Groundwater Storage Changes Research Articles

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.

Quantifying Water Consumption through the Satellite Estimation of Land Use/Land Cover and Groundwater Storage Changes in a Hyper-Arid Region of Egypt

One of the areas that show the most visible effects of human-induced land alterations is also the world’s most essential resource: water. Decision-makers in arid regions face considerable difficulties in providing and maintaining sustainable water resource management. However, developing appropriate and straightforward approaches for quantifying water use in arid/hyper-arid regions is still a formidable challenge. Meanwhile, a better knowledge of the effects of land use land cover (LULC) changes on natural resources and environmental systems is required. The purpose of this study was to quantify the water consumption in a hyper-arid region (New Valley, Egypt) using two different approaches—LULC based on optical remote sensing data and groundwater storage changes based on Gravity Recovery Climate Experiment (GRACE) satellite data—and to compare and contrast the quantitative results of the two approaches. The LULC of the study area was constructed from 1986 to 2021 to identify the land cover changes and investigate the primary water consumption patterns. The analysis of groundwater storage changes utilized two GRACE mascon solutions from 2002 to 2021 in New Valley. The results showed an increase in agricultural areas in New Valley’s oases. They also showed an increased in irrigation water usage and a continuous decrease in the groundwater storage of New Valley. The overall water usage in New Valley for domestic and irrigation was calculated as 18.62 km3 (0.93 km3/yr) based on the LULC estimates. Moreover, the groundwater storage changes of New Valley were extracted using GRACE and calculated to be 19.36 ± 7.96 km3 (0.97 ± 0.39 km3/yr). The results indicated that the water use calculated from LULC was consistent with the depletion in groundwater storage calculated by applying GRACE. This study provides an essential reference for regional sustainability and water resource management in arid/hyper-arid regions.

Combining downscaled-GRACE data with SWAT to improve the estimation of groundwater storage and depletion variations in the Irrigated Indus Basin (IIB)

The growth of agricultural production systems is a major driver of groundwater depletion worldwide. Balancing groundwater supply and food production requires localized understanding of groundwater storage and depletion variations in response to diverse cropping systems and surface water availability for irrigation. While advances through Gravity Recovery and Climate Experiment (GRACE) have facilitated estimating the groundwater storage (GWS) changes in recent years, the coarse resolution of GRACE data hinders the characterization of GWS variation hotspots. Herein, we present a novel spatial water balance approach to improve the distributed estimation of groundwater storage and depletion changes at a spatial scale that can detect the hotspots of GWS variation. We used a mixed geographically weighted regression (MGWR) model to downscale GRACE Level-3 data from coarse resolution (1° × 1°) to fine scale (1 km × 1 km) based on high resolution environmental variables. We then combined the downscaled GRACE-based GWS variations with results from a calibrated Soil and Water Assessment Tool (SWAT) model. We demonstrate an application of the approach in the Irrigated Indus Basin (IIB). Between 2002 and 2019, total loss of groundwater reserves varied in the IIB's 55 canal command areas with the highest loss observed in Dehli Doab by >50 km3 followed by 7.8–49 km3 in the upstream, and 0.77–7.77 km3 in the downstream canal command areas. GWS declined by −325.55 mm/year at Dehli Doab, followed by −186.86 mm/year at BIST Doab, −119.20 mm/year at BARI Doab, and −100.82 mm/year at JECH Doab. The rate of groundwater depletion is increasing in the canal command areas of Delhi Doab and BIST Doab by 0.21–0.35 m/year. Larger groundwater depletion in some canal command areas (e.g., RACHNA, BIST Doab, and Delhi Doab) is associated with the rice-wheat cropping system, low rainfall, and low flows from tributaries.

The Importance of Legislative Reform to Enable Adaptive Management of Water Resources in a Drying Climate

In South Australia’s Eyre Peninsula, groundwater provides 85% of the region’s reticulated water supply. Fresh groundwater resides within shallow karstic limestone aquifers recharged by incident rainfall. Water levels are very responsive to short-term climate variability and are at risk of sustained decline due to long-term drying trends and the further rainfall declines indicated by projections of future climate, thereby increasing risk to water security and groundwater-dependent ecosystems. In 2009, a new adaptive resource management approach was enabled through legislative reform that better addresses climate variability, particularly where aquifer robustness is low. This allows the volume of water available for licensed allocations to be varied annually depending on the current condition of the aquifer resources. A three-tiered trigger level policy varies the rate at which water allocations are limited in proportion to monitored changes in groundwater storage. The three trigger thresholds are specified for each discrete groundwater resource, based on levels of risk. We now have more than five years of observations and practice of this approach to learn of its efficacy and consequences for water users, the water resources, and the environment. It has proved to be an effective way to deal with the uncertainties in how and when climate may change and how water management principles can effectively respond. Our case study provides an example of the importance of legislative reform to enable adaptive water resource management to effectively tackle the challenges of water planning in a drying climate.

Monitoring Groundwater Storage Changes in a Karst Aquifer using Superconducting Gravimeter OSG-066 at the Lijiang Station in China

Monitoring Groundwater Storage Changes in a Karst Aquifer using Superconducting Gravimeter OSG-066 at the Lijiang Station in China

Analysis of groundwater changes (2003–2020) in the North China Plain using geodetic measurements

Study regionNorth China Plain (NCP), China. Study focusThis paper investigates the spatiotemporal characteristics of groundwater storage (GWS) changes in the NCP during 2003–2020 using Gravity Recovery and Climate Experiment (GRACE) and Global Navigation Satellite System (GNSS) measurements with a focus on the subperiods separated by the implementation (December 2014) of the South-to-North Water Diversion Middle Route Project (MRP). New hydrological insights for the regionGRACE-derived results demonstrate that GWS depleted at rates of − 1.66 ± 0.17 cm/yr and − 2.76 ± 0.55 cm/yr for subperiods of 2003–2014 and 2015–2020, respectively, suggesting that GWS continued decreasing after the MRP. However, the spatial distribution of average monthly GWS shows an evident southwest-northeast band distribution, and the GWS depletion in the northwest has been alleviated compared with the southeast. The GNSS stations in the Beijing area show accelerated increasing rates of vertical displacements, while those in the plain areas of Hebei and Tianjin show continuous declining rates, implying that GWS in the Beijing area recovered faster after the MRP, while the GWS depletion in plain areas of Hebei and Tianjin continued. In addition, precipitation is the main driving factor to GWS variability during the whole study period, with grey correlation coefficients of 0.66–0.75. This strong correlation explains the significant reduction in GWS caused by the large precipitation deficit in 2019, which leads to a larger GWS depletion rate for the 2015–2020 period.

Comparison of the shallow groundwater storage change estimated by a distributed hydrological model and GRACE satellite gravimetry in a well-irrigated plain of the Haihe River basin, China

Groundwater is a key component of the global hydrological cycle. Quantitative approaches to estimate the spatial and temporal changes in groundwater storage (GWS) could provide solid foundation for sustainable management. This study made a comprehensive comparison of GWS changes based on two independent approaches: the Soil and Water Assessment Tool (SWAT) hydrological model and Gravity Recovery and Climate Experiment (GRACE) satellite mission data. The feasibility of using these two estimation methods to quantify shallow groundwater variations in the Haihe River basin of China, a globally representative overexploited area, was assessed. The GRACE-derived GWS changes were calculated based on independent component analysis and a joint inversion method. The results showed that the GRACE-estimated shallow GWS anomalies during 2003–2012 agree well with the SWAT-simulated data. For the variations in the long-term GWS depletion rate, we found that heterogeneity at 22 subbasin (approximately 500 to 1500 km2) scales was detected by the GRACE data, and these results were spatially coherent with the SWAT simulations based on the distributed modeling units and multiple inputs. The GRACE estimates could adequately replicate the general trends of GWS depletion in the winter wheat (Triticum aestivum L.) growth period and recovery in the summer maize (Zea mays L.) growth period. Specifically, comparisons between GRACE-based and SWAT-simulated results indicated that the signals of pumping for winter wheat irrigation in several key water requirement periods in April, May and November were identified by GRACE, as were seasonal changes in shallow GWS (e.g., an obvious depletion in spring). Despite an overestimation of the scope of the fluctuations in GRACE estimates compared with those of SWAT simulations from the perspective of the water flux, the effective identification of the impact of anthropogenic exploitation on shallow GWS in GRACE suggests that this monitoring tool could be applied in the groundwater management of this plain. This study demonstrated a good agreement between SWAT-simulated and GRACE-based shallow GWS changes from the perspectives of long-term trends and variations in crop seasons. Additionally, satisfactory consistency was observed from the perspective of the water flux in this semiarid and semihumid study region considering the influence of human activities.

Application of Time-Variable Gravity to Groundwater Storage Fluctuations in Saudi Arabia

In the Middle East, water shortage is becoming more and more serious due to the development of agriculture and industry and the increase in population. Saudi Arabia is one of the most water-consuming countries in the Middle East, and urgent measures are needed. Therefore, we integrated data from Gravity Recovery and Climate Experiment (GRACE), and other relevant data to estimate changes in groundwater storage in Saudi Arabia. The findings are as follows: 1) Average annual precipitation (AAP) was calculated to be 76.4, 90, and 72 mm for the entire period, Period I (April 2002 to March 2006) and Period II (April 2006 to July 2016), respectively. 2) The average TWS variation was estimated to be −7.94 ± 0.22, −1.39 ± 1.35, and −8.38 ± 0.34 mm/yr for the entire period, Period I and Period II, respectively. 3) The average groundwater storage was estimated to be +1.56 ± 1.35 mm/yr during Period I. 4) The higher average groundwater depletion rate was calculated to be −6.05 ± 0.34 mm/yr during Period II. 5) Both soil texture and surface streams in the study area promote lateral flow and carry surface water to the Arabian Gulf and the Red Sea. 6) During Period II, average annual recharge rates were estimated to be +9.48 ± 2.37 and +4.20 ± 0.15 km3 for Saudi Arabia and the Saq aquifer, respectively. 7) This integrated approach is an informative and cost-effective technique to assess the variability of groundwater resources in large areas more efficiently.

Evaluating Groundwater Storage Change and Recharge Using GRACE Data: A Case Study of Aquifers in Niger, West Africa

Accurately assessing groundwater storage changes in Niger is critical for long-term water resource management but is difficult due to sparse field data. We present a study of groundwater storage changes and recharge in Southern Niger, computed using data from NASA Gravity Recovery and Climate Experiment (GRACE) mission. We compute a groundwater storage anomaly estimate by subtracting the surface water anomaly provided by the Global Land Data Assimilation System (GLDAS) model from the GRACE total water storage anomaly. We use a statistical model to fill gaps in the GRACE data. We analyze the time period from 2002 to 2021, which corresponds to the life span of the GRACE mission, and show that there is little change in groundwater storage from 2002–2010, but a steep rise in storage from 2010–2021, which can partially be explained by a period of increased precipitation. We use the Water Table Fluctuation method to estimate recharge rates over this period and compare these values with previous estimates. We show that for the time range analyzed, groundwater resources in Niger are not being overutilized and could be further developed for beneficial use. Our estimated recharge rates compare favorably to previous estimates and provide managers with the data required to understand how much additional water could be extracted in a sustainable manner.

Quantitative separation of the local vadose zone water storage changes using the superconductive gravity technique

The quantitative study of vadose zone water (i.e., the water contained in the rock formation between the ground surface and the groundwater level) is of vital importance for understanding the groundwater flow mechanism, mass exchange, and circulation. However, the vadose zone is closely related to the saturated zone, and it is difficult to directly observe the vadose zone water, causing the quantitative separation of the vadose zone water storage to be a challenging problem. Considering the insufficient spatial resolution of Gravity Recovery and Climate Experiment (GRACE), we draw on the practice of converting the observed groundwater level changes into the water storage changes in the study of groundwater storage changes using GRACE data, and propose a new method for implementing the quantitative separation of vadose zone water storage changes based on a single superconducting gravimeter and groundwater level observation data. This new method is validated with measurement data collected using a superconducting gravimeter (GWR-C032) at the National Geodetic Observation Station in Wuhan, China (Wuhan SG Station) and groundwater level observation data obtained during the same period. The calculation results show that the changes in the vadose zone water storage obtained using the superconductive gravity technique are in good agreement with that obtained using the local hydrological modeling method, indicating that the quantitative separation of the changes in the local vadose zone water storage can be achieved using superconductive gravity and groundwater level observations. During the study period from May 2008 to April 2010, the peak-to-peak changes of the vadose zone water storage at Wuhan SG Station reached 1580 mm, and the seasonal variations were the inverse of the variations of the total groundwater storage changes and the saturated zone water storage changes, indicating that the vadose zone plays a pronounced role in slowing down the total groundwater storage changes.

Groundwater storage change and driving factor analysis in north china using independent component decomposition

North China (NC) is faced groundwater shortage in the past decades. To understand the characteristics of Groundwater Storage (GWS) change in NC, GWS Anomaly (GWSA) is analyzed by using Independent Component Analysis (ICA) with the high-resolution time-variable gravity field model Tongji-RegGrace2019 and hydrological models. According to the spatiotemporal characteristics of Independent Components (ICs), the driving factors and corresponding driving mechanism of GWS changes are further investigated. Results show that the GWS in NC is decreased with a rate of −0.87 ± 0.04 cm/yr from January 2004 to December 2015 and the rate increased to −3.71 ± 0.49 cm/yr from January 2014 to December 2015. Among the first four ICs of GWSA, the first and second ICs (IC1 and IC2) cooperatively reflect long-term and intra-annual GWS changes caused by water consumption of coal mining and agricultural irrigation in northern and southern Shanxi province, with the correlation coefficients of −0.91 and −0.85, respectively. IC3 indicates the signal of semi-annual GWS change related to agricultural irrigation water consumption in southern Hebei province, with a correlation coefficient of −0.85. Besides, IC4 suggests the effect of monsoon precipitation and evaporation in front of Taihang Mountain. Hence, the driving factors, including uneven spatiotemporal distribution of precipitation, intense seasonal evaporation, severe loss by coal mining, coupled with exhaustive exploitation for irrigation, jointly restrict the GWS rise and fall at different time nodes.

Improving groundwater storage change estimates using time-lapse gravimetry with Gravi4GW

Time-lapse gravimetry (repeat microgravity measurement) is a powerful tool for monitoring temporal mass distribution variations, including seasonal and long-term groundwater storage changes (GWSC). This geophysical method for measuring changes in gravity (Δg) is potentially applicable to any groundwater system. Here, I present Gravi4GW, a Python tool for the site-adapted calculation of β, the conversion factor between Δg and GWSC (also known as "topographic admittance"). Alpine catchments, in particular, are ideal target sites as they are highly sensitive to climate variations and can experience significant GWSC, while often lacking groundwater monitoring infrastructure. Therefore, to illustrate the usage of Gravi4GW, I investigate a detailed example of an alpine catchment and examine spatial variations and the effects of depth assumptions. This novel and accessible tool is designed to be useful in both the planning and data processing stages of time-lapse gravimetric field studies.

San Andreas Fault Stress Change Due To Groundwater Withdrawal in California's Central Valley, 1860‐2010

Changes in groundwater storage in California's Central Valley (CV) are considered partly responsible for vertical uplift surrounding the southern CV and for stress changes on nearby faults. Questions remain regarding the distribution of stress on the central San Andreas fault (CSAF) from recent and historical drawdown of the CV aquifer over the past 150 years (1860–2010). We combine groundwater storage change estimates for the 2006–2010 drought with a three-dimensional finite element model to estimate Coulomb failure stress change (ΔCFS) on the CSAF. We combine a simple parameterization of historical hydraulic head change for 1860–1960 with a CV hydrological model (1961–2003) to estimate visco-elastic effects on ΔCFS total and 2010 rate. We find that ΔCFS on the CSAF correlates positively with shallow seismicity and low frequency earthquakes for both short-term and long term stressing. Unloading uplift rates surrounding the southern CV only partially account for the observed vertical GPS rates.

Quantifying Changes in Groundwater Storage and Response to Hydroclimatic Extremes in a Coastal Aquifer Using Remote Sensing and Ground-Based Measurements: The Texas Gulf Coast Aquifer

With the increasing vulnerability of groundwater resources, especially in coastal regions, there is a growing need to monitor changes in groundwater storage (GWS). Estimations of GWS have been conducted extensively at regional to global scales using GRACE and GRACE-FO observations. The major goal of this study was to evaluate the applicability of uninterrupted monthly GRACE-derived terrestrial water storage (TWSGRACE) records in facilitating detection of long- and short-term hydroclimatic events affecting the GWS in a coastal area. The TWSGRACE data gap was filled with reconstructed values from multi-linear regression (MLR) and artificial neural network (ANN) models and used to estimate changes in GWS in the Texas coastal region (Gulf Coast and Carrizo–Wilcox Aquifers) between 2002 and 2019. The reconstructed TWSGRACE, along with soil moisture storage (SMS) from land surface models (LSMs), and surface water storage (SWS) were used to estimate the GRACE-derived GWS (GWSGRACE), validated against the GWS estimated from groundwater level observations (GWSwell) and extreme hydroclimatic event records. The results of this study show: (1) Good agreement between the predicted TWSGRACE data gaps from the MLR and ANN models with high accuracy of predictions; (2) good agreement between the GWSGRACE and GWSwell records (CC = 0.56, p-value < 0.01) for the 2011–2019 period for which continuous GWLwell data exists, thus validating the approach and increasing confidence in using the reconstructed TWSGRACE data to monitor coastal GWS; (3) a significant decline in the coastal GWSGRACE, at a rate of 0.35 ± 0.078 km3·yr−1 (p-value < 0.01), for the 2002–2019 period; and (4) the reliable applicability of GWSGRACE records in detecting multi-year drought and wet periods with good accuracy: Two drought periods were identified between 2005–2006 and 2010–2015, with significant respective depletion rates of −8.9 ± 0.95 km3·yr−1 and −2.67 ± 0.44 km3·yr−1 and one wet period between 2007 and 2010 with a significant increasing rate of 2.6 ± 0.63 km3·yr−1. Thus, this study provides a reliable approach to examine the long- and short-term trends in GWS in response to changing climate conditions with significant implications for water management practices and improved decision-making capabilities.

Recent Changes in Groundwater and Surface Water in Large Pan-Arctic River Basins

Surface and groundwater in large pan-Arctic river basins are changing rapidly. High-quality estimates of these changes are challenging because of the limits on the data quality and time span of satellite observations. Here, the term pan-Arctic river refers to the rivers flowing to the Arctic Ocean basin. In this study, we provide a new evaluation of groundwater storage (GWS) changes in the Lena, Ob, Yenisei, Mackenzie and Yukon River basins from the GRACE total water storage anomaly product, in situ runoff, soil moisture form models and a snow water equivalent product that has been significantly improved. Seasonal Trend decomposition using Loess was utilized to obtain trends in GWS. Changes in surface water (SW) between 1984 and 2019 in these basins were also examined based on the Joint Research Centre Global Surface Water Transition data. Results suggested that there were great GWS losses in the North American river basins, totaling approximately −219 km3, and GWS gains in the Siberian river basins, totaling ~340 km3, during 2002–2017. New seasonal and permanent SWs are the primary contributors to the SW transition, accounting for more than 50% of the area of the changed SW in each basin. Changes in the Arctic hydrological system will be more significant and various in the case of rapid and continuous changes in permafrost.

Numerical modeling of changes in groundwater storage and nitrate load in the unconfined aquifer near a river receiving reclaimed water.

Reclaimed water (RW) has been widely used as an alternative water resource to recharge rivers in mega-city Beijing. At the same time, the RW also recharges the ambient aquifers through riverbank filtration and modifies the subsurface hydrodynamic system and hydrochemical characteristics. To assess the impact of RW recharge on the unconfined groundwater system, we conducted a 3D groundwater flow and solute transport model based on 10years of sequenced groundwater monitoring data to analyze the changes of the groundwater table, Cl- loads, and NO3-N loads in the shallow aquifer after RW recharge to the river channel. The results show that the groundwater table around the river channel elevated by about 3-4m quickly after RW recharge from Dec. 2007 to Dec. 2009, and then remained stable due to the continuous RW infiltration. However, the unconfined groundwater storage still declined overall from 2007 to 2014 due to groundwater exploitation. The storage began to recover after groundwater extraction reduction, rising from 3.76 × 108 m3 at the end of 2014 to 3.85 × 108 m3 at the end of 2017. Cl- concentrations varied from 5-75mg/L before RW recharge to 50-130mg/L in 2years (2007-2009), and then remained stable. The zones of the unconfined groundwater quality affected by RW infiltration increased from 11.7 km2 in 2008 to 26.7 km2 in 2017. Cl- loads in the zone increased from 1.8×103t in 2008 to 3.8 × 103 t in 2017, while NO3-N loads decreased from 29.8 t in 2008 to 11.9 t in 2017 annually. We determined the maximum area of the unconfined groundwater quality affected by RW, and groundwater outside this area not affected by RW recharge keeps its original state. The RW recharge to the river channel in the study area is beneficial to increase the groundwater table and unconfined groundwater storage with lesser environmental impacts.

Evaluation of Groundwater Storage Depletion Using GRACE/GRACE Follow-On Data with Land Surface Models and Its Driving Factors in Haihe River Basin, China

Groundwater storage (GWS) in the Haihe River Basin (HRB), which is one of the most densely populated and largest agricultural areas in China, is of great importance for the ecosystem environment and socio-economic development. In recent years, large-scale overexploitation of groundwater in HRB has made it one of the global hotspots of GWS depletion. In this study, monthly GWS variations in HRB from 2003 to 2020 were estimated using the Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On (GRACE-FO) data in combination with three land surface models (LSMs) from the Global Land Data Assimilation System (GLDAS). The results show the following: (1) HRB suffered extensive GWS depletion from 2003 to 2020, which has been aggravated since 2014, with a mean rate of 1.88 cm·yr−1, which is equivalent to a volume of 6 billion m3·yr−1. The GWS depletion is more serious in the plain zone (−2.36 cm·yr−1) than in the mountainous zone (−1.63 cm·yr−1). (2) Climate changes are excluded from the reasons for GWS depletion due to annual precipitation and evaporation being close to normal throughout the period. In addition, GWS changes show a low correlation with meteorological factors. (3) The consumption of groundwater for irrigation and land use/cover changes have been confirmed to be the dominant factors for GWS depletion in HRB. (4) The effects of inter-basin water transfer projects cannot be obviously observed using the GRACE and GRACE-FO; more inter-basin water transfers are needed for recovering the GWS in HRB. Therefore, it is imperative to control groundwater exploitation and develop a more economical agricultural irrigation structure for the sustainability of groundwater resources in HRB.

Quantification and possible causes of declining groundwater resources in the Euro-Mediterranean region from 2003 to 2020

Groundwater resources in Euro-Mediterranean countries provide a large part of the population’s water supply and are affected to varying degrees by anthropogenic use and climatic impacts. In many places, significant groundwater-level declines have already been observed, indicating an imbalance between natural groundwater recharge and groundwater abstraction. The extent of changes in groundwater storage (GWS) in the period 2003–2020 is quantified for the Euro-Mediterranean region using the latest data from the Gravity Recovery and Climate Experiment (GRACE/GRACE-FO) satellite mission and recently reanalyzed ERA5-Land climate data from the European Centre for Medium-Range Weather Forecasts. The results are set in relation to the prevailing climate, the regional hydrogeological setting, and annual groundwater recharge and abstractions on country level. Analysis of the mean annual trends over the study period shows significant decreases in GWS in many countries of Europe, Northern Africa and the entire Arabian Peninsula. Overall, there are significantly negative trends in about 70% of the study region. The mean of the trends across the Euro-Mediterranean region is –2.1 mm/year. The strongest negative trends in GWS per country are observed in Iraq and Syria (–8.8 and –6.0 mm/year, respectively), but also countries in central and eastern Europe are affected by depleting aquifers. The results are a clear indicator of the already medium-term groundwater stress in the Euro-Mediterranean region, which is expected to increase in the future, and demonstrate the need for adapted strategies for sustainable groundwater management on a transregional scale in the context of climate change and population growth.

Characterization of the hydro-geological regime of fractured aquifers in Benin (West-Africa) using multi-satellites and models

Study regionFractured aquifers in Benin have emerged in the past decades as a critical resource because of the increasing domestic water needs, agriculture, and livestock breeding. Hence this study was undertaken in the poorly gauged and complex basement aquifers beneath the 9th latitude. Study focusTo aid water resources management and sustainability, this study combined satellite remote sensing products with outputs from hydrological models and in situ measurement to assess the spatial and temporal patterns of water storage changes, including groundwater. The fractured aquifers’ hydrogeological regime is further analysed through monitoring wells’ series interpretation. New hydrological insights for the regionResults show considerable inter-annual variations in Terrestrial Water Storage (TWS) during the study period (2003–2019). Two uptrend intervals characterized by wet hydrological seasons from 2006 to 2013 and 2016–2019 with respectively, Sen’s slope values of 1.16 mm/yr and 4.47 mm/yr were observed. However, two downtrend intervals represented by dry hydrological seasons with Sen’s slopes of − 1.01 mm/yr (2003–2006) and − 1.19 mm/yr (2013–2016) were also observed. The inter-annual seasonality in land water storage components showed that unlike the 4 climatic seasons, the hydrogeological regime is affected by 3 seasons each year. The first dry season takes place from January to April-May (−29.1 ± 17.7 mm/yr) followed by the wet season (April-May to October-November: 41.6 ± 26.4 mm/yr) and the last dry period, which is stronger (November to December: −53.7 ± 31.6 mm/yr). Soil moisture (SM) and canopy water content (CW) declined during the 2008–2016 period, coinciding with decline in Tree cover areas, which dropped by 25.5% (3086.7 sqkm). Further from 2007 to 2014, the four monitoring wells have seen the groundwater level increased by, 1.44 m/yr, 2.0 m/yr, 3.2 m/yr, and 0.56 m/yr for the Affon, Zou, Okpara and Couffo sub-basins, respectively. The retrieved TWS and Water ‐ Global Assessment and Prognosis groundwater storage (WaterGap-GWS) sub-domains based on statistical decomposition was used to identify hidden patterns to improve knowledge on the climatic and hydrological conditions, which affected land water storage. Moreover, TWS, GWS and SM lost water 51%, 57% and 47% of the time, respectively, during the period and the storage potential is lower for the aquifers compared to the unsaturated zone. Precipitation is recognized as a primary driver of GWS (r = 0.69; α = 0.05) in the region and a change in GWS is more likely to drive the CW and affect the vegetation greenness. Moreover, the anthropogenic pressure exacerbates Tree cover loss. Additionally, the Extra Tree regressor performs best in the groundwater storage change ∆GWS prediction with a strong and positive Pearson Correlation Coefficient of 0.92. The basement aquifers assessment using the aforementioned approach has potential to support large-scale monitoring efforts for more reliable water resources management in the region.