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The article presents the authors' reply to S. Neuman's comments which advocates the need for an adequate hydrogeological characterization of aquifers. It provides a brief summary of their work that was overlooked by Neuman's comment which include a critical reexamination of the field data collected by L.W. Gelhar et al especially those referred to as of medium or low reliability and the addition of new data what were again screened for reliability.

The paper compares a theory for immiscible displacement based on distinguishing percolating and nonpercolating fluid parts with experimental observations from multistep outflow experiments. The theory was published in 2006 in Physica A, volume 371, pages 209-225; the experiments were published in 1991 in Water Resources Research, volume 27, pages 2113. The present paper focuses on hysteretic phenomena resulting from repeated cycling between drainage and imbibition processes in multistep pressure experiments. Taking into account, the hydraulic differences between percolating and nonpercolating fluid parts provides a physical basis to predict quantitatively the hysteretic phenomena observed in the experiment. While standard hysteretic extensions of the traditional theory are nonlocal in time the theory used in this paper is local in time. Instead of storing the pressure and saturation history, it requires only the current state of the system to reach the same quantitative agreement. [ABSTRACT FROM AUTHOR]

This paper introduces the Water Resources Research special section on Uncertainty Assessment in Surface and Subsurface Hydrology. Over the past years, hydrological literature has seen a large increase in the number of papers dealing with uncertainty. In this article, we present an overview of the different sources of uncertainty and the different types of problems associated with uncertainty assessment. It is argued here that clarity about which part of the large field of uncertainty research is addressed by a given research activity would already help guide discussions within the hydrological community. We present an introduction to the differences between the more classical frequentist approach to uncertainty and Bayesian approaches and between probabilistic and nonprobabilistic approaches. Bayesian approaches allow for inclusion of more subjective expert knowledge and would be more appropriate where less 'hard' data are available. Any underlying assumptions need to be made very clear to the end user. Finally, a brief classification of the articles of the special section is presented. [ABSTRACT FROM AUTHOR]

Training image-based geostatistical methods are increasingly popular in groundwater hydrology even if existing algorithms present limitations that often make real-world applications difficult. These limitations include a computational cost that can be prohibitive for high-resolution 3-D applications, the presence of visual artifacts in the model realizations, and a low variability between model realizations due to the limited pool of patterns available in a finite-size training image. In this paper, we address these issues by proposing an iterative patch-based algorithm which adapts a graph cuts methodology that is widely used in computer graphics. Our adapted graph cuts method optimally cuts patches of pixel values borrowed from the training image and assembles them successively, each time accounting for the information of previously stitched patches. The initial simulation result might display artifacts, which are identified as regions of high cost. These artifacts are reduced by iteratively placing new patches in high-cost regions. In contrast to most patch-based algorithms, the proposed scheme can also efficiently address point conditioning. An advantage of the method is that the cut process results in the creation of new patterns that are not present in the training image, thereby increasing pattern variability. To quantify this effect, a new measure of variability is developed, the merging index, quantifies the pattern variability in the realizations with respect to the training image. A series of sensitivity analyses demonstrates the stability of the proposed graph cuts approach, which produces satisfying simulations for a wide range of parameters values. Applications to 2-D and 3-D cases are compared to state-of-the-art multiple-point methods. The results show that the proposed approach obtains significant speedups and increases variability between realizations. Connectivity functions applied to 2-D models transport simulations in 3-D models are used to demonstrate that pattern continuity is preserved. [ABSTRACT FROM AUTHOR]

The hyporheic zone, defined by shallow subsurface pathways through river beds and banks beginning and ending at the river, is an integral and unique component of fluvial systems. It hosts myriad hydrologically controlled processes that are potentially coupled in complex ways. Understanding these processes and the connections between them is critical since these processes are not only important locally but integrate to impact increasingly larger scale biogeochemical functioning of the river corridor up to the river network scale. Thus, the hyporheic zone continues to be a growing research focus for many hydrologists for more than half the history of Water Resources Research. This manuscript partly summarizes the historical development of hyporheic zone hydrologic science as gleaned from papers published in Water Resources Research, from the birth of the concept of the hyporheic zone as a hydrologic black box (sometimes referred to as transient storage zone), to its adolescent years of being torn between occasionally competing research perspectives of interrogating the hyporheic zone from a surface or subsurface view, to its mature emergence as an interdisciplinary research field that employs the wide array of state-of-the-art tools available to the modern hydrologist. The field is vibrant and moving in the right direction of addressing critical fundamental and applied questions with no clear end in sight in its growth. There are exciting opportunities for scientists that are able to tightly link the allied fields of geology, geomorphology, hydrology, geochemistry, and ecology to tackle the many open problems in hyporheic zone science. [ABSTRACT FROM AUTHOR]

Hillslope hydrological investigations in humid regions to date have focused mostly on runoff generation during events. The few papers that have also examined groundwater recharge processes associated with subsurface stormflow production have found strong linkages between episodes of runoff and recharge to the aquifer. But the range of climate, vegetation, and geological conditions examined thus far has been limited. Here we explore how geologic characteristics, timing of subsurface stormflow, and hydroclimatic conditions relate to the timing of bedrock groundwater recharge at the well‐studied Maimai watershed. We hypothesized that recharge would be determined by subsurface stormflow frequency in this system with high rainfall and little seasonality of the hydrologic response. Unexpectedly, isotopic analysis and noble gas measurements indicated that recharge occurred almost exclusively during winter months despite previous work at Maimai showing subsurface stormflow occurs in all seasons and rainfall‐runoff ratios are high year‐round. A sprinkler and dye experiment conducted directly on open bedrock identified groundwater recharge mechanisms and rates, and a simple empirical recharge model suggests almost 90% of recharge occurred from only 55% of annual precipitation. We found no correlation between the timing and magnitude of groundwater recharge and total precipitation, direct runoff or subsurface stormflow. The catchment effectively converted rainfall to runoff during all seasons, but the unfractured low permeability bedrock (2.3 × 10−8 m/s) required long durations of extended catchment wetness for appreciable recharge to occur—a condition satisfied only during winter months with lower evapotranspiration. These findings suggest the need to better understand the geologic controls of recharge in headwaters. Key Points: Despite minimal seasonality in rainfall, runoff generation, or the rainfall‐runoff ratio, there is strong seasonality in groundwater rechargeExtended periods of high catchment wetness, rather than storm events, were the primary driver of groundwater recharge at the Maimai watershedUnfractured low permeability bedrock and an efficient preferential flow network explain why runoff generation and recharge were uncorrelated [ABSTRACT FROM AUTHOR]

While in catchment and hillslope hydrology a more nuanced approach is now taken to streamflow recession analysis, in the context of major aquifers it is commonly still assumed that the groundwater head recession rate will take exponential form, an idea originally proposed in the 19th Century. However it is shown here that, in early times, the groundwater head recession in a major aquifer should take an almost straight line form with a rate approximately equal to the long-term recharge rate divided by the aquifer storage coefficient. The length of this phase can be estimated from an analytical expression derived in the paper which depends on the aquifer diffusivity, length scale, and the position of the monitoring point. A transitional phase then leads to an exponential phase after some critical time which is independent of the position of the monitoring point. Major aquifers in a state of periodic quasi-steady state are expected to have rates of groundwater flux recession which deviate little from the average rate of groundwater recharge. Where quasi-exponential groundwater declines are observed in nature, their form may be diagnostic of particular types of aquifer properties and/or boundary effects, such as proximity to drainage boundaries, variations in transmissivity with hydraulic head, storage changes due to pumping, nonequilibrium flow at a range of spatial and temporal scales, and variations in specific yield with depth. Recession analysis has applicability to a range of groundwater problems and is powerful way of gaining insight into the hydrologic functioning of an aquifer. [ABSTRACT FROM AUTHOR]

The article presents a commentary on the paper "Plume Spreading in Groundwater by Stretching and Folding," published in "Water Resources Research" journal which proposed a methodology for the acceleration of plume spreading with punctuated injection protocols. The authors question the statement in the paper regarding the idealization of the timing and orientation of the reinjection of fluid particles.

Data from laboratory experiments on a 143 cm tall and 14.5 cm diameter column, packed with Wedron sand with varied constant upper boundary fluxes and water table velocities for both falling and rising water tables are used to validate a finite water-content vadose zone simulation methodology. The one-dimensional finite water-content Talbot and Ogden (2008) (T-O) infiltration and redistribution method was improved to simulate groundwater table dynamic effects and compared against the numerical solution of the Richards equation using Hydrus-1D. Both numerical solutions agreed satisfactorily with time series measurements of water content. Results showed similar performance for both methods, with the T-O method on average having higher Nash-Sutcliffe efficiencies and smaller absolute biases. Hydrus-1D was more accurate in predicting deponding times in the case of a falling water table, while Hydrus-1D and the T-O method had similar errors in predicted ponding times in the case of a rising water table in six of nine tests. The improved T-O method was able to predict general features of vadose zone moisture dynamics with moving water table and surface infiltration using an explicit, mass-conservative formulation. The advantage of an explicit formulation is that it is numerically simple, using forward Euler solution methodology, and is guaranteed to converge and to conserve mass. These properties make the improved T-O method presented in this paper a robust and computationally efficient alternative to the numerical solution of the Richards equation in hydrological modeling applications involving groundwater table dynamic effects on vadose zone soil moistures. [ABSTRACT FROM AUTHOR]

Although we have an intuitive understanding of the behavior and functions of groundwater in the Earth's critical zone at the scales of a column (atmosphere-plant-soil-bedrock), along a toposequence (ridge to valley), and across a small catchment (up to third-order streams), this paper attempts to assess the relevance of groundwater to understanding large-scale patterns and processes such as represented in global climate and Earth system models. Through observation syntheses and conceptual models, evidence are presented that groundwater influence is globally prevalent, it forms an environmental gradient not fully captured by the climate, and it can profoundly shape critical zone evolution at continental to global scales. Four examples are used to illustrate these ideas: (1) groundwater as a water source for plants in rainless periods, (2) water table depth as a driver of plant rooting depth, (3) the accessibility of groundwater as an ecological niche separator, and (4) groundwater as the lower boundary of land drainage and a global driver of wetlands. The implications to understanding past and future global environmental change are briefly discussed, as well as critical discipline, scale, and data gaps that must be bridged in order for us to translate what we learn in the field at column, hillslope and catchment scales, to what we must predict at regional, continental, and global scales. [ABSTRACT FROM AUTHOR]

This series of papers addresses the transport of reactive solutes in groundwater. In part 1, the time-dependent effective retardation factor [ABSTRACT FROM AUTHOR]

This series of papers addresses the transport of sorbing solutes in groundwater. In part 2, plume dispersion, as quantified by the particle displacement variance [ABSTRACT FROM AUTHOR]

The location of a point on the landscape within a stream network (hydrologic position) can be an important predictive measure in hydrology. Hydrologic position is defined here by two metrics: lateral position and distance from stream to divide, both measured horizontally. Lateral position (dimensionless) is the relative position of a point between the stream and its watershed divide. Distance from stream to divide (units of length) is an indicator of position within a watershed: generally small near a confluence and generally large in headwater areas. Watersheds and watershed divides are defined here by Thiessen polygons rather than topographic divides. Lateral position and distance from stream to divide are also defined in the context of hydrologic order. Hydrologic order "n" is defined as the network of streams, and associated divides, of order n and higher. And given that a point can have different positions in different hydrologic orders the term multiorder hydrologic position (MOHP) is used to describe the ensemble of hydrologic positions. MOHP was mapped across the conterminous United States for nine hydrologic orders at a spatial resolution of 30 m (about 8.7 billion pixels). There are 18 metrics for each pixel. Four case studies are presented that use MOHP metrics as explanatory factors in random forest machine learning models. The case studies show that lower order MOHP metrics can serve as indicators of hydrologic process while higher‐order metrics serve as indicators of location. MOHP is shown to have utility as a predictor variable across a large range of scales (50,000 to 8,000,000 km2). Plain Language Summary: In hydrology, as in other endeavors, location matters. This study presents a new type of data that describes the location of a point on the landscape in the context of the network of streams that are present across the continental United States. The new data are presented as maps, and different patterns can be recognized in different areas of the United States. The patterns that can be seen also vary as one looks more or less closely at an area. The patterns that are present in these maps are shown to be useful for the purposes of mapping water resources. Key Points: Multiorder hydrologic position (lateral position and distance from stream to divide) quantifies the location of a point on the landscapeMultiorder hydrologic position is mapped for nine hydrologic orders at a 30‐m resolution for the conterminous United StatesMultiorder hydrologic position is used in machine learning models for the purposes of physiographic and hydrologic mapping [ABSTRACT FROM AUTHOR]

This paper presents a multicontinuum approach to model fractal temporal scaling of catchment response in hydrological systems. The temporal scaling of discharge is quantified in frequency domain by the transfer function [ABSTRACT FROM AUTHOR]

When plants are both sensitive to salt levels in the root zone and able to modify the soil salt balance, changes in vegetation cover may affect the local hydrologic conditions and favor the accumulation of salt within different parts of the soil profile. In such cases a salt-vegetation feedback may exist, whereby both a state with vegetation cover, deep water table, and low salinity and a state with sparse or no vegetation, shallow water table, and high salinity can be stable. In this paper, we develop a modeling framework to relate vegetation-soil salinity feedbacks to the emergence of multiple stable states in the underlying dynamics. This model is used to simulate various scenarios involving changes in forcing parameters of salinity-vegetation dynamics using data from the Murray-Darling Basin. Results show the presence of a strong feedback resulting in bistable dynamics for a wide range of environmental conditions, which has the effect of reducing the resilience of plant ecosystems and the productivity of agricultural systems for areas where such a feedback can occur. [ABSTRACT FROM AUTHOR]

The Gravity Recovery and Climate Experiment (GRACE, 2003–2017) and its Follow‐On mission GRACE‐FO (2018‐now) provide global estimates of the vertically integrated Terrestrial Water Storage Changes (TWSC). Since 2015, the Soil Moisture Active Passive (SMAP) radiometer observes global L‐band brightness temperatures, which are sensitive to near‐surface soil moisture. In this study, we introduce our newly developed Constrained Bayesian (ConBay) optimization approach to merge the TWSC of GRACE/GRACE‐FO along with SMAP soil moisture data into the ∼10 km resolution W3RA water balance model. ConBay is formulated based on two hierarchical multivariate state‐space models to (I) separate land hydrology compartments from GRACE/GRACE‐FO TWSC, and (II) constrain the estimation of surface soil water storage based on the SMAP data. The numerical implementation is demonstrated over the High Plain (HP) aquifer in the United States between 2015 and 2021. The implementation of ConBay is compared with an unconstrained Bayesian formulation, and our validations are performed against in‐situ USGS groundwater level observations and the European Space Agency (ESA)'s Climate Change Initiative (CCI) soil moisture data. Our results indicate that the single GRACE/GRACE‐FO assimilation improves particularly the groundwater compartment. Adding SMAP data to the ConBay approach controls the updates assigned to the surface storage compartments. For example, correlation coefficients between the ESA CCI and the ConBay‐derived surface soil water storage (0.8) that are considerably higher than those derived from the unconstrained experiment (−0.3) in the North HP. The percentage of updates introduced to the W3RA groundwater storage is also decreased from 64% to 57%. Key Points: A Constrained Bayesian (ConBay) optimization approach is introduced to integrate GRACE/GRACE‐FO and SMAP data into a modelThe ConBay integration of GRACE/GRACE‐FO and SMAP controls the updates assigned to the surface soil and groundwater compartmentsThe ConBay approach considerably reduced the phase shift differences between the model‐derived groundwater storage and USGS observation [ABSTRACT FROM AUTHOR]

This work presents a new conservative finite-volume numerical solution for the two-dimensional groundwater flow (Boussinesq) equation, which can be used for investigations of hillslope subsurface flow processes and simulations of catchment hydrology. The Boussinesq equation, which is integrated for each grid element, can take account of the local variations of topography and soil properties within the individual elements. The numerical method allows for wetting and drying of the water table, without "ad hoc assumptions." The stability and convergence of the method is shown to be guaranteed a priori by the properties of the solver itself, even with respect to the boundary conditions, an aspect that has been neglected in previous literature. The numeric solutions are validated against some approximate analytical solutions and compared to those of another (1-D) numerical solver of the Boussinesq equation. The solver capabilities are further explored with simulations of the Panola experimental hillslope where the bedrock topography, which is accurately known, causes complex wetting and drying patterns; in this situation, the importance of a two-dimensional description of subsurface flows to obtain properly simulated discharges becomes clear. Finally, a comparison is made between the results of the presented algorithm and the output of the GEOtop hydrological distributed model, which simulates variably saturated soils; the findings of the comparison are discussed. [ABSTRACT FROM AUTHOR]

Though the essence of the scientific literature is to be a repository of unaffiliated truths, scientific advancement fundamentally stems from the insights and efforts of individuals. This dichotomy can hide exemplars for young scholars of how to contribute to scientific understanding. This section of Water Resources Research addresses eminently successful strategies for advancement of the science of hydrology by exploring the ramifications of the work from Drs. Wilfried Brutsaert and Jean-Yves Parlange, colleagues who made many of the most significant contributions to the understanding of hydrologic processes of the last 50 years. The generous scope of the special section follows the key areas of their contributions, but the content looks forward from their work. Important and novel results span solute transport, infiltration, streamflow generation, and evaporation. [ABSTRACT FROM AUTHOR]

Small mountain lakes function as temporary storage basins for rain and snowmelt‐derived water. Many small lakes lose water seasonally, but questions remain about the processes involved and effects on watershed hydrology. Evaporation and groundwater outflow from lakes may influence baseflow in streams, hydrologic connections among lakes, and water fluxes from a watershed. To evaluate the role of small mountain lakes in watershed hydrology and the dominant pathways of water loss, we studied the water balances of four shallow, closed‐basin, subalpine lakes in southern Wyoming that lose up to 99% of their volumes between early summer and late fall. We tested the performance of seven evaporation models, compared observed rates of water loss with simulations of evaporation and drainage, and conducted geophysical surveys to evaluate the hydrologic environment between lakes. Our results show that groundwater outflow, rather than evaporation, can dominate water loss and cause closed‐basin mountain lakes to be ephemeral. Groundwater fluxes may contribute to varied rates and timing of water loss from the lakes. Evaporation accounted for 14% of water loss in a lake that overlays thin (<0.5 m) sediments and fractured bedrock and 83% in a lake underlain by >3 m of sediments and clay. Gradual recharge of groundwater (<18,000 m3·km−2·day−1) from each study lake likely helps sustain baseflow in streams once snowmelt has subsided. Total water loss from closed‐basin, subalpine lakes may therefore help to maintain baseflow of rivers in late summer, but their impact varies based on geological context and snowmelt availability. Key Points: Groundwater outflow dominates seasonal water loss (as much as 86%) from some closed‐basin, subalpine lakesSubsurface characteristics control seasonal water lossGroundwater outflow influences watershed hydrology [ABSTRACT FROM AUTHOR]

Wetlands provide crucial habitats, are critical in the global carbon cycle, and act as key biogeochemical and hydrological buffers. The effectiveness of these services is mainly controlled by hydrological processes, which can be highly variable both spatially and temporally due to structural complexity and seasonality. Spatial analysis of 2-D geoelectrical monitoring data integrated into the interpretation of conventional hydrological data has been implemented to provide a detailed understanding of hydrological processes in a riparian wetland. A two-layered hydrological system was observed in the peat. In the lower part of the peat, upwelling of deeper groundwater from underlying deposits was considered the driver for a 30% increase in peat resistivity during Winter/Spring. In Spring/Summer there was a 60% decrease in resistivity in the near-surface peats due to plant transpiration and/or microbial activity. Water exchange between the layers only appeared to be initiated following large drops in the encircling surface water stage. For the first time, we demonstrated that automated interpretation of geoelectrical data can be used to quantify ground movement in the vertical direction. Here, we applied this method to quantify shrink-swell of expandable soils, affecting hydrological parameters, such as, porosity and permeability. This study shows that an integrated interpretation of hydrological and geophysical data can significantly improve the understanding of wetland hydrological processes. Potentially, this approach can provide the basis for the evaluation of ecosystem services and may aid in the optimization of wetland management strategies. [ABSTRACT FROM AUTHOR]

Recent climate change has reduced the spatial extent and thickness of permafrost in many discontinuous permafrost regions. Rapid permafrost thaw is producing distinct landscape changes in the Taiga Plains of the Northwest Territories, Canada. As permafrost bodies underlying forested peat plateaus shrink, the landscape slowly transitions into unforested wetlands. The expansion of wetlands has enhanced the hydrologic connectivity of many watersheds via new surface and near-surface flow paths, and increased streamflow has been observed. Furthermore, the decrease in forested peat plateaus results in a net loss of boreal forest and associated ecosystems. This study investigates fundamental processes that contribute to permafrost thaw by comparing observed and simulated thaw development and landscape transition of a peat plateau-wetland complex in the Northwest Territories, Canada from 1970 to 2012. Measured climate data are first used to drive surface energy balance simulations for the wetland and peat plateau. Near-surface soil temperatures simulated in the surface energy balance model are then applied as the upper boundary condition to a three-dimensional model of subsurface water flow and coupled energy transport with freeze-thaw. Simulation results demonstrate that lateral heat transfer, which is not considered in many permafrost models, can influence permafrost thaw rates. Furthermore, the simulations indicate that landscape evolution arising from permafrost thaw acts as a positive feedback mechanism that increases the energy absorbed at the land surface and produces additional permafrost thaw. The modeling results also demonstrate that flow rates in local groundwater flow systems may be enhanced by the degradation of isolated permafrost bodies. [ABSTRACT FROM AUTHOR]

The age distribution of water in hydrologic systems has received renewed interest recently, especially in relation to watershed response to rainfall inputs. The purpose of this contribution is first to draw attention to existing theories of age distributions in population dynamics, fluid mechanics and stochastic groundwater, and in particular to the McKendrick-von Foerster equation and its generalizations and solutions. A second and more important goal is to clarify that, when hydrologic fluxes are modeled by means of time-varying stochastic processes, the age distributions must themselves be treated as random functions. Once their probabilistic structure is obtained, it can be used to characterize the variability of age distributions in real systems and thus help quantify the inherent uncertainty in the field determination of water age. We illustrate these concepts with reference to a stochastic storage model, which has been used as a minimalist model of soil moisture and streamflow dynamics. [ABSTRACT FROM AUTHOR]

Recent advances in geophysical methods have been increasingly exploited as inverse modeling tools in groundwater hydrology. In particular, several attempts to constrain the hydrogeophysical inverse problem to reduce inversion errors have been made using time-lapse geophysical measurements through both coupled and uncoupled (also known as sequential) inversion approaches. Despite the appeal and popularity of coupled inversion approaches, their superiority over uncoupled methods has not been proved conclusively; the goal of this work is to provide an objective comparison between the two approaches within a specific inversion modeling framework based on the ensemble Kalman filter (EnKF). Using EnKF and a model of Lagrangian transport, we compare the performance of a fully coupled and uncoupled inversion method for the reconstruction of heterogeneous saturated hydraulic conductivity fields through the assimilation of ERT-monitored tracer test data. The two inversion approaches are tested in a number of different scenarios, including isotropic and anisotropic synthetic aquifers, where we change the geostatistical parameters used to generate the prior ensemble of hydraulic conductivity fields. Our results show that the coupled approach outperforms the uncoupled when the prior statistics are close to the ones used to generate the true field. Otherwise, the coupled approach is heavily affected by 'filter inbreeding' (an undesired effect of variance underestimation typical of EnKF), while the uncoupled approach is more robust, being able to correct biased prior information, thanks to its capability of capturing the solute travel times even in presence of inversion artifacts such as the violation of mass balance. Furthermore, the coupled approach is more computationally intensive than the uncoupled, due to the much larger number of forward runs required by the electrical model. Overall, we conclude that the relative merit of the coupled versus the uncoupled approach cannot be assumed a priori and should be assessed case by case. [ABSTRACT FROM AUTHOR]

Detailed numerical analyses of flow and transport were used to investigate the effect of spatially connected features on transport in three-dimensional (3-D), spatially heterogeneous, combined vadose zone-groundwater flow systems. Formations with spatially connected fine-textured and coarse-textured features (F-formation and C-formation, respectively), representing the10th and the 90th percentiles of the distributions of the formation's hydraulic parameters, respectively, were considered here. Results of the analyses suggest that in steady state flow, when the unsaturated zone of the combined flow domains is relatively wet, as compared with a Multivariate-Gaussian (MG) formation, spatially connected features may reduce the solute first arrival time, particularly in the C-formation, and may enhance the spreading of the solute breakthrough, particularly in the F-formation. The effect of the spatially connected features on the hydrological response, however, decreases as the unsaturated zone becomes drier. The latter result stems from the decrease in the fraction of the water-filled, pore-space occupied by the connected structures, with decreasing water content. The latter finding also explains the result that the response of more realistic, combined flow systems, whose unsaturated zone is associated with relatively low, intermittent water contents, is essentially independent of the spatially connected features of the formations, regardless of their soil texture. [ABSTRACT FROM AUTHOR]

We study the effect of background flow on the dissolution and transport of carbon dioxide (CO2) during geological storage in saline aquifers, and include the processes of diffusion, advection, and free convection. We develop a semianalytical model that captures the evolution of the dissolution in the absence of free convection. Using the semianalytical solution, we determine scaling relations for the steady rate of dissolution that follow either [ABSTRACT FROM AUTHOR]

Assessing the uncertainties and understanding the deficiencies of climate models are fundamental to developing adaptation strategies. The objective of this study is to understand how well Coupled Model Intercomparison-Phase 5 (CMIP5) climate model simulations replicate ground-based observations of continental drought areas and their trends. The CMIP5 multimodel ensemble encompasses the Climatic Research Unit (CRU) ground-based observations of area under drought at all time steps. However, most model members overestimate the areas under extreme drought, particularly in the Southern Hemisphere (SH). Furthermore, the results show that the time series of observations and CMIP5 simulations of areas under drought exhibit more variability in the SH than in the Northern Hemisphere (NH). The trend analysis of areas under drought reveals that the observational data exhibit a significant positive trend at the significance level of 0.05 over all land areas. The observed trend is reproduced by about three-fourths of the CMIP5 models when considering total land areas in drought. While models are generally consistent with observations at a global (or hemispheric) scale, most models do not agree with observed regional drying and wetting trends. Over many regions, at most 40% of the CMIP5 models are in agreement with the trends of CRU observations. The drying/wetting trends calculated using the 3 months Standardized Precipitation Index (SPI) values show better agreement with the corresponding CRU values than with the observed annual mean precipitation rates. Pixel-scale evaluation of CMIP5 models indicates that no single model demonstrates an overall superior performance relative to the other models. [ABSTRACT FROM AUTHOR]

Recent U.S. Supreme Court rulings have limited federal protections for geographically isolated wetlands (GIWs) except where a 'significant nexus' to a navigable water body is demonstrated. Geographic isolation does not imply GIWs are hydrologically disconnected; indeed, wetland-groundwater interactions may yield important controls on regional hydrology. Differences in specific yield (Sy) between uplands and inundated GIWs drive differences in water level responses to precipitation and evapotranspiration, leading to frequent reversals in hydraulic gradients that cause GIWs to act as both groundwater sinks and sources. These reversals are predicted to buffer surficial aquifer dynamics and thus base flow delivery, a process we refer to as landscape hydrologic capacitance. To test this hypothesis, we connected models of soil moisture, upland water table, and wetland stage to simulate hydrology of a low-relief landscape with GIWs, and explored the influences of total wetland area, individual wetland size, climate, and soil texture on water table and base flow variation. Increasing total wetland area and decreasing individual wetland size substantially decreased water table and base flow variation (e.g., reducing base flow standard deviation by as much as 50%). GIWs also decreased the frequency of extremely high and low water tables and base flow deliveries. For the same total wetland area, landscapes with fewer (i.e., larger) wetlands exhibited markedly lower hydrologic capacitance than those with more (i.e., smaller) wetlands, highlighting the importance of small GIWs to regional hydrology. Our results suggest that GIWs buffer dynamics of the surficial aquifer and stream base flow, providing an indirect but significant nexus to the regional hydrologic system. [ABSTRACT FROM AUTHOR]

We explore the possibility of using remotely sensed soil moisture data and in situ discharge observations to calibrate a large-extent hydrological model. The model used is PCR-GLOBWB-MOD, which is a physically based and fully coupled groundwater-land surface model operating at a daily basis and having a resolution of 30 arc sec (about 1 km at the equator). As a test bed, we use the combined Rhine-Meuse basin (total area: about 200,000 km2), where there are 4250 point-scale observed groundwater head time series that are used to verify the model results. Calibration is performed by simulating 3045 model runs with varying parameter values affecting groundwater head dynamics. The simulation results of all runs are evaluated against the remotely sensed soil moisture time series of SWI (Soil Water Index) and field discharge data. The former is derived from European Remote Sensing scatterometers and provides estimates of the first meter profile soil moisture content at 30 arc min resolution (50 km at the equator). From the evaluation of these runs, we then introduce a stepwise calibration approach that considers stream discharge first, then soil moisture, and finally verify the resulting simulation to groundwater head observations. Our results indicate that the remotely sensed soil moisture data can be used for the calibration of upper soil hydraulic conductivities determining simulated groundwater recharge of the model. However, discharge data should be included to obtain full calibration of the coupled model, specifically to constrain aquifer transmissivities and runoff-infiltration partitioning processes. The stepwise approach introduced in this study, using both discharge and soil moisture data, can calibrate both discharge and soil moisture, as well as predicting groundwater head dynamics with acceptable accuracy. As our approach to parameterize and calibrate the model uses globally available data sets only, it opens up the possibility to set up large-extent coupled groundwater-land surface models in other basins or even globally. [ABSTRACT FROM AUTHOR]

Correlations among observation errors are typically omitted when calculating observation weights for model calibration by inverse methods. We explore the effects of omitting these correlations on estimates of parameters, predictions, and uncertainties. First, we develop a new analytical expression for the difference in parameter variance estimated with and without error correlations for a simple one-parameter two-observation inverse model. Results indicate that omitting error correlations from both the weight matrix and the variance calculation can either increase or decrease the parameter variance, depending on the values of error correlation ( ρ) and the ratio of dimensionless scaled sensitivities ( rdss). For small ρ, the difference in variance is always small, but for large ρ, the difference varies widely depending on the sign and magnitude of rdss. Next, we consider a groundwater reactive transport model of denitrification with four parameters and correlated geochemical observation errors that are computed by an error-propagation approach that is new for hydrogeologic studies. We compare parameter estimates, predictions, and uncertainties obtained with and without the error correlations. Omitting the correlations modestly to substantially changes parameter estimates, and causes both increases and decreases of parameter variances, consistent with the analytical expression. Differences in predictions for the models calibrated with and without error correlations can be greater than parameter differences when both are considered relative to their respective confidence intervals. These results indicate that including observation error correlations in weighting for nonlinear regression can have important effects on parameter estimates, predictions, and their respective uncertainties. [ABSTRACT FROM AUTHOR]

There is widespread recognition that the groundwater-surface water interface can have significant influence on the pattern and form of the transfer of nutrient-rich groundwater to rivers. Characterizing and quantifying this influence is critical for successful management of water resources in many catchments, particularly those threatened by rising nitrate levels in groundwater. Building on previous experimental investigations in one such catchment: the River Leith, UK, we report on a multimeasurement, multiscale program aimed at developing a conceptualization of groundwater-surface water flow pathways along a 200 m reach. Key to this conceptualization is the quantification of vertical and horizontal water fluxes, which is achieved through a series of Darcian flow estimates coupled with in-stream piezometer tracer dilution tests. These data, enhanced by multilevel measurements of chloride concentration in riverbed pore water and water-borne geophysical surveying, reveal a contrast in the contribution of flow components along the reach. In the upper section of the reach, a localized connectivity to regional groundwater, that appears to suppress the hyporheic zone, is identified. Further downstream, horizontal (lateral and longitudinal) flows appear to contribute more to the total subsurface flow at the groundwater-surface water interface. Although variation in hydraulic conductivity of the riverbed is observed, localized variation that can account for the spatial variability in flow pathways is not evident. The study provides a hydrological conceptualization for the site, which is essential for future studies which address biogeochemical processes, in relation to nitrogen retention/release. Such a conceptualization would not have been possible without a multiexperimental program. [ABSTRACT FROM AUTHOR]

The western United States is marked by limited water resources and a fast-growing population. Increasing climate variability as well as a growing demand on water resources highlights the need for improved understanding of connections between regional climate, surface water dynamics, and groundwater recharge. The current study focuses on the linkages between climate variability and groundwater levels in Calleguas Creek watershed located in southern California. The Calleguas Creek groundwater system serves as a critical source of water supply for agricultural and industrial use. Precipitation time series and groundwater levels were analyzed throughout the Calleguas Creek watershed for the period 1975-2004. Water level variability was analyzed for over 311 individual wells with a subset of 20 wells selected for further analysis. A correlation matrix was computed to establish well locations (or groups) with similar hydrologic behavior. Prewhitening methods were used to evaluate the effect of time series autocorrelation on the test statistics for trend detection using the Mann-Kendall test. Both climate and selected groundwater level (well) data were subjected to frequency analysis using fast Fourier transform. The time series of precipitation, the El Niño-Southern Oscillation (ENSO) index, and well levels were analyzed. A strong persistence was observed in the groundwater level time series, ranging from 66% to 99%. Results suggest the existence of significant periodicities between 2.0 and 7.0 years in both the precipitation and the well level data that are partially coincident with ENSO modes. A decadal oscillation was also observed in the well level data, which partially corresponds with the Pacific Decadal Oscillation. Assessment of the complex interactions between climate variability and groundwater levels will facilitate improved water resources planning and management in water-stressed regions where marginal changes in hydrologic budgets have significant implications. [ABSTRACT FROM AUTHOR]

This study investigates the effect of climate change on a groundwater-influenced ecosystem on a hill slope consisting of two vegetation types, one adapted to wet and one adapted to dry soil conditions. The individual effects of changes in precipitation, temperature, and atmospheric CO2 concentration are compared to the combined effect of these factors. Change in atmospheric conditions is based on the Netherlands. Projected climate change is obtained from an ensemble of nested global and regional climate models (GCMs and RCMs), representing the Intergovernmental Panel on Climate Change Special Report on Emissions Scenarios A2 scenario for 2100. For each GCM-RCM combination, change factors were determined and transferred to a stochastic weather generator. All projections show higher temperatures and less annual precipitation. Simulations were performed using an ecohydrological model, consisting of a dynamic soil-plant-atmosphere-continuum model that is fully coupled to a variably saturated hydrological model, using the stochastic weather data as input. Model results show that increasing atmospheric CO2 concentration results in higher biomasses because of higher water use efficiency and a decrease in evaporation downslope where vegetation growth is light limited. The change in precipitation regime (drier summers, wetter winters) causes a decreased biomass of especially the dry-adapted species and increased upslope groundwater recharge, resulting in groundwater rise and an upward shift of wet-adapted vegetation. Temperature rise results in decreased biomass because respiration increases stronger than carbon assimilation, while increased transpiration causes drier soils and a prolonged period of water-limited growth. The combined effect is dominated by the increase in temperature and change in precipitation regime, causing decreased biomass throughout. Surprisingly, the effect on groundwater level depends on the degree by which precipitation distribution changes within the year, showing a drop at a small change and a rise when change is larger. This study thus shows that climate change effects on hydrology and vegetation are far from straightforward and call for fully coupled ecohydrological models and upslope-downslope interaction. [ABSTRACT FROM AUTHOR]

Local water storage changes (WSC) are a key component of many hydrological issues, but their quantification is associated with a high level of uncertainty. High precision in situ gravity measurements are influenced by these WSC. This study evaluates the influence of local WSC (estimated using hydrological techniques) on gravity observations at the Geodetic Observatory Wettzell, Germany. WSC are comprehensively measured in all relevant storage components, namely groundwater, saprolite, soil, topsoil, and snow storage, and their gravity response is calculated. Total local WSC are derived, and uncertainties are assessed. With the exception of snow, all storage components have gravity responses of the same order of magnitude and are therefore relevant for gravity observations. The comparison of the total gravity response of local WSC to the gravity residuals obtained from a superconducting gravimeter shows similarities in both short-term and seasonal dynamics. A large proportion of the gravity residuals can be explained by local WSC. The results demonstrate the limitations of measuring total local WSC using hydrological methods and the potential use of in situ temporal gravity measurements for this purpose. Nevertheless, due to their integrative nature, gravity data must be interpreted with great care in hydrological studies. [ABSTRACT FROM AUTHOR]

We consider parameters determined by the inversion of slug-test head recovery data with the homogeneous-parameter model of Cooper et al. [1967] to be weighted spatial averages of transmissivity and storage defined at a smaller scale. We quantify the spatial averaging using a power-average spatial filter expression. We determine the form of the filter function and the power exponent using numerically simulated slug-test data. The filter function that describes how smaller-scale transmissivity is weighted by slug tests displays an approximate 1/ r2 behavior, with r the radial distance from the well. The radius of the cylinderical volume that is averaged by the slug test is inversely proportional to the square root of the storage coefficient (larger averaging volume with smaller storage). The power exponent grows from −0.19 to 0.345 as the ratio of the characteristic scale of the heterogeneity to the characteristic scale of the averaging volume grows, although a power exponent of zero, corresponding to geometric averaging, provides good results for most simulations. Our results show that while slug tests are useful to estimate transmissivity, they have dubious value for estimating storage coefficients. We find that the transmissivity estimate is unbiased and does not appear to be strongly influenced by storage properties. The storage coefficient estimate is, however, strongly influenced by the transmissivity and is biased. We investigate the interaction between storage coefficient and transmissivity by examining an analytical slug-test model that contains two annular zones, each with distinct transmissivity and storage coefficient. [ABSTRACT FROM AUTHOR]