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Spectral perturbation techniques have been used previously to derive integral expressions for dispersive mixing in concentration-dependent transport in three-dimensional, heterogeneous porous media, where fluid density and viscosity are functions of solute concentration. Whereas earlier work focused on evaluating longitudinal dispersivity in isotropic media and incorporating the result in a mean one-dimensional transport model, the emphasis of this paper is on evaluation of the complete dispersion tensor, including the more general case of anisotropic media. Approximate analytic expressions for all components of the macroscopic dispersivity tensor are derived, and the tensor is shown to be asymmetric. The tensor is separated into its symmetric and antisymmetric parts, where the symmetric part is used to calculate the principal components and principal directions of dispersivity, and the antisymmetric part of the tensor is shown to modify the velocity of the solute body compared to that of the background fluid. An example set of numerical simulations incorporating the tensor illustrates the effect of density-coupled dispersivity on a sinking plume in an aquifer. The simulations show that the effective transverse vertical spreading in a sinking plume to be significantly greater than would be predicted by a standard density-coupled transport model that does not incorporate the coupling in the dispersivity tensor. [ABSTRACT FROM AUTHOR]

This paper presents a methodology to invert tracer arrival times and to incorporate travel time data in the delineation of well capture zones. Within a Bayesian framework the observed arrival times are used to obtain probability-based weights for each realization of the hydraulic conductivity field. Realizations that closely reproduce the observed arrival times are more likely to represent the 'true' or real conductivity field than realizations yielding worse predictions, and are consequently characterized by a higher conditional probability. In the prediction of the capture zones the realizations are weighted by their respective probability. We combine the arrival time data with conductivity measurements and head observations to delineate stochastic capture zones. The conductivity measurements update the prior distributions specified for the unknown structural parameters of the conductivity field, and are used as conditioning data in the generation of conditional conductivity fields. The parameter values used to generate the conductivity realizations are sampled by Monte Carlo from the updated parameter distributions. The head and travel time observations are subsequently used to assign probability-based weights to the conductivity realizations by applying Bayes' theorem. The hyperparameters of the error model are assumed unknown, and their effect is accounted for by marginalization. A synthetic flow setup consisting of a single extraction well in a fully confined aquifer under a regional gradient is used to illustrate the method. We evaluate the relative worth of the different data types by introducing them sequentially in the inverse methodology. Results indicate that the different data types are complementary and that the travel time data are essential to improve the predictions of the capture zones. This is confirmed by the results for the case where uncertainty in the homogeneous porosity is accounted for. [ABSTRACT FROM AUTHOR]

This paper presents a novel approach to the representation of pore-scale exclusion processes, based on the truncation of the distribution of local dispersive displacements in a random-walk particle model. This approach increases the mean velocity of colloidal-sized particles relative to inert solute tracers, and decreases the apparent dispersion. An equivalent continuum model, with modified velocity and dispersion parameters, is also derived. Both the particle and the equivalent continuum models are applied to the results of laboratory experiments on bacterial transport in intact cores from a research site near Oyster, Virginia. The particle-based model requires only modest truncation (8% maximum) of the particle velocity distribution function to closely reproduce the significant observed decrease in bacterial arrival times relative to a bromide tracer. The approach provides a conceptually appealing and consistent means of incorporating the exclusion process into groundwater transport models. [ABSTRACT FROM AUTHOR]

In the Parowan Valley of Utah, groundwater levels have declined by as much as 30 m over the past 50 years with accompanying subsidence rates of up to 5 cm/year. Traditional methods to estimate groundwater storage change use a combination of groundwater level and storativity estimates, but there is often considerable uncertainty in these. In this study, we demonstrate a new method that relies on a combination of geodetic data from InSAR, as well as groundwater level and pumping data, to estimate both the total groundwater storage loss and the percentages of storage loss in fine‐ and coarse‐grained layers within an aquifer system. We find that when aggregated over all of Parowan Valley, fine‐ and coarse‐grained layers account for roughly equal portions of the total groundwater storage loss. However, in confined aquifers, fine‐grained layers account for most of the storage loss. This has important implications on the source of groundwater in depleting aquifer systems, as many models do not account for fine‐grained layers as a source of water. We find that in the Parowan Valley, the aquifer depletion is roughly 12.5% of the volume of pumped groundwater, meaning that the remainder of pumped groundwater is sourced from net inflow. This study presents the first method that combines geodetic and in situ groundwater data to provide estimates of groundwater storage change that account for both coarse‐ and fine‐grained intervals, which are typically present in significant amounts in the major unconsolidated aquifer systems of the world. Plain Language Summary: As surface water becomes more scarce, groundwater represents an important source of freshwater. However, it is being depleted in many basins of the western US and world. The Parowan Valley, in southwest Utah, has seen groundwater levels decline by as much as 30 m over the past 50 years. This has resulted in subsidence rates of up to 5 cm/year. Monitoring groundwater storage loss is critical for implementing sustainable groundwater management plans. Subsidence can be measured with a satellite data set called InSAR (Interferometric Synthetic Aperture Radar) with high accuracy. This can then be coupled with ground‐based measurements to improve our understanding of groundwater storage loss. We implement this method in the Parowan Valley, and find that roughly half of the estimated groundwater storage loss comes from consolidation of fine‐grained materials within the aquifer, which are typically ignored in groundwater budgets. We also find that groundwater depletion is roughly 12.5% of the volume of pumped groundwater, meaning that if pumping were reduced by that amount, depletion of groundwater would likely cease. However, changing recharge and inflow of groundwater could result in renewed depletion in the future (5–10 years), and re‐assessing the water budget regularly is crucial for effective management. Key Points: New method developed to estimate groundwater storage loss using InSAR, groundwater‐level and pumping dataFine‐grained layers account for most aquifer storage loss in confined aquifersWater lost from fine‐grained layers is a significant but unsustainable source of water for confined aquifers [ABSTRACT FROM AUTHOR]

Groundwater abstraction and drainage are considered to be the main drivers for the salinization of low‐lying coastal groundwater systems, while the role of past boundary conditions is less clear. In this study, 3‐D paleo‐hydrogeological variable‐density groundwater flow and salt transport modeling ("paleo‐modeling") is applied to reconstruct the evolution of groundwater salinities during the Holocene, that is, the last 9,000 years, in Northwestern Germany. Novel aspects of this study include the consideration of highly resolved time‐variant boundary conditions in a 3‐D paleo‐modeling framework, for example, sea‐level rise, surface elevation and coastline changes, development of drainage networks and groundwater abstraction, as well as the quantification of isolated processes impacting salinization. Results show that salinization was a function of sea‐level rise from 9000 BP until 1300 CE. The creation of the dike line ∼1300 CE set the starting point for increasing anthropogenic control of the hydro(geo)logical system: changes in surface elevation and drainage of low‐lying marshes have become main drivers for salinization after 1600 CE when peat was artificially degenerated. Moreover, changes in the dike line caused by storm floods impacted the salinities. Model results for 2020 CE match well with present‐day salinity observations. Yet, salinization will continue in the future, as the hydro(geo)logical system has not reached an equilibrium. The presented paleo‐modeling framework can be viewed as a blueprint for similar low‐lying coastal groundwater systems, influenced by marine transgression and human development. Thereby, it enables the reconstruction of meaningful present‐day salinity distributions, serving as a vital basis for modeling future groundwater systems in a changing climate. Key Points: Salinization of a low‐lying coastal groundwater system during the Holocene, that is, 9000 BP until present‐day, was investigatedSea‐level rise and the evolution of paleogeography were major controls on groundwater salinization during most of the timeSurface elevation changes due to land cultivation, as well as the development of drainage networks, control salinization since ∼1600 CE [ABSTRACT FROM AUTHOR]

Groundwater in shallow aquifers is commonly used for community water supply in cold climates. Shallow groundwaters are inherently susceptible to contamination from land‐use and surface water intrusion threatening drinking water usage. We used a large‐scale snapshot data set of stable water isotopes from shallow glaciofluvial aquifers used for drinking water supply in Northern Finland to assess surface water intrusion risks and recharge conditions. This data set was supplemented by long‐term stable water isotope precipitation data, Geographic Information System proximity analysis and multivariate statistics. The isotope analysis suggest that a warm season contributes about 60% to the total annual precipitation in the region. This is reflected in the aquifers isotopic composition as it represents an approximately equal mixture of warm and cold season precipitation. Groundwater isotope data normalized to precipitation inputs by line‐conditioned excess (lc‐excess) was used to flag the water supply wells impacted by surface water intrusions. The proximity analysis showed some of the wells may be affected by intrusions from gravel pit ponds, lakes and peatland drainage. On the larger scale, the wells in coastal areas were more likely to have evaporated water (intrusion) compared to inland regions of Northern Finland due to lower water availability and the presence of man‐made structures. This application of stable water isotopes with lc‐excess is a useful approach not only for recharge studies but also within water management for supply well surface water contamination risk assessment. Plain Language Summary: Shallow unconfined aquifers are commonly used for water supply in cold climates. These groundwater resources are inherently susceptible to contamination, especially in areas where lake, stream or other surface water can seep into the groundwater. To assess surface water intrusion risks and groundwater recharge conditions, we sampled and analyzed a comprehensive part of drinking water supply wells along with precipitation in Northern Finland for stable water isotopes of oxygen and hydrogen, The isotope analysis suggested that the source of groundwater to be an approximately equal mixture of warm and cold season precipitation. Isotope data was used to flag the water supply wells impacted by surface water intrusions. Spatial analysis showed some of the wells may be affected by intrusions from gravel pit ponds, lakes and forestry ditches. The wells in coastal areas were more likely to have intrusion compared to inland regions of Northern Finland due to lower water availability and more dense land‐use. Our stable water isotope analysis is useful not only for hydrological studies but also for water management in water supply well contamination risk assessment. Key Points: Groundwater in shallow aquifers in Northern Finland is an approximately equal mixture of warm and cold season precipitationLine‐conditioned excess (lc‐excess) can be used to identify surface water intrusions into aquifers and recharge modeStable water isotopes can be used in water supply well surface water contamination risk assessments [ABSTRACT FROM AUTHOR]

Agriculture is the largest user of water in the United States. Yet, we do not understand the spatially resolved sources of irrigation water use (IWU) by crop. The goal of this study is to estimate crop‐specific IWU from surface water withdrawals (SWW), total groundwater withdrawals (GWW), and nonrenewable groundwater depletion (GWD). To do this, we employ the PCR‐GLOBWB 2 global hydrology model to partition irrigation information from the U.S. Geological Survey Water Use Database to specific crops across the Continental United States (CONUS). We incorporate high‐resolution input data on agricultural production and climate within the CONUS to obtain crop‐specific irrigation estimates for SWW, GWW, and GWD for 20 crops and crop groups from 2008 to 2020 at county spatial resolution. Over the study period, SWW decreased by 20%, while both GWW and GWD increased by 3%. On average, animal feed (alfalfa/hay) uses the most irrigation water across all water sources: 33 from SWW, 13 from GWW, and 10 km3/yr from GWD. Produce used less SWW (43%), but more GWW (57%), and GWD (27%) over the study time‐period. The largest changes in IWU for each water source between the years 2008 and 2020 are: rice (SWW decreased by 71%), sugar beets (GWW increased by 232%), and rapeseed (GWD increased by 405%). These results present the first national‐scale assessment of irrigation by crop, water source, and year. In total, we contribute nearly 2.5 million data points to the literature (3,142 counties; 13 years; 3 water sources; and 20 crops). Key Points: A national database of crop‐specific irrigation water use by source is developedAnimal feed uses the most irrigation water compared to other crops across all water sourcesRice decreased surface water use by 71%, sugar beets increased groundwater use by 232%, and rapeseed increased groundwater depletion by 405% from 2008 to 2020 [ABSTRACT FROM AUTHOR]

To assess the sustainability of rice production systems, it is essential to quantify its impact on the environment and energy flow. To analyze the energy flow and greenhouse gas emissions, a total of 200 paddy fields were selected based on the water source utilized for irrigation (river as the surface water source and well as the underground water source), transplanting methods (traditional and mechanical) and grain yield (low‐ and high‐yielding rice cultivars) based on a factorial experimental design (2 × 2 × 2 = 8 conditions). Data were collected in 2020 from 200 rice growers in northern Iran, using a face‐to‐face questionnaire survey. Results showed that the highest total energy use efficiency (EUE) (5.26 GJ ha−1) was observed under the surface water source + traditional + low‐yielding rice system. Although the total EUE in surface water source was higher than that in groundwater source (4.98 vs. 2.90). Water consumption by high‐yielding rice cultivars was higher, as compared to the low‐yielding ones. Electricity (35.2%) and nitrogen fertilizer (31.5%) inputs were the highest energy input for the groundwater and surface water sources, respectively. Average shares of the renewable and non‐renewable energies used in agroecosystems were 25.70% and 74.30% respectively, while the shares of the direct and indirect energies were 60.36% and 39.64%, respectively. The calculated global warming potential value in the paddy fields of northern Iran ranged from 878 to 2,666 kg eqCO2 ha−1. Amongst the studied conditions, the surface water source + traditional + high‐yielding rice system had the highest carbon efficiency and carbon sustainability index level. Overall, surface water source conditions were found to be a more environmentally‐friendly approach in northern Iran. These results, thus, demonstrated that there is a huge potential for improving energy efficiency in the studied conditions, especially for high‐yielding rice cultivars irrigated by surface water sources. These findings could help decision‐makers to better understand the linkage between various conditions, which is a prerequisite to any planning effort for future management strategies at national and international levels. Key Points: Energy inputs and energy use efficiency in rice production ranged from 36.7 to 78.3 GJ ha−1 and 1.33 to 2.52 in northern Iran, respectivelyElectricity and nitrogen fertilizer were the most energy‐intensive inputs under groundwater and surface water systems, respectivelyGlobal warming potential was lower under surface water than groundwater source [ABSTRACT FROM AUTHOR]

The article presents a reply by David C. Mays and Roseanna M. Neupauer to a commentary on their paper "Plume Spreading in Groundwater by Stretching and Folding," published in "Water Resources Research" journal. They agree with the commentary that relaxing the assumption of reinjection at a deterministic angle does not remove chaos from periodic injection and extraction schemes. They relate the concerns regarding reinjection in groundwater remediation.

The common‐pool nature of groundwater resources creates incentives to over pump that contribute to their rapid global depletion. In transboundary aquifers, users are separated by a territorial border and might face substantially different economic and hydrogeologic conditions that can alternatively dampen or amplify incentives to over pump. We develop a theoretical model that couples principles of game theory and groundwater flow to capture the combined effect of well locations and user asymmetries on pumping incentives. We find that heterogeneities across users (here referred to as asymmetries) in terms of either energy cost, groundwater profitability or aquifer response tend to dampen incentives to over pump. However, combinations of two or more types of asymmetry can substantially amplify common‐pool overdraft, particularly when the same user simultaneously faces comparatively higher costs (or aquifer response) and profitability. We use this theoretical insight to interpret the emergence of the Disi agreement between Saudi Arabia and Jordan in association with the Disi‐Amman water pipeline. By using bounded non‐dimensional parameters to encode user asymmetries and groundwater connectivity, the theory provides a tractable generalized framework to understand the premature depletion of shared aquifers, whether transboundary or not. Key Points: Economic and hydrogeologic differences between users affect common‐pool externalities in shared aquifersWhen combined, asymmetries in energy cost, groundwater profitability and aquifer response can exacerbate incentives to overpumpChanging asymmetry conditions might have facilitated the world's first distance‐based groundwater treaty [ABSTRACT FROM AUTHOR]

In urban Indonesia, more than 40 million people rely on groundwater self‐supply, but the extent to which self‐supply delivers safe water and the associated risk factors for fecal contamination remain unclear. This study quantified Escherichia coli (E. coli) for 511 self‐supply sources and at point‐of‐use for 173 households in the Indonesian cities of Bekasi and Metro. A structured questionnaire collected information about the household, water sources, and potential contamination sources. Univariate and multivariate logistic regression analysis examined risk factors for fecal contamination. E. coli was detected in 66% of sources, including 55% of boreholes, 64% of protected dug wells, and 82% of unprotected dug wells. Widespread boiling of water meant microbial quality improved significantly between source and point‐of‐use, with E. coli detected in 30% of self‐supply samples at point‐of‐use. Unprotected dug wells were significantly more likely to be contaminated than boreholes. In Bekasi, the analysis found a significant association between presence of E. coli and sanitation systems located within 10 m of the groundwater source. In Metro, poorer households had significantly higher odds of contamination than wealthier households. Other significant factors included shallower borehole depths in Bekasi, use of a rope and bucket, and absence of a concrete platform in Metro. In Bekasi, E. coli concentration at source was significantly associated with water quality at point‐of‐use. Risk of fecal contamination could be reduced by supporting households to invest in improved protection, and by facilitating promotion for safe household water treatment. Support for self‐supply improvements should be weighed against the expansion and improvement of piped water services. Key Points: Risk factors for fecal contamination of groundwater self‐supply assessed in two Indonesian citiesContamination associated with lower socio‐economic status, proximity to sanitation, and lack of well protectionWidespread boiling of self‐supplied water significantly improves microbial quality between source and point‐of‐use [ABSTRACT FROM AUTHOR]

Globally, groundwater represents a critical natural resource that is affected by changes in natural supply and renewal, as well as by increasing human demand and consumption. However, despite its critical role, groundwater is difficult to accurately quantify as it is beneath the Earth surface. Here, we review several state‐of‐the‐art remote sensing techniques useful for local‐ to global‐scale groundwater monitoring and assessment, including proxies for groundwater extraction. These include inferring changes in subsurface water from mass changes using gravitational measurements, and analyzing changes in the Earth surface height using Interferometric Synthetic Aperture Radar, Light Detection and Ranging, Airborne Electromagnetic Systems, and satellite altimetry. Remote sensing information is often used in tandem with ground‐based observations such as hydraulic head in wells, Global Navigational Satellite System monitoring, and numerical modeling to complement the space‐based approaches. In the future, fusing different remote sensing techniques capable of operating in various environments will yield additional insight on the state and rate of use for groundwater across the globe. Key Points: Accurately measuring and monitoring groundwater storage and fluxes is critical for water, food, and energy securityRemote sensing approaches such as gravitational measurements, Interferometric Synthetic Aperture Radar, Global Navigational Satellite System, lidar altimetry, and Airborne Electromagnetic Systems can yield indirect yet valuable information about groundwaterFusing multiple remotely sensed data sets or employing other tools such as numerical models increase the applicability of individual approaches [ABSTRACT FROM AUTHOR]

Drought early warning systems (DEWSs) aim to spatially monitor and forecast risk of water shortage to inform early, risk‐mitigating interventions. However, due to the scarcity of in situ monitoring in groundwater‐dependent arid zones, spatial drought exposure is inferred using maps of satellite‐based indicators such as rainfall anomalies, soil moisture, and vegetation indices. On the local scale, these coarse‐resolution proxy indicators provide a poor inference of groundwater availability. The improving affordability and technical capability of modern sensors significantly increases the feasibility of taking direct groundwater level measurements in data‐scarce, arid regions on a larger scale. Here, we assess the potential of in situ monitoring to provide a localized index of hydrological drought in Somaliland. We find that calibrating a lumped groundwater model with a short time series of groundwater level observations substantially improves the quantification of local water availability when compared to satellite‐based indices. By varying the calibration length, we find that a 5‐week period capturing both wet and dry season conditions provides most of the calibration capacity. This suggests that short monitoring campaigns are suitable for improving estimations of local water availabilities during drought. Short calibration periods have practical advantages, as the relocation of sensors enables rapid characterization of a large number of wells. These well simulations can supplement continuous in situ monitoring of strategic point sources to setup large‐scale monitoring systems with contextualized and localized information on water availability. This information can be used as early warning evidence for the financing and targeting of early actions to mitigate impacts of hydrological drought. Key Points: Advancements in sensing technologies give renewed feasibility to in situ groundwater monitoring in data‐scarce, drought‐prone countriesCalibrating groundwater models with short observation records (weeks) substantially improves on satellite‐based drought exposure indicatorsImproved water availability assessment with in situ sensors provides opportunities for better drought early warning and early action [ABSTRACT FROM AUTHOR]

This study presents the analytical expressions of drainable and fillable porosity for layered soils under shallow groundwater environments. The expressions of drainable porosity λd and fillable porosity λf for two‐layered soils are first derived with water table depth change ∆d→0 under dynamic soil moisture conditions. The expressions of λd and λf consider the dynamic soil moisture conditions through the assumption of vertical fluxes in the unsaturated zone, varying as functions of water table elevation h. The analytical expressions of λd and λf have the same formula form but different values under evapotranspiration and recharge conditions, which is also verified using the numerical calculation. The case comparisons indicate that the vertical distribution of soil layers and the fluxes have significant effects on the performance of λd and λf. Our proposed expressions could effectively characterize the effects of soil layering and vertical fluxes on λd and λf. They could improve the accuracy of the previous expressions for single‐layer soils, and complement the lack of description for layered soils under dynamic soil moisture conditions in previous expressions. Furthermore, under the hydrostatic assumption (static equilibrium of soil water pressure distribution) in the unsaturated zone, the expressions of drainable porosity λd* and fillable porosity λf* for two‐layered soils with large water table fluctuations are derived based on λd and λf (asterisks in λd* and λf* is to distinguish them from λd and λf derived based on ∆d→0). Verification with drainage experimental data indicates that the proposed expressions can be extended to well capture the λd* and λf* changes for two‐layered soils. Application scenarios suggest that our analytical expressions of drainable and fillable porosity are useful to improve the application performance of related subsurface modeling and groundwater estimation for layered soils under shallow water table environments. Key Points: Analytical expressions of drainable and fillable porosity for layered soils were proposed under vertical fluxes conditionsExpressions of drainable/fillable porosity for layered soils with large water table fluctuations were derived under hydrostatic conditionsExpressions were useful to improve the application performance of subsurface modeling and groundwater estimation for layered soils [ABSTRACT FROM AUTHOR]

Groundwater dynamics in continental rift zone settings remain poorly understood because of the spatial heterogeneity in flow, storage, and recharge dynamics. The Central Kenya Rift is an excellent example where, though groundwater is important for domestic, irrigation, and geothermal energy exploitation, its hydrogeological properties remain largely unknown. Existing conceptual groundwater models assume flow from the high‐elevation, humid rift flanks to the low‐elevation, semiarid rift floor, but the role of the faults that fracture the aquifers is commonly unaccounted for. We applied geochemical, isotopic (δ18O, δD, 87Sr/86Sr, 3H‐3He), multivariate statistical methods, and knowledge of geological structures to determine recharge sources, flow, and residence times to revise the conceptual flow model. Results show that groundwater is primarily recharged by meteoric waters, river input, and Lake recharge. The faults impart a control on the groundwater flow within four sub‐compartments. Major differences in flow patterns exist between the eastern and western rift flanks: surface and groundwater transfer from the eastern flanks to the rift floor occurs via relay ramp structures, and flow on the west side takes place laterally from the high escarpment. Although 3H‐3He dating shows that the age of groundwater ranges from a few to >>50 years, most of the groundwater in the rift‐floor area is free of 3H and was recharged before the 1960s. Hence, we propose that these areas receive episodic recharge and represent the most sensitive groundwater resources in the rift. To inform sustainable groundwater development, a robust monitoring network is required to capture the heterogeneous groundwater dynamics. Key Points: Fault ramps are important groundwater transfer zones within the rift blocksFault displacements exert control on recharge and groundwater availability within the rift floorMajority of the water samples from the rift floor lack tritium, meaning that active recharge is minimal [ABSTRACT FROM AUTHOR]

Many irrigated agricultural areas seek to prolong the lifetime of their groundwater resources by reducing pumping. However, it is unclear how lagged responses, such as reduced groundwater recharge caused by more efficient irrigation, may impact the long‐term effectiveness of conservation initiatives. Here, we use a variably saturated, simplified surrogate groundwater model to: (a) analyze aquifer responses to pumping reductions, (b) quantify time lags between reductions and groundwater level responses, and (c) identify the physical controls on lagged responses. We explore a range of plausible model parameters for an area of the High Plains aquifer (USA) where stakeholder‐driven conservation has slowed groundwater depletion. We identify two types of lagged responses that reduce the long‐term effectiveness of groundwater conservation, recharge‐dominated and lateral‐flow‐dominated, with vertical hydraulic conductivity (KZ) the major controlling variable. When high KZ allows percolation to reach the aquifer, more efficient irrigation reduces groundwater recharge. By contrast, when low KZ impedes vertical flow, short term changes in recharge are negligible, but pumping reductions alter the lateral flow between the groundwater conservation area and the surrounding regions (lateral‐flow‐dominated response). For the modeled area, we found that a pumping reduction of 30% resulted in median usable lifetime extensions of 20 or 25 years, depending on the dominant lagged response mechanism (recharge‐ vs. lateral‐flow‐dominated). These estimates are far shorter than estimates that do not account for lagged responses. Results indicate that conservation‐based pumping reductions can extend aquifer lifetimes, but lagged responses can create a sizable difference between the initially perceived and actual long‐term effectiveness. Key Points: The long‐term effectiveness of pumping reduction‐based groundwater conservation is dependent on lagged processesVertical hydraulic conductivity (KZ) controls if lagged responses are lateral‐flow dominated or recharge‐dominatedIgnoring lagged processes overestimates aquifer lifetime by 32 and 133 years in lateral‐flow and recharge‐dominated settings, respectively [ABSTRACT FROM AUTHOR]

Shrimp aquaculture has expanded rapidly in coastal zones worldwide over the past few decades. Saline water stored in shrimp farm ponds can infiltrate into the underlying aquifer causing groundwater salinization and increased submarine groundwater discharge (SGD) to coastal water. However, little research has assessed salinization resulting from these shrimp ponds. To understand the impacts of shrimp farm irrigation on groundwater salinization and SGD, we numerically simulated a series of aquaculture management scenarios in a two‐dimensional conceptual coastal aquifer using a coupled surface‐subsurface approach. We characterized sensitivities to pond water salinity, pond water depth, and farm width. Salinization was assessed by three indicators (salinized area, infiltrated salt mass, and recovery rate), and three SGD indicators were evaluated (fresh SGD, saline SGD, and saltwater circulation rate). Our results show that pond water depth is the primary control on the mass of saltwater infiltration while farm width is the primary control for recovery rate. Pond water salinity and depth affect both fresh and saline SGD. We show that aquaculture is a previously unrecognized mechanism of salinization affecting coastal aquifer vulnerability and SGD. A regional graphical information system analysis shows transformation into aquacultural ponds could introduce considerable SGD variability spatially and temporally. These findings will enable coastal managers to better evaluate groundwater vulnerability in regions with expanding onshore aquaculture and demonstrates the impact of aquaculture on coastal groundwater resources and the need for further study to understand the impact of aquaculture across Asia and the globe. Plain Language Summary: Understanding coastal groundwater salinity and flow is important for managing limited fresh groundwater resources and protecting precious estuarine environments, especially in densely populated coastal areas. Coastal groundwater studies have shown the impacts of diverse natural hydrogeological driving forces and settings. Recently, global expansion of coastal aquaculture ponds has raised environmental concerns about contaminants discharging to surface waters, but less attention has focused on groundwater flow. We developed conceptual models of coastal shrimp ponds to simulate how aquaculture impacts groundwater salinity and discharge into nearby coastal waterbodies. Our results show that coastal aquaculture ponds can contaminate surficial aquifers with saline water and increase discharge of groundwater and solutes to the ocean. This work improves the understanding of human‐induced saltwater intrusion with scientific implications for coastal groundwater practitioners and estuary management. Key Points: Coastal shrimp pond farms cause vertical saltwater intrusion and increased submarine groundwater dischargePond depth controls vertical extent of saltwater intrusion; farm width controls recovery ratesPond water salinity and depth affect character of both fresh and saline submarine groundwater discharge [ABSTRACT FROM AUTHOR]

The interaction between groundwater flow and river flow plays an important role in the bank erosion process, but field measurements on riparian groundwater flows remain very limited, especially in large river systems. Three monitoring wells of groundwater flow were constructed at two typical sites of the Middle Yangtze River (MYR) in 2021, with the hydrographs of the riparian groundwater level and river stage being obtained. Results indicated that (a) the variation in the groundwater level generally followed with the river stage during the flood peak period, but decoupled from the river stage during the recession period; (b) the delayed response of the groundwater level to river stage variation was quantified by the ratio of their change rates, which linearly increased with the magnitude of the river stage, but decreased with the change rate of the river stage as a loose power function; and (c) the increasing ground water level resulted in a 61% loss of the cohesion of upper cohesive bank soil and a complete loss of the apparent cohesion of the lower noncohesive bank soil. The major mechanism underlying the influences of the groundwater flow on bank erosion in the MYR is probably the development of pore water pressure that changes the forces acting on soil block and reduces the soil shear strength, whereas seepage erosion is seemingly not intensive. The current study can provide a direct evidence for the response of the riparian groundwater level variation to the river stage change in a large river system. Key Points: Riparian groundwater level hydrographs were measured for the first time at two sites of the Middle Yangtze RiverDelayed response of groundwater level to river stage change was quantitatively illustratedIntegrated impacts of river stage and groundwater level changes on bank erosion were investigated [ABSTRACT FROM AUTHOR]

Unsuccessful drillings are issues in groundwater exploration using electrical resistivity profiling (ERP) and vertical electrical sounding (VES). Many geophysical companies spend a lot of money without obtaining the flow rate (FR) required during the campaigns for drinking water supply (CDWS). To solve this problem, we applied the support vector machines (SVMs) to real‐world data to predict the FRs before any drilling operations. First, from the ERP and VES, the features such as shape, type, power, magnitude, pseudo‐fracturing index, and ohmic‐area were defined including the geology of the survey area. Second, the FRs were categorized into four classes (dry: FR0 (FR = 0), unsustainable: FR1 (0 < FR ≤ 1), and productive boreholes: FR2 (1 < FR ≤ 3) and FR3 (FR > 3 m3/hr)) and associated with the features to compose two separated data sets: a multiclass data set (D $\mathcal{D}$) for common prediction during the CDWS and a binary data set Db ${\mathcal{D}}_{b}$ (FR < FR2, FR ≥ FR2) addressed to the population living in a rural area. Features were vectorized and data were transformed before feeding to the SVM algorithms. As a result, the SVM models performed 77% of good predictions on D $\mathcal{D}$ and 83% on Db ${\mathcal{D}}_{b}$. Better performances with the optimal hyper‐parameters in D $\mathcal{D}$ (81.61%) and Db ${\mathcal{D}}_{b}$ (87.36%) were achieved using the polynomial and radial basis function kernels respectively. Furthermore, the learning curves have shown that the performance scores on D $\mathcal{D}$ can be improved if larger training data becomes available (275 test samples at least) while it is not necessarily so for Db ${\mathcal{D}}_{b}$. As a benefit, the proposed approach could minimize the rate of unsuccessful drillings during future CDWS. Plain Language Summary: The electrical resistivity profiling and the vertical electrical sounding are cheap geophysical subsurface imaging methods. They are most preferred to find groundwater during the campaigns of drinking water supply, especially in developing countries. However, despite the use of both methods, the numerous unsuccessful drillings, due to their wrong locations, have considerably increased the budget of the project and thereby limiting the number of boreholes previously intended for the population. To solve this problem, a technique was developed using one famous method of artificial intelligence called support vector machines to predict the flow rate (FR) before the drilling operations. To check the efficiency of the proposed approach, the technique was tested with data from a region in the northern part of Cote d'Ivoire (West Africa), which faced a considerable water shortage. The results show 77% capability to predict an accurate FR and 83% when the problem is addressed to the population living in a rural area. Henceforth, the proposed technique can be used to select the right locations expecting to give the recommended FR to minimize the rate of unsuccessful drillings, and indirectly reduce the problem of water scarcity. Key Points: The support vector machines (SVMs) are used to predict groundwater flow rate (FR) before the drilling operationsPrediction data are gathered from electrical profiling, vertical electrical sounding, and boreholesThe SVM models correctly perform 77% and 83% of FR predictions for multiclass and binary data sets respectively [ABSTRACT FROM AUTHOR]

Combined geological, hydrogeological, and geochemical controls on the arsenic concentration of contaminated aquifers in SE Asia were explored by two‐dimensional (2‐D) reactive transport modeling of data sets from Bangladesh, Cambodia, and Vietnam. For each site, the field data are summarized and used to create a conceptual 2‐D reactive transport model that elucidates characteristic features influencing the groundwater arsenic concentration. Comparison of models for Bangladesh and Vietnam indicates that fine‐grained layers overlying young sandy aquifers generate shallow high arsenic groundwater because low vertical groundwater velocities allow sufficient time for kinetic As release from the sediment. The low vertical groundwater velocity below major river channels, predicted by the model, also creates long groundwater residence times, leading to high arsenic groundwater. Young aquifer sediments release more arsenic than older sediments, and alternating young and older sediments create complex patterns of high and low arsenic groundwater. Over time, floodplain basins will subside, and river channels migrate, causing sedimentation and erosion on the floodplain while creating local environments with evolving hydrogeology and groundwater geochemistry. We have developed a three‐step model for the evolution of the Red River floodplain with sedimentation and shifting channels over the last 6000 years. The results show comparable timescales between the dynamics of arsenic release and of river migration, causing complex groundwater As distributions, comprising geochemical palinopsia of long vanished rivers. Plain Language Summary: More than 100 million people in Southeast Asia are affected by too high arsenic concentrations in their groundwater. One of the issues that makes this difficult to handle is that the concentration of arsenic varies enormously within very short distances, making it difficult to find and exploit groundwater low in arsenic. The paper shows how this can be a consequence of complex interactions between the release of arsenic from rust coatings on the sand and clay due to microbes obtaining energy from dissolving the rust using organic matter, the rebinding of arsenic to the rust, the groundwater flow, and the dynamically changing river delta system, leaving chemical afterimages of ancient rivers in the groundwater. To show this, a number of well‐studied sites are analyzed using 2‐D computer calculations that include all the mentioned processes including a geochemical mathematical description of the processes as they change over time as the reactivity of the organic matter and the rust decreases. Key Points: Characteristic examples of high As groundwater can be modeled by 2‐D reactive transport models using PHAST with a common geochemical modelThe 2‐D reactive transport models may include effects of geological development with shifting channels, erosion, and depositionThe observed variability in the As distribution is a function of the sedimentological history, hydrogeology, and age‐dependent release of As [ABSTRACT FROM AUTHOR]

Groundwater use underpins much economic production. The unsustainable use of groundwater threatens environmental flows in surface waters, sustainable development, and future food security. The connection between agricultural trade and groundwater depletion has been recently highlighted, but how groundwater depletion supports the production of industrial and tertiary goods, trade, and consumption remains less well understood. Here, we present the first analysis of groundwater use and depletion embedded in the complete supply chain of China (including primary, secondary, and tertiary products). We use a multiregion input‐output analysis coupled with the high‐resolution groundwater use modeling to track groundwater depletion from production to end consumer. Our modeling results show that groundwater depletion occurred primarily in water scarce North China for agricultural production, but the depleted resource was then incorporated throughout the supply chain and dispersed across Chinese and international consumers. ∼64 billion m3 yr−1 (±1 billion m3 yr−1) groundwater was depleted in China, in which more than a half was from Xinjiang, Hebei, Henan and Heilongjiang Provinces. Approximately 40% of the groundwater depletion can be traced to interprovincial transfer (21 billion m3 yr−1) and export (4.8 billion m3 yr−1). The hot spots for final consumption of groundwater depletion were major cities in both North and South China. Importantly, over 60% of the groundwater depletion was embodied in industrial and tertiary products for final consumption, highlighting the importance of tracing groundwater through the full economy. Groundwater depletion represents a long‐term risk to supply chains, and policy‐makers can use this understanding to sustainably manage groundwater and diversify supply chains. Plain Language Summary: Groundwater is a critical resource that is under increased pressure due to unsustainable exploitation. Groundwater resources underpin both national supply chains and global trade, but how groundwater supports the full supply chain of a nation remains unresolved. This study traces the groundwater depletion incorporated in the complete supply chain (including industrial and tertiary products, not just primary goods) of China through interprovincial transfers and international exports. The results show how groundwater depletion, which is concentrated in North China, is distributed to domestic and international consumers of final goods. International importers receive China's groundwater in the receipt of industrial products. We highlight that groundwater depletion represents a long‐term risk to supply chains. Key Points: Groundwater depletion is traced using a multi‐region input‐output analysis coupled with the high‐resolution groundwater use modelingAbout 21 billion m3 yr−1 groundwater depletion, primarily from water scarce Northern‐China, is virtually transferred domesticallyGroundwater depletion supporting export to the Rest of the World is estimated to be 4.8 billion m3 yr−1 [ABSTRACT FROM AUTHOR]

We develop a physics‐informed machine learning approach for large‐scale data assimilation and parameter estimation and apply it for estimating transmissivity and hydraulic head in the two‐dimensional steady‐state subsurface flow model of the Hanford Site given synthetic measurements of said variables. In our approach, we extend the physics‐informed conditional Karhunen‐Loéve expansion (PICKLE) method to modeling subsurface flow with unknown flux (Neumann) and varying head (time‐dependent Dirichlet) boundary conditions. We demonstrate that the PICKLE method is comparable in accuracy with the standard maximum a posteriori (MAP) method, but is significantly faster than MAP for large‐scale problems. Both methods use a mesh to discretize the computational domain. In MAP, the parameters and states are discretized on the mesh; therefore, the size of the MAP parameter estimation problem directly depends on the mesh size. In PICKLE, the mesh is used to evaluate the residuals of the governing equation, while the parameters and states are approximated by the truncated conditional Karhunen‐Loéve expansions with the number of parameters controlled by the smoothness of the parameter and state fields, and not by the mesh size. For a considered example, we demonstrate that the computational cost of PICKLE increases near linearly (as N1.15) with the number of grid nodes N, while that of MAP increases much faster (as N3.28). We also show that once trained for one set of Dirichlet boundary conditions (i.e., one river stage), the PICKLE method provides accurate estimates of the hydraulic head for any value of the Dirichlet boundary conditions (i.e., for any river stage). Plain Language Summary: We present a novel physics‐informed machine learning framework for parameter and state estimation in large‐scale natural systems's models. Using the Hanford Site as an example, we demonstrate that the proposed framework is comparable in accuracy with the standard parameter estimation methods but is significantly faster than these methods. Key Points: The modified physics‐informed machine learning PICKLE method for large‐scale data assimilation is proposedPICKLE is orders of magnitude faster than traditional a posteriori probability method for the considered high‐resolution Hanford modelTrained for one set of boundary conditions, the PICKLE method can model data for different values of the boundary conditions [ABSTRACT FROM AUTHOR]

Estimating subsurface hydraulic properties using the groundwater (GW) response to Earth tides (ET) and atmospheric pressure is an alternative approach to pumping or slug tests (passive vs. active methods). Yet testing the applicability of models under different subsurface conditions and comparing results with traditional hydraulic methods are lacking. We first review the assumptions of analytical models used to evaluate the GW response to ET and their applicability in unconfined to confined conditions. Second, we develop a robust approach to select the right model based on amplitude and phase pattern of the diurnal and semi‐diurnal tides. Third, based on earlier works we develop an approach to derive the hydraulic conductivity of the screened interval using the GW response to atmospheric tides, here named "atmospheric slug test" (AST). We estimate transmissivity and storativity at three shallow aquifers in Cambodia and compare the results with subsurface properties derived from pumping tests (PT). Transmissivity values from AST and PT are in good agreement. However, we show that storativity values derived from ET show large discrepancies if borehole skin effects are ignored. Further, the GW response to ET exhibits a strong decay in amplitude with frequency while maintaining close to zero and positive phase shifts. When supplemented with the calculated transmissivity values, none of the analytical models was able to reproduce this frequency dependent behavior. Our work emphasizes the need for evaluating passive methods robustness under different subsurface conditions. Further work is required to understand the frequency dependent GW response to natural or artificial forces. Key Points: Using the phase shift sign and the M2/01 ratio we show how to select the right model for groundwater responses to Earth tidesBased on previous work, we develop a new "atmospheric slug test" to estimate the hydraulic conductivity of the screened subsurface intervalDiscrepancies between hydraulic results are attributed to matrix compressibility and frequency dependence of subsurface properties [ABSTRACT FROM AUTHOR]

The current theory for defining the occurrence of riparian lenses (i.e., buoyant, lenticular‐shaped fresh groundwater bodies overlying saline groundwater in riparian zones) is largely based on steady‐state analyses, which neglect the transient dynamics expected in real‐world settings. In this study, the transience of riparian lens movement is investigated for the first time, considering a fully penetrating, gaining river (i.e., the river receives groundwater influxes), and rapid variations in hydraulic boundary conditions. Controlled laboratory experiments and numerical modeling are used to determine the timescales associated with lens movements. Both numerical and experimental timescales show asymmetric behavior in the growth and decline of lenses, whereby growing lenses involve longer timescales than those of shrinking lenses. Inverse relationships between timescales and final hydraulic gradients are observed, such that higher final hydraulic gradients involved shorter timescales. Results also revealed a time lag in the movement of the lens tip (i.e., where the lens and water table intersect) relative to the lens toe (i.e., intersection of the lens with the riverbank). Riparian lenses have near‐identical timescales regardless of whether the river water or aquifer hydraulic head varies, and rather, timescales are more sensitive to the final hydraulic gradient. Application of typical parameter ranges for the River Murray (South Australia) floodplains reveals timescales of years to more than a decade to expand the riparian lens by tens of meters. Insights into riparian lenses transience attained from the current study build on the understanding of transience obtained for other mixed‐convective hydrogeological processes, such as seawater intrusion and retreat. Key Points: Laboratory experiments and numerical modeling of riparian lens transience show growing lenses have longer timescales than shrinking lensesTimescales for lens movements of tens of meters are expected to range from years to more than a decade for field‐scale conditionsThe lens is slower to grow (and shrink) laterally compared to the time required to increase (and decrease) in thickness [ABSTRACT FROM AUTHOR]

Working with airborne electromagnetic (AEM) data acquired in the Kaweah Subbasin in the Central Valley of California, USA, we developed a new approach for imaging the bedrock surface and the confining Corcoran Clay layer. Our approach, which incorporated the prior knowledge of the targets for the improved imaging, included multiple L2‐norm and Lp‐norm inversions as well as an interpolation process. The major improvement in imaging the targets was made in the Lp‐norm inversion step by incorporating prior knowledge. For the Corcoran Clay, pairs of resistivity and driller's logs at two wells guided the selection of the primary resistivity model and were used to increase the accuracy of the estimated Corcoran Clay thickness. The bedrock surface was poorly constrained by well data in the existing groundwater model, appearing as a flat surface. Acquired AEM data covered most of the area, so we had higher confidence in the obtained map of the bedrock surface at depths ranging from 15 to 160 m. There was relatively good agreement between the location of the Corcoran Clay in the AEM data (depth ranging from 50 to 130 m and thickness ranging from 3 to 25 m) and the existing groundwater model, with both depth and thickness showing ∼15% relative difference. The AEM data provided information about the continuity of the Corcoran Clay that is challenging to capture in the well data. The locations of the bedrock and Corcoran Clay were used in a structurally constrained inversion to improve the imaging of the smaller‐scale resistivity structure. Plain Language Summary: A groundwater model is a representation of a groundwater system used to both understand the properties and processes of the subsurface, and to manage the groundwater resource. An important step in developing a groundwater model is describing the large‐scale structure, the "plumbing system" that controls the groundwater flow. Given the size of many groundwater models, obtaining an accurate description of the large‐scale structure can be a challenging task. The traditional approach has involved the drilling of wells. While wells can provide accurate information at the location of the wells, the spatial sampling of the subsurface that can be accomplished through the drilling of wells is limited. An airborne electromagnetic (AEM) method provides an alternative by using a geophysical measurement to map out the electrical resistivity to depths of hundreds of meters. Working with AEM data acquired in the Kaweah Subbasin in the Central Valley of California, we were able to image the large‐scale structure—the bedrock surface and a thick confining clay layer. The key improvement was made by incorporating prior knowledge into the inversion of the data. Once we had obtained the large‐scale structure, we found that this could be used to improve the imaging of the smaller‐scale structure. Key Points: Airborne electromagnetic (AEM) data were used to map out the large‐scale structure in the Kaweah subbasin in California's Central ValleyA new approach to the inversion of AEM data improved the accuracy in estimates of the bedrock surface and the confining Corcoran Clay layerThe defined large‐scale structure was used in a structurally‐constrained inversion to improve the imaging of smaller‐scale structure [ABSTRACT FROM AUTHOR]

Assessing water travel times in hyporheic and adjacent alluvial sediments is important to quantify exchange rates, biogeochemical turnover, and pollution dynamics across groundwater‐surface water interfaces and in floodplain aquifers. Heat and 222Rn are useful natural tracers for this purpose. While heat transport is commonly simulated via the convection‐conduction equation, the transport of 222Rn is often simulated assuming purely advective transport and a homogenous distribution of 222Rn production rates across the model domain. In the present study, we explicitly model the production, transport, and radioactive decay of 222Rn after surface water infiltration into an alluvial aquifer with the numerical models MODFLOW‐NWT and MT3D‐USGS. Using field observations, numerical modeling, and laboratory experiments, we show that 222Rn production rates vary substantially in the alluvial aquifer of a lowland river. Distributions of mean river‐to‐groundwater travel time in the alluvial aquifer, estimated via a Bayesian approach, shifted considerably depending on whether 222Rn and time series of groundwater heads and temperature were jointly inverted and whether 222Rn production rates were allowed to vary spatially. With distance to the river, differences between the median river‐to‐groundwater travel time and apparent 222Rn ages increased by factors ranging from 1.4 at 4 m to 11.9 at 59 m, a finding that highlights the need to simulate 222Rn transport explicitly. The joint inversion of 222Rn, groundwater heads, and temperature reduced uncertainty associated with mean travel times, suggesting that the uncertainty introduced by spatially varying 222Rn production rates can at least partly be compensated by using a combination of natural tracers. Plain Language Summary: Rivers and groundwater bodies exchange water back and forth. Knowing the time that water spends traveling from rivers to certain points in groundwater is important for water management, particularly in situations where drinking water is produced via bank filtration: It allows to predict what happens to natural and toxic compounds that come from the river and to assess the quantity of water moving from rivers to aquifers. Here, we present a modeling approach to estimate travel times of groundwater originating from a nearby river. The approach uses the open‐source software packages MODFLOW‐NWT and MT3D‐USGS and a combination of natural tracers, such as the noble gas 222Rn, which is produced in the subsurface with varying rates but decays with a constant rate constant. We demonstrate that by using both 222Rn and other natural tracers, such as heat, we estimate travel times from rivers to points in groundwater with less uncertainty. Key Points: A flexible approach to explicitly model the transport of 222Rn in saturated porous media using MODFLOW‐NWT and MT3D‐USGS is presentedSpatially varying 222Rn production rates, found to vary by a factor of two at relatively small spatial scales (<100 m), are consideredA joint inversion of hydraulic heads, temperature, and 222Rn reduced the high degree of uncertainty in the estimated groundwater travel times [ABSTRACT FROM AUTHOR]

The current theory for defining the occurrence of riparian lenses (i.e., buoyant, lenticular‐shaped fresh groundwater bodies overlying saline groundwater in riparian zones) is largely based on steady‐state analyses, which neglect the transient dynamics expected in real‐world settings. In this study, the transience of riparian lens movement is investigated for the first time, considering a fully penetrating, gaining river (i.e., the river receives groundwater influxes), and rapid variations in hydraulic boundary conditions. Controlled laboratory experiments and numerical modeling are used to determine the timescales associated with lens movements. Both numerical and experimental timescales show asymmetric behavior in the growth and decline of lenses, whereby growing lenses involve longer timescales than those of shrinking lenses. Inverse relationships between timescales and final hydraulic gradients are observed, such that higher final hydraulic gradients involved shorter timescales. Results also revealed a time lag in the movement of the lens tip (i.e., where the lens and water table intersect) relative to the lens toe (i.e., intersection of the lens with the riverbank). Riparian lenses have near‐identical timescales regardless of whether the river water or aquifer hydraulic head varies, and rather, timescales are more sensitive to the final hydraulic gradient. Application of typical parameter ranges for the River Murray (South Australia) floodplains reveals timescales of years to more than a decade to expand the riparian lens by tens of meters. Insights into riparian lenses transience attained from the current study build on the understanding of transience obtained for other mixed‐convective hydrogeological processes, such as seawater intrusion and retreat. Key Points: Laboratory experiments and numerical modeling of riparian lens transience show growing lenses have longer timescales than shrinking lensesTimescales for lens movements of tens of meters are expected to range from years to more than a decade for field‐scale conditionsThe lens is slower to grow (and shrink) laterally compared to the time required to increase (and decrease) in thickness [ABSTRACT FROM AUTHOR]

In 1986, a seminal data set from the Canadian Forces Base (C.F.B.) Borden aquifer, Ontario, Canada, was published, illustrating, in unprecedented detail, the spatial distribution of hydraulic conductivity (K). Among many contributions attached to that data set was a geostatistical examination of field‐based data for comparison with theoretical predictions of macro‐dispersivity. However, that work treated K as a static parameter and the sole source of flow variability. Here, point velocity probes (PVPs) are used to extend the earlier work by collecting a novel high‐resolution data set of groundwater velocity (v) measurements in the C.F.B. Borden aquifer. Velocity is a dynamic parameter of fundamental importance, closely tied to solute dispersion. Over 400 velocity measurements were collected along a transect perpendicular to flow, analyzed geostatistically, and compared with the analysis of the B‐B′ cross‐section of K reported by Sudicky (1986), https://doi.org/10.1029/wr022i013p02069. The PVP measurements exhibited geostatistics similar to those previously estimated by Sudicky (1986), https://doi.org/10.1029/wr022i013p02069. This finding suggests, as implicitly assumed in the previous work, the distribution of v is primarily controlled by K. PVPs also provided a novel, high‐resolution data set of groundwater flow directions. Since the ultimate objective of aquifer characterization includes the definition of velocity fields, this work not only extends the 1986 work, but also demonstrates a viable alternative for characterizing flow patterns in aquifers—with the advantage that direct v measurements reflect variability in porosity and hydraulic gradient, as well as K. This could ultimately be advantageous at sites with greater heterogeneity or other dynamic hydrogeological variabilities more pronounced than those at the Borden site. Plain Language Summary: Transport of contaminants in the ground is dominated by groundwater flow. This is typically investigated using conventional methods based on water‐level measurements and other indirect techniques. This article outlines an alternative method for investigating groundwater flow by direct measurements of groundwater velocity. A chief advantage of the approach is a reduction of measurement uncertainty. Geostatistics describing aquifer structure based on the two methods were shown to be similar. This illustrates the advantages of making direct velocity measurements are not offset by a loss of data quality or geostatistical value. This new method may have application in the characterization and monitoring of contaminants transporting by groundwater or undergoing clean‐up efforts. Key Points: Geostatistics of direct groundwater velocity measurements are consistent with those from hydraulic conductivity at Canadian Forces Base BordenCorrelation lengths of groundwater velocity magnitude and direction shown to be similarHigh‐resolution velocity data set shows plume spreading (i.e., dispersion), without Fickian dispersion, based solely on advective processes [ABSTRACT FROM AUTHOR]

Groundwater runoff is an important component of river runoff and is sensitive to climate change. However, it is difficult to differentiate groundwater runoff from total river runoff by current base flow separation methods. Here, we propose a physically based model with a two‐pass digital filter for separating groundwater runoff from streamflow time series. The proposed method is based on the fundamental theory of surface–groundwater interactions under a variable groundwater recharge process that is highly associated with river stages. Our results show that the groundwater flow recession constant and the base flow recession constant can be determined separately by performing variogram analysis on long‐term streamflow time series data. For watersheds with areas of 103–104 km2, the groundwater flow recession constant differs from the typical values of the base flow recession constant by an order of magnitude or more. Therefore, the developed method can be used to quantify groundwater runoff dynamics on a temporal scale spanning tens of years. Unlike other approaches to base flow separation, our proposed method can reveal the effects of climate change on groundwater runoff. Key Points: A physically based model with a two‐pass digital filter for groundwater runoff separation from streamflow time series was proposedGroundwater flow recession constant and the base flow recession constant can be determined separately using variogram analysisThe proposed method is able to reveal the effects of climate change on groundwater runoff [ABSTRACT FROM AUTHOR]

Subsurface pipe failures in cold regions are generally believed to be exacerbated by differential strain in shallow soils induced by seasonal freeze and thaw cycles. The transient stress–strain fields resulting from soil water phase change may influence the occurrence of local buried pipe breaks including those related to urban water mains. This work proposes that freezing‐induced frost loading results in uneven stress–strain distributions along the buried water mains placing them at risk of bending, breaking, and/or leaking. A coupled thermal‐hydraulic‐mechanical (THM) model was developed to illustrate the interactions among moisture, temperature, and stress–strain fields within variably saturated freezing soils. Several typical cases involving highly frost‐susceptible and lower frost‐susceptible soils underlying roadbeds were examined. Results show that the magnitude of frost‐induced compressive stress and strain changes between different frost‐susceptible soils can vary significantly. Such substantial differences in stress–strain fields would increase the breakage risk of water mains buried within different types of soils. Furthermore, even water mains buried within soils with low frost‐susceptibility are at risk when additional sources of soil water exist and are available to migrate to the freezing front. To reduce the risk of damage to buried pipe‐like infrastructure, such as municipal water mains, from soil freezing phenomena, the selected backfill material should have fairly consistent frost susceptibility or a broad zone of transition should be considered between materials with significantly different frost susceptibility. In addition, buried pipes should be kept as far away from external sources of subsurface water as possible considering the potential for the water source to exacerbate the level of risk to the pipe. Key Points: Subsurface pipe failures in cold regions are exacerbated by freezing‐induced differential stress–strain fieldsLocal sources of water available to feed ice growth increases the breakage risk of water mainsThe magnitude of frost‐induced compressive stresses and negative strains may vary significantly between different intermingled soil types [ABSTRACT FROM AUTHOR]

Defects in sanitary‐sewer infrastructure enable exchange of large volumes of fluids to and from the environment. The intrusion of rainfall and groundwater into sanitary sewers is called inflow and infiltration (I&I). Though long recognized in the assessment of sewers, the impacts of I&I on streamflow within urban watersheds are unknown. We quantified rainfall‐derived I&I (RDI&I), groundwater infiltration (GI), and total I&I using measured flows within sanitary‐sewer pipes serving four watersheds near Atlanta, Georgia, USA. Flows were monitored in pipes that parallel local stream channels and compared with streamflow measured at nearby gauging sites. Freshwater diverted into the sewer system due to I&I ranged from 24% to 36% of the flow measured within individual pipes. The RDI&I was the smaller component of I&I, ranging from 4.2 to 9.8 mm per year among watersheds. The GI was typically an order of magnitude greater than RDI&I, ranging from 24 to 41 mm per year among watersheds with annual stream discharge of approximately 500 mm. The I&I occurring at specific moments in time commonly represented 0%–20% of the flow measured in the adjacent stream. The enhancement of low flows in streams that could be achievable if I&I were abated ranges from as much as 6%–36% across watersheds. Our discussion presents explanations for the seasonality of I&I and associated impacts on streamflow in urban watersheds, while identifying important sources of remaining uncertainty. Our results support the conclusion that I&I substantially reduces flows in urban streams, especially low flows during dry weather. Plain Language Summary: Water infrastructure degrades over time, allowing for the movement of large volumes of fluids to and from the environment. Inflow is the flow of precipitation into sanitary‐sewer pipes through, for example, leaky manholes and gutter downspouts. Infiltration is the flow of groundwater into the same pipes through fractures, leaky joints, and other defects. They cause overflows of untreated sewage that endanger humans and ecosystems. Inflow and infiltration also represent large diversions of fresh water away from local streams. We examined rates of flow in four sanitary‐sewer pipes and quantified the inflow and infiltration. We compared these estimates to the rates of flow in nearby streams. We find that inflow and infiltration of fresh water into sanitary‐sewer pipes commonly represent 20%–40% of the flow in the pipes. Most of that water emanates from aquifers with a smaller fraction supplied by precipitation. If the inflow and infiltration were abated, and that fresh water instead flowed to stream channels, then the flows in those channels during dry periods could be enhanced by as much as 6%–36%. This is important in urban watersheds, where streamflow during dry weather is already diminished. Inflow and infiltration represent infrastructure‐mediated flow pathways that substantially alter the water cycle in urban watersheds. Key Points: Large volumes of precipitation and ground water flow into sanitary‐sewer pipesThis undesirable diversion of water substantially reduces flow in urban streamsAbatement of this problem could enhance low flows in urban streams by 6%–36% [ABSTRACT FROM AUTHOR]

Specific yield (Sy) is one of the most important aquifer parameters in groundwater models with large storage changes. However, it has received limited attention from the modeling community and there are few, if any, approaches that can be widely applied for determination of Sy at the scale required for regional aquifer studies. Despite the limitations of parameter estimation methods and of direct measurements of aquifer storage properties in the field, the large uncertainty in model Sy values is typically ignored and Sy is incorrectly assumed to be a known quantity. These practices can introduce errors into model predictions of aquifer budget components, such as recharge and storage depletion. In turn, aquifer management and planning can be greatly impacted, as estimates of aquifer responses to pumping changes are highly dependent on those simulated budget components. In this work, we use a groundwater model of a portion of the High Plains aquifer in the central United States to illustrate the impacts of inappropriately large Sy values on model predictions and the related water management ramifications. Building on our recent work, we propose a data‐driven approach for determining specific yield for regional groundwater models. Results demonstrate that this new approach can improve model reliability and lead to more robust predictions of aquifer responses to management scenarios. Key Points: Specific yield values are uncertain in many models of unconsolidated aquifersModels with inappropriate specific yield values can produce unreliable aquifer budgets and responses under future management scenariosA new approach is proposed for estimating the spatial distribution of specific yield for regional groundwater models [ABSTRACT FROM AUTHOR]

Studies of coastal groundwater dynamics often assume two‐dimensional (2D) flow and transport along a shore‐perpendicular cross‐section. We show that along‐shore movement of groundwater may also be significant in heterogeneous coastal aquifers. Simulations of groundwater flow and salt transport incorporating different geologic structure show highly three‐dimensional (3D) preferential flow paths. The along‐shore movement of groundwater on average accounts for 40%–50% of the total flowpath length in both conduit‐type (e.g., volcanic) heterogeneous aquifers and statistically equivalent (e.g., deltaic) systems generated with sequential indicator simulation (SIS). Our results identify a critical role of three‐dimensionality in systems with connected high‐permeability geological features. 3D conduit features connecting land and sea cause more terrestrial groundwater flow through the inland boundary and intensify water exchange along the land‐sea interface. Therefore, conduits increase the rate of SGD compared to equivalent homogeneous, SIS and corresponding 2D models. In contrast, in SIS‐type systems, less‐connected high‐permeability features produce mixing zones and SGD nearer to shore, with comparable rates in 3D and 2D models. Onshore, 3D heterogeneous cases have longer flowpaths and travel times from recharge to discharge compared to 2D cases, but offshore travel times are much shorter, particularly for conduit‐type models in which flow is highly preferential. Flowpath lengths and travel times are also highly variable in 3D relative to 2D for all heterogeneous simulations. The results have implications for water resources management, biogeochemical reactions within coastal aquifers, and subsequent chemical fluxes to the ocean. Plain Language Summary: The findings of this study provide insight into the complex patterns of groundwater flow under the influence of geologic variability in coastal aquifers. In coastal regions, studies of solute transport processes mainly rely on an assumption of 2D groundwater flow and solute transport in the shore‐perpendicular direction. Our results reveal that groundwater does not only flow toward the sea, it also can flow along‐shore, especially in aquifers with features that connect the onshore and offshore. This affects exchange and mixing between fresh and saline groundwater, which can strongly impact delivery of contaminants and nutrients to sensitive nearshore marine ecosystems. Results highlight the importance of characterizing the geology of coastal aquifers and representing it in models of groundwater flow and contaminant transport. Key Points: Geologic heterogeneity causes substantial along‐shore movement of groundwater in coastal heterogeneous aquifersAlong‐shore groundwater flow in heterogeneous aquifers increases variability of travel length and time of groundwater prior to dischargeConnected, high‐permeability features increase the rate of SGD compared to equivalent homogeneous, SIS, and corresponding 2D models [ABSTRACT FROM AUTHOR]

This study focuses on the development of a new model that can be used not only to forecast the groundwater depth but also to quantify the uncertainty of forecasts. Therefore, we built a hybrid of Self‐Exciting Threshold Autoregressive‐Generalized Autoregressive Conditional Heteroscedasticity model, namely SETAR‐GARCH model to obtain point and interval forecasts of groundwater depth. In addition, single SETAR model and a hybrid of SETAR and Kernel Density Estimation model, referred as SETAR‐KDE, were compared. Two average monthly groundwater depth of Xi'an and Baoji cities in Shaanxi, China, were used to evaluate the models. The study results demonstrate that the SETAR‐GARCH model achieves satisfactory point and interval forecasting performance and reduces the uncertainty of interval forecasting. Furthermore, we found and proved a significant finding that the GARCH model does not have any influence on the point forecasting, but it can achieve superior performance of interval forecasting by combined with SETAR model. This study may provide a new idea for nonstationary and nonlinear groundwater depth prediction and further provide a reference for the design and management of water systems in the changing environment. Key Points: A new hybrid model was built and achieves satisfactory point and interval forecasting performance for the groundwater depthThe Generalized Autoregressive Conditional Heteroscedasticity (GARCH) model does not have any influence on the point predictionThe GARCH model reduces the uncertainty of interval prediction [ABSTRACT FROM AUTHOR]

Groundwater is by far the largest unfrozen freshwater resource on the planet. It plays a critical role as the bottom of the hydrologic cycle, redistributing water in the subsurface and supporting plants and surface water bodies. However, groundwater has historically been excluded or greatly simplified in global models. In recent years, there has been an international push to develop global scale groundwater modeling and analysis. This progress has provided some critical first steps. Still, much additional work will be needed to achieve a consistent global groundwater framework that interacts seamlessly with observational datasets and other earth system and global circulation models. Here we outline a vision for a global groundwater platform for groundwater monitoring and prediction and identify the key technological and data challenges that are currently limiting progress. Any global platform of this type must be interdisciplinary and cannot be achieved by the groundwater modeling community in isolation. Therefore, we also provide a high‐level overview of the groundwater system, approaches to groundwater modeling and the current state of global groundwater representations, such that readers of all backgrounds can engage in this challenge. Plain Language Summary: Groundwater is an important part of the water cycle but we are still working on the best ways to include it in global models. This study provides an overview of the state of the science for groundwater modeling and outlines a road map for what is needed to improve global groundwater models. Key Points: A global groundwater framework is needed to address critical gaps in our understanding and predictive capacity of the hydrologic cycleWe envision a framework that will combine observations and models to provide spatially and temporally continuous groundwater informationThe proposed framework could improve predictability in existing models and provide valuable new information for water management [ABSTRACT FROM AUTHOR]

Groundwater models always involve high‐dimensional parameters, which makes computationally tractable data assimilation using surrogate models very challenging. To address this issue, one common practice is to employ dimension reduction (DR) techniques. Nevertheless, traditional DR methods are usually implemented based on prior parameter statistics, that is, without considering the inherent system dynamics. Here, we show that when significant difference in parameter sensitivity exists, further efficiency can be achieved by adopting a supervised DR method, that is, the active subspace (AS) method. To avoid non‐trivial efforts in calculating the gradient information needed in the standard AS method, a cluster‐based gradient‐free AS (GFAS) method is developed in this study. By combining GFAS with Gaussian process regression, a surrogate model for the CPU‐demanding groundwater model can be adaptively constructed to accelerate data assimilation. Furthermore, a compensation scheme is proposed to cope with uncertainty underestimation caused by DR. The developed approach is tested with numerical experiments and field cases, which illustrated that the new approach is more efficient than the previously developed unsupervised ones by incorporating sensitivity information. Although an iterative ensemble smoother is employed in this study, the proposed method can also be used in other data assimilation approaches, such as Markov chain Monte Carlo and ensemble Kalman filter. Key Points: The active subspace method reduces the dimensionality by incorporating sensitivity informationA cluster‐based gradient‐free method is proposed for searching the active subspaceA compensation scheme is proposed to alleviate uncertainty underestimation [ABSTRACT FROM AUTHOR]

This study explores groundwater management policies and the effect of modeling assumptions on the projected performance of those policies. The study compares an optimal economic allocation for groundwater use subject to streamflow constraints, achieved by a central planner with perfect foresight, with a uniform tax on groundwater use and a uniform quota on groundwater use. The policies are compared with two modeling approaches, the Optimal Control Model (OCM) and the Multi-Agent System Simulation (MASS). The economic decision models are coupled with a physically based representation of the aquifer using a calibrated MODFLOW groundwater model. The results indicate that uniformly applied policies perform poorly when simulated with more realistic, heterogeneous, myopic, and self-interested agents. In particular, the effects of the physical heterogeneity of the basin and the agents undercut the perceived benefits of policy instruments assessed with simple, single-cell groundwater modeling. This study demonstrates the results of coupling realistic hydrogeology and human behavior models to assess groundwater management policies. The Republican River Basin, which overlies a portion of the Ogallala aquifer in the High Plains of the United States, is used as a case study for this analysis. [ABSTRACT FROM AUTHOR]

The author comments on the article "Groundwater depletion in the Middle East from GRACE with implications for transboundary water management in the Tigris-Euphrates-Western Iran Region," by Katalyn A. Voss and colleagues. The author states that using satellite-based information along with Geographical Information System and Remote Sensing techniques on basin wide water resources development will determine water use. The author says that data time series of major surface water bodies were used.

Many deep aquifers overlain by barrier formations in the continental US are used as geological repositories for wastewaters coproduced from hydrocarbon exploration. This practice is to protect shallow groundwater following the US Environmental Protection Agency's regulation. Implicit in such practice is the assumption that deep aquifers overlain by mudstones or shales are confined so that the injected fluids will not migrate upward to contaminate shallow groundwater. However, no systematic test of this hypothesis has been made. Here we invert the groundwater response to both the M2 and the O1 tides and to the barometric pressure across a large (2.046 × 106 km2) geologic regime, the North China Platform, to systematically evaluate the hydraulic parameters as functions of depth and time without a priori assumption. Our result, the first of such inversion, shows no depth dependence of aquifer confinement to a depth of 3,400 m and that deep confined aquifers overlain by barrier formations may become leaky after distant earthquakes. It suggests that monitoring of aquifer confinement may be needed to ensure if the targeted deep aquifer for wastewater injection is really confined. The results may be timely and of global significance because the practice of hydrofracking for natural gas and the deep injection for the disposal of the coproduced wastewaters in the US may soon be adopted by other countries, such as China. Plain Language Summary: A great amount of wastewaters coproduced from hydrocarbon exploration in the mid‐continental US has been injected into deep aquifers. Such practice has been based upon an implicit assumption that deep aquifers confined by aquitards are fully confined so that the injected toxic water will not leak back to contaminate shallow groundwater or the environment. This assumption, however, has never been carefully tested. Here we test this hypothesis using groundwater data from wells over a large geological regime, the North China Platform, as a function of depth and time. Our results show no clear dependence of aquifer confinement on depth, that significant leakage occurs even in the deepest studied aquifer (>3400 m) confined by thick clay/mud aquitards, and that an initially confined aquifer became may leaky after a distant earthquake. Because the practice of hydrofracking for natural gas and deep injection for the disposal of the coproduced wastewaters may soon be adopted by other countries, such as China, our study is timely and may be of global significance. Key Points: Systematic evaluation of the confinement of aquifers in the North China Platform at different depths down to 3.4 kmNovel revealing most of the studied aquifers leak, and has no dependence on aquifer depth or thickness of the overlain geologic barrierBoth the M2 and O1 tidal waves are used in the inversion of aquifer parameters without a prior assumption [ABSTRACT FROM AUTHOR]

Accelerating warming, changes in the amount, timing, and form of precipitation, and rapidly growing populations highlight the need for improved predictions of snowmelt‐driven water supplies. Although decadal‐scale trends in reduced streamflow are common, minimal progress has been made in improving streamflow prediction on the annual time scales on which management decisions are made. Efficient allocation of dwindling supplies requires incorporating rapidly evolving knowledge of streamflow generation into parsimonious models capable of improving prediction on seasonal, annual, and multiyear time scales of water resource management. We address this need using long‐term streamflow and climate records in 12 catchments averaging 90 years of observations and totaling more than 1,080 site‐years of data. These catchments experience similar regional climate forcing each year, but are diverse enough to represent broad ranges in precipitation, temperature, vegetation, and geology characteristic of much of the western US. We find that January baseflow across all catchments exhibits a coherent, quasi‐decadal periodicity that presumably is indicative of groundwater response to decadal climate. Although the direct contribution of this discharge to streamflow is small, interannual variability in groundwater discharge is a consistently strong predictor of runoff efficiency suggesting that antecedent groundwater storage alters precipitation routing to streamflow. Incorporating antecedent groundwater storage with precipitation and melt dynamics in multiple linear regression models reduces uncertainty in annual runoff from approximately 40% to <5%. These simple models, using readily available data, provide immediately useful tools for water managers to anticipate and respond to streamflow variability on time scales of 1 to 10 years. Plain Language Summary: Climate change is increasing the frequency and severity of drought in western North America highlighting the need to improve how water supplies are managed. Although the issues are complex, one of the largest challenges in efficient and equitable water resource management is the high spatial and temporal variability in runoff efficiency. Runoff efficiency, also termed water yield, is the fraction of annual precipitation that exits a catchment as surface water discharge. These values vary from less than 20% to greater than 70% across the western US; more importantly for water supply is that interannual water yield from any one catchment may vary ±30%. This variability is highlighted by Spring runoff 2020 in the upper Colorado River Basin where annual snowfall was close to average but annual runoff was 50%–60% of normal. Our results demonstrate that a major driver of variability in runoff efficiency is a regionally coherent, quasi‐decadal periodicity in groundwater storage reflected in mid‐winter baseflow. Incorporating interannual storage variability into a statistical model of streamflow reduces uncertainty in water yield by roughly half. Importantly, the ability to quantify storage in mid‐winter allows resource managers to anticipate and plan for annual water supply months before snowmelt begins. Key Points: Spatially coherent temporal patterns in winter discharge indicate quasi‐decadal periodicity in groundwater storage in headwater catchmentsThis variability in antecedent catchment water storage is a primary determinant of water yield during the subsequent spring snowmeltVariability in mid‐winter baseflow allows water managers to anticipate and plan for the upcoming runoff season months before melt begins [ABSTRACT FROM AUTHOR]

Groundwater depletion is a major threat to agricultural and municipal water supply in California's Central Valley. Recent droughts during 2007–2009 and 2012–2016 exacerbated chronic groundwater depletion. However, it is unclear how much groundwater storage recovered from drought‐related overdrafts during post‐drought years, and how climatic conditions and water management affected recovery times. We estimated groundwater storage change in the Central Valley for April 2002 through September 2019 using four methods: GRACE satellite data, a water balance approach, a hydrologic simulation model, and monitoring wells. We also evaluated the sensitivity of drought recovery to different climate scenarios (recent climate ± droughts and future climate change scenarios: 20 GCMs and 2 RCPs) using water balance method and statistical sampling of historical climate data. Estimated Central Valley groundwater loss during the two droughts ranged from 19 km3 (2007–2009) to 28 km3 (2012–2016) (median of four methods). Median aquifer storage recovery was 34% and 19% of the overdraft during the 2010–2011 and 2017–2019 post‐drought years, respectively. Numerical experiments show that recovery times are sensitive to climate forcing, with longer recovery times for a future climate scenario that replicate historical climatology relative to historical forcing with no droughts. Overdraft recovery times decrease by ∼2× with implementation of pumping restrictions (30th to 50th percentiles of historical groundwater depletion) to constrain groundwater depletion relative to no restrictions with a no‐drought future climatology. This study highlights the importance of considering water management implications for future drought recoveries within the context of climate change scenarios. Plain Language Summary: California's Central Valley has experienced chronic groundwater depletion over the past few decades, the rate of which was amplified by droughts in 2007–2009 and 2012–2016. There is limited knowledge as to how much of the drought‐caused groundwater depletion has recovered during post‐drought years and how climate and water management affect overdraft recovery times. We address these issues by estimating groundwater storage changes using four methods and conducting numerical experiments with varying climatic conditions and water management options. We find that less than one‐third of the groundwater overdraft from the most recent droughts was recovered during post‐drought years. Projected overdraft recovery times vary greatly depending on the climate scenarios and water management strategies, and future droughts are likely to cause overdrafts from which recovery is unlikely given the current level of groundwater extractions. However, management measures such as capping groundwater pumping could reduce recovery times by a factor of two or more depending on the groundwater extraction cap and post‐drought climate. Key Points: Groundwater storage recovery during post‐drought periods ranged from 34% (2007–2009 drought) to 19% (2012–2016 drought)Projected drought recovery times decrease by a factor of 3.6–7.8 with post‐drought periods containing no drought years or wet years onlyOverdraft recovery time decreased by a factor of 2 with implementation of modest pumping restrictions under no‐drought post‐drought climate [ABSTRACT FROM AUTHOR]

Rapid infiltration following precipitation may result in groundwater contamination from surface contaminants or pathogens. In fractured rock, contaminants can migrate rapidly to points of groundwater withdrawals. In contrast to the temporal availability of groundwater quality chemical indicators, meteorological and groundwater level observations are available in real‐time to estimate time‐varying recharge, which can act as a surrogate to identify periods of rapid infiltration that may indicate contamination susceptibility. Estimating recharge using methods, such as base‐flow recession, unsaturated infiltration models, or water‐table fluctuations (WTF), cannot capitalize on currently available technologies and telecommunication infrastructure to conduct real‐time recharge estimation at scales relevant to characterizing rapid infiltration. We present a linear, physics‐based state‐space (SS) model of one‐dimensional infiltration to estimate recharge, which includes preferential and diffuse‐flow to the water table. The model can take advantage of real‐time data for water‐table altitude, precipitation, and evapotranspiration. Model parameters are calibrated over an observation period, and the Kalman Filter (KF) is subsequently applied to continuously update the observed (water‐table altitude) and unobserved (groundwater recharge) system states and predict future states as new data become available. The SS/KF algorithm is demonstrated at the Masser Groundwater Recharge Site in Pennsylvania, USA and comparisons are made with recharge estimates from WTF methods. Model results indicate that the frequency of observations (daily vs. sub‐daily) dictates the allocation between preferential and diffuse flow. Additionally, because infiltration processes encompass many nonlinearities, model parameters estimated from observation periods need to be updated at least seasonally to account for changing recharge conditions. Key Points: A state‐space model interprets time series for water table altitude and meteorological inputs to estimate and forecast rechargePreferential and diffuse flow through the unsaturated zone are conceptualized using a precipitation rate thresholdThe frequency of observations dictates the recharge allocation between preferential and diffuse flow through the unsaturated zone [ABSTRACT FROM AUTHOR]

Two approaches to augment irrigation water supply are managed aquifer recharge (MAR) and on‐farm reservoirs with tail‐water recovery (OFS‐TWR). We explore the joint use of MAR and OFS‐TWR to sustain groundwater and agricultural income with climate variability by farmer risk preference. We find that MAR use declines slightly with greater risk aversion and declines rapidly with MAR cost. The use of OFS‐TWR increases with risk aversion. The use of MAR increases income by a larger percentage for risk‐neutral than very risk‐averse producers. Across risk aversion degree, the use of MAR leads to a percentage increase in income that is double the percentage increase in the standard deviation of income (i.e., income risk). At a low cost of MAR, there is little difference in MAR use between a very risk‐averse and a risk‐neutral producer across scenarios of crop profit margin and the initial depth to water. Large differences in OFS‐TWR use between a very risk‐averse and a risk‐neutral producer occur when there are relatively high crop profit margin, low initial water depth, or a low discount rate. Key Points: Optimal use managed aquifer recharge and on‐farm reservoirs with tail‐water recovery were computed in a stochastic programming modelThe results show managed aquifer recharge use is most sensitive to cost while surface reservoirs are sensitive to climate uncertaintyManaged aquifer recharge is similar for risk neutral and risk averse farmers when the cost of managed aquifer recharge is low [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]

The past decade has seen significant progress in characterizing uncertainty in environmental systems models, through statistical treatment of incomplete knowledge regarding parameters, model structure, and observational data. Attention has now turned to the issue of model structural adequacy (MSA, a term we prefer over model structure "error"). In reviewing philosophical perspectives from the groundwater, unsaturated zone, terrestrial hydrometeorology, and surface water communities about how to model the terrestrial hydrosphere, we identify several areas where different subcommunities can learn from each other. In this paper, we (a) propose a consistent and systematic "unifying conceptual framework" consisting of five formal steps for comprehensive assessment of MSA; (b) discuss the need for a pluralistic definition of adequacy; (c) investigate how MSA has been addressed in the literature; and (d) identify four important issues that require detailed attention--structured model evaluation, diagnosis of epistemic cause, attention to appropriate model complexity, and a multihypothesis approach to inference. We believe that there exists tremendous scope to collectively improve the scientific fidelity of our models and that the proposed framework can help to overcome barriers to communication. By doing so, we can make better progress toward addressing the question "How can we use data to detect, characterize, and resolve model structural inadequacies?" [ABSTRACT FROM AUTHOR]

Three interlinked processes drive groundwater balances in diverse regions globally : (1) groundwater-irrigation intensification, (2) electrical energy supply for agriculture, and (3) climatic variability. Mexico's water-energy-climate nexus offers generic lessons because of its water scarcity and institutional reforms followed in other emerging economies. This paper analyzes data for 280 aquifers in Mexico, all registered water users, population projections, 2010-2100 precipitation and temperature projections for A1B and A2 emissions scenarios from 15 general circulation models, and 1999-2009 agricultural electricity use. Under A2 emissions, aquifers with negative balances will increase from 92 to 130 in number between 2010 and 2100, and the national groundwater deficit will increase by 21.3 km3. Under A2 and medium-variant population growth (which peaks midcentury), negative-balance aquifers will increase from 92 to 133, and the national groundwater deficit will increase by 22.4 km3. Agricultural power pricing offers a nexus-based policy tool to address aquifer depletion, an opportunity that was lost with the 2003 reduction in nighttime tariffs. Under A2, medium-variant population, and simulated 2% real annual increases in agricultural power tariffs, negative-balance aquifers will increase from 92 to 111, and the national groundwater deficit will increase by 17.5 km3 between 2010 and 2100. Regulatory and user-based groundwater management initiatives indicate growing awareness of aquifer depletion; however, the long-term outlook points to continued depletion. This raises the need to harness nexus-based policy options, i.e., increasing agricultural power tariffs, eliminating reduced nighttime tariffs, enforcing legislation linking groundwater extraction to power use, and limiting new power connections for groundwater wells. [ABSTRACT FROM AUTHOR]

This paper develops a general mathematical model for describing head fluctuations in an aquifer of long but narrow islands subject to a dual tide effect. The upper boundary condition of the aquifer is represented by an equation combining the simplified free surface equation with a leakage term. Such an equation is considered as a general expression representing the upper boundary condition of a confined, unconfined, or leaky confined aquifer. The closed-form solution of the model represented by two series terms is developed by the direct Fourier method and finite Fourier sine transform. One of the series can reduce to a closed-form expression by means of contour integral and residue theorem. If the width of the island is very large, this solution gives the predicted head almost the same as that of the solution for an aquifer subject to a single tide effect. It is found that the presence of an upper aquitard produces significant vertical flow in the lower leaky confined aquifer even if the aquitard permeability is low. Neglecting such vertical flow may result in an overestimate of hydraulic head in the leaky confined aquifer. The attenuation factor and phase lag predicted from the present solution subject to the dual tide effect agree well with those estimated from 57 day head fluctuation data observed at Garden Island, Australia. [ABSTRACT FROM AUTHOR]

Dynamic data for parameter estimation in groundwater hydrology are often strongly sensitive to averages of porosity and permeability over extended regions, but only weakly sensitive to the properties of individual gridblocks. Consequently, updates of gridblock parameters from the ensemble Kalman filter (EnKF) will be subject to substantial error due to spurious correlations when the size of the ensemble is small. It has been shown that adaptive localization methods can reduce the magnitude of spurious correlations and regularize updates from the EnKF. In this paper we show that when data are sensitive to an average of parameters over extended regions, adaptive localization methods may eliminate true, but weak, correlations. For this type of data, there appears to be a relationship between the spatial scale of the model variables and the sensitivity of observations to the variables. By introducing a multiscale expansion with adaptive localization, we are able to increase the sensitivity to variables corresponding to large spatial scales and more accurately compute their magnitudes, while eliminating updates to variables corresponding to small spatial scales with weak correlations. Three examples with distinct features are used to compare the new method of combining adaptive localization with wavelet parameterization to the case of no localization and adaptive localization with standard gridblock parameterization. We show that the Kalman gain estimated using the wavelet parameterization with adaptive localization is less biased and has smaller root-mean-square error for the problems in which spatial averaging is important and the Kalman gain has large-scale features. [ABSTRACT FROM AUTHOR]

Adapting to climate change is just one among many challenges facing river managers. The response will involve balancing the long-term water demands of society with the changing needs of the environment in sustainable and cost effective ways. This paper describes a modeling framework for evaluating the sensitivity of low river flows to different configurations of abstraction licensing under both historical climate variability and expected climate change. A rainfall-runoff model is used to quantify trade-offs among environmental flow (e-flow) requirements, potential surface and groundwater abstraction volumes, and the frequency of harmful low-flow conditions. Using the River Itchen in southern England as a case study it is shown that the abstraction volume is more sensitive to uncertainty in the regional climate change projection than to the e-flow target. It is also found that "smarter" licensing arrangements (involving a mix of hands off flows and "rising block" abstraction rules) could achieve e-flow targets more frequently than conventional seasonal abstraction limits, with only modest reductions in average annual yield, even under a hotter, drier climate change scenario. [ABSTRACT FROM AUTHOR]

Large-scale groundwater pumping or deep fluid injection in a multilayered subsurface system may generate pressure perturbation not only in the target formation(s), but also in over- and underlying units. Hydraulic communication in the vertical direction may occur via diffuse leakage through aquitards and/or via focused leakage through leaky wells. Existing analytical solutions for pressure perturbation and fluid flow in such systems consider either diffuse leakage or focused leakage, but never in combination with each other. In this study, we developed generalized analytical solutions that account for the combined effect of diffuse and focused leakage. The new solutions solve for pressure changes in a system of N aquifers with alternating leaky aquitards in response to fluid injection/extraction with any number, N1, of injection/pumping (active) wells, and passive leakage/recharge in any number, NL, of leaky wells. The equations of horizontal groundwater flow in the aquifers are coupled by the vertical flow equations in the aquitards and by the flow continuity equations in the leaky wells. The solution methodology, described in detail in this paper, involves transforming the transient flow equations into the Laplace domain; decoupling the resulting ordinary differential equations (ODEs) coupled by diffuse leakage via eigenvalue analysis; solving a system of NL × N linear algebraic equations for the unknown rates of flow through leakage wells; and superposing the solution of pressure buildup/drawdown in aquifers and aquitards resulting from flow in the N1 active and NL leaky wells. Verification of the new methodology was achieved by comparison with existing analytical solutions for diffuse leakage and for focused leakage, and against a numerical solution for combined diffuse and focused leakage. Application to an eight-aquifer system with leaky aquitards and one leaky well demonstrates the usefulness and efficiency of the approach, and illustrates the pressure behavior over a spectrum of leakage scenarios and parameters. [ABSTRACT FROM AUTHOR]