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

The Cambodia-Mekong River Delta Aquifer, a vital transboundary aquifer in the Lower Mekong Region of Southeast Asia, faces escalating challenges due to excessive groundwater extraction for agriculture and domestic purposes. In response, this study utilizes the Gravity Recovery and Climate Experiment (GRACE) satellite estimates, combined with land-surface-model and remote-sensing datasets to estimate groundwater storage anomalies (GWSA) across the aquifer where traditional monitoring is limited. The study further evaluates the consistency of GRACE-derived data in comparison to both localized in situ measurements and a global-scale hydrological model. Additionally, the spatio-temporal trends in groundwater depletion over a 14-year span (2003–2016) were mapped. The results reveal a good agreement between GRACE-derived GWSA, PC-Raster Global Water Balance (PCR-GLOBWB) model outputs, and observed in situ measurements, thereby underscoring the pivotal role of satellite observations in comprehensively assessing groundwater resources within the aquifer. The findings expose a concerning downward trend, with groundwater storage declining at a rate of ~0.68 cm/year, resulting in a total volume loss of 18.28 km³ over the 14-year span. Notably, the depletion rate is higher in the coastal regions of the Mekong Delta and certain areas within the Tonle Sap Basin. Discrepancies between GRACE and observed GWSA are attributed to multiple factors, including the absence of local signals, intricate hydrogeological dynamics, limitations in specific yield and storage estimations, and the uneven distribution of monitoring wells in the region. This research emphasizes the potential of GRACE estimates to supplement in situ observations on a regional scale, establishing a critical foundation for transboundary groundwater management strategies.