Groundwater Storage Research Articles

Ambiguities in implementing regulations for sustainable groundwater resources development in Vietnam with a case study of Tra Vinh province in Mekong Delta

Study regionThe study region is the coastal Tra Vinh province located in Mekong Delta, Vietnam Study focusThe study aims to clarify the ambiguities in implementing Vietnam's regulation on restrictions of groundwater abstraction in Mekong Delta. Three ambiguities were identified, namely, how should half of the saturated thickness of the unconfined aquifer be computed; is the water level depth of 30 m for the confined aquifer sufficient to prevent seawater intrusion and land subsidence; and why is the interference of water use rights of neighbouring provinces not considered. Numerical groundwater flow, saltwater transport and land subsidence were constructed for Tra Vinh province to assess water level requirements from Vietnam regulation and important constraints for groundwater sustainability. The model results show that although all scenarios satisfy groundwater level requirements from the Vietnam regulation, only the mitigation scenario combined with reducing abstraction and increasing recharge can lead to sustainable development. New hydrological insights for the regionThe current groundwater abstraction in Tra Vinh province has caused groundwater storage depletion and induced seawater intrusion, land subsidence, and captured groundwater inflow from neighbouring provinces. When the business as usual continues, seawater may intrude 4–5 km inland and cumulative land subsidence may reach 1.5 m by 2100. Reduction of current abstraction by 50% supplemented with the increase of recharge by 1.5 times in the dunes can lead to sustainable development.

Twenty-Year Spatiotemporal Variations of TWS over Mainland China Observed by GRACE and GRACE Follow-On Satellites

Terrestrial water storage (TWS) is a pivotal component of the global water cycle, profoundly impacting water resource management, hazard monitoring, and agriculture production. The Gravity Recovery and Climate Experiment (GRACE) and its successor, the GRACE Follow-On (GFO), have furnished comprehensive monthly TWS data since April 2002. However, there are 35 months of missing data over the entire GRACE/GFO observational period. To address this gap, we developed an operational approach utilizing singular spectrum analysis and principal component analysis (SSA-PCA) to fill these missing data over mainland China. The algorithm was demonstrated with good performance in the Southwestern River Basin (SWB, correlation coefficient, CC: 0.71, RMSE: 6.27 cm), Yangtze River Basin (YTB, CC: 0.67, RMSE: 3.52 cm), and Songhua River Basin (SRB, CC: 0.66, RMSE: 7.63 cm). Leveraging two decades of continuous time-variable gravity data, we investigated the spatiotemporal variations in TWS across ten major Chinese basins. According to the results of GRACE/GFO, mainland China experienced an average annual TWS decline of 0.32 ± 0.06 cm, with the groundwater storage (GWS) decreasing by 0.54 ± 0.10 cm/yr. The most significant GWS depletion occurred in the Haihe River Basin (HRB) at −2.07 ± 0.10 cm/yr, significantly substantial (~1 cm/yr) depletions occurred in the Yellow River Basin (YRB), SRB, Huaihe River Basin (HHB), Liao-Luan River Basin (LRB), and Southwest River Basin (SWB), and moderate losses were recorded in the Northwest Basin (NWB, −0.34 ± 0.03 cm/yr) and Southeast River Basin (SEB, −0.24 ± 0.10 cm/yr). Furthermore, we identified that interannual TWS variations in ten basins of China were primarily driven by soil moisture water storage (SMS) anomalies, exhibiting consistently and relatively high correlations (CC > 0.60) and low root-mean-square errors (RMSE < 5 cm). Lastly, through the integration of GRACE/GFO and Global Land Data Assimilation System (GLDAS) data, we unraveled the contrasting water storage patterns between northern and southern China. Southern China experienced drought conditions, while northern China faced flooding during the 2020–2023 La Niña event, with the inverse pattern observed during the 2014–2016 El Niño event. This study fills in the missing data and quantifies water storage variations within mainland China, contributing to a deeper insight into climate change and its consequences on water resource management.

Satellite-based estimates of declining groundwater storage in the transboundary Cambodia-Mekong River Delta Aquifer of the Lower Mekong region, Southeast Asia

Satellite-based estimates of declining groundwater storage in the transboundary Cambodia-Mekong River Delta Aquifer of the Lower Mekong region, Southeast Asia

Evaluating groundwater balance and its spatial Distribution as part of NASA GRACE mission for Blue Nile River (Ethiopia, East Africa)

The paper introduces a methodology for estimating groundwater balance on the example of the Blue Nile basin by means of NASA GRACE space technologies. The international Gravity Recovery and Climate Experiment (GRACE) was launched in 2002 to measure time-space variations in the Earth's gravity field. It is a system for remote determination of changes in gravitational force associated with variations of mass within the Earth. The study involves a gravity recovery and climate experiment together with The Global Land Data Assimilation System to calculate groundwater storage as applied to the Blue Nile River basin, based on processing satellite data and spatial localization of the Blue Nile River using an optimal window function with specific scan line. Groundwater storage and its dynamics were investigated from 2003 to 2010. Considering surface water storage, surface runoff and soil moisture, the authors estimated groundwater budget in the Blue Nile River basin. According to the obtained results, the average groundwater storage in the area under study comprised 1100.0 mm/year. Due to a lack of land hydrological studies, verification of the data obtained was not carried out, but was recommended as a matter for future research in this area.

The GWR model-based regional downscaling of GRACE/GRACE-FO derived groundwater storage to investigate local-scale variations in the North China Plain

Groundwater storage and depletion fluctuations in response to groundwater availability for irrigation require understanding on a local scale to ensure a reliable groundwater supply. However, the coarser spatial resolution and intermittent data gaps to estimate the regional groundwater storage anomalies (GWSA) prevent the Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On (GARCE-FO) mission from being applied at the local scale. To enhance the resolution of GWSA measurements using machine learning approaches, numerous recent efforts have been made. With a focus on the development of a new algorithm, this study enhanced the GWSA resolution estimates to 0.05° by extensively investigating the continuous spatiotemporal variations of GWSA based on the regional downscaling approach using a regression algorithm known as the geographically weighted regression model (GWR). First, the modified seasonal decomposition LOESS method (STL) was used to estimate the continuous terrestrial water storage anomaly (TWSA). Secondly, to separate GWSA from TWSA, a water balance equation was used. Third, the continuous GWSA was downscaled to 0.05° based on the GWR model. Finally, spatio-temporal properties of downscaled GWSA were investigated in the North China Plain (NCP), China's fastest-urbanizing area, from 2003 to 2022. The results of the downscaled GWSA were spatially compatible with GRACE-derived GWSA. The downscaled GWSA results are validated (R = 0.83) using in-situ groundwater level data. The total loss of GWSA in cities of the NCP fluctuated between 2003 and 2022, with the largest loss seen in Handan (−15.21 ± 7.25 mm/yr), Xingtai (−14.98 ± 7.25 mm/yr), and Shijiazhuang (−14.58 ± 7.25 mm/yr). The irrigated winter-wheat farming strategy is linked to greater groundwater depletion in several cities of NCP (e.g., Xingtai, Handan, Anyang, Hebi, Puyang, and Xinxiang). The study's high-resolution findings can help with understanding local groundwater depletion that takes agricultural water utilization and provide quantitative data for water management.

Assessment of groundwater reservoir and the influence of its characteristics on water dynamic control.

Groundwater reservoirs play an important role in regional water volume balance. In order to ensure the efficiency of a groundwater reservoir and obtain its maximum value, it is necessary to figure out its characteristics, such as minimum water level, normal water level, and maximum water level. The characteristics not only determined the storage capacity but also influenced the water allocation and utilization. Therefore, this paper took a groundwater reservoir built in an alluvial fan as an example to assess its storage capacity and the influence on water dynamic control. The alluvial fan was located in the front of Lao River, where there were an alluvial-diluvial fan, groundwater storage space, and natural impervious boundary. Therefore, it was an ideal place for constructing a groundwater reservoir. Through theoretical analysis and numerical simulation, the storage capacity was determined, and the influence of groundwater reservoir characteristics on water dynamic control was analyzed. On this basis, the water-supply method in the research area was confirmed. This paper will present a new method for repairing the groundwater system and protecting the ecological environment.

Combined monthly GRACE-FO gravity fields for a Global Gravity-based Groundwater Product

SUMMARY The Combination Service for Time-variable Gravity fields (COST-G) operationally provides combinations of monthly Earth gravity field models derived from observations of the microwave ranging instrument of the GRACE Follow-on (GRACE-FO) satellite mission, applying the quality control and combination methodology originally developed by the Horizon 2020 project European Gravity Service for Improved Emergency Management for the data of the GRACE satellites. In the frame of the follow-up Horizon 2020 project Global Gravity-based Groundwater Product (G3P), the GRACE-FO combination is used to derive global grids of groundwater storage anomalies. To meet the user requirements and achieve optimal signal-to-noise ratio, the combination has been further developed and extended to incorporate: • new time-series based on the alternative accelerometer transplant product generated in the frame of the project by the Institute of Geodesy at the Graz University of Technology, which specifically improves the estimation of the C30 coefficient and also reduces the noise at medium to short wavelengths, and • the new time-series AIUB–GRACE-FO–RL02 of monthly GRACE-FO gravity fields, which is derived at the Astronomical Institute of the University of Bern by applying empirical noise modelling techniques. The COST-G quality control confirms the consistency of the contributing GRACE-FO time-series concerning the signal amplitude of seasonal hydrology in large river basins and the secular mass change in polar regions, but it also indicates rather diverse noise characteristics. The difference in the noise levels is taken into account in the combination process by relative weights derived by variance component estimation on the solution level. The weights are expected to be inverse proportional to the noise levels of the individual gravity field solutions. However, this expectation is violated when applying the weighting scheme as developed for the GRACE combination. The reason is found in the high-order coefficients of the gravity field, which are poorly determined from the low–low range-rate observations due to the observation geometry and suffer from aliasing due to the malfunctioning accelerometer onboard one of the GRACE-FO satellites. Hence, for the final G3P-combination a revised weighting scheme is applied where the gravity field coefficients beyond order 60 are excluded from the determination of the weights. The quality of the combined gravity fields is assessed by comparison of the noise content and the signal-to-noise ratio with the individual time-series. Independent validation is provided by the COST-G validation centre at the GFZ German Research Centre for Geosciences, where orbit fits of the low-flying Gravity and steady-state Ocean Circulation Explorer satellite are performed that confirm the high quality of the combined GRACE-FO gravity fields. By the end of the G3P project, the new combination scheme is implemented by COST-G as the new COST-G–GRACE-FO–RL02 and continued to be used for the operational GRACE-FO combination.

Multi-sensor observations for monitoring groundwater depletion and land subsidence

Study regionThe Kabudarahang Plain and the Razan-Qahavand Plain. Study focusImproper use of water resources has reduced groundwater levels and created land subsidence (LS) in many plains of Iran. The aim and innovation of this research are to study multi-sensor observations for LS and groundwater depletion and explore the relationships of the involved variables with high confidence. The gravity recovery and climate experiment (GRACE) observations can be used to evaluate water storage changes at the Earth’s surface. GRACE has stripe errors, leakage and various noises that multilevel 3D wavelet decomposition (M3WD) has been suggested to mitigate noises and downscale for small scale. This study has investigated the interferometric synthetic-aperture radar (InSAR) of Sentinel-1 images from October 2014 to September 2019, the GRACE data from March 2002 to July 2016, and groundwater hydrograph (GH) from 2014 to 2020. New hydrological insight for the regionThe maximum LS rate, obtained from small baseline subset-differential of InSAR is 20 mm/year at the Kabudarahang Plain (KP) and 30 mm/year at Razan-Qahavand Plain (RQP). The groundwater storage variations (ΔGW) have a decreasing trend of 78.45 ± 0.2 million cubic meters/year. The GH for the KP and RQP shows a downward trend of 3.25 and 1.81 m/year, respectively. Based on the outcomes, the M3WD can increase the correlation of ΔGW with other sensors by 15 %. Also, validation between sensors with normalized cross-correlation has remarkable compatibility. The multi-sensor study of ΔGW and LS revealed various dimensions with high reliability and can facilitate the water resource management.

Lateral flow in bedrock dominates baseflow and flood flow in a catchment underlain with permeable sedimentary rock in the high‐relief Chichibu Mountains

AbstractDetermining the mechanisms of runoff generation is essential to land and water resource conservation, but lateral flow processes in high‐relief mountain areas remain unclear due to the difficulty to observe and measure these processes. This study demonstrates that lateral flow in bedrock dominates both high and low flows in a catchment underlain with permeable sedimentary bedrock in the high‐relief Chichibu Mountains. Through measurement of precipitation, discharge, soil, and stream temperatures and tracers (SiO2 and δ18O) in the first‐order stream Bakemonozawa (0.51 km2) and comparison of its water balance with the nested Kawamata catchment (93.58 km2) i, we showed that 831–1012 mm/year or 47%–53% of annual precipitation leaked from Bakemonozawa. The streamflow was discharged from an average depth of 2.3 m or deeper, which were deeper than the measured deepest soil near the gauge. Furthermore, stream temperature decreased significantly during a large summer storm, demonstrating that the relative contribution of the deep bedrock flowpath increased. Large isotopic variation in precipitation was dampened in the stream, suggesting that most baseflow and some higher flows arose from groundwater storage with a constant isotopic ratio due to dispersion and mixing. The discharge–SiO2 relationship suggested at least two groundwater sources, namely, shallow and deep bedrock groundwater, with different runoff responses. Discharge increased dramatically when the catchment became wet, coinciding with the deep bedrock groundwater contribution. Further research is needed to clarify the hydrological characteristics of bedrock.

Exploiting Earth Observations to Enable Groundwater Modeling in the Data-Sparse Region of Goulbi Maradi, Niger

Groundwater modeling is a useful tool for assessing sustainability in water resources planning. However, groundwater models are difficult to construct in regions with limited data availability, areas where planning is most crucial. We illustrate how remote sensing data can be used with limited in situ data to build and calibrate a regional groundwater model in the Goulbi Maradi alluvial aquifer in southern Niger in Western Africa. We used data from the NASA Gravity Recovery and Climate Experiment (GRACE) satellite mission to estimate recharge rates, the primary source of water to the aquifer. We used the groundwater storage changes obtained from GRACE data from 2009 to 2021 to establish an overall water budget. We used this water budget to back-calculate groundwater withdrawals from pumping in the region. There are only very limited historic data on withdrawals. This approach allowed us to calibrate the model and use it as a predictive tool to analyze the impact of various assumptions about future recharge and groundwater extraction patterns associated with the development of groundwater infrastructure in the region. The results indicate that water extraction from the Goulbi Maradi alluvial aquifer is sustainable, even if current groundwater extraction is increased by up to 28%.

An appraisal of the local‐scale spatio‐temporal variations of drought based on the integrated GRACE/GRACE‐FO observations and fine‐resolution FLDAS model

AbstractThe gravity recovery and climate experiment (GRACE) observations have so far been utilized to detect and trace the variations of hydrological extremes worldwide. However, applying the coarse resolution GRACE estimates for local‐scale analysis remains a big challenge. In this study, a new version of the fine resolution (1 km) Famine early warning systems network Land Data Assimilation System (FLDAS) model data was integrated into a machine learning model along with the GRACE data to evaluate the subbasin‐scale variations of water storage, and drought. With a correlation of and a root mean square error (RMSE) of of its results, the downscaling model turned out to be very successful in modelling the finer resolution variations of TWSA. The water storage deficit (WSD) and Water Storage Deficit Index (WSDI) were used to determine the episodes and severity of drought events. Accordingly, two severe droughts (January 2008 to March 2009 and September 2019 to December 2020) were discerned in the Kizilirmak Basin (KB) located in Central Türkiye. The characterization of droughts was evaluated based on WSDI, scPDSI, and model‐based drought indices of the soil moisture storage percentile (SMSP) and groundwater storage percentile (GWSP). The results indicated discrepancies in the drought classes based on different indices. However, the WSDI turned out to be more correlated with GWSP, suggesting its high ability to monitor groundwater droughts as well.

Soil water infiltration characteristics of reforested areas in the paleo-periglacial eastern Liaoning mountainous regions, China

Plantation forests (PF) and natural secondary forests (NSF) are the primary reforestation approaches. The establishment of PF can affect forest hydrological processes by changing soil structure. To date, few studies have focused on these changes and the effects on hydrological processes for the paleo-periglacial landform. To reveal reforestation approaches effects on water infiltration, including soil water infiltration capacity, retention capacity, and waterflow path pattern, we conducted field dye-tracer investigations with rainfall and laboratory infiltration experiments for the paleo-periglacial landform of eastern Liaoning mountains, China. The results showed that (1) Soil physical properties (including total porosity (TP), capillary porosity (CP), non-capillary porosity (NCP), initial soil water content (IWC), field water capacity (FWC)) and root abundance (RA) decreased with soil depth in both PF and NSF, while the soil bulk density (BD) and distribution of gravel content showed opposite changes. (2) Establishment of PF reduced the infiltration capacity and water retention capacity in the 0–20 cm layer, but enhanced the water retention capacity in 20–30 cm layer. Low IWC was conducive to increase soil water content (SWC) after infiltration. (3) Infiltration capacity parameters (including saturated hydraulic conductivity (Ks), SWC, difference between SWC and IWC (SWC–IWC), dye coverage ratio (DC)) were significantly correlated with BD, TP, CP, NCP, FWC, and fine roots RA (P < 0.05). Better connectivity gravels were more conducive to water infiltration. (4) Preferential flow was the main infiltration type, but with different waterflow paths pattern, with the 'funnel', 'finger' shape for PF, NSF, respectively. Increasing infiltration could increase flow path connectivity. Our findings show that soil physical properties, roots, and gravel occurrence affected soil infiltration, and different reforestation approaches had varying impacts on soil infiltration, water redistribution, transportation, and storage of surface and groundwater, improving the understanding of ecohydrological processes and effects of water resources management in forest ecosystems of paleo-periglacial landform.

Using Tritium Isotope in Groundwater to Estimate the Reservoir Capacity at Erbil, Northern Iraq

Tritium is a radioactive isotope of hydrogen in water molecule can be applied in tracer studies to estimate water residence time. The tritium values of the groundwater in Erbil and the reservoir's capacity were the main subjects of this investigation. Three sub-basins, including, Kapran (720 km2), Central (1315 km2), and Bashtapa (625 km2) constructs Erbil City's groundwater basin. 27 groundwater samples from three sub-basins were taken for oxygen-18, tritium analysis and measurement of electrical conductivity. Tritium values divided the groundwater reservoirs into three possible groups: pre-bomb groundwaters that has ages older than 70 years due to lower tritium values, contaminated groundwater and mixture between young and old groundwaters. It is adequate to suppose that there were several substantial subsurface retention reservoirs in Dashty-Hawler basin. For instance, if two wells from the Kapan sub-basin were used as a point of reference, well No. 1 in Kalak region would have a tritium value of 1.7 ± 0.72 TU having groundwater that was approximately 16.2 years old. In contrast, the well No. 6 at Grd-Jutyar has a lower tritium value equal to 0.72 ± 0.68 TU indicating a greater residence time of 31.6 years. These two wells have differing reservoir sizes, and well No. 6 has a longer retention basin to store groundwater than well No. 1 According to these findings, younger groundwater and smaller retention basins were indicated by greater tritium levels, whereas older groundwater and larger retention basins were noted by lower tritium levels.

A dynamical downscaling method of groundwater storage changes using GRACE data

Study regionThe Northwest inland region of China. Study focusThe use of Gravity Recovery and Climate Experiment (GRACE) satellite data for assessing groundwater storage changes (GWSC) is rapidly expanding; however, dealing with such low spatial resolution data is always a challenge. In the present study, a dynamical downscaling method based on groundwater flow processes was proposed to simulate GWSC. Firstly, a hypothetical model constructed by MODFLOW package used to evaluate the efficiency of the developed model. Then, a part of the northwest inland region of China was taken as a real example to verify the applicability of the model. Finally, the downscaled results are used to estimate the groundwater storage changes in the study area. New hydrological insights for the regionThe spatiotemporal change of GWSC from the proposed model in a hypothetical conceptual model matched well with those from the MODFLOW package. The average correlation coefficient between downscaled GWSC and observed groundwater level is over 0.54, and downscaled GWSC showed the reasonable heterogeneity in the study area. From 2008–2011, the area with a decline in groundwater storage accounted for about 12% of the total area, while it increased to 21.8% from 2012 to 2016. The proposed model has the potential to be applied for the large-scale evaluation of GWSC and to improve the understanding of GWSC in remote areas.

Correlating Groundwater Storage Change and Precipitation in Alabama, United States from 2000–2021 by Combining the Water Table Fluctuation Method and Statistical Analyses

The complexity of aquifers poses a challenge for fully comprehending the impact of climate change on groundwater. In this study, we employed a suite of hydrological and statistical methods, including the water table fluctuation (WTF) method, wavelet analysis, the Hurst exponent, and temporal trend analysis, to assess groundwater storage (GWS) changes and their correlation with precipitation in Alabama, located in the southeastern United States. These approaches were used to evaluate the temporal variability of GWS as derived from well data and large-scale model estimates that incorporated satellite observations. The results unveiled a nuanced and regionally variable relationship between GWS changes and precipitation over the past two decades. While the Mann–Kendall test did not reveal any statistically significant overarching trends in GWS changes, Sen’s slope analysis indicated subtle regional variations, including a minor decline of −0.2 mm/year for GWS in southern Alabama and modest increases of 0.5 mm/year and 0.38 mm/year in the western and northern regions, respectively, from 2000–2021. Wavelet coherence analysis showed significant co-variation between GWS and precipitation in cycles ranging from 8 to 32 months, suggesting potential cyclic or intermittent influences. Furthermore, we detected strong persistence within the groundwater system using the Hurst exponent, indicating the substantial temporal memory impact. These findings are useful for developing effective groundwater management strategies in a changing climate.

Determination of Aquitard Storage from Pumping Tests in Leaky Aquifers

The management of groundwater resources requires a thorough understanding of groundwater flow and storage. Aquitards, in particular, can store large amounts of groundwater, but are generally often overlooked or ignored in groundwater surveys. In this study, we show how a pumping test performed in a leaky aquifer can be used to estimate the storage coefficient of an overlying aquitard. A new analytical solution is presented for a constant-rate pumping test in a leaky aquifer taking into account the water released from storage in the aquitard. The utility of the method is demonstrated by a practical application using data from a pumping test conducted in a semi-confined sandy aquifer overlain by an aquitard consisting of a mixture of clay and sand. The results obtained show that the solution enables the accurate calculation of the storage properties of the aquitard.

Identifying potential hotspots of groundwater-climate interaction in the Great Artesian Basin, Australia

Groundwater aquifers around the world are experiencing stress due to the composite influence of climate change and excessive groundwater withdrawal. Despite this influence, there is still insufficient knowledge on climate-groundwater interaction dynamics in complex aquifers like Australia's Great Artesian Basin. This understanding can inform improved management, which can in turn, increase resilience of groundwater system to on-going groundwater withdrawal and climate change. The main aim of this research was to assess the impacts of climatic change on groundwater storage (GWS) changes over the Great Artesian Basin (GAB) using a range of multi-scale indicators, including Gravity Recovery and Climate Experiment (GRACE)-derived groundwater observations and standardized precipitation evapotranspiration indices (SPEI). Rotated principal component analysis (rPCA) was used to identify climatic hotspots (where GWS variation is particularly sensitive or vulnerable to the impacts of climate change), and correlation analyses were used to assess the relationship between these hotspots and changes in groundwater over space and time. The localized spatial pattern via rPCA was uncovered in the GAB's southeastern region, where annual variations in GWS exhibited conspicuous peaks in 2002, 2014, and 2015. Another finding was that the southeast region of the GAB experienced higher GWS variations as compared to other sub-basins during 2007–2008, which resulted in low groundwater recharge. These findings underscore the variations in GWS within the GAB, emphasizing its responsiveness to climatic variations (e.g., rainfall contributes to recharge of groundwater system and drought intensifies water extraction and restricts discharge). Moreover, this study found a substantial negative correlation (r = −0.7), between SPEI at 12-month scale and GWS variation in the southern GAB region. This relationship highlights the vulnerability of groundwater storage in the southern GAB region to prolonged drought conditions. Furthermore, this study identified positive correlations (r = 0.4 to 0.8), between key climate indices such as the El Niño-Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO) with GWS anomalies in the southeast and northern GAB regions. These correlations underscore the pivotal roles played by ENSO and PDO in shaping GWS variations in these areas, highlighting the need to incorporate an understanding of these climatic processes into water resource management strategies. This consideration is crucial for ensuring sustainable water management practices that can adapt to the influence of these climate phenomena. These findings provide essential insights into the dynamics of groundwater in the face of changing environmental conditions, with implications for sustainable water management.

Monitoring hydrological changes with satellite data: Spring thaw's effect on soil moisture and groundwater in seasonal Freezing-Thawing zones

Due to the dense vegetation coverage in mountainous areas and difficulties in field monitoring in mid-high-latitude freezing-thawing zones, satellite and remote sensing technologies can be used to monitor long-term global hydrological changes. This study uses multiple satellite data of snow water equivalent (SWE), soil moisture (SM), and groundwater storage anomaly (GWSA) to analyse their spatial–temporal changes in the seasonal freezing-thawing Tumen River Basin. Water transformation during the thawing period is determined through groundwater level fluctuations and correlation analysis methods; thus, the recharge effects of snowmelt and seasonally frozen soil thawing on soil moisture and groundwater are identified. The snow begins to melt in February, and snow water equivalent decreases gradually from upstream to downstream after reaching the peak. Furthermore, seasonally frozen soil melts gradually from March. Soil moisture downstream is higher than that in the upper and middle reaches. Additionally, the GWSA increases gradually in the thawing period, which is high in mountainous areas and low in plains. The Spearman correlation coefficient among ΔSWE, ΔSM, and ΔGWSA is more significant than 0.5 and dependent on altitude. With snow and frozen soil melting, vertical infiltration and meltwater percolation increase soil moisture and GWSA. This phenomenon is predominant at low altitudes (<600 m) and pore phreatic groundwater in the middle and lower reaches. However, snowmelt is the dominant factor in groundwater recharge at high-altitude upper reaches. For seasonal freezing-thawing zones, a close water transformation was observed between snow melting, soil water thawing, and groundwater in spring. Thus, the environmental and ecological impact of spring snowmelt and floods in freezing-thawing zones can be monitored using multi-source data.

Spatial and temporal downscaling schemes to reconstruct high-resolution GRACE data: A case study in the Tarim River Basin, Northwest China

Climate change and excessive exploitation of water resources exert pressure on groundwater supply and the ecosystem in drylands. Although The Gravity Recovery and Climate Experiment (GRACE) satellites has demonstrated the feasibility of quantifying global groundwater storage variations, monitoring regional-scale groundwater has been challenging due to the coarse resolution of GRACE. Previous GRACE downscaling studies focused on develop new algorithms based on the perspective of pixel spatial correlation to improve resolution, which cannot better capture the temporal evolution of GRACE data effectively. In this study, we employ the semi-supervised variational autoencoder (SSVAER) algorithm and the multi-scale geographically weighted regression (MGWR) model to establish two different downscaling schemes: pixel temporal continuity downscaling and pixel spatial correlation downscaling. These schemes achieve spatial resolution downscaling of GRACE-derived groundwater storage anomalies (GWSA) from 0.5° to 0.1°. Additionally, the applicability of the PCR-GLOBWB model in drylands is verified. Furtherly, the spatiotemporal distribution patterns of GWSA are analyzed. The results show that (1) Both the temporal and spatial downscaling methods produced consistent results, with data correlations ranged from 0.94 to 0.98 observed in over 80 % of the range before and after downscaling; (2) The groundwater storage change rate in the northern Tarim River Basin (TRB) is 25 times greater than the model results, while in other regions, the average deviation is 2.6 times; (3) The two schemes enhance the correlation (0.27) between GWSA and groundwater level anomaly (GWLA) to 0.59 and 0.52, respectively, with a three-month lag in GWSA relative to GWLA. The temporal downscaling approach exhibited higher CC and lower RMSE, outperforming the spatial downscaling approach. The high-resolution results in this study can well complement groundwater level prediction efforts in arid regions and provide quantitative information for local water resource management.

Groundwater Storage Variations in the Main Karoo Aquifer Estimated Using GRACE and GPS

The Gravity Recovery and Climate Experiment (GRACE) provided valuable insights into variations in Groundwater Storage (GWS). However, the sensitivity of utilizing Global Positioning System (GPS) time series displacement data for detecting changes in GWS remains a subject of ongoing discussion. In order to estimate the spatiotemporal GWS, we selected a vertical displacement from 65 GPS stations located in the Main Karoo Aquifer (MKA). We performed total water storage (TWS) inversion on GPS vertical displacement components; after that, we deducted surface water components based on the Global Land Data Assimilation System (GLDAS) from January 2013 to December 2021. Additionally, for validation, we compared our GWS estimates with the GRACE-derived GWS and observed GWS values derived from the WaterGAP Global Hydrology Model (WGHM) compartments. We discovered that the TWS and GWS trends derived from GPS and GRACE exhibited similar behaviors with trend values overestimated by GRACE and WGHM. Our findings demonstrate relatively typical behavior between GPS and GRACE in the first and second principal component behaviors (PCs) and empirical orthogonal function (EOF) loadings (or spatial patterns). With a contribution of 71.83% to GPS-derived GWS variability and 68.92% to GRACE-derived GWS variability, EOF-1 is a relatively potent factor. For Principal Components PC1 and PC2, the GRACE and GPS PCs have correlation coefficients of 0.75 and 0.84, respectively. Finally, with higher temporal resolution, GPS can perform the same task as GRACE in hydrological applications. In addition, GPS can add important and valuable information to assess regional GWS change.