Unraveling the Link between Rainfall and Groundwater: A Regional Approach

Groundwater serves as an unsung hero in the worldwide freshwater crisis, supporting agriculture, sustaining communities, and mitigating the effects of climate variability. India leads the world in groundwater consumption. It extracts approximately 250 km³  annually, surpassing the combined withdrawals of China and the United States. Groundwater extraction is expected to escalate in the coming future due to agricultural demands, thereby stressing the already over-exploited groundwater reserves. These findings emphasize the critical need for in-depth research on groundwater systems. The present study focuses on the agro-ecological zones (AEZs) of India, as classified by the National Bureau of Soil Survey and Land Use Planning (NBSS&LUP). AEZs are characterized by unique climatic, soil, and hydrological properties, providing an ideal framework for analyzing groundwater trends at a regional scale. The intricate relationship between rainfall and groundwater levels across different agro-ecological zones was analyzed. The Mann-Whitney U test results reveal significant (p < 0.05) differences in groundwater-levels between normal and dry (deficient rainfall) years in Zones 3, 10, 16, and 19, as well as between normal and wet (excess rainfall) years in Zones 3, 10, 11, 15, 16, and 17, highlighting the pronounced impact of rainfall variability on groundwater availability in these regions. A decline in water table over the two decades (1996-2016) is observed in 57.42% of the total geographical area. Furthermore, regression analysis demonstrated strong correlations (r > 0.7) between annual rainfall and post-monsoon groundwater levels in ten out of the eighteen AEZs considered for the analysis. In addition, Zone 11 ‘Central Highlands’ and Zone 16 ‘Deccan Plateau (Karnataka)’ exhibited stronger correlations at a lag of 1 month, highlighting the delayed response of groundwater to rainfall in these regions. It was also observed that the total area where groundwater extraction during monsoon exceeds recharge, expands from 0.68% in 1996, to 1.21% in 2006, and to 3.89% in 2016. The findings of this study emphasize the need for adaptive, zone-specific strategies to ensure sustainable groundwater management under the changing climate and socio-economic conditions.