Natural and anthropogenic drivers of the lost groundwater from the Ganga River basin

Ganga river basin is the most populated and among the worst water-stressed river basins in the world. The basin contributes to 40% of India's gross domestic product. Despite the Ganga basin's cultural, heritage, and economic importance, the interplay among the crucial factors that make the basin one of the global groundwater depletion hotspots is not well understood. Using observations from wells and Gravity Recovery and Climate Experiment satellites and simulations from a hydrological model, here we show that the Ganga river basin has lost 226.57 ± 25.22 km3 groundwater during 2002-2016, which is about 20 times the storage capacity of the largest (Indira Sagar) reservoir in India. A significant (p-value < 0.05) decline (∼11%) in the summer monsoon (June-September) during 1951-2016, severe and frequent droughts (2009, 2014, 2015), and groundwater pumping for irrigation have contributed to groundwater depletion. However, the non-renewable groundwater abstraction is the most significant (relative contribution = 80%) contributor to the groundwater depletion in the basin. Renewable groundwater pumping contributed to only 20% of the total groundwater depleted during the 2002-2016 period. Severe and frequent droughts in the basin pose a double whammy of reducing groundwater recharge and increasing withdrawal. Changes in cropping patterns, groundwater metering, and improved water use efficiency are needed to reduce the non-renewable groundwater abstraction for irrigation, which is crucial for water sustainability in the basin.