Groundwater Storage Changes Research Articles

Spatial variation in groundwater depletion across China under multiple stresses

Groundwater, an essential component of the hydrological cycle, has been subject to intense pressure worldwide over recent decades, such that reserves have changed appreciably, challenging our ability to deliver on water resource protection goals. While the changes in storage can be quantified, there is little information about what drives the changes in groundwater storage (GWS) and how the impacts of those drivers vary spatially and temporally. In this study, the spatial and temporal variations in GWS across China from 2002 to 2016 are studied, and how GWS was influenced by natural and anthropogenic drivers at multiple scales is examined using Gravity Recovery and Climate Experiment (GRACE) satellite data. The national-scale GWS declined continuously at a rate of −0.27 cm/a over the study period. Interesingly, the difference in the GWS change between the south (increasing) and the north (decreasing) was decreasing, while the gap between the east (increasing) and the west (decreasing) was becoming wider. The main drivers of GWS changes at the national and regional scales were precipitation, agricultural water consumption, and the total population, while industrial water consumption, domestic water consumption, and evapotranspiration were important at the regional and provincial scales. Policies recently implemented to protect GWS have been effective. Given the potential for GWS changes to have impacts at multiple scales, policies and goals should address the drivers of GWS changes at different scales.

Characterization of Changes in Groundwater Storage in the Lachlan Catchment, Australia, Derived From Observations of Surface Deformation and Groundwater Level Data

AbstractGlobal Positioning System (GPS) deformation measurements were combined with groundwater level data to examine the spatiotemporal variability of groundwater storage in the Lachlan catchment located in central New South Wales (Australia). After correcting for effects of glacial isostatic adjustment, non‐tidal oceanic and atmospheric loading as well as hydrologic loading using existing models, we show that the seasonal and interannual variability of ground deformation and hydraulic head level data, extracted using wavelet time‐frequency analysis, exhibits an in‐phase behavior, indicating that the observed surface deformation is the poroelastic response to groundwater pressure change in aquifer system. Combination of GPS displacement and groundwater level change enables the estimation of elastic skeletal specific storage coefficients, which were then used for estimating groundwater storage changes. The estimated groundwater storage changes clearly reflect the four climate events of the Lachlan catchment since 1996: (a) the Millennium drought over 1996–2009, (b) the 2011–2012 La Nina and two significant floods in 2012 and 2016, (c) the drought conditions from mid‐2017 to late‐2019, and (d) the return of La Nina conditions since early 2020. We also found annual and long‐term groundwater storage variations of respectively and over the period 2012–2021. Moreover, we show that groundwater level fluctuations can be predicted from GPS displacement measurements and storage coefficients with sufficient accuracy (80% correlation and 70% RMS reduction when compared in terms of seasonal cycle). This study provides essential information that can contribute to future groundwater planning, management, and control over the Australian continent.

Considerations on the water supply of the aquifer-wetland systems in Colombia from the GRACE satellites data interpretation

[Introduction]: Thanks to the recent data provided by the GRACE satellites, it is possible to establish a prelimi- nary estimate of changes in groundwater storage (GWS), thus offering a new and unprecedented tool for studying the dynamics of aquifer-wetland systems. [Objective]: As a result of the trends observed in GRACE for the Magdalena-Cauca basin in Colombia, this study investigates the water availability of the aquifer-wetland systems. [Methodology]: For this purpose, a map overlay analysis was performed using GRACE data to estimate ground- water recharge and discharge for different basins and aquifer-wetland systems. [Results]: Considering the trends in groundwater storage (GWS), we obtained water volumes and translated these into recharge and discharge esti- mates of the aquifer-wetland systems identified in the Magdalena-Cauca basin. These values show a total recharge between 2002 and 2010 of 284.65 mm, followed by a loss of 490.68 mm between 2011 and 2017. Furthermore, good correspondence was observed between the ENSO and GWS events, especially during the strong 2010-2011 La Niña and 2015-2016 El Niño events, which caused severe impacts in the country and that were particularly evi- dent in the Mojana wetland area located in the north of the basin. [Conclusions]: Although these results have yet to be validated with continuous monitoring data, the results presented in this study raise questions on the formulation of better policies and management of water resources that promote future water security.

Water Storage Variation and Its Possible Causes Detected by GRACE in the Volta River Basin

This study applies Gravity Recovery and Climate Experiment (GRACE) data and the WaterGAP (Water Global Analysis and Prognosis) Global Hydrology Model (WGHM) to investigate the influence of the Bui reservoir operation on water storage variation within the Volta River Basin (VRB). Variation in groundwater storage anomalies (GWSA) was estimated by combining GRACE-derived terrestrial water storage anomalies (TWSA), radar altimetry records, imagery-derived reservoir (Lake Volta and Bui) surface water storage anomalies (SWSA), and Global Land Data Assimilation System (GLDAS)-simulated soil moisture storage anomalies (SMSA) from 2002 to 2016. Results showed that TWSA increased (1.30 ± 0.23 cm/year) and decreased (−0.82 ± 0.27 cm/year) during 2002–2011 and 2011–2016, respectively, within VRB, matching previous TWSA investigations in this area. It revealed that the multi-year averages of monthly GRACE-derived TWSA changes in 2011–2016 displayed an overall increasing trend, indicating storage increase in regional hydrology; while the Lake Volta water storage changes decreased. The GRACE-minus-WGHM residuals display an increasing trend in VRB water storage during the Bui reservoir impoundment during 2011–2016. The observed trend compares well with the estimated Bui reservoir SWSA, indicating that GRACE solutions can retrieve the true amplitude of large mass changes happening in a concentrated area, though Bui reservoir is much smaller than the resolution of GRACE global solutions. It also revealed that GWSA were almost stable from 2002 to 2006, before increasing and decreasing during 2006–2011 and 2012–2016 with rates of 2.67 ± 0.34 cm/year and −1.80 ± 0.32 cm/year, respectively. The observed trends in the GRACE-derived TWSA and GWSA changes are generally attributed to the hydro-meteorological conditions. This study shows that the effects of strong El-Niño Southern Oscillation events on the GWSA interannual variability within the VRB is short-term, with a lag of 6 months. This study specifically showed that the Bui reservoir operation significantly affects the TWSA changes and provides knowledge on groundwater storage changes within the VRB.

Spatiotemporal Variation and Driving Analysis of Groundwater in the Tibetan Plateau Based on GRACE Downscaling Data

The special geographical environment of the Tibetan Plateau makes ground observation of Ground Water Storage (GWS) changes difficult, and the data obtained from the GRACE gravity satellites can effectively solve this problem. However, it is difficult to investigate the detailed GWS changes because of the coarser spatial resolution of GRACE data. In this paper, we constructed a 0.1° resolution groundwater storage anomalies (GWSA) dataset on the Tibetan Plateau from 2002 to 2020 based on a phased statistical downscaling model and analyzed the spatiotemporal variation and driving factors of the GWSA in order to better study the changes of GWS on the Qinghai Tibet Plateau. The results show that: (1) In the Tibetan Plateau and 12 sub-basins, the GWSA before and after downscaling show a very high correlation in time series and relatively good performance in spatial consistency, and the downscaled GWSA indicate a consistent trend with the measured groundwater level. (2) The GWSA on the Tibetan Plateau shows a downward trend (−0.45 mm/yr) from 2002 to 2020, and the variation trend of the GWSA in the Tibetan Plateau shows significant spatial heterogeneity. (3) The GWSA changes in the Tibetan Plateau are mainly dominated by natural factors, but the influence of human activities in individual sub-basins can not be ignored. Among the teleconnection factors, El Nino-Southern Oscillation Index (ENSO) has the greatest influence on the GWSA on the Tibetan Plateau.

Spatiotemporal Analysis of Groundwater Storage Changes, Controlling Factors, and Management Options over the Transboundary Indus Basin

Intensive groundwater abstraction has augmented socio-economic development worldwide but threatens the sustainability of groundwater resources. Spatiotemporal analysis of groundwater storage changes is a prerequisite to sustainable water resource management over river basins. To estimate the groundwater storage changes/anomalies (GWCs) in the Indus River Basin (IRB), where observation wells are sparse, Gravity Recovery and Climate Experiment, the Global Land Data Assimilation System, and the WaterGAP Hydrological Model data were employed. The groundwater storage changes and controlling factors were investigated at three tier levels (TTLs), i.e., the basin, river reach, and region, to explore their implications on regional water resource management and provide management options at each level. Overall, the IRB groundwater declined from January 2003 to December 2016, with a relatively higher rate during 2003–2009 than during 2010–2016. Spatially, according to a reach-specific analysis, 24%, 14%, and 2% of the upper, middle, and lower reaches of the IRB, respectively, were indicated by a ‘severe groundwater decline’ over the entire period (i.e., 2003–2016). The GRACE-based GWCs were validated with in situ data of two heterogeneous regions, i.e., Kabul River Basin (KRB) and Lower Bari Doab Canal (LBDC). The analysis showed a correlation (R2) of 0.77 for LBDC and 0.29 for KRB. This study’s results reveal that climatic variations (increase in evapotranspiration); anthropogenic activities, i.e., pumping for irrigation; and water allocations in these regions mainly drive the groundwater storage changes across the Indus Basin.

Threat of land subsidence to the groundwater supply capacity of a multi-layer aquifer system

Study regionThe study area was the Cangzhou area in the central-eastern North China Plain (NCP). Study focusCompaction of the aquifer system leads to a change in hydraulic parameters, including the storage coefficient and hydraulic conductivity. This further reduces its groundwater storage and supply capacity. This study aimed to evaluate the non-linear variation in hydraulic parameters and feedback to the aquifer system during land subsidence. The aquifer system in Cangzhou has suffered severe land subsidence caused by groundwater overexploitation and was chosen to demonstrate this effect. A one-dimensional non-linear compaction and water drainage model was developed considering the changes in the storage coefficient and hydraulic conductivity. New hydrological insightsBased on the physical characteristics of the Cangzhou aquifer system, a compaction model was applied to establish a quantitative relationship between deep groundwater abstraction, hydraulic parameter change, groundwater storage, and supply capacity change during land subsidence. Results show that the inelastic storage coefficient was reduced by 0.005–0.032 for the deep aquifer system in the subsiding area. This represents a reduction of 220 million m3 of groundwater that can be exploited per meter decline in the confined water level. The permanent loss of groundwater storage capacity is estimated to be 9.41 billion m3. These research results have important reference significance for the scientific evaluation of aquifer water supply capacity change in subsiding regions.

Downscaling WGHM-Based Groundwater Storage Using Random Forest Method: A Regional Study over Qazvin Plain, Iran

Climate change, urbanization, and a growing population have led to a rapid increase in groundwater (GW) use. As a result, monitoring groundwater changes is essential for water managers and decision-makers. Due to the lack of reliable and insufficient in situ information, remote sensing and hydrological models may be counted as alternative sources to assess GW storage changes on regional and global scales. However, often, these hydrological models have a low spatial resolution for water-related applications on a small scale. Therefore, the main purpose of this study is to downscale the GW storage anomaly (GWSA) of the WaterGAP Global Hydrology Model (WGHM) from a coarse (0.5 degrees) to a finer spatial resolution (0.1 degrees) using fine spatial resolution auxiliary datasets (0.1 degrees), such as evaporation (E), surface (SRO), subsurface runoff (SSRO), snow depth (SD), and volumetric soil water (SWVL), from the ERA5-Land model, as well as the global precipitation (Pre) measurement (GPM-IMERG) product. The Qazvin Plain in central Iran was selected as the case study region, as it faces a severe decline in GW resources. Different statistical regression models were tested for the GWSA downscaling to find the most suitable method. Moreover, since different water budget components (such as precipitation or storage) are known to have temporal lead or lag relative to each other, the approach also incorporates a time shift factor. The most suitable regression model with the highest skill score during the training-validation was selected and applied to predict the final 0.1-degree GWSA. The downscaled results showed high agreement with the in situ groundwater levels over the Qazvin Plain on both interannual and monthly time scales, with a correlation coefficient of 0.989 and 0.62, respectively. Moreover, the downscaled product represents clear proof that the developed downscaling technique is able to learn from high-resolution auxiliary data to capture GWSA features at a higher spatial resolution. The major benefit of the proposed method lies in the utilization of only the auxiliary data that are available with global coverage and are free of charge, while not requiring in situ GW records for training or prediction. Therefore, the proposed downscaling technique can potentially be applied at a global scale and to aquifers in other geographical regions.

Enhanced Understanding of Groundwater Storage Changes under the Influence of River Basin Governance Using GRACE Data and Downscaling Model

The low spatial resolution of Gravity Recovery and Climate Experiment (GRACE) data limits their application in practical groundwater resource management. To overcome this limitation, this study developed a dynamic downscaling method based on a model using groundwater storage anomaly (GWSA) data to study groundwater storage changes in an inland arid region. The groundwater storage model was calibrated using publicly accessible data at a spatial resolution of 1°. The constructed model had a satisfactory fitting effect in both the calibration and validation periods, with correlation coefficients over 0.60, in general, and a root mean square error of less than 1.00 cm equivalent water height (EWH). It was found that the hydraulic gradient coefficient was the most sensitive parameter, whereas the boundary condition had an obvious influence on the simulated GWSA compared to the different forcing data. The model was then refined at a higher resolution (0.05°) using driving data to obtain downscaled GWSA data. The downscaled results had a similar pattern to the GRACE-derived GWSA and reflected the spatial heterogeneity across the basin scale and subregion scales. The downscaled GWSA shows that the groundwater storage had an overall downward trend during the period from 2003 to 2019 and the annual decline rates ranged from 0.22 to 0.32 cm/year in four subregions. A four-month time lag between the field-observed and downscaled GWSA was observed downstream of the study area. This study provides an applicable method for assessing groundwater storage changes for groundwater management at the local scale.

Runoff response to changing environment in Loess Plateau, China: Implications of the influence of climate, land use/land cover, and water withdrawal changes

The influence of climate change and human interference bring high nonstationary characteristics that are different from the original long-term stable state. So that, investigating runoff variation and attribution under changing environments is important for implementing adaptive management of global water resources, especially on the interannual scale. In this study, the attribution of measured runoff change in the Wuding River Basin (WRB) of the Loess Plateau, China, was studied using the elasticity method based on the Budyko framework and groundwater storage change analysis on an interannual scale. The change in the measured runoff driven factors was divided into three categories: climate change, human-induced land use/land cover (HI-LULC) change, and water withdrawal change. The results show that the contribution rate of climate change was the largest (with the mean value 54.07%), followed by the HI-LULC change (with the mean value 43.85%), and the minimum is WW change (with the mean value 2.08%). Climate change had a maximum contribution to runoff change, of which precipitation was dominant, followed by relative humidity, net radiation, and wind speed; air temperature had a minimal impact. In addition, the ecological change tendency was analyzed by combining HI-LULC and NDVI data. The analysis results of ecological change tendency indicate that the policy implementation of soil and water erosion control caused the ecological restoration to some extent, but also led to a decrease in runoff. This study provides scientific and useful information for the conservation of water and soil, runoff prediction, and sustainable water resource management in the WRB, and enriches the knowledge of contribution analysis of runoff variation which can be referenced by other basins across the globe.

Solid Water Melt Dominates the Increase of Total Groundwater Storage in the Tibetan Plateau

AbstractUnderstanding how groundwater storage (GWS) responds to climate change is essential for water resources management and future water availability in the Tibetan Plateau (TP). However, the dominant factor controlling long‐term GWS changes remains unclear and its responses to climate change are not well understood. Here we combined multi‐source datasets including in‐situ measurements, satellite observations, global models, and reanalysis products to reveal that GWS increased at 5.59 ± 1.44 Gt/yr during 2003–2016 while showing spatial heterogeneities with increasing trends in northern TP and glacial regions and declining trends in central and southern TP. The accelerated transformation from solid water (glaciers, snow, and permafrost; −17.72 ± 1.53 Gt/yr) into liquid water provide more recharge to groundwater, dominating the total GWS increase. This study contributes to a better understanding of the hydrological cycle under climate change and provides key information for projecting water availability under different future scenarios in the TP.

Linking Land Use Land Cover change to global groundwater storage

Groundwater storage is facing the constant threat of over-exploitation and irreversible depletion, often attributed to agricultural and industrial usage as well as human mismanagement. While several methodologies, varying from well logs to gravity recovery data, have been successfully adopted over the years to track and mitigate groundwater loss, Land Use and Land Cover (LULC) has never been quantified to evaluate groundwater storage and variability. LULC change alters the hydrological connectivity between the surface and subsurface water. Towards this, we employed a decision tree based Machine Learning model to (a) identify hydrological and terrestrial drivers affecting groundwater resources, (b) predict shallow and deep groundwater variability, (c) rank the drivers according to their impact on groundwater distribution, and (d) understand groundwater distribution as a function of LULC change. The model was developed globally, and then extended to basinal scale observations in the Indus, Ganga and Brahmaputra rivers of the Indian subcontinent. Model output has helped to (a) compute the ‘infiltration index’ associated with each Land Cover, (b) equate cropland expansion among the three basins with shallow and deep groundwater storage and (c) link LULC-groundwater change to crop yield. RCP 2.6 crop yield estimates for the 21st century proves detrimental to Indian food and freshwater security, given the strong coupling of groundwater-LULC among the three basins and how Land Cover change translates to groundwater storage.

Hydro-Geophysical Evaluation of the Regional Variability of Senegal’s Terrestrial Water Storage Using Time-Variable Gravity Data

The Gravity Recovery and Climate Experiment (GRACE) satellite data retrieval experiment has been instrumental in characterizing the global fluctuations in terrestrial water storage (ΔTWS) over the past 20 years. Given the limited availability of hydrological data, GRACE measurements are frequently combined with other climatic models, standardized precipitation index (SPI), and standardized temperature index (STI) data to examine the likelihood of such impacts on hydrology and calculate the groundwater storage changes (ΔGWS). The characterization of the intensity and variability of drought events has been identified based on the Terrestrial Water Storage Deficit Index (TWSI), derived from GRACE mass concentration blocks (mascons) over Senegal during the studied period (April 2002–December 2021). The results are: (1) The average annual precipitation (AAP) rate for the entire period was calculated at 692.5 mm/yr. (2) The GRACE-derived ΔTWS variations were calculated at +0.89 ± 0.34, +0.07 ± 0.36, +1.66 ± 1.20, and +0.63 ± 0.08 cm/yr for Periods I (April 2002–December 2009), II (January 2010–December 2017), III (January 2018–December 2021), and the entire period (April 2002–December 2021), respectively. (3) The ΔGWS changes were estimated to be +0.89 ± 0.31, +0.085 ± 0.33, +1.64 ± 1.11, and +0.63 ± 0.08 cm/yr for Periods I, II, II, and the entire period, respectively. (4) There is good agreement in some years and seasons according to the investigation of the link between the GRACE dataset, STI, and SPI. (5) Senegal’s groundwater storage is increasing at a rate of 0.63 ± 0.08 cm/yr (1.24 ± 0.16 km3/yr) between April 2002 and December 2021. (6) Considering the yearly extraction rates of 1.13 ± 0.11 cm/yr (2.22 ± 0.22 km3/yr), an average recharge rate of +1.76 ± 0.14 cm/yr (+3.46 ± 0.28 km3/yr) was calculated for the studied area. The integrated strategy is instructive and economical.

Land Subsidence and Aquifer‐System Storage Loss in Central Mexico: A Quasi‐Continental Investigation With Sentinel‐1 InSAR

AbstractAquifers play an important role in addressing water needs worldwide. When overexploited, they may lose storage and compact, causing land subsidence and impacts on urban landscapes. Using Sentinel‐1 satellite imagery, we perform the largest ever‐made Interferometric Synthetic Aperture Radar (InSAR) survey over Mexico, across a 700,000 km2 area hosting >85.2 million inhabitants. We estimate present‐day subsidence rates for yet unmapped and well‐known hotspots (e.g., −45 cm/year in Mexico City), and compute compaction volumes at >300 aquifer‐systems (up to −60 hm3/year at Mexico City Metropolitan Area). InSAR‐derived aquifer‐system compaction generally correlates well with groundwater deficits, extractions and storage changes from management reports. Semi‐theoretical relationships for the whole Central Mexico and hydrological‐administrative regions VII, VIII, and XIII, enable the assessment of compaction rates and volumes resulting from groundwater exploitation. These could be used to inform groundwater management strategies towards adaptation to climate change and future needs of a growing population.

Changes in nonlinearity and stability of streamflow recession characteristics under climate warming in a large glaciated basin of the Tibetan Plateau

Abstract. The accelerated climate warming in the Tibetan Plateau after 1997 has profound consequences in hydrology, geography, and social wellbeing. In hydrology, the change in streamflow as a result of changes in dynamic water storage that originated from glacier melt and permafrost thawing in the warming climate directly affects the available water resources for societies of the most populated nations in the world. In this study, annual streamflow recession characteristics are analyzed using daily climate and hydrological data during 1980–2015 in the Yarlung Zangbo River basin (YRB) of the southern Tibetan Plateau. The recession characteristics are examined in terms of dQ/dt = − aQb and the response/sensitivity of streamflow to changes in groundwater storage. Major results show that climate warming has significantly increased the nonlinearity of the response (b) and streamflow stability [log(a)] in most subbasins of the YRB. These changes in the recession characteristics are attributed to the opposite effects of increases in the available water storage and recession timescale on the recession. Climate warming has increased subbasin water storage considerably due to more recharge from accelerated glacier melting and permafrost thawing after 1997. Meanwhile, the enlarged storage lengthens recession timescales and thereby decreases the sensitivity of discharge to storage. In the recession period when recharge diminished, increased evaporation and the decreased buffering effect of frost soils under warmer temperatures accelerate the initial recession of streamflow. By contrast, enlarged storage and lengthened recession timescales slow down the recession. While reservoir regulations in some basins have helped reduce and even reverse some of these climate warming effects, this short-term remedy can only function before the solid water storage is exhausted should the climate warming continue.

Methods for Characterizing Groundwater Resources with Sparse In Situ Data

Accurate characterization of groundwater resources is required for sustainable management. Due to the cost of installing monitoring wells and challenges in collecting and managing in situ data, groundwater data are sparse—especially in developing countries. In this study, we demonstrate an analysis of long-term groundwater storage changes using temporally sparse but spatially dense well data, where each well had as few as one historical groundwater measurement. We developed methods to synthetically estimate groundwater table elevation (WTE) times series by clustering wells using two different methods; a uniform grid and k-means-constrained clustering to create pseudo-wells. These pseudo-wells had a more complete groundwater level time history, which we then temporally and spatially interpolated to analyze groundwater storage changes in an aquifer. We demonstrated these methods on the Beryl-Enterprise aquifer in Utah, USA, where other researchers quantified the groundwater storage depletion rate, and the wells had a large number of historical measurements. We randomly used one measurement per well and showed that our methods yielded storage depletion rates similar to published values. We applied the method to a region in southern Niger where wells had only one measurement per well, and showed that our estimated groundwater storage change trend reasonably matched that which was calculated using GRACE satellite data.

Characterization of Aquifer System and Groundwater Storage Change Due to South-to-North Water Diversion Project at Huairou Groundwater Reserve Site, Beijing, China, Using Geodetic and Hydrological Data

Groundwater overexploitation is a critical issue in the North China Plain (NCP), resulting in groundwater level decline and surface subsidence for the last half-century. This problem, however, has been greatly alleviated by the South-to-North Water Diversion (SNWD) Project since 2015. Monitoring of this process has been steadily improved in recent years using water level and geodetic observations. Here, we characterize the water storage change at the Huairou groundwater reserve site (HGRS) in Beijing due to the SNWD by combining Interferometric Synthetic Aperture Radar (InSAR) data of the Sentinel-1 satellites, continuous Global Positioning System (GPS) data, and well water level data observed during the same time. InSAR observations revealed subsidence up to ~400 mm in the Beijing plain but uplift at ~40 mm in the HGRS during 2015–2019, and more than 70% of the uplift occurred from October 2018 to January 2019. By integrating the most significant uplift deformation during October 2018 to January 2019 with water level observations at the same time, we estimated the storativity of the confined aquifer system at HGRS as 1.68–7.82×10−3, weighing in the correction for effective stress and surface deformation for various situations. Based on the estimated aquifer storativity and the observed water level change in the unconfined and confined aquifer, the recharged water storage for the confined and unconfined aquifers was estimated as 1.20–1.39×107 m3 and ~2.86×108 m3 from 6 October 2018 to 22 January 2019, respectively, which is about 4% and 91% of the surface water recharge through river channels in the same period due to the SNWD Project. Our study demonstrates that integration of geodetic and hydrological data can provide crucial information for the assessment of groundwater circulation and assistance of groundwater management.

Evaluating dynamics of GRACE groundwater and its drought potential in Taihang Mountain Region, China

The Taihang Mountain Region (TMR) of China is an important headwater area that provides substantial water resources supporting the social and ecological stability of the North China Plain. Currently, groundwater resources in the TMR are rapidly declining, but the degree of groundwater storage (GWS) depletion and the driving mechanism of groundwater drought remain poorly understood. This study combined the Gravity Recovery and Climate Experiment (GRACE) and Global Land Data Assimilation System (GLDAS) datasets to thoroughly analyze GWS changes in the TMR and the dominant driving factors during 2003–2016. In addition, a new index—the groundwater drought potential index (GDPI)—was established to quantify the potential of groundwater drought. We found that GRACE-derived terrestrial water storage in the TMR experienced a significant reduction (-21.82 mm/yr, p < 0.01) during 2003–2016, with a similar reduction in GWS (-21.22 mm/yr, p < 0.01). Human-induced impacts exerted a decisive role in GWS decreases in this region, with a relative contribution rate of 96% compared to the climatic contribution (4%). Climatic impacts on GWS were primarily observed in the southeastern TMR, where climate change appeared to be unbeneficial to groundwater recovery and exacerbated local groundwater depletion. Specifically, the groundwater drought potential was entirely increasing, indicating a worsened climate condition that precipitation increasingly failed to fill the groundwater deficit. Actions should be taken urgently to address the local groundwater crisis and to maintain water security. Our proposed GDPI not only reflected the degree of groundwater depletion but also revealed the relationship between groundwater drought and precipitation, making the index capable of being applied in drought forecasting. This study delivers a new perspective to better understand the mechanisms of GWS change and groundwater drought, and the results are expected to provide a valuable reference for groundwater management and water sustainability goals.

Reconciling regional water diversion and urban growth policies to protect groundwater across a large urban region in China

The overexploitation of groundwater (GW) and the associated environmental consequences are a cause for concern worldwide. Despite widespread unease about replenishing GW through, for example, water diversion schemes, there have been few comprehensive impact assessments, particularly for large urban areas that are undergoing rapid development to achieve regional goals. Using gravity recovery and climate experiment (GRACE) satellite data, we quantified the changes in GW storage from 2002 to 2017 and the reasons for the changes across a large-scale urban agglomeration in China (Beijing-Tianjin-Hebei, BTH) with a serious GW deficit. We found that, at the aggregate scale, from 2002 to 2017, the GW storage over the entire BTH declined at an average annual rate of 1.20 cm, but the rate of decline decreased markedly after 2010. At the city scale, the rates of decline in the GW storage between different cities varied but were strongly spatially correlated. The spatial correlation pattern was inverted from north to south and corresponded with the unbalanced diversion of water between cities from the South-to-North Water Transfer Project and was also affected by the policy of transferring industrial water from Beijing to the central and southern plains of BTH. Human-related factors were the most significant drivers of the changes in GW storage and, of these, agricultural water consumption had the most effect on the changes in GW storage. To protect GW supplies in large-scale urban agglomerations, regionally diverted water allocations should be coordinated with other development strategies, such as the regional industry distribution and transfer policy and agricultural water use management.

Analyses of groundwater storage change using GRACE satellite data in the Usutu-Mhlatuze drainage region, north-eastern South Africa

Study regionThe Usutu-Mhlatuze Water Management Area located in northern KwaZulu-Natal province, South Africa. Study focusRecent studies have indicated a steady decline in groundwater levels in the Usutu-Mhlatuze Water Management Area. However, the absence of representative aquifer storage parameter values including storativity and specific yield, lack of sufficient groundwater monitoring wells and inconsistent and erratic groundwater level observation data from existing limited networks make it difficult to understand groundwater storage changes in the region. Therefore, this study analyses groundwater storage change of the primary and secondary aquifers within the Usutu-Mhlatuze Water Management Area using GRACE satellite derived terrestrial water storage data, Global Land Data Assimilation System (GLDAS) soil moisture data and in-situ measured surface water storage information. New hydrological insights for the regionThe GRACE derived groundwater storage anomalies for the Usutu-Mhlatuze Water Management Area showed good agreement with in-situ groundwater storage anomalies observed from the limited groundwater level monitoring piezometers in the primary and secondary aquifers. The goodness of fit (R2) between the GRACE derived and the observed groundwater storage changes in the primary and secondary aquifers were 0.79 and 0.74, respectively. The GRACE derived groundwater storage change data for the entire study period (between 2002 and 2020) indicated that the primary and secondary aquifers experienced a groundwater storage loss of 925 × 106 m3 and 3614 × 106 m3, respectively.