Space-time evolution of land subsidence in the National Capital Region of India using ALOS-1 and Sentinel-1 SAR data: Evidence for groundwater overexploitation

Land subsidence due to groundwater extraction is increasingly becoming a major scientific and societal issue. In this study, ALOS-1 (2007-10) and Sentinel-1 (Nov.2014-19) datasets are utilized to compute vertical land subsidence in the National Capital Region (NCR) of India. Two major subsidence zones located in Dwarka (12 km2) and Gurgaon (1 km2) area, showing subsidence rate up to 6.0 cm/year, are observed during 2007-10. Between Nov.2014-19, while a substantial reduction in the size (by - 6 km2) and subsidence rate (by - 3 cm/ year) is observed for the zone in Dwarka, significant enlargement (by - 11 km2) and enhanced subsidence rate (by 5 cm/year) are observed for the zone in Gurgaon. An emerging subsidence zone (4. 5 km2) subsiding at 3.6 cm/year is also detected in the Faridabad area. The deceleration of subsidence in the Dwarka area is due to stabilization of groundwater level post-2014. Residual compaction of the aquifer system in the Dwarka area halted after Sept. 2016, and at present, it shows elastic deformation. In Gurgaon and Faridabad, dominantly negative linear trends in vertical displacement, lack of seasonal signals, and ongoing groundwater depletion indicate continuous inelastic deformation of the aquifer system. The preferential disposition of subsidence zones on the western part of Delhi ridge indicates a dominantly confined aquifer system and aquifer compartmentalization by the Delhi ridge. Results presented here demonstrate that precise subsidence monitoring is vital to identify hotspots of groundwater exploitation, analyze aquifer characteristics at higher resolution and prepare precautionary and mitigatory measures for impending groundwater-related hazards.