We report on a field test of a transportable version of a superconducting gravimeter (SG) intended for groundwater storage monitoring. The test was conducted over a 6‐month period at a site adjacent ...to a well in the recharge zone of the karstic Edwards Aquifer, a major groundwater resource in central Texas. The purpose of the study was to assess requirements for unattended operation of the SG in a field setting and to obtain a gravimetric estimate of aquifer specific yield. The experiment confirmed successful operation of the SG, but water level changes were small (<0.3 m) leading to uncertainty in the estimate of specific yield. Barometric pressure changes were the dominant cause of both water level variations and non‐tidal gravity changes. The specific yield estimate (0.26) is larger than most published values and dependent mainly on low frequency variations in residual gravity and water level time series.
Satellite monitoring of changes in terrestrial water storage provides invaluable information regarding the basin‐scale dynamics of hydrological systems where ground‐based records are limited. In the ...Bengal Basin of Bangladesh, we test the ability of satellite measurements under the Gravity Recovery and Climate Experiment (GRACE) to trace both the seasonality and trend in groundwater storage associated with intensive groundwater ion for dry‐season irrigation and wet‐season (monsoonal) recharge. We show that GRACE (CSR, GRGS) datasets of recent (2003 to 2007) groundwater storage changes (ΔGWS) correlate well (r = 0.77 to 0.93, p value < 0.0001) with in situ borehole records from a network of 236 monitoring stations and account for 44% of the total variation in terrestrial water storage (ΔTWS); highest correlation (r = 0.93, p value < 0.0001) and lowest root‐mean‐square error (<4 cm) are realized using a spherical harmonic product of CSR. Changes in surface water storage estimated from a network of 298 river gauging stations and soil‐moisture derived from Land Surface Models explain 22% and 33% of ΔTWS, respectively. Groundwater depletion estimated from borehole hydrographs (−0.52 ± 0.30 km3 yr−1) is within the range of satellite‐derived estimates (−0.44 to −2.04 km3 yr−1) that result from uncertainty associated with the simulation of soil moisture (CLM, NOAH, VIC) and GRACE signal‐processing techniques. Recent (2003 to 2007) estimates of groundwater depletion are substantially greater than long‐term (1985 to 2007) mean (−0.21 ± 0.03 km3 yr−1) and are explained primarily by substantial increases in groundwater ion for the dry‐season irrigation and public water supplies over the last two decades.
Key Points
Validates satellite measurements of groundwater storage changes in the humid tropics
Evaluates the ability of various GRACE products to trace groundwater storage changes
Demonstrates the critical but often ignored contribution of surface water storage
GRACE gravity variation recovery and InSAR-derived ground displacement data show promise in supporting and assessing groundwater management policies. However, GRACE system's resolution is too low, ...and the inversion of InSAR data into volume of groundwater storage loss requires extensive and often unavailable lithological data. Here we present how InSAR can be used to constrain and spatially focus GRACE-derived groundwater mass loss to depletion areas, reducing the gap between the GRACE scale and the typical water management scales. While we highlight the tremendous potential of a fully geodetic, quantitative, and high resolution mapping of groundwater storage loss, we also point out the crucial need for producing guidelines on the proper GRACE solution to use for any study area and/or application.
In order to illustrate the GRACE/InSAR combination procedure, we present a case study in Central Mexico, where groundwater depletion of ~5000Million Cubic Meters per year (MCM/yr) is reported by the water governance agencies and is well documented in the scientific literature. However, in this region, not all GRACE solutions provide reasonable groundwater depletion estimates. Using two of them, an inversion is performed to focus the groundwater-related GRACE signal over different mass distribution maps. Several mass distributions are tested, including two from InSAR-derived aquifer compaction mapping. The results show that the regions of Mexico City and Bajio, an agricultural and industrial corridor 250km North of Mexico City, are the main contributors to the regional groundwater depletion. The mass distribution map produced directly from InSAR leads to results closer to official groundwater budgets than the others tested.
•Mapping groundwater depletion solely with independent geodetic observations•Fusion of high-resolution InSAR data within GRACE mass estimation•Improved estimation of groundwater storage change from a set of small aquifers•GRACE-derived groundwater mass changes are focused to the water management scale.•Discrepancies among GRACE products limit its integration into water management
Proliferation of different total water storage (TWS) change products from the Gravity Recovery and Climate Experiment (GRACE) satellites, including the newly released mascon solution, warrants ...detailed analysis of their uncertainties and an urgent need to optimize different products for obtaining an elevated understanding of TWS changes globally. The three cornered hat method is used to quantify uncertainties in TWS changes from GRACE observations, land surface models, and global hydrological models, indicating that the WaterGap Global Hydrological Model (WGHM)-based TWS changes show the lowest uncertainty over sixty basins covering a range of climate settings and levels of human activities globally. Bayesian model averaging (BMA) using WGHM TWS output for training (2003–2006) is subsequently used to merge TWS changes from various GRACE products. Results indicate that the BMA-based TWS changes show the highest consistency with the WGHM output for the validation period (2007–2009) in terms of the highest medium of the Nash-Sutcliffe Efficiency (NSE) coefficient of 0.714 among all TWS change products for the sixty basins. The mascon solution shows a medium of NSE of 0.682, higher than other GRACE TWS change products. Analysis of spatiotemporal variability in BMA-based TWS changes and the mascon solution indicates that higher depletion rates for the 13-year period (Apr 2002–Mar 2015) occurred over major aquifers due to groundwater withdrawals for irrigation (e.g., Tigris, Central Valley, Ganges, upper Arkansas, and Indus), basins subject to great glacier and snow melting (e.g., Yukon, Fraser, and eastern Ganges), the north Caspian Sea (e.g., Don and Ural), and the Caspian Sea. Significant increasing trends in TWS are found over west (e.g., Gambia and Niger) and South Africa (e.g., Zambezi), South America (e.g., Essequibo), North America (e.g., Koksoak and Missouri), central India (e.g., Narmada and Godavari), the north Tibetan Plateau, and the middle Yangtze River basin. Empirical Orthogonal Function decomposition is used to investigate spatiotemporal variations in the GRACE mascon solution-based TWS changes during the study period, showing a detailed pattern of increasing and decreasing long-term trends, interannual and seasonal variations in TWS over the global land surface.
•The three cornered hat method is used to quantify uncertainties in TWS changes.•Bayesian model averaging (BMA) and WGHM are used to optimize GRACE TWS changes.•BMA-based TWS changes perform best in all TWS change products from GRACE satellites.•Trends, interannual and seasonal variability in TWS changes globally are examined.
Several researchers have postulated that, under a changing climate due to anthropogenic forcing, an intensification of the water cycle is already under way. This is usually related to increases in ...hydrological fluxes as precipitation (P), evapotranspiration (E), and river discharge (R). It is under debate, however, whether such observed or reconstructed flux changes are real and on what scales. Large‐scale increase or decrease of the flux deficit (P‐E‐R), i.e., flux changes that do not compensate, would lead to acceleration or deceleration of water storage anomalies potentially visible in Gravity Recovery and Climate Experiment (GRACE) data. In agreement with earlier studies, we do find such accelerations in global maps of gridded GRACE water storage anomalies over 2003–2012. However, these have been generally associated with interannual and decadal climate variability. Yet we show that even after carefully isolating and removing the contribution of El Niño that partially masks long‐term changes, using a new method, accelerations of up to 12 mm/yr2 remain in regions such as Australia, Turkey, and Northern India. We repeat our analysis with flux fields from two global atmospheric reanalyses that include land surface models (ERA‐Interim and MERRA‐Land). While agreeing well with GRACE on shorter time scales, they fall short in displaying long‐term trends corresponding to GRACE accelerations. We hypothesize that this may be due to time‐varying biases in the reanalysis fluxes as noticed in other studies. We conclude that even though its data record is short, GRACE provides new information that should be used to constrain future reanalyses toward a better representation of long‐term water cycle evolution.
Key Points
A new method for removing ENSO‐related interannual variability is presented
GRACE sees regions of distinct water storage acceleration, even after removing an “ENSO” mode
Atmospheric‐land surface reanalyses largely fail to reproduce corresponding flux trends
The Gravity Recovery And Climate Experiment (GRACE) satellite mission provides time-variable gravity fields that are commonly used to study regional and global terrestrial total water storage (TWS) ...changes. These estimates are superimposed by different error sources such as the north–south stripes in the spatial domain and spectral/spatial leakage errors, which should be reduced before use in hydrological applications. Although different filtering methods have been developed to mitigate these errors, their performances are known to vary between regions. In this study, a Kernel Fourier Integration (KeFIn) filter is proposed, which can significantly decrease leakage errors over (small) river basins through a two-step post-processing algorithm. The first step mitigates the measurement noise and the aliasing of unmodelled high-frequency mass variations, and the second step contains an efficient kernel to decrease the leakage errors. To evaluate its performance, the KeFIn filter is compared with commonly used filters based on (i) basin/gridded scaling factors and (ii) ordinary basin averaging kernels. Two test scenarios are considered that include synthetic data with properties similar to GRACE TWS estimates within 43 globally distributed river basins of various sizes and application of the filters on real GRACE data. The KeFIn filter is assessed against water flux observations through the water balance equations as well as in-situ measurements. Results of both tests indicate a remarkable improvement after applying the KeFIn filter with leakage errors reduced in 34 out of the 43 assessed river basins and an average improvement of about 23.38% in leakage error reduction compared to other filters applied in this study.
•GRACE data are superimposed by different error sources like spectral/spatial leakage.•Kernel Fourier Integration (KeFIn) filter is proposed to decrease leakage errors.•The KeFIn filter works exceptionally well over (small) river basins.•GRACE filtered products better estimate changes of the terrestrial water storage.
•Geostatistical inversion of head and flux data recorded under steady-state conditions.•For few observations, flux data is more informative than head data.•For many observations, the inversion ...results are comparable regardless of data type.•For equal number of observations, individual and joint inversion perform similarly.•The pumping borehole boundary condition affects individual, but not joint inversion.
Hydraulic tomography is a state-of-the-art method for inferring hydraulic conductivity fields using head data. We employed geostatistical inversion using synthetically generated head and flux data individually and jointly in a steady-state experiment. We designed 96 inversion scenarios to better understand the relative merits of each data type. For the typical case of a small number of observation points, we find that flux data provide a better resolved hydraulic conductivity field compared to head data when considering data with similar signal-to-noise ratios. This finding is further confirmed by a resolution analysis. When considering a high number of observation points, the estimated fields are of similar quality regardless of the data type. In terms of borehole boundary conditions, the best setting for flux and head data are constant head and constant rate, respectively, while joint inversion results are insensitive to the borehole boundary type. When considering the same number of observations, the joint inversion of head and flux data does not offer advantages over individual inversions. When considering the same number of observation points and, hence, twice as many observations, the joint inversion performs better than individual inversions. The findings of this paper are useful for future planning and design of hydraulic tomography tests comprising flux and head data.
In the last decade, remote sensing of the temporal variation of ground level and gravity has improved our understanding of groundwater dynamics and storage. Mass changes are measured by GRACE ...(Gravity Recovery and Climate Experiment) satellites, whereas ground deformation is measured by processing synthetic aperture radar satellites data using the InSAR (Interferometry of Synthetic Aperture Radar) techniques. Both methods are complementary and offer different sensitivities to aquifer system processes. GRACE is sensitive to mass changes over large spatial scales (more than 100,000 km
). As such, it fails in providing groundwater storage change estimates at local or regional scales relevant to most aquifer systems, and at which most groundwater management schemes are applied. However, InSAR measures ground displacement due to aquifer response to fluid-pressure changes. InSAR applications to groundwater depletion assessments are limited to aquifer systems susceptible to measurable deformation. Furthermore, the inversion of InSAR-derived displacement maps into volume of depleted groundwater storage (both reversible and largely irreversible) is confounded by vertical and horizontal variability of sediment compressibility. During the last decade, both techniques have shown increasing interest in the scientific community to complement available in situ observations where they are insufficient. In this review, we present the theoretical and conceptual bases of each method, and present idealized scenarios to highlight the potential benefits and challenges of combining these techniques to remotely assess groundwater storage changes and other aspects of the dynamics of aquifer systems.
Droughts and floods alternately occur over a large karst plateau (Yun–Gui Plateau) in Southwest China. Here we show that both the frequency and severity of droughts and floods over the plateau are ...intensified during the recent decade from three-decade total water storage anomalies (TWSA) generated by Gravity Recovery and Climate Experiment (GRACE) satellite data and artificial neural network (ANN) models. The developed ANN models performed well in hindcasting TWSA for the plateau and its three sub-regions (i.e., the upper Mekong River, Pearl River, and Wujiang River basins), showing coefficients of determination (R2) of 0.91, 0.83, 0.76, and 0.57, respectively. The intensified climate extremes are indicative of large changes in the hydrological cycle and brought great challenges in water resource management there. The TWSA of the plateau remained fairly stable during the 1980s, and featured an increasing trend at a rate of 5.9±0.5mm/a in the 1990s interspersed extreme flooding in 1991 and during the second half of the 1990s. Since 2000, the TWSA fluctuated drastically, featuring severe spring droughts from 2003 to 2006, the most extreme spring drought on record in 2010, and severe flooding in 2008. The TWSA of the upper Mekong has decreased by ~100mm (~15km3) compared with that at the end of the 1990s. In addition to hindcasting TWSA, the developed approach could be effective in generating future TWSA and potentially bridge the gap between the current GRACE satellites and the GRACE Follow-On Mission expected to launch in 2017.
•An ANN model is developed to extend GRACE total water storage changes back to 1979.•Trends in three-decade total water storage changes in SW China are examined.•The most extreme drought in 2010 and severe flooding in 2008 are examined.•The developed approach is able to hindcast and predict total water storage change.
Groundwater flow depends on the heterogeneity of hydraulic properties whose field characterization is challenging. Recently developed active‐Distributed Temperature Sensing (DTS) experiments offer ...the possibility to directly measure groundwater fluxes resulting from heterogeneous flow fields. Here, based on fundamental principles and numerical simulations, two interpretation methods of active‐DTS experiments are proposed to estimate both the porous media thermal conductivities and the groundwater fluxes in sediments. These methods rely on the interpretation of the temperature increase measured along a single heated fiber‐optic (FO) cable and consider heat transfer processes occurring both through the FO cable itself and through the porous media. The first method relies on the Moving Instantaneous Line Source model that reproduces the temperature increase and provides estimates of thermal conductivity and groundwater flux as well as an evaluation of the temperature rise due to the FO cable. The second method, based on the graphical identification of three characteristic times, provides complementary estimates of flux, fully independent of the effect of the FO cable. Sandbox experiments provide an experimental validation of the interpretation methods, demonstrate the excellent accuracy of groundwater flux estimates (<5%), and highlight the complementarity of both methods. Active‐DTS experiments allow investigating groundwater fluxes over a large range spanning 1 × 10−6−5 × 10−2 m/s, depending on the duration of the experiment. Considering the applicability of active‐DTS experiments in different contexts, we propose a general experimental framework for the application of both interpretation methods in the field, making active‐DTS field experiments especially promising for many subsurface applications.
Key Points
Numerical and experimental validation of two methods to interpret active‐Distributed Temperature Sensing (DTS) experiments in sediments
Determination of thermal conductivity and groundwater flux with low uncertainties
Definition of the applicability, range of measurements, and limits of active‐DTS experiments
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