Integrating Meteorological and GRACE-Based Indices to Assess Groundwater Drought Under Climate Change in Data-Scarce Mediterranean Basins

Groundwater is a vital resource for socio-economic sustainability ye is increasingly threatened by climate change and rising anthropogenic pressures. Lebanon, and particularly the Al Assi River Basin (ARB), experiences recurrent droughts, but limited meteorological and groundwater observations hinder effective water resources monitoring and management. This study employs satellite-derived datasets to investigate the relationship between hydrological and hydrogeological drought indicators across Lebanon, with emphasis on the ARB. The Standardized Precipitation Index (SPI) and the Standardized Precipitation and Evapotranspiration Index (SPEI) were computed from CHIRPS precipitation and CHIRTS-ERA5 temperature data at multiple accumulation periods. Groundwater drought was quantified using the Groundwater Drought Index (GGDI), derived from GRACE terrestrial water storage anomalies. SPI, SPEI, and GGDI were harmonized at a 10 km resolution, and correlation analyses, including lagged correlations, were conducted to examine their temporal relationship. Results show that SPEI at 18-month accumulation exhibits the highest correlation with GGDI (R ≈ 0.70) at zero lag, reflecting the response of the multi-layered ARB aquifer system. Future SPEI projections were derived using bias-corrected outputs from the CNRM-ALADIN53 regional climate model under RCP 4.5 and RCP 8.5 scenarios. Using the regression model resulting from the strongest correlation, future GGDI projections were generated, highlighting a progressive increase in drought-affected areas, with the most severe groundwater deficits expected under RCP 8.5 by the late 21st century. These findings underscore the need for integrated water-resource management that combines climate-informed planning, satellite-based monitoring, and strengthened in-situ hydrogeological networks to improve resilience to future climatic extremes in data-scarce regions.