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Bias correction (BC) of General Circulation Models (GCMs) variables is a common practice when it is being used for climate impact assessment studies at regional scales. The present study proposes a bias correction method (LR-Reg) that first adjusts the original GCM precipitation for local lapse rate corrections and later bias corrects the lapse rate-adjusted GCMs precipitation data with linear regression coefficients. We evaluated LR-Reg BC method in comparison to Linear Scaling (LS) and Quantile Mapping (QMap) BC methods, and NASA’s downscaled NEX data for Monsoon Asia region. This study used Coupled Model Intercomparison Project Phase 6 (CMIP6)-based MIROC6 GCM precipitation with historical and projected shared socio-economic pathways (SSP) scenarios (SSP245 and SSP585) datasets. The BC comparison results show that the relative percentage reduction in mean absolute error (MAE) values of LR-Reg over LS-BC was up to 10–30% while this relative reduction in MAE values of LR-Reg was 30–50% over QMap-BC and 75–100% over NASA’s NEX-data. The future projected precipitation over Monsoon Asia during dry season shows more decreased precipitation by up to 100% mostly in the south Asia while during wet season shows more increased precipitation by up to 50% mostly in the northeastern China and in the Himalayan belts with respect to the baseline condition (1970–2005). The results on the average precipitation per 0.25 degree increase in latitude analysis shows that the maximums of average monsoon precipitation during baseline period occur at 0 and 25 degree latitudes while the projected monsoon precipitation during both SSP scenarios occurs at 10 and 20 degree latitudes which clearly shows an inward shift in the latitude axis for the projected precipitation in the Monsoon 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.

Groundwater exploitation for different sectors in Cambodia is expanding. Groundwater levels have already begun to decline in some parts of the country. Monitoring and assessing groundwater storage (GWS) change, aquifer stress and aquifer resilience will support the proper planning and management of the country’s groundwater resources; however, information regarding groundwater in Cambodia is currently scarce. Thus, GWS change in Cambodia over the 15 years from April 2002 to March 2017 was assessed using remote-sensing-based Gravity Recovery and Climate Experiment (GRACE) and Global Land Data Assimilation System (GLDAS) datasets, with a comprehensive validation of the GRACE-derived groundwater storage anomaly (GWSA) with respect to in-situ field-based observations. The current study also investigated the impact of surface water storage (SWS) change in Tonle Sap Lake, South-East Asia’s largest freshwater lake, on deriving the GWS change in Cambodia. The groundwater aquifer stresses (GAS), and aquifer resilience (AR) were also evaluated. The validation results were promising, with the correlation coefficient between satellite-based estimations and ground-based measurements ranging from 0.82 to 0.88 over four subbasins. The overall decreasing rate of GWS was found to be –0.63 mm/month, with two basins having the highest declining rate of more than 1.4 mm/month. Meanwhile, the aquifer experiencing stress during the dry season had a very low ability to quickly recover from these stresses. These findings emphasise that appropriate management is urgently needed to ensure the sustainability of the groundwater resource system in this country.