochastic modeling has not yet permeated into practitioners?
- Authors: X. Sanchez-Vila; D. Fernàndez-Garcia
Abstract: We address modern topics of Stochastic Hydrogeology from their potential relevance to real modeling efforts at the field scale. While the topics of stochastic hydrogeology and numerical modelling have become routine in hydrogeological studies, non-deterministic models have not yet permeated into practitioners. We point out a number of limitations of stochastic modelling when applied to real applications and comment on the reasons why stochastic models fail to become an attractive alternative for practitioners. We specifically separate issues corresponding to flow, conservative transport and reactive transport. The different topics addressed are: emphasis on process modeling, need for upscaling parameters and governing equations, relevance of properly accounting for detailed geological architecture in hydrogeological modeling, and specific challenges of reactive transport. We end up by concluding that the main responsible for non-deterministic models having not yet permeated in industry can be fully attributed to researchers in stochastic hydrogeology. This article is protected by copyright. All rights reserved.
- Estimation of three-phase relative permeability by simulating fluid
dynamics directly on rock-microstructure images
- Authors: F. Jiang; T. Tsuji
Abstract: Given the world's growing demand for energy, a combination of geological CO2 sequestration and enhanced oil recovery (EOR) technologies is currently regarded as a promising solution, as it would provide a means of reducing carbon emissions into the atmosphere while also leading to the economic benefit of simultaneously recovering oil. The optimization of injection strategies to maximize CO2 storage and increase the oil recovery factors requires complicated pore-scale flow information within a reservoir system consisting of coexisting oil, water, and CO2 phases. In this study, an immiscible three-phase lattice-Boltzmann (LB) model was developed to investigate the complicated flow state with interaction between water, oil, and CO2 systems in porous media. The two main mechanisms of oil remobilization, namely, double-drainage and film flow, can be captured by our model. The estimation of three-phase relative permeability is proposed using the digital rock physics (DRP) simulations. The results indicate that the relative permeability of CO2 as calculated using our steady-state method is not sensitive to the initial oil fraction if the oil distribution is originally uniform. Baker's empirical model [Baker, 1988] was tested and found to be able to provide a good prediction of the three-phase relative permeability data. Our numerical method provides a new tool for accurately predicting three-phase relative permeability data directly based on micro-CT rock images. This article is protected by copyright. All rights reserved.
- Push-pull tracer tests: Their information content and use for
characterizing non-Fickian, mobile-immobile behavior
- Authors: Scott K. Hansen; Brian Berkowitz, Velimir V. Vesselinov, Daniel O'Malley, Satish Karra
Abstract: Path reversibility and radial symmetry are often assumed in push-pull tracer test analysis. In reality, heterogeneous flow fields mean that both assumptions are idealizations. To understand their impact, we perform a parametric study which quantifies the scattering effects of ambient flow, local-scale dispersion and velocity field heterogeneity on push-pull breakthrough curves and compares them to the effects of mobile-immobile mass transfer (MIMT) processes including sorption and diffusion into secondary porosity. We identify specific circumstances in which MIMT overwhelmingly determines the breakthrough curve, which may then be considered uninformative about drift and local-scale dispersion. Assuming path reversibility, we develop a continuous time random walk-based interpretation framework which is flow-field agnostic and well suited to quantifying MIMT. Adopting this perspective, we show that the radial flow assumption is often harmless: to the extent that solute paths are reversible, the breakthrough curve is uninformative about velocity field heterogeneity. Our interpretation method determines a mapping function (i.e. subordinator) from travel time in the absence of MIMT to travel time in its presence. A mathematical theory allowing this function to be directly “plugged into” an existing Laplace-domain transport model to incorporate MIMT is presented and demonstrated. Algorithms implementing the calibration are presented and applied to interpretation of data from a push-pull test performed in a heterogeneous environment. A successful four-parameter fit is obtained, of comparable fidelity to one obtained using a million-node 3D numerical model. Finally, we demonstrate analytically and numerically how push-pull tests quantifying MIMT are sensitive to remobilization, but not immobilization, kinetics. This article is protected by copyright. All rights reserved.
- Strengths and weaknesses of temporal stability analysis for monitoring and
estimating grid-mean soil moisture in a high-intensity irrigated
- Authors: Youhua Ran; Xin Li, Rui Jin, Jian Kang, Michael H. Cosh
Abstract: Monitoring and estimating grid-mean soil moisture is very important for assessing many hydrological, biological, and biogeochemical processes and for validating remotely sensed surface soil moisture products. Temporal stability analysis (TSA) is a valuable tool for identifying a small number of representative sampling points to estimate the grid-mean soil moisture content. This analysis was evaluated and improved using high-quality surface soil moisture data that were acquired by a wireless sensor network in a high-intensity irrigated agricultural landscape in an arid region of northwestern China. The performance of the TSA was limited in areas where the representative error was dominated by random events, such as irrigation events. This shortcoming can be effectively mitigated by using a stratified TSA (STSA) method, proposed in this paper. In addition, the following methods were proposed for rapidly and efficiently identifying representative sampling points when using TSA. (1) Instantaneous measurements can be used to identify representative sampling points to some extent; however, the error resulting from this method is significant when validating remotely sensed soil moisture products. Thus, additional representative sampling points should be considered to reduce this error. (2) The calibration period can be determined from the time span of the full range of the grid-mean soil moisture content during the monitoring period. (3) The representative error is sensitive to the number of calibration sampling points, especially when only a few representative sampling points are used. Multiple sampling points are recommended to reduce data loss, improve the likelihood of representativeness at two scales. This article is protected by copyright. All rights reserved.
- A generalized non-Darcian radial flow model for constant rate test
- Authors: Ming-Ming Liu; Yi-Feng Chen, Jia-Min Hong, Chuang-Bing Zhou
Abstract: Models used for data interpretation of constant rate tests (CRTs) are commonly derived with the assumption of Darcian flow in an idealized geometry, hence disregarding the non-Darcian nature of fluid flow and the complexity of flow geometry. In this study, an Izbash's law-based analytical model is proposed by means of Laplace transform and linearization approximation for interpretation of non-Darcian flow in a generalized radial formation where the flow dimension may become fractional between 1 and 3. The source storage and skin effects are also considered in the model development. The proposed model immediately reduces to Barker's  model for Darcian flow in the generalized radial formation and to Wen's [2008a] model for non-Darcian flow in a two-dimensional confined aquifer. A comparison with numerical simulations shows that the proposed model behaves well in low non-Darcian flow condition or at late times. The proposed model is finally applied for data interpretation of the constant rate pumping tests performed at Ploemeur [Le Borgne et al., 2004], showing that the estimated hydraulic properties (i.e. hydraulic conductivity, specific storage coefficient, non-Darcy exponent and the dimension of flow geometry) are well representative of the hydrogeologic conditions on the field scale at the test site after the exploitation of groundwater. The proposed model is an extension of the generalized radial flow (GRF) model, which would be of significance in the problem of choosing an appropriate dimension of flow geometry in which non-Darcian flow occurs. This article is protected by copyright. All rights reserved.
- Global evaluation of new GRACE mascon products for hydrologic applications
- Authors: Bridget R. Scanlon; Zizhan Zhang, Himanshu Save, David N. Wiese, Felix W. Landerer, Di Long, Laurent Longuevergne, Jianli Chen
Abstract: Recent developments in mascon (mass concentration) solutions for GRACE (Gravity Recovery and Climate Experiment) satellite data have significantly increased the spatial localization and amplitude of recovered terrestrial Total Water Storage anomalies (TWSA); however, land hydrology applications have been limited. Here we compare TWSA from Apr. 2002 through Mar. 2015 from (1) newly released GRACE mascons from the Center for Space Research (CSR-M) with (2) NASA JPL mascons (JPL-M), and with (3) CSR Tellus gridded spherical harmonics rescaled (sf) (CSRT-GSH.sf) in 176 river basins, ∼60% of the global land area. Time series in TWSA mascons (CSR-M and JPL-M) and spherical harmonics are highly correlated (mostly >0.9). The signal from long-term trends (up to ±20 mm/yr) is much less than that from seasonal amplitudes (up to 250 mm). Net long-term trends, summed over all 176 basins, are similar for CSR and JPL mascons (66–69 km3/yr) but are lower for spherical harmonics (∼14 km3/yr). Long-term TWSA declines are found mostly in irrigated basins (- 41 to -69 km3/yr). Seasonal amplitudes agree among GRACE solutions, increasing confidence in GRACE-based seasonal fluctuations. Rescaling spherical harmonics significantly increases agreement with mascons for seasonal fluctuations, but less for long-term trends. Mascons provide advantages relative to spherical harmonics, including (1) reduced leakage from land to ocean increasing signal amplitude, and (2) application of geophysical data constraints during processing with little empirical post-processing requirements, making it easier for non-geodetic users. Results of this product intercomparison should allow hydrologists to better select suitable GRACE solutions for hydrologic applications. This article is protected by copyright. All rights reserved.
- Innovative framework to simulate the fate and transport of
non-conservative constituents in urban combined-sewer catchments
- Authors: V. M. Morales; J. C. Quijano, A. Schmidt, M. H. Garcia
Abstract: We have developed a probabilistic model to simulate the fate and transport of non-conservative constituents in urban watersheds. The approach implemented here extends previous studies that rely on the Geomorphological Instantaneous Unit Hydrograph concept to include non-conservative constituents. This is implemented with a factor χ that affects the transfer functions and therefore accounts for the loss (gain) of mass associated with the constituent as it travels through the watershed. Using this framework we developed an analytical solution for the dynamics of dissolved oxygen (DO) and biochemical oxygen demand (BOD) in urban networks based on the Streeter and Phelps model. This model breaks-down the catchment into a discreet number of possible flow paths through the system, requiring less data and implementation effort than well-established deterministic models. Application of the model to one sewer catchment in the Chicago area with available BOD information proved its ability to predict the BOD concentration observed in the measurements. In addition, comparison of the model with a calibrated Storm Water Management Model (SWMM) of another sewer catchment from the Chicago area showed that the model predicted the BOD concentration as well as the widely accepted SWMM. The developed model proved to be a suitable alternative to simulate the fate and transport of constituents in urban catchments with limited and uncertain input data. This article is protected by copyright. All rights reserved.
- Discussion of “Estimation of composite hydraulic resistance in
ice-covered alluvial streams”, by Zare et al. (2016) WRR 52(2):
- Authors: Spyros Beltaos
Abstract: Inconsistencies between basic hydraulics and hypotheses advanced by the authors are identified. They are attributed primarily to weaknesses in the analysis of velocity measurements in the ice-controlled flow layer and suggestions for improvement are offered. This article is protected by copyright. All rights reserved.
- Mapping soil water dynamics and a moving wetting front by spatiotemporal
inversion of electromagnetic induction data
- Authors: J. Huang; F.A. Monteiro Santos, J. Triantafilis
Abstract: Characterization of the spatio-temporal distribution of soil volumetric water content (θ) is fundamental to agriculture, ecology and earth science. Given the labour intensive and inefficient nature of determining θ, apparent electrical conductivity (ECa) measured by electromagnetic induction has been used as a proxy. A number of previous studies have employed inversion algorithms to convert ECa data to depth-specific electrical conductivity (σ) which could then be correlated to soil θ and other soil properties. The purpose of this study was to develop a spatio-temporal inversion algorithm which accounts for the temporal continuity of ECa. The algorithm was applied to a case study where time-lapse ECa was collected on a 350-m transect on 7 different days on an alfalfa farm in the USA. Results showed that the approach was able to map the location of moving wetting front along the transect. Results also showed that the spatio-temporal inversion algorithm was more precise (RMSE = 0.0457 cm3/cm3) and less biased (ME = -0.0023 cm3/cm3) as compared with the non-spatio-temporal inversion approach (0.0483 cm3/cm3 and ME = -0.0030 cm3/cm3, respectively). In addition, the spatio-temporal inversion algorithm allows for a reduced set of ECa surveys to be used when non abrupt changes of soil water content occur with time. To apply this spatio-temporal inversion algorithm beyond low induction number condition, full solution of the EM induction phenomena can be studied in the future. This article is protected by copyright. All rights reserved.
- Infiltration of fine sediment mixtures through poorly sorted immobile
- Authors: Francisco Núñez-González
Abstract: Percolation of fine sediment is a common process in gravel-bed rivers, which often exhibit extended and overlapping grain size distributions of the bed and the supplied fine sediment. Yet, existing sediment infiltration theory assumes well-sorted fine material with smaller grain size than the bed pores, and as such, is not suitable for many situations encountered in gravel-bed streams. Previous developments for infiltration of uniform material are here generalized to consider poorly-sorted sediment mixtures. Governing equations and a numerical solution to model the vertical distribution of infiltrating sediment are presented. The equations are solved as a function of a trapping coefficient, dependent on the relative size of infiltrating fines in relation to bed material. A method is developed to generate equivalent grain size distributions to calculate the trapping coefficient, when grain sizes of the infiltrating and bed materials overlap. Moreover, a bed cutoff size is defined and computed with a semi-empirical packing-porosity model, to distinguish particles smaller than the bed pores. Published experimental data are used to test the new model and calibrate the trapping coefficient. It is shown that this coefficient is highly sensible to the fine and coarse tails of fine and coarse materials grain size distributions. Accordingly, calibrated values of the coefficient are set as a function of a mean size ratio, computed from the geometric mean of the tails of the size distributions. Incorporating this relation, the model performed well in reproducing indirect observations of sediment infiltration from experiments reported in the literature. This article is protected by copyright. All rights reserved.
- Observed and simulated hydrologic response for a first-order catchment
during extreme rainfall three years after wildfire disturbance
- Authors: Brian A. Ebel; Francis K. Rengers, Gregory E. Tucker
Abstract: Hydrologic response to extreme rainfall in disturbed landscapes is poorly understood because of the paucity of measurements. A unique opportunity presented itself when extreme rainfall in September 2013 fell on a headwater catchment (i.e.
- Quantifying the importance of spatial resolution and other factors through
global sensitivity analysis of a flood inundation model
- Authors: James Thomas Steven Savage; Francesca Pianosi, Paul Bates, Jim Freer, Thorsten Wagener
Abstract: Where high resolution topographic data are available, modellers are faced with the decision of whether it is better to spend computational resource on resolving topography at finer resolutions or on running more simulations to account for various uncertain input factors (e.g. model parameters). In this paper we apply Global Sensitivity Analysis to explore how influential the choice of spatial resolution is when compared to uncertainties in the Manning's friction coefficient parameters, the inflow hydrograph, and those stemming from the coarsening of topographic data used to produce Digital Elevation Models (DEMs). We apply the hydraulic model LISFLOOD-FP to produce several temporally and spatially variable model outputs that represent different aspects of flood inundation processes, including flood extent, water depth and time of inundation. We find that the most influential input factor for flood extent predictions changes during the flood event, starting with the inflow hydrograph during the rising limb before switching to the channel friction parameter during peak flood inundation, and finally to the floodplain friction parameter during the drying phase of the flood event. Spatial resolution and uncertainty introduced by resampling topographic data to coarser resolutions are much more important for water depth predictions, which are also sensitive to different input factors spatially and temporally. Our findings indicate that the sensitivity of LISFLOOD-FP predictions is more complex than previously thought. Consequently, the input factors that modellers should prioritise will differ depending on the model output assessed, and the location and time of when and where this output is most relevant. This article is protected by copyright. All rights reserved.
- Response to the discussion of “Estimation of composite hydraulic
resistance in ice-covered alluvial streams”
- Authors: Soheil Ghareh Aghaji Zare; Stephanie A. Moore, Colin D. Rennie, Ousmane Seidou, Habib Ahmari, Jarrod Malenchak
Abstract: Previously utilized techniques for analysis of flow velocity in ice controlled zone are confirmed to be valid. Potential sources of error regarding the estimation of energy grade line slope are corrected considering the comments by the discussant. A modified version of the equation for composite roughness calculation originally introduced in the paper is presented. The revised new method then is tested against the other available methods and its accuracy is evaluated. Other assumptions and analyses presented in the paper are also tested and validated. This article is protected by copyright. All rights reserved.
- Quantifying an aquifer nitrate budget and future nitrate discharge using
field data from streambeds and well nests
- Authors: Troy E. Gilmore; David P. Genereux, D. Kip Solomon, Kathleen. M. Farrell, Helena Mitasova
Abstract: Novel groundwater sampling (age, flux, and nitrate) carried out beneath a streambed and in wells was used to estimate (1) the current rate of change of nitrate storage, dSNO3/dt, in a contaminated unconfined aquifer, and (2) future [NO3-]FWM (the flow-weighted mean nitrate concentration in groundwater discharge) and fNO3 (the nitrate flux from aquifer to stream). Estimates of dSNO3/dt suggested that at the time of sampling (2013) the nitrate storage in the aquifer was decreasing at an annual rate (mean = -9 mmol/m2year) equal to about one-tenth the rate of nitrate input by recharge. This is consistent with data showing a slow decrease in the [NO3-] of groundwater recharge in recent years. Regarding future [NO3-]FWM and fNO3, predictions based on well data show an immediate decrease that becomes more rapid after ∼5 years before leveling out in the early 2040s. Predictions based on streambed data generally show an increase in future [NO3-]FWM and fNO3 until the late 2020s, followed by a decrease before leveling out in the 2040s. Differences show the potential value of using information directly from the groundwater – surface water interface to quantify the future impact of groundwater nitrate on surface water quality. The choice of denitrification kinetics was similarly important; compared to zero-order kinetics, a first-order rate law levels out estimates of future [NO3-]FWM and fNO3 (lower peak, higher minimum) as legacy nitrate is flushed from the aquifer. Major fundamental questions about non-point-source aquifer contamination can be answered without a complex numerical model or long-term monitoring program. This article is protected by copyright. All rights reserved.
- A travel-time based approach to model kinetic sorption in highly
heterogeneous porous media via reactive hydrofacies
- Authors: Michael Finkel; Peter Grathwohl, Olaf A. Cirpka
Abstract: We present a semi-analytical model for the transport of solutes being subject to sorption in porous aquifers. We couple a travel-time based model of advective transport with a spherical diffusion model of kinetic sorption in non-uniform material mixtures. The model is formulated in the Laplace domain and transformed to the time domain by numerical inversion. By this, three-dimensional transport of solutes undergoing mass transfer between aqueous and solid phases can be simulated very efficiently. The model addresses both hydraulic and reactive heterogeneity of porous aquifers by means of hydrofacies, which function as homogeneous but non-uniform subunits. The total exposure time to each of these subunits controls the magnitude of sorption effects, whereas the particular sequence of facies through which the solute passes is irrelevant. We apply the model to simulate the transport of phenanthrene in a fluvio-glacial aquifer, for which the hydrofacies distribution is known at high resolution, the lithological composition of each facies has been analyzed, and sorption properties of the lithological components are available. Taking the fully resolved hydrofacies-model as reference, we evaluate different approximations referring to lower information levels, reflecting shortcomings in typical modeling projects. The most important feature for a good description of both the main breakthrough and tailing of phenanthrene is the non-uniformity of the porous medium. While spatial heterogeneity of chemical properties might be neglected without introducing a large error, an approximation of the facies' composition in terms of a uniform substitute material considerably compromises the quality of the modeling result. This article is protected by copyright. All rights reserved.
- Catchments' hedging strategy on evapotranspiration for climatic
- Authors: Chi Zhang; Wei Ding, Yu Li, Yin Tang, Dingbao Wang
Abstract: In this paper, we test the hypothesis that natural catchments utilize hedging strategy for evapotranspiration and water storage carryover with uncertain future precipitation. The hedging strategy for evapotranspiration in catchments under different levels of water availability is analytically derived with marginal utility principle. It is found that there exists hedging between evapotranspiration for present and future only with a portion of water availability. Observation data sets of 160 catchments in the United States covering the period from 1983 to 2003 demonstrate the existence of hedging in catchment hydrology and validate the proposed hedging strategy. We also find that more water is allocated to carryover storage for hedging against the future evapotranspiration deficit in the catchments with larger aridity indexes or with larger variability in future precipitation, i.e., long-term climate and precipitation variability control the degree of hedging. This article is protected by copyright. All rights reserved.
- Panel regressions to estimate low-flow response to rainfall variability in
- Authors: Maoya Bassiouni; Richard M. Vogel, Stacey A. Archfield
Abstract: Multicollinearity and omitted-variable bias are major limitations to developing multiple linear regression models to estimate streamflow characteristics in ungaged areas and varying rainfall conditions. Panel regression is used to overcome limitations of traditional regression methods, and obtain reliable model coefficients, in particular to understand the elasticity of streamflow to rainfall. Using annual rainfall and selected basin characteristics at 86 gaged streams in the Hawaiian Islands, regional regression models for three stream classes were developed to estimate the annual low-flow duration discharges. Three panel-regression structures (random effects, fixed effects, and pooled) were compared to traditional regression methods, in which space is substituted for time. Results indicated that panel regression generally was able to reproduce the temporal behavior of streamflow and reduce the standard errors of model coefficients compared to traditional regression, even for models in which the unobserved heterogeneity between streams is significant and the variance inflation factor for rainfall is much greater than 10. This is because both spatial and temporal variability were better characterized in panel regression. In a case study, regional rainfall elasticities estimated from panel regressions were applied to ungaged basins on Maui, using available rainfall projections to estimate plausible changes in surface-water availability and usable stream habitat for native species. The presented panel-regression framework is shown to offer benefits over existing traditional hydrologic regression methods for developing robust regional relations to investigate streamflow response in a changing climate. This article is protected by copyright. All rights reserved.
- Non-Darcian flow experiments of shear-thinning fluids through rough-walled
- Authors: Antonio Rodríguez de Castro; Giovanni Radilla
Abstract: Understanding non-Darcian flow of shear-thinning fluids through rough-walled rock fractures is of vital importance in a number of industrial applications such as hydrogeology or petroleum engineering. Different laws are available to express the deviations from linear Darcy law due to inertial pressure losses. In particular, Darcy's law is often extended through addition of quadratic and cubic terms weighted by two inertial coefficients depending on the strength of the inertia regime. The relations between the effective shear viscosity of the fluid and the apparent viscosity in porous media when inertial deviations are negligible were extensively studied in the past. However, only recent numerical works have investigated the superposition of both inertial and shear-thinning effects, finding that the same inertial coefficients obtained for non-Darcian Newtonian flow apply in the case of shear-thinning fluids. The objective of this work is to experimentally validate these results, extending their applicability to the case of rough-walled rock fractures. To do so, flow experiments with aqueous polymer solutions have been conducted using replicas of natural fractures, and the effects of polymer concentration, which determine the shear rheology of the injected fluid, have been evaluated. Our findings show that the experimental pressure loss-flow rate data for inertial flow of shear-thinning fluids can be successfully predicted from the empirical parameters obtained during non-Darcian Newtonian flow and Darcian shear-thinning flow in a given porous medium. This article is protected by copyright. All rights reserved.
- Effects of an emergent vegetation patch on channel reach bathymetry and
stability during repeated unsteady flows
- Authors: Kevin A. Waters; Joanna Crowe Curran
Abstract: While research into the interaction between in-channel vegetation, flow, and bed sediment has increased in recent years, there is still a need to understand how unsteady flows affect these processes, particularly in terms of channel bed adjustments. In this study, flume experiments tested two flood hydrograph sizes run over sand/gravel and sand/silt beds to evaluate reach scale impacts of a mid-channel vegetation patch of variable stem density on channel bathymetry and stability. Alternating flood hydrographs with periods of low, steady flow created flow sequences reflective of an extended unsteady flow regime, thereby simulating time scales consisting of multiple flood events. Digital elevation models provided detailed measurements of channel change following each flood event to enable analysis over each unsteady flow sequence. The vegetation patch created characteristic channel bathymetries dependent on sediment mixture and patch density that in all cases resulted in a more variable bed structure than channels without a patch. Reach scale stability, quantified based on net volumetric bed change, only occurred with a sparse patch in the low flood sequence, corresponding with little variation in surface composition and structure. In most other cases, scour measured at the patch prevented stability at the reach scale, especially in the finer substrate. Overall, findings show that a channel may only adjust to a stable bathymetry upon addition of a mid-channel vegetation patch within a limited range of flow regimes and patch stem densities, and that for the experimental conditions tested here, in-stream patches generally did not enhance reach scale bed stability. This article is protected by copyright. All rights reserved.
- Reorganization of river networks under changing spatiotemporal
precipitation patterns: An optimal channel network approach
- Authors: Armaghan Abed-Elmdoust; Mohammad-Ali Miri, Arvind Singh
Abstract: We investigate the impact of changing non-uniform spatial and temporal precipitation patterns on the evolution of river networks. To achieve this, we develop a two-dimensional optimal channel network (OCN) model with a controllable rainfall distribution to simulate the evolution of river networks, governed by the principle of minimum energy expenditure, inside a prescribed boundary. We show that under non-uniform precipitation conditions, river networks reorganize significantly toward new patterns with different geomorphic and hydrologic signatures. This reorganization is mainly observed in the form of migration of channels of different orders, widening or elongation of basins as well as formation and extinction of channels and basins. In particular, when the precipitation gradient is locally increased, the higher-order channels, including the mainstream river, migrate toward regions with higher precipitation intensity. Through pertinent examples, the reorganization of the drainage network is quantified via stream parameters such as Horton-Strahler and Tokunaga measures, order-based channel total length and river long profiles obtained via simulation of three-dimensional basin topography, while the hydrologic response of the evolved network is investigated using metrics such as hydrograph and power spectral density of simulated streamflows at the outlet of the network. In addition, using OCNs, we investigate the effect of orographic precipitation patterns on multi-catchment landscapes composed of several interacting basins. Our results show that network-inspired methods can be utilized as insightful and versatile models for directly exploring the effects of climate change on the evolution of river drainage systems. This article is protected by copyright. All rights reserved.
- Sand infiltration into a gravel bed: A mathematical model
- Authors: A. Herrero; C. Berni
Abstract: Fine sediment infiltration into a river bed is a physical process affected by different human actions and has several environmental, socioeconomic and river morphology consequences. A theoretical model is proposed herein aiming to reproduce the fine sediment content depth profile resulting from the infiltration of fine sediment into an initially clean gravel bed. The model is based on the probability of infiltrating particles to be trapped in a pore throat formed by three bed particles. The model is tested against previous experimental results and is found to reproduce adequately the occurrence of the two infiltration mechanisms reported by previous studies: bridging and unimpeded static percolation. Theoretical depth profiles are found to underestimate fine sediment content at the bed subsurface (below 2-3 gravel diameter depth) compared to the laboratory results. This may be due to hyporheic flow that is not taken into account in our model. In flow experiments, the particles previously infiltrated and deposited might be destabilized by pore water flow and their fall down to the bed might be magnified. This article is protected by copyright. All rights reserved.
- A generalized threshold model for computing bedload grain-size
- Authors: Alain Recking
Abstract: For morphodynamic studies, it is important to compute not only the transported volumes of bedload, but also the size of the transported material. A few bedload equations compute fractional transport (i.e., both the volume and grain size distribution), but many equations compute only the bulk transport (a volume) with no consideration of the transported grain sizes. To fill this gap, a method is proposed to compute the bedload grain size distribution separately to the bedload flux. The method is called the Generalized Threshold Model (GTM), because it extends the flow competence method for threshold of motion of the largest transported grain size to the full bed surface grain size distribution. This was achieved by replacing dimensional diameters with their size indices in the standard hiding function, which offers a useful framework for computation, carried out for each indices considered in the range [1, 100]. New functions are also proposed to account for partial transport. The method is very simple to implement and is sufficiently flexible to be tested in many environments. In addition to being a good complement to standard bulk bedload equations, it could also serve as a framework to assist in analyzing the physics of bedload transport in future research. This article is protected by copyright. All rights reserved.
- An analytical approach for the simulation of flow in a heterogeneous
confined aquifer with a parameter zonation structure
- Authors: Ching-Sheng Huang; Hund-Der Yeh
Abstract: This study introduces an analytical approach to estimate drawdown induced by well extraction in a heterogeneous confined aquifer with an irregular outer boundary. The aquifer domain is divided into a number of zones according to the zonation method for representing the spatial distribution of a hydraulic parameter field. The lateral boundary of the aquifer can be considered under the Dirichlet, Neumann or Robin condition at different parts of the boundary. Flow across the interface between two zones satisfies the continuities of drawdown and flux. Source points, each of which has an unknown volumetric rate representing the boundary effect on the drawdown, are allocated around the boundary of each zone. The solution of drawdown in each zone is expressed as a series in terms of the Theis equation with unknown volumetric rates from the source points. The rates are then determined based on the aquifer boundary conditions and the continuity requirements. The estimated aquifer drawdown by the present approach agrees well with a finite element solution developed based on the Mathematica function NDSolve. As compared with the existing numerical approaches, the present approach has a merit of directly computing the drawdown at any given location and time and therefore takes much less computing time to obtain the required results in engineering applications. This article is protected by copyright. All rights reserved.
- Adaptive measurements of urban runoff quality
- Authors: Brandon P. Wong; Branko Kerkez
Abstract: An approach to adaptively measure runoff water quality dynamics is introduced, focusing specifically on characterizing the timing and magnitude of urban pollutographs. Rather than relying on a static schedule or flow-weighted sampling, which can miss important water quality dynamics if parameterized inadequately, novel Internet-enabled sensor nodes are used to autonomously adapt their measurement frequency to real-time weather forecasts and hydrologic conditions. This dynamic approach has the potential to significantly improve the use of constrained experimental resources, such as automated grab samplers, which continue to provide a strong alternative to sampling water quality dynamics when in-situ sensors are not available. Compared to conventional flow- or time-weighted sampling schemes, which rely on preset thresholds, a major benefit of the approach is the ability to dynamically adapt to features of an underlying hydrologic signal. A 28 km2 urban watershed was studied to characterize concentrations of total suspended solids (TSS) and total phosphorus. Water quality samples were autonomously triggered in response to features in the underlying hydrograph and real-time weather forecasts. The study watershed did not exhibit a strong first flush and intra-event concentration variability was driven by flow acceleration, wherein the largest loadings of TSS and total phosphorus corresponded with the steepest rising limbs of the storm hydrograph. The scalability of the proposed method is discussed in the context of larger sensor network deployments, as well the potential to improving control of urban water quality. This article is protected by copyright. All rights reserved.
- On the correlation of water vapor and CO2: Application to flux
partitioning of evapotranspiration
- Authors: Wen Wang; James A. Smith, Prathap Ramamurthy, Mary Lynn Baeck, Elie Bou-Zeid, Todd M. Scanlon
Abstract: The partitioning of evapotranspiration (ET) between plant transpiration (Et) and direct evaporation (Ed) presents one of the most important and challenging problems for characterizing ecohydrological processes. The exchange of water vapor (q) and CO2 (c) are closely coupled in ecosystem processes and knowledge of their controls can be gained through joint investigation of q and c. In this study we examine the correlation of water vapor and CO2 (Rqc) through analyses of high frequency time series derived from eddy covariance measurements collected over a suburban grass field in Princeton, NJ during a two-year period (2011-2013). Rqc at the study site exhibits pronounced seasonal and diurnal cycles, with maximum anticorrelation in June and maximum decorrelation in January. The diurnal cycle of Rqc varies seasonally and is characterized by a near-symmetric shape with peak anticorrelation around local noon. Wavelet and spectral analyses suggest that q and c are jointly transported for most eddy scales (1-200 m), which is important for flux-variance ET partitioning methods (e.g. Scanlon and Sahu ). The diurnal cycle of the transpiration fraction (ratio of Et to total ET) exhibits an asymmetric diurnal cycle, especially during the warm season, with peak values occurring in the afternoon. These ET partitioning results give similar diurnal and seasonal patterns compared with numerical simulations from the Noah Land Surface Model using the Jarvis canopy resistance formulation. This article is protected by copyright. All rights reserved.
- Predicting shifts in rainfall-runoff partitioning during multiyear
drought: Roles of dry period and catchment characteristics
- Authors: Margarita Saft; Murray C. Peel, Andrew W. Western, Lu Zhang
Abstract: While the majority of hydrological prediction methods assume that observed interannual variability explores the full range of catchment response dynamics, recent cases of prolonged climate drying suggest otherwise. During the ∼decade-long Millennium drought in south-eastern Australia significant shifts in hydrologic behaviour were reported. Catchment rainfall-runoff partitioning changed from what was previously encountered during shorter droughts, with significantly less runoff than expected occurring in many catchments. In this article we investigate the variability in the magnitude of shift in rainfall-runoff partitioning observed during the Millennium drought. We re-evaluate a large range of factors suggested to be responsible for the additional runoff reductions. Our results suggest that the shifts were mostly influenced by catchment characteristics related to pre-drought climate (aridity index and rainfall seasonality) and soil and groundwater storage dynamics (pre-drought interannual variability of groundwater storage and mean solum thickness). The shifts were amplified by seasonal rainfall changes during the drought (spring rainfall deficits). We discuss the physical mechanisms that are likely to be associated with these factors. Our results confirm that shifts in the annual rainfall-runoff relationship represent changes in internal catchment functioning, and emphasise the importance of cumulative multiyear changes in the catchment storage for runoff generation. Prolonged drying in some regions can be expected in the future, and our results provide an indication of which catchments characteristics are associated with catchments more susceptible to a shift in their runoff response behaviour. This article is protected by copyright. All rights reserved.
- Modeling the influence of preferential flow on the spatial variability and
time-dependence of mineral weathering rates
- Authors: Sachin Pandey; Harihar Rajaram
Abstract: Inferences of weathering rates from laboratory and field observations suggest significant scale and time dependence. Preferential flow induced by heterogeneity (manifest as permeability variations or discrete fractures) has been suggested as one potential mechanism causing scale/time dependence. We present a quantitative evaluation of the influence of preferential flow on weathering rates using reactive transport modeling. Simulations were performed in discrete fracture networks (DFNs) and correlated random permeability fields (CRPFs), and compared to simulations in homogeneous permeability fields. The simulations reveal spatial variability in the weathering rate, multi-dimensional distribution of reactions zones, and the formation of rough weathering interfaces and corestones due to preferential flow. In the homogeneous fields and CRPFs, the domain-averaged weathering rate is initially constant as long as the weathering front is contained within the domain, reflecting equilibrium-controlled behavior. The behavior in the CRPFs was influenced by macrodispersion, with more spread-out weathering profiles, an earlier departure from the initial constant rate and longer persistence of weathering. DFN simulations exhibited a sustained time-dependence resulting from the formation of diffusion-controlled weathering fronts in matrix blocks, which is consistent with the shrinking core mechanism. A significant decrease in the domain-averaged weathering rate is evident despite high remaining mineral volume fractions, but the decline does not follow a 1/√t dependence, characteristic of diffusion, due to network scale effects and advection-controlled behavior near the inflow boundary. The DFN simulations also reveal relatively constant horizontally-averaged weathering rates over a significant depth range, challenging the very notion of a weathering front. This article is protected by copyright. All rights reserved.
- Modeling sediment transport after ditch network maintenance of a forested
- Authors: K. Haahti; H. Marttila, L. Warsta, T. Kokkonen, L. Finér, H. Koivusalo
Abstract: Elevated suspended sediment (SS) loads released from peatlands after drainage operations and the resulting negative effect on the ecological status of the receiving water bodies have been widely recognized. Understanding the processes controlling erosion and sediment transport within the ditch network forms a prerequisite for adequate sediment control. While numerous experimental studies have been reported in this field, model based assessments are rare. This study presents a modeling approach to investigate sediment transport in a peatland ditch network. The transport model describes bed erosion, rain-induced bank erosion, floc deposition, and consolidation of the bed. Coupled to a distributed hydrological model, sediment transport was simulated in a 5.2 ha forestry-drained peatland catchment for two years after ditch cleaning. Comparing simulation results to measured SS concentrations suggested that the loose peat material, produced during excavation, contributed markedly to elevated SS concentrations immediately after ditch cleaning. Both snowmelt and summer rainstorms contributed critically to annual loads. Springtime peat erosion during snowmelt was driven by ditch flow whereas during summer rainfalls, bank erosion by raindrop impact was identified as an important process. Relating modeling results to observed spatial topographic changes in the ditch network was challenging and the results were difficult to verify. Nevertheless, the model has potential to identify risk areas for erosion. The results demonstrate that modeling is effective in separating the importance of different processes and complements pure experimental approaches. Modeling results can aid planning and designing efficient sediment control measures and guide the focus of experimental studies. This article is protected by copyright. All rights reserved.
- Arbitrarily Complex Chemical Reactions on Particles
- Authors: David A. Benson; Diogo Bolster
Abstract: Previous particle-tracking (PT) algorithms for chemical reaction conceptualize each particle being composed of one species. Reactions occur by either complete or partial birth/death processes between interacting particles. Here we extend the method by placing any number of chemical species on each particle. The particle/particle interaction is limited to mass exchange. After exchange, reactions of any sort are carried out independently on each particle. The novel components of the algorithms are verified against analytic solutions where possible. This article is protected by copyright. All rights reserved.
- Multidecadal dynamics of alternate bars in the Alpine Rhine River
- Authors: Luca Adami; Walter Bertoldi, Guido Zolezzi
Abstract: We report on a multi-decadal analysis of alternate bar dynamics in a 41.7 km reach of the Alpine Rhine River, which represents an almost unique example of a regulated river with fixed levees, straight reaches and regular bends in which alternate gravel bars spontaneously formed and migrated for more than a century. The analysis is based on freely available Landsat imagery, which provided an accurate and frequent survey of the dynamics of the alternate bar configuration since 1984. Bars were characterized in terms of wavelength, migration, and height. Longitudinal and temporal patterns are investigated as a function of flood occurrence and magnitude and in relation to the presence of local planform discontinuities (bends and ramps) that may affect their dynamics. Bars in the upper part of the reach are mostly steady and relatively long (about 13 channel widths); bars in the lower part of the reach are migrating and shorter (about 9 channel widths). Bar height is rather uniform along the reach, ranging between 3 to 4 m. The temporally long hydrological dataset allowed the investigation of bar migration during flood events, showing that bars migrate faster for intermediate floods. The observed relationship between bar migration and wavelength was consistent with linear theories for free migrating and steady forced bars in straight channels. The comparison of theories with observations highlights the key role of theories to support interpretation of observations, for a better understanding of the morphodynamic processes controlling bar formation and dynamics. This article is protected by copyright. All rights reserved.
- Estimating surface turbulent heat fluxes from land surface temperature and
soil moisture observations using the particle batch smoother
- Authors: Yang Lu; Jianzhi Dong, Susan C. Steele-Dunne, Nick van de Giesen
Abstract: Surface heat fluxes interact with the overlying atmosphere and play a crucial role in meteorology, hydrology and climate change studies, but in-situ observations are costly and difficult. It has been demonstrated that surface heat fluxes can be estimated from assimilation of land surface temperature (LST). One approach is to estimate a neutral bulk heat transfer coefficient (CHN) to scale the sum of turbulent heat fluxes, and an evaporative fraction (EF) that represents the partitioning between fluxes. Here, the newly developed particle batch smoother (PBS) is implemented. The PBS makes no assumptions about the prior distributions and is therefore well-suited for non-Gaussian processes. It is also particularly advantageous for parameter estimation by tracking the entire prior distribution of parameters using Monte Carlo sampling. To improve the flux estimation on wet or densely vegetated surfaces, a simple soil moisture scheme is introduced to further constrain EF, and soil moisture observations are assimilated simultaneously. This methodology is implemented with the FIFE 1987 and 1988 data sets. Validation against observed fluxes indicates that assimilating LST using the PBS significantly improves the flux estimates at both daily and half-hourly time scales. When soil moisture is assimilated, the estimated EFs become more accurate, particularly when the surface heat flux partitioning is energy-limited. The feasibility of extending the methodology to use remote sensing observations is tested by limiting the number of LST observations. Results show that flux estimates are greatly improved after assimilating soil moisture, particularly when LST observations are sparse. This article is protected by copyright. All rights reserved.
- Groundwater flow and heterogeneous discharge into a seepage lake: Combined
use of physical methods and hydrochemical tracers
- Authors: J. Kazmierczak; S. Müller, B. Nilsson, D. Postma, J. Czekaj, E. Sebok, S. Jessen, S. Karan, C. Stenvig Jensen, K. Edelvang, P. Engesgaard
Abstract: Groundwater discharge into a seepage lake was investigated by combining flux measurements, hydrochemical tracers, geological information and a telescopic modelling approach using first two-dimensional (2D) regional then 2D local flow and flow path models. Discharge measurements and hydrochemical tracers supplement each other. Discharge measurements yield flux estimates, but rarely provide information about the origin and flow path of the water. Hydrochemical tracers may reveal the origin and flow path of the water, but rarely provide any information about the flux. While aquifer interacting with the lake remained under seemingly steady state conditions across seasons, a high spatial and temporal heterogeneity in the discharge to the lake was observed. The results showed that part of the groundwater flowing from the west passes beneath the lake and discharges at the eastern shore, where groundwater springs and high discharge zones (HDZs) are observed at the lake bottom and at seepage faces adjacent to the lake. In the 2D cross-section, surface runoff from the seepage faces delivers 64% of the total groundwater inputs to the lake, and a 2 m wide offshore HDZ delivers 13%. Presence of HDZs may control nutrient fluxes to the lake. This article is protected by copyright. All rights reserved.
- Effects of climate change on deep-water oxygen and winter mixing in a deep
lake (Lake Geneva) – Comparing observational findings and modeling
- Authors: Robert Schwefel; Adrien Gaudard, Alfred Wüest, Damien Bouffard
Abstract: Low concentrations of dissolved oxygen remain a global concern regarding the ecological health of lakes and reservoirs. In addition to high nutrient loads, climate-induced changes in lake stratification and mixing represent additional anthropogenic meanace resulting in decreased deep-water oxygen levels. The analysis of 43 years of monitoring data from Lake Geneva shows no decreasing trend neither in the areal hypolimnetic mineralization rate, nor in the extent of hypoxia. Instead, hypoxic conditions are predominantly controlled by deep mixing in winter and much less by the trophic variations over the past decades. To reproduce winter mixing, the one-dimensional hydrodynamic model SIMSTRAT was specially adapted to deep lakes and run for several climate scenarios. The simulations predicted a decrease in the maximum winter mixing depth from an average of ∼172 m for 1981–2012 to ∼136 m and ∼127 m in response to predicted atmospheric temperatures between 2045–2076 and 2070–2101 according to Intergovernmental Panel on Climate Change scenarios. Concurrently, events with complete homogenization of temperature and oxygen in winter will decrease by ∼50%. Consequently, the hypolimnetic oxygen concentrations will significantly decrease. These results demonstrate that changes in deep mixing can have stronger impact than eutrophication on the deep-water oxygen levels of oligomictic lakes. This article is protected by copyright. All rights reserved.
- Coupled modeling of storm surge and coastal inundation: A case study in
New York City during Hurricane Sandy
- Authors: Jie Yin; Ning Lin, Dapeng Yu
Abstract: In this paper we describe a new method of modeling coastal inundation arising from storm surge by coupling a widely used storm surge model (ADCIRC) and an urban flood inundation model (FloodMap). This is the first time the coupling of such models is implemented and tested using real events. The method offers a flexible and efficient procedure for applying detailed ADCIRC storm surge modeling results along the coastal boundary (with typical resolution of ∼100 m) to FloodMap for fine resolution inundation modeling (< 5 m). The coastal inundation during Hurricane Sandy was simulated at both the city (New York City) and sub-regional (lower Manhattan) scales with various resolutions. Results obtained from the ADCIRC and coupled ADCIRC -FloodMap simulations were compared with the recorded (High Water Marks) and derived (inundation extent based on the planar method) data from FEMA. At the city scale, coupled ADCIRC -FloodMap modeling demonstrates improved prediction over ADCIRC modeling alone for both the extent and depth of inundation. The advantage of the coupled model is further illustrated in the sub-regional modeling, using a mesh resolution of 3 m which is substantially finer than the inland mesh resolution used by ADCIRC (> 70 m). In further testing, we explored the effects of mesh resolution and roughness specification. Results agree with previous studies that fine resolution is essential for capturing intricate flow paths and connectivity in urban topography. While the specification of roughness is more challenging for urban environments, it may be empirically optimized. The successful coupling of ADCIRC and FloodMap models for fine-resolution coastal inundation modeling unlocks the potential for undertaking large numbers of probabilistically-based synthetic surge events for street-level risk analysis. This article is protected by copyright. All rights reserved.
- Rock fracture grouting with microbially induced carbonate precipitation
- Authors: James M. Minto; Erica MacLachlan, Gráinne El Mountassir, Rebecca J. Lunn
Abstract: Microbially induced carbonate precipitation has been proposed for soil stabilization, soil strengthening and permeability reduction as an alternative to traditional cement and chemical grouts. In this paper we evaluate the grouting of fine aperture rock fractures with calcium carbonate, precipitated through urea hydrolysis, by the bacteria Sporosarcina pasteurii. Calcium carbonate was precipitated within a small-scale and a near field-scale (3.1 m2) artificial fracture consisting of a rough rock lower surfaces and clear polycarbonate upper surfaces. The spatial distribution of the calcium carbonate precipitation was imaged using time-lapse photography and the influence on flow pathways revealed from tracer transport imaging. In the large-scale experiment, hydraulic aperture was reduced from 276 μm to 22 μm, corresponding to a transmissivity reduction of 1.71x10−5 m2/s to 8.75x10−9 m2/s, over a period of 12 days under constantly flowing conditions. With a modified injection strategy a similar three orders of magnitude reduction in transmissivity was achieved over a period of three days. Calcium carbonate precipitated over the entire artificial fracture with strong adhesion to both upper and lower surfaces and precipitation was controlled to prevent clogging of the injection well by manipulating the injection fluid velocity. These experiments demonstrate that microbially induced carbonate precipitation can successfully be used to grout a fracture under constantly flowing conditions and may be a viable alternative to cement based grouts when a high level of hydraulic sealing is required and chemical grouts when a more durable grout is required. This article is protected by copyright. All rights reserved.
- Reassessing the MADE direct-push hydraulic conductivity data using a
revised calibration procedure
- Authors: Geoffrey C. Bohling; Gaisheng Liu, Peter Dietrich, James J. Butler
Abstract: In earlier work, we presented a geostatistical assessment of high-resolution hydraulic conductivity (K) profiles obtained at the MADE site using direct-push (DP) methods. The profiles are derived from direct-push injection logger (DPIL) measurements that provide a relative indicator of vertical variations in K with a sample spacing of 1.5 cm. The DPIL profiles are converted to K profiles by calibrating to the results of direct-push permeameter (DPP) tests performed at selected depths in some of the profiles. Our original calibration used a linear transform that failed to adequately account for an upper limit on DPIL responses in high-K zones and noise in the DPIL data. Here we present a revised calibration procedure that accounts for the upper limit and noise, leading to DPIL K values that display a somewhat different univariate distribution and a lower lnK variance (5.9±1.5) than the original calibration values (6.9±1.8), although each variance estimate falls within the other's 95% confidence interval. Despite the change in the univariate distribution, the autocorrelation structure and large-scale patterns exhibited by the revised DPIL K values still agree well with those exhibited by the flowmeter data from the site. We provide the DPIL and DPP data, along with our calibrated DPIL K values, in the supplemental materials. This article is protected by copyright. All rights reserved.
- Bounded fractional diffusion in geological media: Definition and
- Authors: Yong Zhang; Christopher T. Green, Eric M. LaBolle, Roseanna M. Neupauer, HongGuang Sun
Abstract: Spatiotemporal Fractional-Derivative Models (FDMs) have been increasingly used to simulate non-Fickian diffusion, but methods have not been available to define boundary conditions for FDMs in bounded domains. This study defines boundary conditions and then develops a Lagrangian solver to approximate bounded, one-dimensional fractional diffusion. Both the zero-value and non-zero-value Dirichlet, Neumann, and mixed Robin boundary conditions are defined, where the sign of Riemann-Liouville fractional derivative (capturing non-zero-value spatial-nonlocal boundary conditions with directional super-diffusion) remains consistent with the sign of the fractional-diffusive flux term in the FDMs. New Lagrangian schemes are then proposed to track solute particles moving in bounded domains, where the solutions are checked against analytical or Eularian solutions available for simplified FDMs. Numerical experiments show that the particle-tracking algorithm for non-Fickian diffusion differs from Fickian diffusion in relocating the particle position around the reflective boundary, likely due to the nonlocal and non-symmetric fractional diffusion. For a non-zero-value Neumann or Robin boundary, a source cell with a reflective face can be applied to define the release rate of random-walking particles at the specified flux boundary. Mathematical definitions of physically meaningful nonlocal boundaries combined with bounded Lagrangian solvers in this study may provide the only viable techniques at present to quantify the impact of boundaries on anomalous diffusion, expanding the applicability of FDMs from infinite domains to those with any size and boundary conditions. This article is protected by copyright. All rights reserved.
- Analysis of vadose zone inhomogeneity towards distinguishing recharge
rates: Solving the nonlinear interface problem with Newton method
- Authors: David R. Steward
Abstract: Recharge from surface to groundwater is an important component of the hydrological cycle, yet its rate is difficult to quantify. Percolation through two-dimensional circular inhomogeneities in the vadose zone is studied where one soil type is embedded within a uniform background, and nonlinear interface conditions in the quasilinear formulation are solved using Newton's method with the Analytic Element Method. This numerical laboratory identifies detectable variations in pathline and pressure head distributions that manifest due to a shift in recharge rate through in a heterogeneous media. Pathlines either diverge about or converge through coarser and finer grained materials with inverse patterns forming across lower and upper elevations; however, pathline geometry is not significantly altered by recharge. Analysis of pressure head in lower regions near groundwater identifies a new phenomenon: its distribution is not significantly impacted by an inhomogeneity soil type, nor by its placement, nor by recharge rate. Another revelation is that pressure head for coarser grained inhomogeneities in upper regions is completely controlled by geometry and conductivity contrasts; a shift in recharge generates a difference Δp that becomes an additive constant with the same value throughout this region. In contrast, shifts in recharge for finer grained inhomogeneities reveal patterns with abrupt variations across their interfaces. Consequently, measurements aimed at detecting shifts in recharge in a heterogeneous vadose zone by deciphering the corresponding patterns of change in pressure head should focus on finer grained inclusions well above a groundwater table. This article is protected by copyright. All rights reserved.
- Impact of hydrologically driven hillslope erosion and landslide occurrence
on soil organic carbon dynamics in tropical watersheds
- Authors: Yannis G. Dialynas; Satish Bastola, Rafael L. Bras, Erika Marin-Spiotta, Whendee L. Silver, Elisa Arnone, Leonardo V. Noto
Abstract: The dynamics of soil organic carbon (SOC) in tropical forests play an important role in the global carbon (C) cycle. Past attempts to quantify the net C exchange with the atmosphere in regional and global budgets do not systematically account for dynamic feedbacks among linked hydrological, geomorphological, and biogeochemical processes, which control the fate of SOC. Here we quantify effects of geomorphic perturbations on SOC oxidation and accumulation in two adjacent wet tropical forest watersheds underlain by contrasting lithology (volcaniclastic rock and quartz diorite) in the Luquillo Critical Zone Observatory. This study uses the spatially-explicit and physically-based model of SOC dynamics tRIBS-ECO (Triangulated Irregular Network-based Real-time Integrated Basin Simulator-Erosion and Carbon Oxidation) and measurements of SOC profiles and oxidation rates. Our results suggest that hillslope erosion at the two watersheds may drive C sequestration or CO2 release to the atmosphere, depending on the forest type and land use. The net erosion-induced C exchange with the atmosphere was controlled by the spatial distribution of forest types. The two watersheds were characterized by significant erosion and dynamic replacement of upland SOC stocks. Results suggest that the landscape underlain by volcaniclastic rock has reached a state close to geomorphic equilibrium, and the landscape underlain by quartz diorite is characterized by greater rates of denudation. These findings highlight the importance of the spatially-explicit and physical representation of C erosion driven by local variation in lithological and geomorphological characteristics and in forest cover. This article is protected by copyright. All rights reserved.
- Hysteretic sediment fluxes in rainfall-driven soil erosion: Particle size
- Authors: Mohsen Cheraghi; Seifeddine Jomaa, Graham C. Sander, D. A. Barry
Abstract: A detailed laboratory study was conducted to examine the effects of particle size on hysteretic sediment transport under time-varying rainfall. A rainfall pattern composed of seven sequential stepwise varying rainfall intensities (30, 37.5, 45, 60, 45, 37.5 and 30 mm h−1), each of 20-mins duration, was applied to a 5-m × 2-m soil erosion flume. The soil in the flume was initially dried, ploughed to a depth of 20 cm and had a mechanically smoothed surface. Flow rates and sediment concentration data for seven particle size classes (< 2, 2-20, 20-50, 50-100, 100-315, 315-1000 and > 1000 µm) were measured in the flume effluent. Clockwise hysteresis loops in the sediment concentration versus discharge curves were measured for the total eroded soil and the finer particle sizes (< 2, 2-20 and 20-50 µm). However, for particle sizes greater than 50 µm, hysteresis effects decreased and suspended concentrations tended to vary linearly with discharge. The Hairsine and Rose (HR) soil erosion model agreed well with the experimental data for the total eroded soil and for the finer particle size classes (up to 50 µm). For the larger particle size classes, the model provided reasonable qualitative agreement with the measurements although the fit was poor for the largest size class (> 1000 µm). Overall, it is found that hysteresis varies amongst particle sizes and that the predictions of the HR model are consistent with hysteretic behavior of different sediment size classes. This article is protected by copyright. All rights reserved.
- Aquifer permeability change caused by a near-field earthquake, Canterbury,
- Authors: H. K. Rutter; S.C. Cox, N. F. Dudley Ward, J. J. Weir
Abstract: The MW 7.1 Darfield (Canterbury) earthquake, 4 September 2010, generated widespread hydrological effects in New Zealand ranging from instantaneous changes of piezometric levels, to more sustained post-seismic changes in spring flow, river discharge and groundwater levels, and increased turbidity and declined yields of water abstracted from wells. Four years later, piezometric levels remained elevated in deeper (>40 m) aquifers along the north-western (upper) side of the Canterbury Plains near the Greendale Fault, with changes in mean piezometric level reaching +13 m. Eigen modelling suggests that sustained high groundwater was not the result of changes in abstraction or land surface recharge. Step-drawdown tests at six wells within 15 km of Greendale Fault were carried out prior to the earthquake, and were re-tested following fault-rupture. Eden-Hazel analysis of discharge/drawdown relationships discriminates potential sources of head losses, and how these changed (or otherwise) as a result of the earthquake. Objective application of Eden-Hazel analysis provided confidence levels for the interpretation, including when step tests provide reliable/unreliable estimates of transmissivity change. Increases in both aquifer losses and well losses were observed in four wells, reflecting both a change in sediment transmissivity and decrease in well efficiency. At two locations, the data were unable to provide results that can be interpreted with confidence. As the majority of local groundwater flow occurs through high permeability open framework gravel lenses, we suggest that reduction in the permeability of these gravels, due to fine-sediment incursion, is the cause of the reduction in transmissivity and increase in well losses. This article is protected by copyright. All rights reserved.
- Comparison of static and dynamic resilience for a multipurpose reservoir
- Authors: Slobodan P. Simonovic; R. Arunkumar
Abstract: Reliability, resilience and vulnerability are the traditional risk measures used to assess the performance of a reservoir system. Among these measures, resilience is used to assess the ability of a reservoir system to recover from a failure event. However, the time independent static resilience does not consider the system characteristics, interaction of various individual components and does not provide much insight into reservoir performance from the beginning of the failure event until the full performance recovery. Knowledge of dynamic reservoir behavior under the disturbance offers opportunities for proactive and/or reactive adaptive response that can be selected to maximize reservoir resilience. A novel measure is required to provide insight into the dynamics of reservoir performance based on the reservoir system characteristics and its adaptive capacity. The reservoir system characteristics include, among others, reservoir storage curve, reservoir inflow, reservoir outflow capacity and reservoir operating rules. The reservoir adaptive capacity can be expressed using various impacts of reservoir performance under the disturbance (like reservoir release for meeting a particular demand, socio-economic consequences of reservoir performance, or resulting environmental state of the river upstream and downstream from the reservoir). Another way of expressing reservoir adaptive capacity to a disturbing event may include aggregated measures like reservoir robustness, redundancy, resourcefulness and rapidity. A novel measure that combines reservoir performance and its adaptive capacity is proposed in this paper and named ‘dynamic resilience'. The paper also proposes a generic simulation methodology for quantifying reservoir resilience as a function of time. The proposed resilience measure is applied to a single multi-purpose reservoir operation and tested for a set of failure scenarios. The dynamic behavior of reservoir resilience is captured using the system dynamics simulation approach, a feedback-based object-oriented method, very effective for modelling complex systems. The results of dynamic resilience are compared with the traditional performance measures in order to identify advantages of the proposed measure. The results confirm that the dynamic resilience is a powerful tool for selecting proactive and reactive adaptive response of a multipurpose reservoir to a disturbing event that cannot be achieved using traditional measures. The generic quantification approach proposed in the paper allows for easy use of dynamic resilience for planning and operations of various civil infrastructure systems. This article is protected by copyright. All rights reserved.
- Physio-climatic controls on vulnerability of watersheds to climate and
land use change across the United States
- Authors: Ankit Deshmukh; Riddhi Singh
Abstract: Understanding how a watershed's physio-climatic characteristics affect its vulnerability to environmental (climatic and land use) change is crucial for managing these complex systems. In this study, we combine the strengths of recently developed exploratory modelling frameworks and comparative hydrology to quantify the relationship between watershed's vulnerability and its physio-climatic characteristics. We propose a definition of vulnerability that can be used by a diverse range of water system managers and is useful in the presence of large uncertainties in drivers of environmental change. This definition is related to adverse climate change and land use thresholds that are quantified using a recently developed exploratory modelling approach. In this way, we estimate the vulnerability of 69 watersheds in the United States to climate and land use change. We explore definitions of vulnerability that describe average or extreme flow conditions, as well as others that are relevant from the point of view of instream organisms. In order to understand the dominant controls on vulnerability, we correlate these indices with watershed's characteristics describing its topography, geology, drainage, climate, and land use. We find that mean annual flow is more vulnerable to reductions in precipitation in watersheds with lower average soil permeability, lower baseflow index, lower forest cover, higher topographical wetness index, and vice-versa. Our results also indicate a potential mediation of climate change impacts by regional groundwater systems. By developing such relationships across a large range of watersheds, such information can potentially be used to assess the vulnerability of ungauged watersheds to uncertain environmental change. This article is protected by copyright. All rights reserved.
- Pore-scale network modeling of microbially induced calcium carbonate
precipitation (MICP): Insight into scale dependence of biogeochemical
- Authors: Chao-Zhong Qin; S. Majid Hassanizadeh, Anozie Ebigbo
Abstract: The engineering of microbially induced calcium carbonate precipitation (MICP) has attracted much attention in a number of applications, such as sealing of CO2 leakage pathways, soil stabilization, and subsurface remediation of radionuclides and toxic metals. The goal of this work is to gain insight into pore-scale processes of MICP and scale dependence of biogeochemical reaction rates. This will help us develop efficient field-scale MICP models. In this work, we have developed a comprehensive pore-network model for MICP, with geochemical speciation calculated by the open-source PHREEQC module. A numerical pseudo-3D micromodel as the computational domain was generated by a novel pore-network generation method. We modeled a three-stage process in the engineering of MICP including the growth of biofilm, the injection of calcium-rich medium, and the precipitation of calcium carbonate. A number of test cases were conducted to illustrate how calcite precipitation was influenced by different operating conditions. In addition, we studied the possibility of reducing the computational effort by simplifying geochemical calculations. Finally, the effect of mass transfer limitation of possible carbonate ions in a pore element on calcite precipitation was explored. This article is protected by copyright. All rights reserved.
- Monitoring groundwater storage changes in complex basement aquifers: An
evaluation of the GRACE satellites over East Africa
- Authors: J. Nanteza; C. R. de Linage, B.F. Thomas, J.S. Famiglietti
Abstract: Although the use of the Gravity Recovery and Climate Experiment (GRACE) satellites to monitor groundwater storage changes has become commonplace, our evaluation suggests that careful processing of the GRACE data is necessary to extract a representative signal especially in regions with significant surface water storage (i.e. lakes/reservoirs). In our study, we use cautiously processed datasets, including GRACE, lake altimetry and model soil moisture, to reduce scaling factor bias and compare GRACE-derived groundwater storage changes to in-situ groundwater observations over parts of East Africa. Over the period 2007-2010, a strong correlation between in-situ groundwater storage change and GRACE-groundwater estimates (Spearman's ρ = 0.6) is found. Piecewise trend analyses for the GRACE-groundwater estimates reveal significant negative storage changes that are attributed to groundwater use and climate variability. Further analysis comparing groundwater and satellite precipitation datasets permits identification of regional groundwater characterization. For example, our results identify potentially permeable and/or shallow groundwater systems underlying Tanzania and deep and/or less permeable groundwater systems underlying the Upper-Nile basin. Regional groundwater behaviors in the semi-arid regions of Northern Kenya are attributed to hydraulic connections to recharge zones outside the sub-basin boundary. Our results prove the utility of applying GRACE in monitoring groundwater resources in hydrologically complex regions that are under-sampled and where policies limit data accessibility. This article is protected by copyright. All rights reserved.
- Modeling the hydrological and mechanical effect of roots on shallow
- Authors: E. Arnone; D. Caracciolo, L. V. Noto, F. Preti, R. L. Bras
Abstract: This study proposes a new methodology for estimating the additional shear strength (or cohesion) exerted by vegetation roots on slope stability analysis within a coupled hydrological-stability model. The mechanical root cohesion is estimated within a Fiber Bundle Model framework that allows for the evaluation of the root strength as a function of stress-strain relationships of populations of fibers. The use of such model requires the knowledge of the root architecture. A branching topology model based on Leonardo's rule is developed, providing an estimation of the amount of roots and the distribution of diameters with depth. The proposed methodology has been implemented into an existing distributed hydrological-stability model able to simulate the dynamics of factor of safety as a function of soil moisture dynamics. The model also accounts for the hydrological effects of vegetation, which reduces soil water content via root water uptake, thus increasing the stability. The entire methodology has been tested in a synthetic hillslope with two configurations of vegetation type, i.e. trees and shrubs, which have been compared to a configuration without vegetation. The vegetation has been characterized using roots data of two mediterranean plant species. The results demonstrate the capabilities of the topological model in accurately reproducing the observed root structure of the analyzed species. For the environmental setting modelled, the effects of root uptake might be more significant than the mechanical reinforcement; the additional resistance depends strictly on the vegetation root depth. Finally, for the simulated climatic environment, landslides are seasonal, in agreement with past observations. This article is protected by copyright. All rights reserved.
- Experimental tests of truncated diffusion in fault damage zones
- Authors: Anna Suzuki; Toshiyuki Hashida, Kewen Li, Roland N. Horne
Abstract: Fault zones affect the flow paths of fluids in groundwater aquifers and geological reservoirs. Fault-related fracture damage decreases to background levels with increasing distance from the fault core according to a power law. This study investigated mass transport in such a fault-related structure using nonlocal models. A column flow experiment is conducted to create a permeability distribution that varies with distance from a main conduit. The experimental tracer response curve is preasymptotic and implies subdiffusive transport, which is slower than the normal Fickian diffusion. If the surrounding area is a finite domain, an upper truncated behavior in tracer response (i.e., exponential decline at late times) is observed. The tempered anomalous diffusion (TAD) model captures the transition from subdiffusive to Fickian transport, which is characterized by a smooth transition from power-law to an exponential decline in the late-time breakthrough curves. This article is protected by copyright. All rights reserved.
- Sediment replenishment: Influence of the geometrical configuration on the
morphological evolution of channel-bed
- Authors: E. Battisacco; M.J. Franca, A.J. Schleiss
Abstract: Dams trap sediment in the upstream reservoir, which may lead to river bed armoring, streambank erosion and failure, channel incision and reduction of the morphological diversity in the downstream river reaches. The replenishment of sediment is a mitigation measure for this problem applied in river reaches downstream of dams. Previously performed field experiments always used one single volume of sediment replenishment. To explore different alternatives, the replenished volume was here divided in four deposits with the motivation to influence also the morphological evolution downstream. Six different geometrical configurations together with three submergence conditions of sediment replenishment were tested for the first time in a laboratory experiment and are herein discussed. The results of the sediment replenishment mitigation technique are described in terms of occupied surface of the flume bed and the temporal evolution of erosion and transportation of introduced sediments. It is shown that, under our experimental conditions, complete submersion of the replenishment volume results in complete erosion of the placed sediment, with a high persistence of the added material along the channel length. The geometrical configuration of the replenishment volume plays a key role for the evolution of bed-forms downstream. Parallel configurations lead to a wider spread of material across the channel. Alternated configurations are suitable to produce sediment clustering and high persistence of placed material in the channel. Observed periodic mounds, considered as the initiating condition for alternate bars, follow a wavelength related to the length of the replenishment when the replenishment volumes are alternating. This article is protected by copyright. All rights reserved.
- Demasking the integrated information of discharge - Advancing sensitivity
analysis to consider different hydrological components and their rates of
- Authors: Bjoern Guse; Matthias Pfannerstill, Abror Gafurov, Nicola Fohrer, Hoshin Gupta
Abstract: Discharge as an integrated representation of all hydrological processes is the most common response variable used in sensitivity analyses. However, due to overlaying effects of all hydrological processes, the sensitivity signal of certain parameters to discharge can be masked. A more informative form of sensitivity analysis can be achieved by investigating how parameter sensitivities are related to individual modeled hydrological components. In our study, the TEDPAS (TEmporal Dynamics of PArameter Sensitivity) methodology is used to calculate daily sensitivities to modeled hydrological components and to detect temporal variations in dominant parameters. As a further enhancement to consider both magnitude and dynamics, temporal variations in parameter dominance are analyzed, both for magnitudes and rates of change of hydrological components. For this purpose, regime curves for parameter sensitivities are constructed. The results demonstrate that sensitivities of parameters increase when using the corresponding hydrological component instead of discharge as response variable. For each hydrological component, seasonal patterns of parameter dominance are detected using both magnitude and rate of change as response variable. Major differences are detected for certain capacity parameters, which are less pronounced using rates of change. Overall, we show that disentangling the diagnostic information hidden in the integrated signal of discharge can lead to a more informative signal regarding the sensitivity of hydrological components. Such advancements in sensitivity analysis can lead to a better understanding of how model parameters control the individual hydrological components in time. This article is protected by copyright. All rights reserved.
- Evolution and persistence of cross-directional statistical dependence
during finite-Péclet transport through a real porous medium
- Authors: Sebastian Most; Branko Bijeljic, Wolfgang Nowak
Abstract: Transport of passive, dissolved compounds in fully-saturated complex porous media frequently exhibits non-Fickian characteristics. One of the most interesting questions is to ascertain the time scales at which it is possible to describe transport as a statistically independent process. Therefore we study the mechanisms for evolution and then the decrease of non-Fickianity as a function of increasing time. Adopting the Lagrangian perspective, we provide a non-linear copula analysis of advective-diffusive processes by analyzing particle trajectories in a real porous media, as provided by direct numerical simulations on the three-dimensional image of Doddington sandstone. First, we analyze the memory effects between time-consecutive particle position increments and cross-dependence between longitudinal and transversal particle position increments as a function of given time increments and time lags between consecutive time increments. Second, we investigate the influence of the Péclet regime on the temporal evolution of dependence. Our main findings are: (a) Cross-dependence between longitudinal and transversal particle position increments is persistent over the investigated range of time increments, even though this aspect has been neglected up to date. (b) Lower Péclet numbers lead to a weaker dependence that is, however, more persistent over time than in higher-Péclet transport regimes. We confirm that non-Fickianity comes from spatial coherence associated with heterogeneities of the velocity field that introduce cross-dependence and memory into the transport process. Overall, we show that memory and cross-dependence are persistent in and among all directions, that the dependence is highly-nonlinear, occurs at different temporal scales, and is dependent on the Péclet number. This article is protected by copyright. All rights reserved.
- Combining snow, streamflow, and precipitation gauge observations to infer
- Authors: Brian Henn; Martyn P. Clark, Dmitri Kavetski, Bruce McGurk, Thomas H. Painter, Jessica D. Lundquist
Abstract: Precipitation data in mountain basins is typically sparse and subject to uncertainty due to difficulties in measurement and capturing spatial variability. Streamflow provides indirect information about basin-mean precipitation, but inferring precipitation from streamflow requires assumptions about hydrologic model structure that influence precipitation amounts. In this study, we test the extent to which using both snow and streamflow observations reduces differences in inferred annual total precipitation, compared to inference from streamflow alone. The case study area is the upper Tuolumne River basin in the Sierra Nevada of California, where distributed and basin-mean snow water equivalent (SWE) estimates have been made using LiDAR as part of the NASA Airborne Snow Observatory (ASO). To reconstruct basin-mean SWE for years prior to the ASO campaign, we test for a robust relationship between SWE estimates from ASO and from snow courses and pillows, which have a longer record. Relative to ASO's distributed SWE observations, point SWE measurements in this part of the Sierra Nevada tend to overestimate SWE at a given elevation, but under-sample high-elevation areas. We then infer precipitation from snow and streamflow, obtained from multiple hydrologic model structures. When included in precipitation inference, snow data reduce by up to one third the standard deviations of the water year total precipitation between model structures, and improve the consistency between structures in terms of the yearly variability in precipitation. We reiterate previous findings that multiple types of hydrologic data improve the consistency of modeled physical processes and help identify the most appropriate model structures. This article is protected by copyright. All rights reserved.
- Numerical simulation of transient groundwater age distributions assisting
land and water management in the Middle Wairarapa Valley, New Zealand
- Authors: MichaelW. Toews; Christopher J. Daughney, Fabien J. Cornaton, Uwe Morgenstern, Ryan D. Evison, Bethanna M. Jackson, Karine Petrus, Doug Mzila
Abstract: This study used numerical models to simulate transient groundwater age distributions using a time-marching Laplace transform Galerkin (TMLTG) technique. First, the TMLTG technique was applied to simple box models configured to match idealized lumped parameter models (LPMs). Even for simple box models, time-varying recharge can generate groundwater age distributions with highly irregular shapes that vary over time in response to individual recharge events. Notably, the transient numerical simulations showed that the breakthrough and mean ages are younger than in the steady flow case, and that this difference is greater for sporadic recharge time series than for more regular recharge time series. Second, the TMLTG technique was applied to a transient numerical model of the 270 km2 Middle Wairarapa Valley, New Zealand. To our knowledge this study is the first application of the TMLTG technique to a real-world example, made possible by the dataset of tritium measurements that exists for the Wairarapa Valley. Results from a transient mean age simulation shows variation from a few days to over a decade in either temporal or spatial dimensions. Temporal variations of mean age are dependent on seasonal climate and groundwater abstraction. Results also demonstrated important differences between the transient age distributions derived from the TMLTG technique compared to the much simpler steady-state LPMs that are frequently applied to interpret age tracer data. Finally, results had direct application to land and water management, for example for identification of land areas where age distributions vary seasonally, affecting the security of groundwater supplies used for drinking water. This article is protected by copyright. All rights reserved.
- Plausibility of freshwater lenses adjacent to gaining rivers: Validation
by laboratory experimentation
- Authors: A.D. Werner; A. Kawachi, T. Laattoe
Abstract: The occurrence of freshwater lenses in saline aquifers adjoining gaining rivers has recently been demonstrated as being theoretically possible by way of analytical solution. However, physical evidence for freshwater lenses near gaining rivers is limited largely to airborne geophysical surveys. This paper presents the first direct observations of freshwater lenses adjacent to gaining rivers, albeit at the laboratory scale, as validation of their plausibility. The experimental conditions are consistent with the available analytical solution, which is compared with laboratory observations of lens extent and the saltwater flow rate, for various hydraulic gradients. Numerical simulation shows that dispersion can account for the small amount of mismatch between the sharp-interface analytical solution and laboratory measurements. Calibration and uncertainty analysis demonstrate that accurate mathematical predictions require calibration to laboratory measurements of the lens. The results provide unequivocal proof that freshwater lenses can persist despite gaining river conditions concordant with theoretical lenses predicted by the analytical solution, at least within the constraints of the experimental setup. This article is protected by copyright. All rights reserved.
- Rescaling the complementary relationship for land surface evaporation
- Authors: R. Crago; J. Szilagyi, R. Qualls, J. Huntington
Abstract: Recent research into the complementary relationship (CR) between actual and apparent potential evaporation has resulted in numerous alternative forms for the CR. Inspired by Brutsaert , who derived a general CR in the form y=function(x), where x is the ratio of potential evaporation to apparent potential evaporation and y is the ratio of actual to apparent potential evaporation, an equation is proposed to calculate the value of x at which y goes to zero, denoted xmin. The value of xmin varies even at an individual observation site, but can be calculated using only the data required for the Penman (1948) equation as expressed here, so no calibration of xmin is required. It is shown that the scatter in x-y plots using experimental data is reduced when x is replaced by X=(x-xmin)/(1-xmin). This rescaling results in data falling along the line y=X, which is proposed as a new version of the CR. While a reinterpretation of the fundamental boundary conditions proposed by Brutsaert  is required, the physical constraints behind them are still met. An alternative formulation relating y to X is also discussed. This article is protected by copyright. All rights reserved.
- Locally conservative groundwater flow in the continuous Galerkin method
using 3-D prismatic patches
- Authors: Qiang Wu; Yingwang Zhao, Yu-Feng F. Lin, Hua Xu
Abstract: A new procedure has been developed to improve the velocity field computed by the continuous Galerkin finite element method (CG). It enables extending the postprocessing algorithm proposed by Cordes and Kinzelbach  to three-dimensional (3-D) models by using prismatic patches for saturated groundwater flow. This approach leverages a dual mesh to preserve local mass conservation and provides interpolated velocities based on consistent fluxes. To develop this 3-D approach, a triangular conservative patch is introduced by computing not only advection fluxes, but also vertical infiltrations, storage changes, and other sink or source terms. This triangular patch is then used to develop a prismatic patch, which consists of subprisms in two layers. By dividing a single two-layer patch into two separate one-layer patches, two dimensional (2-D) algorithms can be applied to compute velocities. As a consequence, each subelement is able to preserve local mass conservation. A hypothetical 3-D model is used to evaluate the precision of streamlines and flow rates generated by this approach and the FEFLOW simulation program. This article is protected by copyright. All rights reserved.
- Kinetics of gravity-driven slug flow in partially wettable capillaries of
varying cross section
- Authors: Alon Nissan; Qiuling Wang, Rony Wallach
Abstract: A mathematical model for slug (finite liquid volume) motion in not-fully-wettable capillary tubes with sinusoidally varying cross-sectional areas was developed. The model, based on the Navier–Stokes equation, accounts for the full viscous terms due to non-uniform geometry, the inertial term, the slug's front and rear meniscus hysteresis effect, and dependence of contact angle on flow velocity (dynamic contact angle). The model includes a velocity-dependent film that is left behind the advancing slug, reducing its mass. The model was successfully verified experimentally by recording slug movement in uniform and sinusoidal capillary tubes with a gray-scale high-speed camera. Simulation showed that tube non-uniformity has a substantial effect on slug flow pattern: in a uniform tube it is monotonic and depends mainly on the slug's momentary mass/length; an undulating tube radius results in non-monotonic flow characteristics. The static non-zero contact angle varies locally in non-uniform tubes owing to the additional effect of wall slope. Moreover, the non-uniform cross-sectional area induces slug acceleration, deceleration, blockage and metastable-equilibrium locations. Increasing contact angle further amplifies the geometry effect on slug propagation. The developed model provides a modified means of emulating slug flow in differently wettable porous media for intermittent inlet water supply (e.g. raindrops on the soil surface). This article is protected by copyright. All rights reserved.
- The influence of an in-network lake on the timing, form, and magnitude of
downstream dissolved carbon and nutrient flux
- Authors: Alexey Kalinin; Tim Covino, Brian McGlynn
Abstract: Within fluvial networks, lakes can be sinks or sources of dissolved organic carbon (DOC) and nutrients, yet the controls over sink-source behavior remain unclear. We investigated the influence that an in-network lake exerted on DOC and nutrient export. Our investigation consisted of: 1) injecting a conservative tracer to determine lake travel times and flow paths; 2) sampling lake inflow, outflow, and surrounding groundwater to determine water and nutrient budgets; and, 3) sampling internal lake profiles to ascertain in-lake physico-chemical patterns through time. Conservative tracer data indicated considerable in-lake retention and combined with inflow-outflow discharge measurements revealed a decoupling of kinematic and solute pulses. Nitrate (NO3) was the dominant form of dissolved inorganic nitrogen (DIN) at lake inflow whereas ammonium (NH4) became the dominant component at lake outflow. The lake was a sink for NO3-N and PO4, but a source for NH4-N, DON, TDN, and DOC. We observed hydrologic controls on DOC concentrations and export patterns, but redox controls on DIN dynamics. Our results indicate that lakes within fluvial networks can be sources of dissolved organic material and reduced nitrogen (NH4) while simultaneously being sinks for NO3-N and PO4-P. Determining controls on sink-source behavior and the cumulative effect of lakes on DOC and nutrient budgets is a necessary first step toward improved understanding of the role of lakes in network- to regional-scale dynamics. This article is protected by copyright. All rights reserved.
- Evolution of wet- and dry-day frequency in the western Amazon basin:
Relationship with atmospheric circulation and impacts on vegetation
- Authors: Jhan Carlo Espinoza; Hans Segura, Josyane Ronchail, Guillaume Drapeau, Omar Gutierrez-Cori
Abstract: This paper documents the spatio-temporal evolution of wet- and dry-day frequency (WDF and DDF) in the western Amazon, its relationships with oceanic and atmospheric variability and possible impact on vegetation. WDF and DDF changed significantly during the 1980-2009 period (p
- Nonstationary decision model for flood risk decision scaling
- Authors: Caitlin M. Spence; Casey M. Brown
Abstract: Hydroclimatic stationarity is increasingly questioned as a default assumption in flood risk management (FRM), but successor methods are not yet established. Some potential successors depend on estimates of future flood quantiles, but methods for estimating future design storms are subject to high levels of uncertainty. Here we apply a Nonstationary Decision Model (NDM) to flood risk planning within the decision scaling framework. The NDM combines a nonstationary probability distribution of annual peak flow with optimal selection of flood management alternatives using robustness measures. The NDM incorporates structural and nonstructural FRM interventions and valuation of flows supporting ecosystem services to calculate expected cost of a given FRM strategy. A search for the minimum-cost strategy under incrementally varied representative scenarios extending across the plausible range of flood trend and value of the natural flow regime discovers candidate FRM strategies that are evaluated and compared through a decision scaling analysis (DSA). The DSA selects a management strategy that is optimal or close to optimal across the broadest range of scenarios or across the set of scenarios deemed most likely to occur according to estimates of future flood hazard. We illustrate the decision framework using a stylized example flood management decision based on the Iowa City flood management system, which has experienced recent unprecedented high flow episodes. The decision scaling analysis indicates a preference for combining infrastructural and non-structural adaptation measures to manage flood risk and makes clear that options-based approaches cannot be assumed to be “no” or “low regret.” This article is protected by copyright. All rights reserved.
- Transferability of hydrological models and ensemble averaging methods
between contrasting climatic periods
- Authors: Ciaran Broderick; Tom Matthews, Robert L. Wilby, Satish Bastola, Conor Murphy
Abstract: Understanding hydrological model predictive capabilities under contrasting climate conditions enables more robust decision making. Using Differential Split Sample Testing (DSST) we analyse the performance of six hydrological models for 37 Irish catchments under climate conditions unlike those used for model training. Additionally, we consider four ensemble averaging techniques when examining inter-period transferability. DSST is conducted using two/three-year non-continuous blocks of (i) the wettest/driest years on record based on precipitation totals, and (ii) years with a more/less pronounced seasonal precipitation regime. Model transferability between contrasting regimes was found to vary depending on the testing scenario, catchment and evaluation criteria considered. As expected, the ensemble average outperformed most individual ensemble members. However, averaging techniques differed considerably in the number of times they surpassed the best individual model-member. Bayesian Model Averaging (BMA) and the Granger-Ramanathan (GRA) method were found to outperform the simple arithmetic mean (SAM) and Akaike Information Criteria Averaging (AICA). Here, GRA performed better than the best individual model in 51% to 86% of cases (according to the Nash-Sutcliffe criterion). When assessing model predictive skill under climate change conditions we recommend (i) setting up DSST to select the best available analogues of expected annual mean and seasonal climate conditions; (ii) applying multiple performance criteria; (iii) testing transferability using a diverse set of catchments and; (iv) using a multi-model ensemble in conjunction with an appropriate averaging technique. Given the computational efficiency and performance of GRA relative to BMA, the former is recommended as the preferred ensemble averaging technique for climate assessment. This article is protected by copyright. All rights reserved.
- Numerical homogenization of the Richards equation for unsaturated water
flow through heterogeneous soils
- Authors: Na Li; Xingye Yue, Li Ren
Abstract: Homogenized equations and the corresponding effective constitutive relations are generally necessary for numerically modeling large-scale unsaturated flow processes in soils. Recently, based on the Kirchhoff transformation and the two-scale convergence theory, a homogenization method for the Richards equation with the Mualem-van Genuchten model has been proposed, with a constant model parameter α relating to the inverse of the air-entry pressure and the soil pore size distribution. The homogenized model is computationally efficient and convenient to use because of its explicit expression. In this study, we generalize this method, allowing α to be a spatially distributed random field and proposing a homogenized Richards equation in the mixed form (θ/h) under the condition that the effective hydraulic conductivity tensor is diagonal. This generalization eliminates the limitation of a constant α in practical applications; the proposed homogenized model is meaningful in most situations because the flow problems are influenced mainly by the diagonal terms of conductivity and the off-diagonal terms are often neglected. Two-dimensional numerical tests are conducted in soil profiles with different degrees of spatial heterogeneity structure to illustrate that the homogenized model can capture the fine-scale flow behaviors on coarse grids effectively. Homogenization for the Richards equation with other two commonly used constitutive relations - the Brooks-Corey model and the Gardner-Russo model - is also illustrated in this study. This article is protected by copyright. All rights reserved.
- Validating reconstruction of snow water equivalent in California's Sierra
Nevada using measurements from the NASA Airborne Snow Observatory
- Authors: Edward H. Bair; Karl Rittger, Robert E. Davis, Thomas H. Painter, Jeff Dozier
Abstract: Accurately estimating basin-wide snow water equivalent (SWE) is the most important unsolved problem in mountain hydrology. Models that rely on remotely sensed inputs are especially needed in ranges with few surface measurements. The NASA Airborne Snow Observatory (ASO) provides estimates of SWE at 50-meter spatial resolution in several basins across the Western US during the melt season. Primarily, water managers use this information to forecast snowmelt runoff into reservoirs; another impactful use of ASO measurements lies in validating and improving satellite-based snow estimates or models that can scale to whole mountain ranges, even those without ground-based measurements. We compare ASO measurements from 2013 to 2015 to four methods that estimate spatially distributed SWE: two versions of a SWE reconstruction method, spatial interpolation from snow pillows and courses, and NOAA's Snow Data Assimilation System (SNODAS). SWE reconstruction downscales energy forcings to compute potential melt, then multiplies those values by satellite-derived estimates of fractional snow-covered area to calculate snowmelt. The snowpack is then built in reverse from the date the snow is observed to disappear. The two SWE reconstruction models tested include one that employs an energy balance calculation of snowmelt, and one that combines net radiation and degree-day approaches to estimate melt. Our full energy balance model, without ground observations, performed slightly better than spatial interpolation from snow pillows, having no systematic bias and 26% mean absolute error when compared to SWE from ASO. Both reconstruction models and interpolation were more accurate than SNODAS. This article is protected by copyright. All rights reserved.
- Connectivity, permeability and channeling in randomly-distributed and
kinematically-defined discrete fracture network models
- Authors: J. Maillot; P. Davy, R. Le Goc, C. Darcel, J.R. de Dreuzy
Abstract: A major use of DFN models for industrial applications is to evaluate permeability and flow structure in hardrock aquifers from geological observations of fracture networks. The relationship between the statistical fracture density distributions and permeability has been extensively studied, but there has been little interest in the spatial structure of DFN models, which is generally assumed to be spatially random (i.e. Poisson). In this paper, we compare the predictions of Poisson DFNs to new DFN models where fractures result from a growth process defined by simplified kinematic rules for nucleation, growth and fracture arrest (Davy et al, 2010, 2013). This so-called ‘kinematic fracture model' is characterized by a large proportion of T-intersections, and a smaller number of intersections per fracture. Several kinematic models were tested and compared with Poisson DFN models with the same density, length and orientation distributions. Connectivity, permeability and flow distribution were calculated for 3D networks with a self-similar power-law fracture length distribution. For the same statistical properties in orientation and density, the permeability is systematically and significantly smaller by a factor of 1.5 to 10 for kinematic than for Poisson models. In both cases, the permeability is well described by a linear relationship with the areal density p32, but the threshold of kinematic models is 50% larger than of Poisson models. Flow channeling is also enhanced in kinematic DFN models. This analysis demonstrates the importance of choosing an appropriate DFN organization for predicting flow properties from fracture network parameters. This article is protected by copyright. All rights reserved.
- Variation of organic matter quantity and quality in streams at critical
zone observatory watersheds
- Authors: Matthew P. Miller; Elizabeth W. Boyer, Diane M. McKnight, Michael G. Brown, Rachel S. Gabor, Carolyn T. Hunsaker, Lidiia Iavorivska, Shreeram Inamdar, Dale W. Johnson, Louis A. Kaplan, Henry Lin, William H. McDowell, Julia N. Perdrial
Abstract: The quantity and chemical composition of dissolved organic matter (DOM) in surface waters influence ecosystem processes and anthropogenic use of freshwater. However, despite the importance of understanding spatial and temporal patterns in DOM, measures of DOM quality are not routinely included as part of large scale ecosystem monitoring programs and variations in analytical procedures can introduce artifacts. In this study, we used consistent sampling and analytical methods to meet the objective of defining variability in DOM quantity and quality and other measures of water quality in streamflow issuing from small forested watersheds located within five Critical Zone Observatory sites representing contrasting environmental conditions. Results show distinct separations among sites as a function of water quality constituents. Relationships among rates of atmospheric deposition, water quality conditions, and stream DOM quantity and quality are consistent with the notion that areas with relatively high rates of atmospheric nitrogen and sulfur deposition and high concentrations of divalent cations result in selective transport of DOM derived from microbial sources, including in-stream microbial phototrophs. We suggest that the critical zone as a whole strongly influences the origin, composition, and fate of DOM in streams. This study highlights the value of consistent DOM characterization methods included as part of long-term monitoring programs for improving our understanding of interactions among ecosystem processes as controls on DOM biogeochemistry. This article is protected by copyright. All rights reserved.
- Diel discharge cycles explained through viscosity fluctuations in riparian
- Authors: Michael Schwab; Julian Klaus, Laurent Pfister, Markus Weiler
Abstract: Diel (also called diurnal) discharge patterns with minima in the afternoon are generally explained by the daily cycle of evapotranspiration, while maxima in the afternoon are often linked to freeze-thaw cycles. In a schistose and forested headwater catchment in Luxembourg, we observed daily discharge maxima in the afternoon, although temperatures remained persistently above zero and vegetation was still in a dormant state. We show that diel water temperature fluctuations - and therefore viscosity fluctuations - in the upper layer of the riparian zone can be an explanation for the observed daily discharge maxima in the afternoon during the dormant season. In the transition period between the dormant and the growing season, the counteracting viscosity and evapotranspiration processes cancel each other out. Subsequently, during the growing season, evapotranspiration is the dominant process guiding the diel discharge pattern; nevertheless, the viscosity effect might still be present, but invisible. We believe this finding also to be of relevance when analyzing daily fluctuations of biogeochemicals in stream water. This article is protected by copyright. All rights reserved.
- Global estimation of effective plant rooting depth: Implications for
- Authors: Yuting Yang; Randall J. Donohue, Tim R. McVicar
Abstract: Plant rooting depth (Zr) is a key parameter in hydrological and biogeochemical models, yet the global spatial distribution of Zr is largely unknown due to the difficulties in its direct measurement. Additionally, Zr observations are usually only representative of a single plant or several plants, which can differ greatly from the effective Zr over a modelling unit (e.g., catchment or grid-box). Here, we provide a global parameterization of an analytical Zr model that balances the marginal carbon cost and benefit of deeper roots, and produce a climatological (i.e., 1982-2010 average) global Zr map. To test the Zr estimates, we apply the estimated Zr in a highly transparent hydrological model (i.e., the Budyko-Choudhury-Porporato (BCP) model) to estimate mean annual actual evapotranspiration (E) across the globe. We then compare the estimated E with both water balance-based E observations at 32 major catchments and satellite grid-box retrievals across the globe. Our results show that the BCP model, when implemented with Zr estimated herein, optimally reproduced the spatial pattern of E at both scales (i.e., R2=0.94, RMSD=74 mm yr−1 for catchments, and R2=0.90, RMSD=125 mm yr−1 for grid-boxes) and provides improved model outputs when compared to BCP model results from two already existing global Zr datasets. These results suggest that our Zr estimates can be effectively used in state-of-the-art hydrological models, and potentially biogeochemical models, where the determination of Zr currently largely relies on biome type-based look-up tables. This article is protected by copyright. All rights reserved.
- A hydro-economic modelling framework for optimal integrated management of
forest and water
- Authors: Alberto García-Prats; Antonio D. del Campo, Manuel Pulido-Velazquez
Abstract: Forests play a determinant role in the hydrologic cycle, with water being the most important ecosystem service they provide in semiarid regions. However, this contribution is usually neither quantified nor explicitly valued. The aim of this study is to develop a novel hydro-economic modelling framework for assessing and designing the optimal integrated forest and water management for forested catchments. The optimization model explicitly integrates changes in water yield in the stands (increase in groundwater recharge) induced by forest management, and the value of the additional water provided to the system. The model determines the optimal schedule of silvicultural interventions in the stands of the catchment in order to maximize the total net benefit in the system. Canopy cover and biomass evolution over time were simulated using growth and yield allometric equations specific for the species in Mediterranean conditions. Silvicultural operation costs according to stand density and canopy cover were modelled using local cost databases. Groundwater recharge was simulated using HYDRUS, calibrated and validated with data from the experimental plots. In order to illustrate the presented modelling framework a case study was carried out in a planted pine forest (Pinus halepensis Mill.) located in south-western Valencia province (Spain).The optimized scenario increased groundwater recharge. This novel modelling framework can be used in the design of a “payment for environmental services” scheme in which water beneficiaries could contribute to fund and promote efficient forest management operations. This article is protected by copyright. All rights reserved.
- Drainage mechanisms in porous media: From piston-like invasion to
formation of corner flow networks
- Authors: Frouke Hoogland; Peter Lehmann, Rajmund Mokso, Dani Or
Abstract: Water drainage from porous media is a highly dynamic process often marked by rapid piston-like air invasion events at the front and other rapid interfacial reconfigurations. Liquid phase entrapped behind the moving front drains at significantly slower rates often via gravity driven flow through corners and crevices. This distribution of slowly draining residual water phase determines the plant available water and biological functioning of soils. The study aims to determine the conditions for the flow regime transition from piston-like invasion at a drainage front to slower corner and film-dominated flow at the pore and sample scale. This transition was observed experimentally for sand and glass beads with fast X-ray tomography, revealing water fragmentation into clusters of full pores interconnected by water in films and corners. The observed liquid morphology at the transition from piston to corner flow was reproduced by a quasi-static pore network model and predicted by percolation theory. The amount of capillary-retained water at flow transition controlling the subsequent drainage dynamics could be reproduced by an idealized star shaped pore whose geometry is deduced from macroscopic properties of the porous medium. Predictions of water content thresholds at flow transitions were in agreement with other critical saturation values associated with cessation of solute diffusion and of internal drainage (at field capacity) highlighting the criticality of water phase continuity disruption for formation of relatively stable unsaturated conditions controlled by slow corner flow that support life in soil. This article is protected by copyright. All rights reserved.
- Drainage dynamics controlled by corner flow: Application of the foam
- Authors: Frouke Hoogland; Peter Lehmann, Dani Or
Abstract: In fast drainage processes water is retained behind the front, defining the plant available water and hydraulic properties of the unsaturated region. In this study we show that the foam drainage equation (FDE) can be applied to predict macroscopic drainage dynamics behind the front because a network of liquid channels controls the liquid flow in both foams and crevices of the pore space. To predict drainage rates at the Darcy scale the FDE is solved numerically after adapting channel geometries and boundary conditions to experimental conditions. The FDE results were in good agreement with measured flow rates behind a drainage front in coarse and fine sand. A notable exception was rapid drainage from fine sand where saturated pore clusters persisted after front passage and drained faster compared to FDE predictions. The dominance of corner capillary flows implied by the good agreement with the FDE formulation could improve the scientific underpinning of the unsaturated hydraulic conductivity function and offers a more realistic view of the geometry of pathways for colloid and pathogen transport in unsaturated media. This article is protected by copyright. All rights reserved.
- On the value of surface saturated area dynamics mapped with thermal
infrared imagery for modeling the hillslope-riparian-stream continuum
- Authors: B. Glaser; J. Klaus, S. Frei, J. Frentress, L. Pfister, L. Hopp
Abstract: The highly dynamic processes within a hillslope-riparian-stream (HRS) continuum are known to affect streamflow generation, but are yet not fully understood. Within this study, we simulated a headwater HRS continuum in western Luxembourg with an integrated hydrologic surface subsurface model (HydroGeoSphere). The model was set up with thorough consideration of catchment-specific attributes and we performed a multi criteria model evaluation (4 years) with special focus on the temporally varying spatial patterns of surface saturation. We used a portable thermal infrared (TIR) camera to map surface saturation with a high spatial resolution and collected 20 panoramic snapshots of the riparian zone (approx. 10 m x 20 m) under different hydrologic conditions. Qualitative and quantitative comparison of the processed TIR panoramas and the corresponding model output panoramas revealed a good agreement between spatiotemporal dynamic model and field surface saturation patterns. A double logarithmic linear relationship between surface saturation extent and discharge was similar for modeled and observed data. This provided confidence in the capability of an integrated hydrologic surface subsurface model to represent temporal and spatial water flux dynamics at small (HRS continuum) scales. However, model scenarios with different parameterizations of the riparian zone showed that discharge and surface saturation were controlled by different parameters and hardly influenced each other. Surface saturation only affected very fast runoff responses with a small volumetric contribution to stream discharge, indicating that the dynamic surface saturation in the riparian zone does not necessarily imply a major control on runoff generation. This article is protected by copyright. All rights reserved.
- Reliable long-range ensemble streamflow forecasts by combining dynamical
climate forecasts: A conceptual runoff model and a staged error model
- Authors: James C. Bennett; QJ Wang, Ming Li, David Robertson, Andrew Schepen
Abstract: We present a new streamflow forecasting system called forecast guided stochastic scenarios (FoGSS). FoGSS makes use of ensemble seasonal precipitation forecasts from a coupled ocean-atmosphere general circulation model (CGCM). The CGCM forecasts are post-processed with the method of calibration, bridging and merging (CBaM) to produce ensemble precipitation forecasts over river catchments. CBaM corrects biases and removes noise from the CGCM forecasts, and produces highly reliable ensemble precipitation forecasts. The post-processed CGCM forecasts are used to force the Wapaba monthly rainfall-runoff model. Uncertainty in the hydrological modelling is accounted for with a 3-stage error model. Stage 1 applies the log-sinh transformation to normalize residuals and homogenize their variance; Stage 2 applies a conditional bias-correction to correct biases and help remove negative forecast skill; Stage 3 applies an autoregressive model to improve forecast accuracy at short lead-times and propagate uncertainty through the forecast. FoGSS generates ensemble forecasts in the form of time series for the coming 12-months.In a case study of two catchments, FoGSS produces reliable forecasts at all lead-times. Forecast skill with respect to climatology is evident to lead-times of about 3 months. At longer lead-times, forecast skill approximates that of climatology forecasts; that is, forecasts become like stochastic scenarios. Because forecast skill is virtually never negative at long lead-times, forecasts of accumulated volumes can be skillful. Forecasts of accumulated 12-month streamflow volumes are significantly skillful in several instances, and ensembles of accumulated volumes are reliable. We conclude that FoGSS forecasts could be highly useful to water managers. This article is protected by copyright. All rights reserved.
- The role of water treatment abstraction in the flux and greenhouse gas
emissions from organic carbon and nitrogen within UK rivers
- Authors: Finlay N.C; Johnson K, F. Worrall
Abstract: The fate of organic matter through watersheds has been shown to be an important component of the global carbon cycle and processes in rivers can rapidly transfer carbon from the terrestrial biosphere to the atmosphere. However, the role of water abstraction in diverting organic matter from freshwater has not been considered. This study used two methods to estimate the amount of organic carbon removed by water treatment processes, firstly, by estimating the amount of carbon that has to be removed given the abstracted volumes and the freshwater composition; and, secondly, estimated from reports of the production and composition of water treatment residuals from water companies. For the UK, the median total organic carbon removed by water abstraction was 46 ktonnes C/yr, this equates to a median per capita value of 0.76 kg C/ca/yr. The median total organic nitrogen removed was 4.0 ktonnes N/yr, equivalent to 0.07 kg N/ca/yr. The removal of TOC by water abstraction represents 1.5% of the total removal rate across UK watersheds. The release of greenhouse gases from UK rivers is now estimated to be between 12754 – 32332 ktonnes CO2eq/yr equivalent to between 55 and 127 tonnes CO2eq/km2/yr with fluvial organic matter between 8800 and 15116 ktonnes CO2eq/yr in the proportion 6:86:8 N2O:CO2:CH4. The emissions factor for 1 tonne of organic carbon entering the UK fluvial network has a median value of 2.95 tonnes CO2eq/yr with a 5th to 95th percentile range of 2.55 to 3.59 tonnes CO2eq/yr. Globally, a per capita values for countries with municipal treated water supply would be 0.8 to 0.86 kg C/ca/yr. This article is protected by copyright. All rights reserved.
- Irrigation water policy analysis using a business simulation game
- Authors: M. Buchholz; G. Holst, O. Musshoff
Abstract: Despite numerous studies on farmers' responses to changing irrigation water policies, uncertainties remain about the potential of water pricing schemes and water quotas to reduce irrigation. Thus far, policy impact analysis is predominantly based upon rational choice models that assume behavioral assumptions, such as a perfectly rational profit-maximizing decision maker. Also, econometric techniques are applied which could lack internal validity due to uncontrolled field data. Furthermore, such techniques are not capable of identifying ill-designed policies prior to their implementation. With this in mind, we apply a business simulation game for ex ante policy impact analysis of irrigation water policies at the farm level. Our approach has the potential to reveal the policy-induced behavioral change of the participants in a controlled environment. To do so, we investigate how real farmers from Germany, in an economic experiment, respond to a water pricing scheme and a water quota intending to reduce irrigation. In the business simulation game, the participants manage a ‘virtual' cash-crop farm for which they make crop allocation and irrigation decisions during several production periods, while facing uncertain product prices and weather conditions. The results reveal that a water quota is able to reduce mean irrigation applications, while a water pricing scheme does not have an impact, even though both policies exhibit equal income effects for the farmers. However, both policies appear to increase the variation of irrigation applications. Compared to a perfectly rational profit-maximizing decision maker, the participants apply less irrigation on average, both when irrigation is not restricted and when a water pricing scheme applies. Moreover, the participants' risk attitude affects the irrigation decisions. This article is protected by copyright. All rights reserved.
- Spatial downscaling of precipitation using adaptable random forests
- Authors: Xiaogang He; Nathaniel W. Chaney, Marc Schleiss, Justin Sheffield
Abstract: This paper introduces Prec-DWARF (Precipitation Downscaling With Adaptable Random Forests), a novel machine-learning based method for statistical downscaling of precipitation. Prec-DWARF sets up a nonlinear relationship between precipitation at fine resolution and covariates at coarse/fine resolution, based on the advanced binary tree method known as Random Forests (RF). In addition to a single RF, we also consider a more advanced implementation based on two independent RFs which yield better results for extreme precipitation. Hourly gauge-radar precipitation data at 0.125° from NLDAS-2 are used to conduct synthetic experiments with different spatial resolutions (0.25°, 0.5° and 1°). Quantitative evaluation of these experiments demonstrates that Prec-DWARF consistently outperforms the baseline (i.e., bi-linear interpolation in this case) and can reasonably reproduce the spatial and temporal patterns, occurrence and distribution of observed precipitation fields. However, Prec-DWARF with a single RF significantly underestimates precipitation extremes and often cannot correctly recover the fine-scale spatial structure, especially for the 1° experiments. Prec-DWARF with a double RF exhibits improvement in the simulation of extreme precipitation as well as its spatial and temporal structures, but variogram analyses show that the spatial and temporal variability of the downscaled fields are still strongly underestimated. Covariate feature importance analysis shows that the most important predictors for the downscaling are the coarse scale precipitation values over adjacent grid cells as well as the distance to the closest dry grid cell (i.e., the dry drift). The encouraging results demonstrate the potential of Prec-DWARF and machine-learning based techniques in general for the statistical downscaling of precipitation. This article is protected by copyright. All rights reserved.
- Colloids and Organic Matter Complexation Control Trace Metal
Concentration‐Discharge Relationships in Marshall Gulch Stream Waters
- Authors: Kyle D. Trostle; J. Ray Runyon, Michael A. Pohlmann, Shelby E. Redfield, Jon Pelletier, Jennifer McIntosh, Jon Chorover
Abstract: This study combined concentration‐discharge analyses (filtration at 0.45 μm), cascade filtrations (at 1.2 μm, 0.4 μm, 0.025 μm) and asymmetrical flow field flow fractionation (AF4) to probe the influence of colloidal carriers (dissolved organic matter and inorganic nanoparticles) on observed concentration‐discharge relationships for trace metals in a 155 hectare forested catchment of the Santa Catalina Mountains Critical Zone Observatory (SCM CZO), Arizona, USA. Many major elements (Na, Mg, Si, K, Ca) show no colloidal influence, and concentration‐discharge relationships for these species are explained by previous work. However, the majority of trace metals (Al, Ti, V, Mn, Fe, Cu, Y, REE, U) show at least some influence of colloids on chemistry when filtered at the standard 0.45 μm cutoff. Concentration‐discharge slopes of trace metals with modest colloidal influence are shallow (∼0.3) similar to that measured for dissolved organic carbon (DOC, 0.24), whereas elements with greater colloidal influence have steeper concentration‐discharge slopes approaching that of Al (0.76), the element with the largest colloidal influence in this study (on average 68%). These findings are further supported by AF4 measurements that show distinct and resolvable pools of low hydrodynamic diameter DOC‐sized material coexistent with larger diameter inorganic colloids, and the ratio of these carriers changes systematically with discharge because the DOC pool has a concentration‐discharge relationship with shallower slope than the inorganic colloidal pool. Together these datasets illustrate that positive concentration‐discharge slopes of trace metals in stream waters may be explained as the relative partitioning of trace metals between DOC and inorganic colloids, with contributions of the latter likely increasing as a result of increased prevalence of macropore flow. This article is protected by copyright. All rights reserved.
- Cumulative Relative Reactivity: A Concept for Modeling Aquifer‐Scale
- Authors: Matthias Loschko; Thomas Wöhling, David L. Rudolph, Olaf A. Cirpka
Abstract: We simulate aquifer‐scale reactive transport using an approach based on travel times and relative reactivity. The latter quantifies the intensity of the chemical reaction relative to a reference reaction rate with identical concentrations and can be interpreted as the strength of electron‐donor (or electron‐acceptor) release by the matrix, scaled by a reference release. In general, the relative reactivity is a spatially variable property reflecting the geology of the formation. In the proposed approach, we track the path of individual water parcels through the aquifer and evaluate the age of the water parcels and the relative reactivity integrated along their trajectories. By switching from spatial discretization to cumulative relative reactivity, advective‐reactive transport can be simulated by solving a single system of ordinary differential equations for each combination of concentrations in the inflow. We test the validity of the approach in a two‐dimensional test case of steady‐state groundwater flow and reactive transport involving aerobic respiration and denitrification. Here we compare steady‐state concentration distributions of the spatially explicit virtual truth, accounting for dispersive mixing, with the approximation based on cumulative relative reactivity and show that the errors introduced by neglecting dispersive mixing are minor if the target quantities are the mass fluxes crossing a control plane or being collected by a well. We further demonstrate the efficiency of the approach in a synthetic three‐dimensional case study. The proposed approach is computationally so efficient, that ensemble runs to assess statistical distributions of concentration time series of reactive solutes become feasible, which is not practical with a spatially explicit model. This article is protected by copyright. All rights reserved.
- Flow reconstructions in the Upper Missouri River Basin using riparian tree
- Authors: Derek M. Schook; Jonathan M. Friedman, Sara L. Rathburn
Abstract: River flow reconstructions are typically developed using tree rings from montane conifers that cannot reflect flow regulation or hydrologic inputs from the lower portions of a watershed. Incorporating lowland riparian trees may improve the accuracy of flow reconstructions when these trees are physically linked to the alluvial water table. We used riparian plains cottonwoods (Populus deltoides, ssp. monilifera) to reconstruct discharge for three neighboring rivers in the Upper Missouri River Basin: the Yellowstone (n = 389 tree cores), Powder (n = 408), and Little Missouri Rivers (n = 643). We used the Regional Curve Standardization approach to reconstruct log‐transformed discharge over the four months in early summer that most highly correlated to tree ring growth. The reconstructions explained at least 57% of the variance in historical discharge and extended back to 1742, 1729 and 1643. These are the first flow reconstructions for the Lower Yellowstone and Powder Rivers, and they are the furthest downstream among Rocky Mountain rivers in the Missouri River Basin. Although mostly free‐flowing, the Yellowstone and Powder Rivers experienced a shift from early‐ to late‐summer flows within the last century. This shift is concurrent with increasing irrigation and reservoir storage, and it corresponds to decreased cottonwood growth. Low‐frequency flow patterns revealed wet conditions from 1870‐1980, a period that includes the majority of the historical record. The 1816‐1823 and 1861‐1865 droughts were more severe than any recorded, revealing that drought risks are underestimated when using the instrumental record alone. This article is protected by copyright. All rights reserved.
- Importance of soil heating, liquid water loss, and vapor flow enhancement
- Authors: Michael D. Novak
Abstract: Field measurements conducted by Cahill and Parlange  are reanalyzed to verify if their conclusion that daytime peak values of 60–70 W m−2 of latent heat flux divergence occurred in the 7–10 cm soil layer of a drying Yolo silt loam when maximum values of surface latent heat flux are estimated to have been about 100 W m−2. The new analyses, as similar to theirs as possible, are validated using a numerical simulation of coupled soil moisture and heat flow based on Philip and de Vries  as a test bed. The numerical simulation is extended to include the flow of air induced by diurnal soil heating and evaporative water loss to verify the flux divergence calculations reported in Parlange et al.  that explained the findings of Cahill and Parlange . It is shown that the conclusions of both of these papers are in error, so that the original version of the Philip and de Vries  theory is consistent with their field measurements after all and the effects of air flow associated with soil heating and liquid water loss (and low frequency barometric pressure variations also considered) are negligible in practice. In an additional investigation enhancement of diffusive vapor flow (first postulated by Philip and de Vries ) and discussed extensively in the literature since is shown to have negligible effects on cumulative evaporation under field conditions. This article is protected by copyright. All rights reserved.
- Excess warming of a central European lake driven by solar brightening
- Authors: M. Schmid; O. Köster
Abstract: Recent trends in summer surface temperatures of many lakes exceed the corresponding air temperature trends. This disagrees with expectations from lake surface heat budgets, which predict that lake surface temperatures should increase by 75 to 90% of the increase in air temperatures. Here we investigate the causes for this excess warming for Lower Lake Zurich, a representative deep and stratified Central European lake, by a combined data analysis and modelling approach. Lake temperatures are simulated using a one‐dimensional vertical model driven by 33 years of homogenized meteorological data. The model is calibrated and validated using an equally long time series of monthly water temperature profiles. The effects of individual forcing parameters are investigated by scenarios where the trends of single variables are retained while those of all other forcing variables are removed. The results show that ∼60% of the observed warming of spring and summer lake surface temperatures were caused by increased air temperature and ∼40% by increased solar radiation. The effects of the trends of all other forcing variables were small. Following projections of climate models, the increasing trends in solar radiation, and consequently the excess warming of lake surface temperatures, are not likely to continue in the future. This article is protected by copyright. All rights reserved.
- Characterization of reactive transport by 3D electrical resistivity
tomography (ERT) under unsaturated conditions
- Authors: Markus Wehrer; Andrew Binley, Lee D. Slater
Abstract: The leaching of nitrate from intensively used arable soil is of major concern in many countries. In this study we show how time lapse electrical resistivity tomography (ERT) can be used to characterize spatially heterogeneous processes of ion production, consumption and transport in soils. A controlled release fertilizer was introduced into an undisturbed soil core in a laboratory lysimeter and subjected to infiltration events. The production of ions resulting from processes associated with nitrification and their transport through the soil core was observed by time lapse ERT and analysis of seepage water samples from a multicompartment sampler. ERT images show development and propagation of a high conductivity plume from the fertilizer source zone. Molar amounts of nitrate produced in and exported from the soil core could be well reproduced by time lapse ERT using a spatial moment analysis. Furthermore, we observed that several shape measures of local breakthrough‐curves (BTCs) of seepage water conductivity and nitrate derived by effluent analyses and BTCs of bulk conductivity derived by ERT are highly correlated, indicating the preservation of spatial differences of the plume breakthrough in the ERT data. Also differences between nitrate breakthrough and a conservative tracer breakthrough can be observed by ERT. However, the estimation of target ion concentrations by ERT is errorbound and the smoothing algorithm of the inversion masks spatial conductivity differences. This results in difficulties reproducing spatial differences of ion source functions and variances of travel times. Despite the observed limitations, we conclude that time lapse ERT can be qualitatively and quantitatively informative with respect to processes affecting the fate of nitrate in arable soils. This article is protected by copyright. All rights reserved.
- Most computational hydrology is not reproducible, so is it really
- Authors: Christopher Hutton; Thorsten Wagener, Jim Freer, Dawei Han, Chris Duffy, Berit Arheimer
Abstract: Reproducibility is a foundational principle in scientific research. Yet in computational hydrology, the code and data that actually produces published results is not regularly made available, inhibiting the ability of the community to reproduce and verify previous findings. In order to overcome this problem we recommend that re‐useable code and formal workflows, which unambiguously reproduce published scientific results, are made available for the community alongside data, so that we can verify previous findings, and build directly from previous work. In cases where reproducing large‐scale hydrologic studies is computationally very expensive and time‐consuming, new processes are required to ensure scientific rigour. Such changes will strongly improve the transparency of hydrological research, and thus provide a more credible foundation for scientific advancement and policy support. This article is protected by copyright. All rights reserved.
- Optimizing embedded sensor network design for catchment‐scale
snow‐depth estimation using LiDAR and machine learning
- Authors: C. A. Oroza; Z. Zheng, S. D. Glaser, D. Tuia, R. C. Bales
Abstract: We evaluate the accuracy of a machine‐learning algorithm that uses LiDAR data to identify optimal ground‐based sensor placements for catchment‐scale snow measurements. Sampling locations that best represent catchment physiographic variables are identified with the Expectation Maximization algorithm for a Gaussian mixture model. A Gaussian process is then used to model the snow depth in a 1‐km2 area surrounding the network, and additional sensors are placed to minimize the model uncertainty. The aim of the study is to determine the distribution of sensors that minimizes the bias and RMSE of the model. We compare the accuracy of the snow‐depth model using the proposed placements to an existing sensor network at the Southern Sierra Critical Zone Observatory. Each model is validated with a 1‐m2 LiDAR‐derived snow‐depth raster from March 14th, 2010. The proposed algorithm exhibits higher accuracy with fewer sensors (8 sensors, RMSE 38.3 cm, bias = 3.49 cm) than the existing network (23 sensors, RMSE 53.0 cm, bias = 15.5 cm) and randomized placements (8 sensors, RMSE 63.7 cm, bias = 24.7 cm). We then evaluate the spatial and temporal transferability of the method using 14 LiDAR scenes from two catchments within the JPL Airborne Snow Observatory. In each region, optimal sensor placements are determined using the first available snow raster for the year. The accuracy in the remaining LiDAR surveys is then compared to 100 configurations of sensors selected at random. We find the error statistics (bias and RMSE) to be more consistent across the additional surveys than the average random configuration. This article is protected by copyright. All rights reserved.
- Merging radar and in situ rainfall measurements: An assessment of
different combination algorithms
- Authors: Mohammad Mahadi Hasan; Ashish Sharma, Fiona Johnson, Gregoire Mariethoz, Alan Seed
Abstract: Merging radar and gauge rainfall estimates is an area of active research. Since rain gauges alone are often limited at representing the complete spatial distribution of rainfall, a combination of radar derived rainfall with spatially interpolated gauge estimates using alternate weighting approaches is investigated. This paper examines several merging methods that differ in the consideration of correlation among the estimation errors, their distribution and the application of dynamic and static weighting. The merging process has been applied to the radar data from Terrey Hills radar located in Sydney, Australia and spatially interpolated gauge rainfall on the same area. The performance of the merging methods is assessed by comparing the combined estimate with the gauge observation. It is however clear from our findings that rainfall estimation from any of the combination approaches assessed contains less error than any of the non‐combination approaches. The results show that the correlation between these two rainfall estimation errors plays a significant role in the performance of the merging methods. The combination method should be chosen depending on the purpose, accuracy of the estimate and complexity of the method. This article is protected by copyright. All rights reserved.
- Runoff of small rocky headwater catchments: Field observations and
- Authors: C. Gregoretti; M. Degetto, M. Bernard, G. Crucil, A. Pimazzoni, G. Devido, M. Berti, A. Simoni, S. Lanzoni
Abstract: In dolomitic headwater catchments, intense rainstorms of short duration produce runoff discharges that often trigger debris flows on the scree slopes at the base of rock cliffs. In order to measure these discharges, we placed a measuring facility at the outlet (elevation 1770 m a.s.l.) of a small, rocky headwater catchment (area ∼ 0.032 km2, average slope ∼ 320%) located in the Venetian Dolomites (North Eastern Italian Alps). The facility consists of an approximately rectangular basin, ending with a sharp‐crested weir. Six runoff events were recorded in the period 2011‐2014, providing a unique opportunity for characterizing the hydrological response of the catchment. The measured hydrographs display impulsive shapes, with an abrupt raise up to the peak, followed by a rapidly decreasing tail, until a nearly constant plateau is eventually reached. This behavior can be simulated by means of a distributed hydrological model if the excess rainfall is determined accurately. We show that using the Soil Conservation Service Curve‐Number (SCS‐CN) method and assuming a constant routing velocity invariably results in an underestimated peak flow and a delayed peak time. A satisfactory prediction of the impulsive hydrograph shape including peak value and timing is obtained only by combining the SCS‐CN procedure with a simplified version of the Horton equation, and simulating runoff routing along the channel network through a matched diffusivity kinematic wave model. The robustness of the proposed methodology is tested through a comparison between simulated and observed timings of runoff or debris flow occurrence in two neighboring alpine basins. This article is protected by copyright. All rights reserved.
- Particle selectivity of sediment deposited over grass barriers and the
effect of rainfall
- Authors: Chengzhong Pan; Lan Ma, John Wainwright
Abstract: Particle selectivity of the sediment deposited over vegetative barriers is of importance to predict sediment transport and particulate pollutant load into surface waters. Grassed barriers with 20%, 40%, 60%, 70% and 90% covers at 15° slope were subjected to silt‐laden inflows in the presence and absence of simulated rainfalls to investigate the sediment deposition processes. The results show that re‐grass of steep croplands can effectively trap eroded sediment from upslope, and the rowed grass barriers can strengthen sediment deposition. The deposition order of sediment particle sizes (μm) follows (>50)> (25‐50)>(10‐25)=((2‐10), and the particle selectivity weakens with increasing grass covers. Clay particles had a similar deposition efficiency to overall sediment, implying the effectiveness of re‐grass in controlling soil nutrient loss. The contribution of grass to total overland flow resistance is almost equivalent to the percentage of grass cover. For steep grassed slopes, raindrop impact significantly decreases sediment deposition, but limitedly affects particle selectivity of deposited sediment and overland flow hydraulics. Both raindrop kinetic energy and stream power available for surface soil contribute to sediment deposition in net deposition areas of grass barriers. These imply that rainfall effect on sediment delivery over vegetated barriers derives from the additional raindrop energy, rather than the variation in runoff hydraulics. These results can help to clarify the effect of raindrop impact on sediment transport and to evaluate the benefit of re‐vegetation in decreasing sediment yield and its particulate nutrient load into surface waters. This article is protected by copyright. All rights reserved.
- Accounting for the influence of vegetation and landscape improves model
transferability in a tropical savannah region
- Authors: Hongkai Gao; Markus Hrachowitz, Nutchanart Sriwongsitanon, Fabrizio Fenicia, Shervan Gharari, Hubert H. G. Savenije
Abstract: Understanding which catchment characteristics dominate hydrologic response and how to take them into account remains a challenge in hydrological modeling, particularly in ungaged basins. This is even more so in non‐temperate and non‐humid catchments, where ‐ due to the combination of seasonality and the occurrence of dry spells ‐ threshold processes are more prominent in rainfall runoff behavior. An example is the tropical savannah, the second largest climatic zone, characterized by pronounced dry and wet seasons and high evaporative demand. In this study, we investigated the importance of landscape variability on the spatial variability of stream flow in tropical savannah basins. We applied a stepwise modeling approach to 23 sub‐catchments of the Upper Ping River in Thailand, where gradually more information on landscape was incorporated. The benchmark is represented by a classical lumped model (FLEXL), which does not account for spatial variability. We then tested the effect of accounting for vegetation information within the lumped model (FLEXLM), and subsequently two semi‐distributed models: one accounting for the spatial variability of topography‐based landscape features alone (FLEXT), and another accounting for both topographic features and vegetation (FLEXTM). In cross‐validation, each model was calibrated on one catchment, and then transferred with its fitted parameters to the remaining catchments. We found that when transferring model parameters in space, the semi‐distributed models accounting for vegetation and topographic heterogeneity clearly outperformed the lumped model. This suggests that landscape controls a considerable part of the hydrological function and explicit consideration of its heterogeneity can be highly beneficial for prediction in ungaged basins in tropical savannah. This article is protected by copyright. All rights reserved.
- Valley and channel networks extraction based on local topographic
curvature and K‐means clustering of contours
- Authors: Milad Hooshyar; Dingbao Wang, Seoyoung Kim, Stephen C. Medeiros, Scott C. Hagen
Abstract: A method for automatic extraction of valley and channel networks from high‐resolution digital elevation models (DEMs) is presented. This method utilizes both positive (i.e., convergent topography) and negative (i.e., divergent topography) curvature to delineate the valley network. The valley and ridge skeletons are extracted using the pixels' curvature and the local terrain conditions. The valley network is generated by checking the terrain for the existence of at least one ridge between two intersecting valleys. The transition from unchannelized to channelized sections (i.e., channel head) in each 1st‐order valley tributary is identified independently by categorizing the corresponding contours using an unsupervised approach based on K‐means clustering. The method does not require a spatially constant channel initiation threshold (e.g., curvature or contributing area). Moreover, instead of a point attribute (e.g., curvature), the proposed clustering method utilizes the shape of contours, which reflects the entire cross‐sectional profile including possible banks. The method was applied to three catchments: Indian Creek and Mid Bailey Run in Ohio, and Feather River in California. The accuracy of channel head extraction from the proposed method is comparable to state‐of‐the‐art channel extraction methods. This article is protected by copyright. All rights reserved.
- Flipping the thin film model: Mass transfer by hyporheic exchange in
gaining and losing streams
- Authors: Alexander H. McCluskey; Stanley B. Grant, Michael J. Stewardson
Abstract: The exchange of mass between a stream and its hyporheic zone, or “hyporheic exchange”, is central to many important ecosystem services. In this paper we show that mass transfer across the streambed by linear mechanisms of hyporheic exchange in a gaining or losing stream can be represented by a thin film model in which: (a) the mass transfer coefficient is replaced with the average Darcy flux of water downwelling into the sediment; and (b) the driving force for mass transfer is “flipped” from normal to the surface (concentration difference across a boundary layer) to parallel to the surface (concentration difference across downwelling and upwelling zones). Our analysis is consistent with previously published analytical, computational, and experimental studies of hyporheic exchange in the presence of stream‐groundwater interactions, and links stream network, advection‐dispersion, and stochastic descriptions of solute fate and transport in rivers. This article is protected by copyright. All rights reserved.
- Evaluation of mercury cycling and hypolimnetic oxygenation in
mercury‐impacted seasonally stratified reservoirs in the Guadalupe River
- Authors: Stephen A. McCord; Marc W. Beutel, Stephen R. Dent, S. Geoffrey Schladow
Abstract: Surface water reservoirs trap inorganic mercury delivered from their watersheds, create conditions that convert inorganic mercury to highly toxic methylmercury (MeHg), and host sportfish in which MeHg bioaccumulates. The Santa Clara Valley Water District (District) actively manages and monitors four mercury‐impaired reservoirs that help to serve communities in South San Francisco Bay, California. The Guadalupe River watershed, which contains three of those reservoirs, also includes the New Almaden mercury‐mining district, the largest historic mercury producer in North America. Monthly vertical profiles of field measurements and grab samples in years 2011‐2013 portray annual cycling of density stratification, dissolved oxygen (DO), and MeHg. Monitoring results highlight the role that hypolimnetic hypoxia plays in MeHg distribution in the water column, as well as the consistent, tight coupling between MeHg in ecological compartments (water, zooplankton and bass) across the four reservoirs. Following the 2011‐2013 monitoring period, the District designed and installed hypolimnetic oxygenation systems (HOS) in the four reservoirs in an effort to repress MeHg buildup in bottom waters and attain regulatory targets for MeHg in water and fish tissue. Initial HOS operation in Calero Reservoir in 2014 enhanced bottom water DO and depressed hypolimnetic buildup of MeHg, but did not substantially decrease mercury levels in zooplankton or small fish. This article is protected by copyright. All rights reserved.
- The direct and indirect effects of watershed land use and soil type on
stream water metal concentrations
- Authors: M. Taka; J. Aalto, J. Virkanen, M. Luoto
Abstract: Identifying the factors controlling stream water pollutants is challenged by the diversity of potential sources, pathways, and processes. This study tests the effects of watershed characteristics on stream water metal concentrations across environmental gradients. By using an extensive data set of 83 watersheds in southern Finland and structural equation modeling (SEM), the direct and indirect effects of land use and soil type on metal concentrations were explored. Both land use and soil type resulted in statistically significant direct effects on metals; for example land use was found to control dissolved metal concentrations, whereas soil type had the strongest links for total metal concentrations. The consideration of indirect correlation further strengthened the effects of soil type up to 50%, thus suggesting the dominant role of soil across land use intensities. Moreover, the results indicate that modified landscapes mediate the effect of natural soil processes in controlling stream metal concentrations. This work highlights the benefits of structural equation model framework, as the underlying paths for water quality are more likely to be identified, compared to traditional regression methods. Thus the implementation of SEM on water quality studies is highly encouraged. This article is protected by copyright. All rights reserved.
- Modeling multidomain hydraulic properties of shrink‐swell soils
- Authors: Ryan D. Stewart; Majdi R. Abou Najm, David E. Rupp, John S. Selker
Abstract: Shrink‐swell soils crack and become compacted as they dry, changing properties such as bulk density and hydraulic conductivity. Multi‐domain models divide soil into independent realms that allow soil cracks to be incorporated into classical flow and transport models. Incongruously, most applications of multi‐domain models assume that the porosity distributions, bulk density, and effective saturated hydraulic conductivity of the soil are constant. This study builds on a recently derived soil shrinkage model to develop a new multi‐domain, dual‐permeability model that can accurately predict variations in soil hydraulic properties due to dynamic changes in crack size and connectivity. The model only requires estimates of soil gravimetric water content and a minimal set of parameters, all of which can be determined using laboratory and/or field measurements. We apply the model to eight clayey soils, and demonstrate its ability to quantify variations in volumetric water content (as can be determined during measurement of a soil water characteristic curve) and transient saturated hydraulic conductivity, Ks (as can be measured using infiltration tests). The proposed model is able to capture observed variations in Ks of one to more than two orders of magnitude. In contrast, other dual‐permeability models assume that Ks is constant, resulting in the potential for large error when predicting water movement through shrink‐swell soils. Overall, the multi‐domain model presented here successfully quantifies fluctuations in the hydraulic properties of shrink‐swell soil matrices, and are suitable for use in physical flow and transport models based on Darcy's Law, the Richards Equation, and the advection‐dispersion equation. This article is protected by copyright. All rights reserved.
- Using web‐based observations to identify thresholds of a person's
stability in a flow
- Authors: L. Milanesi; M. Pilotti, B. Bacchi
Abstract: Flood risk assessment and mitigation are important tasks that should take advantage of rational vulnerability models to increase their effectiveness. These models are usually identified through a relevant set of laboratory experiments. However, there is growing evidence that these tests are not fully representative of the variety of conditions that characterize real flood hazard situations. This paper suggests a citizen science based and innovative approach to obtain information from web resources for the calibration of people's vulnerability models.A comprehensive study employing commonly‐used web engines allowed the collection of a wide set of documents showing real risk situations for people impacted by flood, classified according to the stability of the involved subjects. A procedure to extrapolate the flow depth and velocity from the video frames is developed and its reliability is verified by comparing the results with observation. The procedure is based on the statistical distribution of the population height employing a direct uncertainty propagation method. The results complement the experimental literature data and conceptual models. The growing availability of online information will progressively increase the sample size of the sample on which the procedure is based and will eventually lead to the identification of a probability surface describing the transition between stability and instability conditions of individuals in a flow. This article is protected by copyright. All rights reserved.
- An extended JBN method of determining unsteady state two‐phase
- Authors: Xiongyu Chen; Amir Kianinejad, David A. DiCarlo
Abstract: Relative permeability is the reduction of permeability of porous media when subjected to multi‐phase flow and a key parameter in subsurface hydrology. The JBN method [Welge, 1952; Johnson et al., 1959] is a well‐known method of obtaining relative permeability, which measures the overall pressure drop and the effluent phase ratio versus time during two‐phase displacements. By assuming no capillary pressure or gravity, the JBN method obtains the relative permeabilities to both phases at the core outlet. Since data across a range of saturations are acquired in a relatively short time, this method is widely used. This work extends the JBN method by having (1) section‐wise pressure drop measurements between the core inlet, four pressure taps on the core and the outlet, (2) local saturation measurements, and (3) local phase fluxes. With these data, the extended JBN method can determine relative permeabilities to both phases at each pressure tap of the core (not just at the core outlet). The JBN extension is shown using a data set where CO2 invades a brine‐filled core. From this it is found that the advantages of the extended JBN method over the regular JBN method are: (1) four times more data are obtained, and (2) data are more accurate because the capillary end effect is experimentally avoided. Avoiding the end effect results in tripling the saturation range, and obtaining relative permeabilities that are consistent with steady‐state measurements and roughly 40% higher than those from the regular JBN method. This article is protected by copyright. All rights reserved.
- Vertical dispersion in vegetated shear flows
- Authors: Simonetta Rubol; Ilenia Battiato, Felipe P. J. de Barros
Abstract: Canopy layers control momentum and solute transport to and from the overlying water surface layer. These transfer mechanisms strongly dependent on canopy geometry, affect the amount of solute in the river, the hydrological retention and availability of dissolved solutes to organisms located in the vegetated layers and are critical to improve water quality. In this work we consider steady‐state transport in a vegetated channel under fully‐developed flow conditions. Under the hypothesis that the canopy layer can be described as an effective porous medium with prescribed properties, i.e. porosity and permeability, we model solute transport above and within the vegetated layer with an advection‐dispersion equation with a spatially variable dispersion coefficient (diffusivity). By means of the Generalized Integral Transform Technique, we derive a semi‐analytical solution for the concentration field in submerged vegetated aquatic systems. We show that canopy layer's permeability affects the asymmetry of the concentration profile, the effective vertical spreading behavior, and the magnitude of the peak concentration. Due to its analytical features, the model has a low computational cost. The proposed solution successfully reproduces previously published experimental data. This article is protected by copyright. All rights reserved.
- Two and a half years of country‐wide rainfall maps using radio links
from commercial cellular telecommunication networks
- Authors: A. Overeem; H. Leijnse, R. Uijlenhoet
Abstract: Although rainfall estimation employing microwave links from cellular telecommunication networks is recognized as a new promising measurement technique, its potential for long‐term large‐scale operational rainfall monitoring remains to be demonstrated. This study contributes to this endeavor by deriving a continuous series of rainfall maps from a large 2.5 year microwave link data set of, on average, 3383 links (2044 link paths) covering The Netherlands (∼3.5 × 104 km2), a midlatitude country (∼5° E, ∼52° N) with a temperate climate. Maps are extensively verified against an independent gauge‐adjusted radar rainfall data set for different temporal (15 min, 1 hour, 1 day, 1 month) and spatial (0.9, 74 km2) scales. The usefulness of different steps in the rainfall retrieval algorithm, i.c. a wet‐dry classification method and a filter to remove outliers, is systematically assessed. A novel dew filter is developed to correct for dew‐induced wet antenna attenuation, which, although a relative underestimation of 6% to 9% is found, generally yields good results. The microwave link rainfall estimation technique performs well for the summer months (June, July, August), even outperforming interpolation of automatic rain gauge data (with a density of ∼1 gauge per 1000 km2), but large deviations are found for the winter months (December, January, February). These deviations are generally expected to be related to frozen or melting precipitation. Hence, our results show the potential of commercial microwave links for long‐term large‐scale operational rainfall monitoring. This article is protected by copyright. All rights reserved.
- Adaptive mixed finite element methods for Darcy flow in fractured porous
- Authors: Huangxin Chen; Amgad Salama, Shuyu Sun
Abstract: In this paper, we propose adaptive mixed finite element methods for simulating the single‐phase Darcy flow in two‐dimensional fractured porous media. The reduced model that we use for the simulation is a discrete fracture model coupling Darcy flows in the matrix and the fractures, and the fractures are modeled by one‐dimensional entities. The Raviart‐Thomas mixed finite element methods are utilized for the solution of the coupled Darcy flows in the matrix and the fractures. In order to improve the efficiency of the simulation, we use adaptive mixed finite element methods based on novel residual‐based a posteriori error estimators. In addition, we develop an efficient upscaling algorithm to compute the effective permeability of the fractured porous media. Several interesting examples of Darcy flow in the fractured porous media are presented to demonstrate the robustness of the algorithm. This article is protected by copyright. All rights reserved.
- A Bayesian hierarchical nonhomogeneous hidden Markov model for multisite
- Authors: C. Bracken; B. Rajagopalan, C. Woodhouse
Abstract: In many complex water supply systems, the next generation of water resources planning models will require simultaneous probabilistic streamflow inputs at multiple locations on an interconnected network. To make use of the valuable multi‐century records provided by tree‐ring data, reconstruction models must be able to produce appropriate multisite inputs. Existing streamflow reconstruction models typically focus on one site at a time, not addressing intersite dependencies and potentially misrepresenting uncertainty. To this end, we develop a model for multisite streamflow reconstruction with the ability to capture intersite correlations. The proposed model is a hierarchical Bayesian nonhomogeneous hidden Markov model (NHMM). A NHMM is fit to contemporary streamflow at each location using lognormal component distributions. Leading principal components of tree rings are used as covariates to model nonstationary transition probabilities and the parameters of the lognormal component distributions. Spatial dependence between sites is captured with a Gaussian elliptical copula. Parameters of the model are estimated in a fully Bayesian framework, in that marginal posterior distributions of all the parameters are obtained. The model is applied to reconstruct flows at 20 sites in the Upper Colorado River Basin (UCRB) from 1473‐1906. Many previous reconstructions are available for this basin, making it ideal for testing this new method. The results show some improvements over regression‐based methods in terms of validation statistics. Key advantages of the Bayesian NHMM over traditional approaches are a dynamic representation of uncertainty and the ability to make long multisite simulations that capture at‐site statistics and spatial correlations between sites. This article is protected by copyright. All rights reserved.
- Analyses of infrequent (quasi‐decadal) large groundwater recharge events
in the northern Great Basin: Their importance for groundwater
availability, use, and management
- Authors: Melissa D. Masbruch; Christine A. Rumsey, Subhrendu Gangopadhyay, David D. Susong, Tom Pruitt
Abstract: There has been a considerable amount of research linking climatic variability to hydrologic responses in the western United States. Although much effort has been spent to assess and predict changes in surface‐water resources, little has been done to understand how climatic events and changes affect groundwater resources. This study focuses on characterizing and quantifying the effects of large, multi‐year, quasi‐decadal groundwater recharge events in the northern Utah portion of the Great Basin for the period 1960 to 2013. Annual groundwater level data were analyzed with climatic data to characterize climatic conditions and frequency of these large recharge events. Using observed water‐level changes and multivariate analysis, five large groundwater recharge events were identified with a frequency of about 11 to 13 years. These events were generally characterized as having above‐average annual precipitation and snow water equivalent and below‐average seasonal temperatures, especially during the spring (April through June). Existing groundwater flow models for several basins within the study area were used to quantify changes in groundwater storage from these events. Simulated groundwater storage increases per basin from a single recharge event ranged from about 115 Mm3 to 205 Mm3. Extrapolating these amounts over the entire northern Great Basin indicates that a single large quasi‐decadal recharge event could result in billions of cubic meters of groundwater storage. Understanding the role of these large quasi‐decadal recharge events in replenishing aquifers and sustaining water supplies is crucial for long‐term groundwater management. This article is protected by copyright. All rights reserved.
- Laboratory flume experiments with the Swiss plate geophone bedload
monitoring system. Part I: Impulse counts and particle size identification
- Authors: Carlos R. Wyss; Dieter Rickenmann, Bruno Fritschi, Jens M. Turowski, Volker Weitbrecht, Robert M. Boes
Abstract: We performed systematic flume experiments using natural bedload particles to quantify the effect of different parameters on the signal registered by the Swiss plate geophone, a bedload surrogate monitoring system. It was observed that the number of impulses computed from the raw signal clearly depends on bed particle size, mean flow velocity, bed roughness, and to a minor extent on particle shape. The centroid frequency of the signal resulting from the collision of a bedload particle against the geophone plate was found to be inversely related to particle size but to be less sensitive to variations in mean flow velocity and bed roughness than the signal amplitude, which is also related to particle size. Combining frequency and amplitude information resulted in a more robust identification of the transported particles size over a wide range of sizes than using amplitude information alone. This article is protected by copyright. All rights reserved.
- Analytical approximations for effective relative permeability in the
- Authors: Avinoam Rabinovich; Boxiao Li, Louis J. Durlofsky
Abstract: We present an analytical method for calculating two‐phase effective relative permeability, krjeff, where j designates phase (here CO2 and water), under steady state and capillary limit assumptions. These effective relative permeabilities may be applied in experimental settings and for upscaling in the context of numerical flow simulations, e.g., for CO2 storage. An exact solution for effective absolute permeability, keff, in two‐dimensional log‐normally distributed isotropic permeability (k) fields is the geometric mean. We show that this does not hold for since log normality is not maintained in the capillary limit phase permeability field (kkrj) when capillary pressure, and thus the saturation field, is varied. Nevertheless, the geometric mean is still shown to be suitable for approximating krjeff when the variance of ln k is low. For high variance cases, we apply a correction to the geometric‐average gas effective relative permeability using a Winsorized mean, which neglects large and small Kj values symmetrically. The analytical method is extended to anisotropically correlated log‐normal permeability fields using power law averaging. In these cases the Winsorized mean treatment is applied to the gas curves for cases described by negative power law exponents (flow across incomplete layers). The accuracy of our analytical expressions for is demonstrated through extensive numerical tests, using low‐ and high‐variance permeability realizations with a range of correlation structures. We also present integral expressions for geometric‐mean and power law average krjeff for the systems considered, which enable derivation of closed‐form series solutions for without generating permeability realizations. This article is protected by copyright. All rights reserved.
- Improving the realism of hydrologic model functioning through multivariate
- Authors: O. Rakovec; R. Kumar, S. Attinger, L. Samaniego
Abstract: Increased availability and quality of near real‐time observations provide the opportunity to improve understanding of predictive skills of hydrologic models. Recent studies have shown the limited capability of river discharge data alone to adequately constrain different components of distributed model parameterizations. In this study, the GRACE satellite‐based total water storage (TWS) anomaly is used to complement the discharge data with the aim to improve the fidelity of mesoscale hydrologic model (mHM) through multivariate parameter estimation. The study is conducted on 83 European basins covering a wide range of hydro‐climatic regimes. The model parameterization complemented with the TWS anomalies leads to statistically significant improvements in (1) discharge simulations during low‐flow period, and (2) evapotranspiration estimates which are evaluated against independent data (FLUXNET). Overall, there is no significant deterioration in model performance for the discharge simulations when complemented by information from the TWS anomalies. However, considerable changes in the partitioning of precipitation into runoff components are noticed by in‐/exclusion of TWS during the parameter estimation. Introducing monthly averaged TWS data only improves the dynamics of streamflow on monthly or longer time scales, which mostly addresses the dynamical behavior of the baseflow reservoir. A cross‐evaluation test carried out to assess the transferability of the calibrated parameters to other locations further confirms the benefit of complementary TWS data. In particular, the evapotranspiration estimates show more robust performance when TWS data are incorporated during the parameter estimation, in comparison with the benchmark model constrained against discharge only. This study highlights the value for incorporating multiple data sources during parameter estimation to improve the overall realism of hydrologic models and their applications over large domains. This article is protected by copyright. All rights reserved.
- Laboratory flume experiments with the Swiss plate geophone bedload
monitoring system. Part II: Application to field sites with direct bedload
- Authors: Carlos R. Wyss; Dieter Rickenmann, Bruno Fritschi, Jens M. Turowski, Volker Weitbrecht, Eric Travaglini, Eric Bardou, Robert M. Boes
Abstract: The Swiss plate geophone is a bedload surrogate monitoring system that had been calibrated in several gravel‐bed streams through field calibration measurements. Field calibration measurements are generally expensive and time consuming, therefore we investigated the possibility to replace it by a flume‐based calibration approach. We applied impulse‐diameter relations for the Swiss plate geophone obtained from systematic flume experiments to field calibration measurements in four different gravel‐bed streams. The flume‐based relations were successfully validated with direct bedload samples from field measurements, by estimating the number of impulses based on observed bedload masses per grain size class. We estimated bedload transport mass by developing flume‐based and stream‐dependent calibration procedures for the Swiss plate geophone system using an additional empirical function. The estimated masses are on average in the range of ±90% of measured bedload masses in the field, but the accuracy is generally improved for larger transported bedload masses. We discuss the limitations of the presented flume‐based calibration approach. This article is protected by copyright. All rights reserved.
- Real‐Time Estimation of Snow Water Equivalent in the Upper Colorado
River Basin using MODIS‐based SWE Reconstructions and SNOTEL data
- Authors: Dominik Schneider; Noah P. Molotch
Abstract: Changes in climate necessitate improved snowpack information to better represent anomalous distributions of snow water equivalent (SWE) and improve water resource management. We estimate the spatial distribution of SWE for the Upper Colorado River basin weekly from January to June 2001‐2012 in quasi‐real‐time by two regression techniques: a baseline regression of in situ operationally measured point SWE using only physiographic information and regression of these in situ points combining both physiographic information and historical SWE patterns from a remote sensing‐based SWE reconstruction model. We compare the baseline regression approach to our new regression in the context of spatial snow surveys and operational snow measuring stations. When compared to independent distributed snow surveys, the new regression reduces the bias of SWE estimates from ‐5.5% to 0.8%, and RMSE of the SWE estimates by 8% from 0.25 m to 0.23 m. Notable improvements were observed in alpine terrain with bias declining from ‐38% to only 3.4%, and RMSE was reduced by 13%, from 0.47 m to 0.41 m. The mean increase in cross‐validated r2 for the new regression compared to the baseline regression is from 0.22 to 0.33. The largest increase in r2 in any one year is 0.19, an 83% improvement. The new regression estimates, on average, 31% greater SWE depth than the baseline regression in areas above 3000 m elevation, which contributes up to 66% of annual SWE volume in the driest year. This indicates that the historical SWE patterns from the reconstruction adds information to the interpolation beyond the physiographic conditions represented by the SNOTEL network. Given that previous works using SWE reconstructions were limited to retrospective analyses by necessity, the work presented here represents an important contribution in that it extends SWE reconstructions to real‐time applications and illustrates that doing so significantly improves the accuracy of SWE estimates. This article is protected by copyright. All rights reserved.
- Sharing the cost of a river basin adaptation portfolios to climate change:
Insights from social justice and cooperative game theory
- Authors: Corentin Girard; Jean‐Daniel Rinaudo, Manuel pulido‐velazquez
Abstract: The adaptation of water resource systems to the potential impacts of climate change requires mixed portfolios of supply and demand adaptation measures. The issue is not only to select efficient, robust and flexible adaptation portfolios but also to find equitable strategies of cost allocation among the stakeholders. Our work addresses such cost allocation problems by applying two different theoretical approaches: social justice and cooperative game theory in a real case study. First of all, a cost‐effective portfolio of adaptation measures at the basin scale is selected using a least‐cost optimization model. Cost allocation solutions are then defined based on economic rationality concepts from cooperative game theory (the Core). Secondly, interviews are conducted to characterize stakeholders' perceptions of social justice principles associated with the definition of alternatives cost allocation rules. The comparison of the cost allocation scenarios leads to contrasted insights in order to inform the decision making process at the river basin scale and potentially reap the efficiency gains from cooperation in the design of river basin adaptation portfolios. This article is protected by copyright. All rights reserved.
- On the turbulent flow structure around an instream structure with
- Authors: Seokkoo Kang; Craig Hill, Fotis Sotiropoulos
Abstract: We investigate the flow dynamics around a rock vane, a widely‐used instream structure for stream restoration, by conducting laboratory flume experiments, and carrying out high‐resolution Large Eddy Simulation (LES) taking advantage of parallel computing. The flume experiments are conducted under fixed‐ and mobile‐bed conditions, where the velocities and bed elevations are measured, respectively. The LES is carried out for the fixed‐bed experiment by directly resolving the details of the rocks that constitute the vane and the individual roughness elements on the channel bed. The LES‐computed mean flow statistics show good agreement with the measurements, and the analysis of the computed flow field reveals the existence of two counter‐rotating secondary flow cells downstream of the vane, which originate from the plunging of the three‐dimensional streamlines onto a lower part of the sidewall downstream of the vane. To further examine the role of the secondary flow cells under a mobile‐bed condition, the LES results are compared with the equilibrium bed elevation measured in the mobile bed experiment. The mobile‐bed experiment reveals the existence of an oblique sand ridge downstream of the vane that is aligned with the line of flow convergence caused by the collision of the two secondary flow cells. The results indicate that the two counter‐rotating cells downstream of the rock vane has a profound impact on the mean flow and bed shear stress as well as on the bed morphodynamics. This article is protected by copyright. All rights reserved.
- Issue Information
- Pages: 7545 - 7547
- Carbon dynamics in the hyporheic zone of a headwater mountain stream in
the Cascade Mountains, Oregon
- Authors: Hayley A. Corson-Rikert; Steven M. Wondzell, Roy Haggerty, Mary V. Santelmann
Pages: 7556 - 7576
Abstract: We investigated carbon dynamics in the hyporheic zone of a steep, forested, headwater catchment western Oregon, USA. Water samples were collected monthly from the stream and a well network during base flow periods. We examined the potential for mixing of different source waters to explain concentrations of DOC and DIC. We did not find convincing evidence that either inputs of deep groundwater or lateral inputs of shallow soil water influenced carbon dynamics. Rather, carbon dynamics appeared to be controlled by local processes in the hyporheic zone and overlying riparian soils. DOC concentrations were low in stream water (0.04–0.09 mM), and decreased with nominal travel time through the hyporheic zone (0.02–0.04 mM lost over 100 h). Conversely, stream water DIC concentrations were much greater than DOC (0.35–0.7 mM) and increased with nominal travel time through the hyporheic zone (0.2–0.4 mM gained over 100 h). DOC in stream water could only account for 10% of the observed increase in DIC. In situ metabolic processing of buried particulate organic matter as well as advection of CO2 from the vadose zone likely accounted for the remaining 90% of the increase in DIC. Overall, the hyporheic zone was a source of DIC to the stream. We suggest that, in mountain stream networks, hyporheic exchange facilitates the transformation of particulate organic carbon buried in floodplains and transports the DIC that is produced back to the stream where it can be evaded to the atmosphere.
- Sequential ensemble-based optimal design for parameter estimation
- Authors: Jun Man; Jiangjiang Zhang, Weixuan Li, Lingzao Zeng, Laosheng Wu
Pages: 7577 - 7592
Abstract: The ensemble Kalman filter (EnKF) has been widely used in parameter estimation for hydrological models. The focus of most previous studies was to develop more efficient analysis (estimation) algorithms. On the other hand, it is intuitively understandable that a well-designed sampling (data-collection) strategy should provide more informative measurements and subsequently improve the parameter estimation. In this work, a Sequential Ensemble-based Optimal Design (SEOD) method, coupled with EnKF, information theory and sequential optimal design, is proposed to improve the performance of parameter estimation. Based on the first-order and second-order statistics, different information metrics including the Shannon entropy difference (SD), degrees of freedom for signal (DFS) and relative entropy (RE) are used to design the optimal sampling strategy, respectively. The effectiveness of the proposed method is illustrated by synthetic one-dimensional and two-dimensional unsaturated flow case studies. It is shown that the designed sampling strategies can provide more accurate parameter estimation and state prediction compared with conventional sampling strategies. Optimal sampling designs based on various information metrics perform similarly in our cases. The effect of ensemble size on the optimal design is also investigated. Overall, larger ensemble size improves the parameter estimation and convergence of optimal sampling strategy. Although the proposed method is applied to unsaturated flow problems in this study, it can be equally applied in any other hydrological problems.
- Inferring changes in water cycle dynamics of intensively managed
landscapes via the theory of time-variant travel time distributions
- Authors: Mohammad Danesh-Yazdi; Efi Foufoula-Georgiou, Diana L. Karwan, Gianluca Botter
Pages: 7593 - 7614
Abstract: Climatic trends and anthropogenic changes in land cover and land use are impacting the hydrology and water quality of streams at the field, watershed, and regional scales in complex ways. In poorly drained agricultural landscapes, subsurface drainage systems have been successful in increasing crop productivity by removing excess soil moisture. However, their hydroecological consequences are still debated in view of the observed increased concentrations of nitrate, phosphorus, and pesticides in many streams, as well as altered runoff volumes and timing. In this study, we employ the recently developed theory of time-variant travel time distributions within the StorAge Selection function framework to quantify changes in water cycle dynamics resulting from the combined climate and land use changes. Our results from analysis of a subbasin in the Minnesota River Basin indicate a significant decrease in the mean travel time of water in the shallow subsurface layer during the growing season under current conditions compared to the pre-1970s conditions. We also find highly damped year-to-year fluctuations in the mean travel time, which we attribute to the “homogenization” of the hydrologic response due to artificial drainage. The dependence of the mean travel time on the spatial heterogeneity of some soil characteristics as well as on the basin scale is further explored via numerical experiments. Simulations indicate that the mean travel time is independent of scale for spatial scales larger than approximately 200 km2, suggesting that hydrologic data from larger basins may be used to infer the average of smaller-scale-driven changes in water cycle dynamics.
- On river-floodplain interaction and hydrograph skewness
- Authors: Ayan S. Fleischmann; Rodrigo C. D. Paiva, Walter Collischonn, Mino V. Sorribas, Paulo R. M. Pontes
Pages: 7615 - 7630
Abstract: Understanding hydrological processes occurring within a basin by looking at its outlet hydrograph can improve and foster comprehension of ungauged regions. In this context, we present an extensive examination of the roles that floodplains play on driving hydrograph shapes. Observations of many river hydrographs with large floodplain influence are carried out and indicate that a negative skewness of the hydrographs is present among many of them. Through a series of numerical experiments and analytical reasoning, we show how the relationship between flood wave celerity and discharge in such systems is responsible for determining the hydrograph shapes. The more water inundates the floodplains upstream of the observed point, the more negatively skewed is the observed hydrograph. A case study is performed in the Amazon River Basin, where major rivers with large floodplain attenuation (e.g., Purus, Madeira, and Juruá) are identified with higher negative skewness in the respective hydrographs. Finally, different wetland types could be distinguished by using this feature, e.g., wetlands maintained by endogenous processes, from wetlands governed by overbank flow (along river floodplains). A metric of hydrograph skewness was developed to quantify this effect, based on the time derivative of discharge. Together with the skewness concept, it may be used in other studies concerning the relevance of floodplain attenuation in large, ungauged rivers, where remote sensing data (e.g., satellite altimetry) can be very useful.
- Inverse modeling of unsaturated flow using clusters of soil texture and
- Authors: Yonggen Zhang; Marcel G. Schaap, Alberto Guadagnini, Shlomo P. Neuman
Pages: 7631 - 7644
Abstract: Characterization of heterogeneous soil hydraulic parameters of deep vadose zones is often difficult and expensive, making it necessary to rely on other sources of information. Pedotransfer functions (PTFs) based on soil texture data constitute a simple alternative to inverse hydraulic parameter estimation, but their accuracy is often modest. Inverse modeling entails a compromise between detailed description of subsurface heterogeneity and the need to restrict the number of parameters. We propose two methods of parameterizing vadose zone hydraulic properties using a combination of k-means clustering of kriged soil texture data, PTFs, and model inversion. One approach entails homogeneous and the other heterogeneous clusters. Clusters may include subdomains of the computational grid that need not be contiguous in space. The first approach homogenizes within-cluster variability into initial hydraulic parameter estimates that are subsequently optimized by inversion. The second approach maintains heterogeneity through multiplication of each spatially varying initial hydraulic parameter by a scale factor, estimated a posteriori through inversion. This allows preserving heterogeneity without introducing a large number of adjustable parameters. We use each approach to simulate a 95 day infiltration experiment in unsaturated layered sediments at a semiarid site near Phoenix, Arizona, over an area of 50 × 50 m2 down to a depth of 14.5 m. Results show that both clustering approaches improve simulated moisture contents considerably in comparison to those based solely on PTF estimates. Our calibrated models are validated against data from a subsequent 295 day infiltration experiment at the site.
- Water pollution and income relationships: A seemingly unrelated partially
- Authors: Mahesh Pandit; Krishna P. Paudel
Pages: 7668 - 7689
Abstract: We used a seemingly unrelated partially linear model (SUPLM) to address a potential correlation between pollutants (nitrogen, phosphorous, dissolved oxygen and mercury) in an environmental Kuznets curve study. Simulation studies show that the SUPLM performs well to address potential correlation among pollutants. We find that the relationship between income and pollution follows an inverted U-shaped curve for nitrogen and dissolved oxygen and a cubic shaped curve for mercury. Model specification tests suggest that a SUPLM is better specified compared to a parametric model to study the income-pollution relationship. Results suggest a need to continually assess policy effectiveness of pollution reduction as income increases.
- Mapping high-resolution soil moisture and properties using distributed
temperature sensing data and an adaptive particle batch smoother
- Authors: Jianzhi Dong; Susan C. Steele-Dunne, Tyson E. Ochsner, Christine E. Hatch, Chadi Sayde, John Selker, Scott Tyler, Michael H. Cosh, Nick van de Giesen
Pages: 7690 - 7710
Abstract: This study demonstrated a new method for mapping high-resolution (spatial: 1 m, and temporal: 1 h) soil moisture by assimilating distributed temperature sensing (DTS) observed soil temperatures at intermediate scales. In order to provide robust soil moisture and property estimates, we first proposed an adaptive particle batch smoother algorithm (APBS). In the APBS, a tuning factor, which can avoid severe particle weight degeneration, is automatically determined by maximizing the reliability of the soil temperature estimates of each batch window. A multiple truth synthetic test was used to demonstrate the APBS can robustly estimate soil moisture and properties using observed soil temperatures at two shallow depths. The APBS algorithm was then applied to DTS data along a 71 m transect, yielding an hourly soil moisture map with meter resolution. Results show the APBS can draw the prior guessed soil hydraulic and thermal properties significantly closer to the field measured reference values. The improved soil properties in turn remove the soil moisture biases between the prior guessed and reference soil moisture, which was particularly noticeable at depth above 20 cm. This high-resolution soil moisture map demonstrates the potential of characterizing soil moisture temporal and spatial variability and reflects patterns consistent with previous studies conducted using intensive point scale soil moisture samples. The intermediate scale high spatial resolution soil moisture information derived from the DTS may facilitate remote sensing soil moisture product calibration and validation. In addition, the APBS algorithm proposed in this study would also be applicable to general hydrological data assimilation problems for robust model state and parameter estimation.