Unravelling the dynamics and causes of land subsidence in the National Capital Territory of Delhi, India, by spaceborne and in situ observations

This paper identified severe groundwater depletion and land subsidence in parts of the National Capital Territory of Delhi (NCT Delhi), India, by long-term groundwater level (GWL) data analysis and spaceborne geodetic observations. For long-term geodetic observation, multi-frequency conventional differential interferometric SAR (DInSAR) and persistent scatterer-based advanced DInSAR supplemented with collateral high-precision GPS and terrestrial levelling were adopted. In DInSAR analyses since 2005, two prominent subsiding areas were identified in NCT Delhi: (1) Dwarka-Palam-Raj Nagar (DPR) and (2) Kapashera-Gurugram (KG) areas. In NCT Delhi, land subsidence was first detected in DPR and KG areas in 2005–2006 and 2008, respectively. Initially, land subsidence increased consistently in both places with an average subsidence rate of 3 cm/year (2005–2006) to 9 cm/year (2010–2011) in the DPR area and 5 cm/year (2008–2009) to 8 cm/year (2010–2011) in the KG area. Later, in the DPR area, land subsidence has sharply reduced in spatial coverage and rate since 2014. In contrast, in the KG area, land subsidence has consistently increased spatially with accelerated subsidence rates of 10 to 13 cm/year. Besides, several new subsiding areas were identified in the advanced DInSAR of the 2014–2019 period. During the overlapping period (2017–2018) of conventional and advanced DInSAR, high-precision GPS, and terrestrial levelling, the subsidence rates were compared; those were found to be closely agreeable. Finally, the nature and cause of subsidence were characterised by analysing GWL decline vs urban built-up growth vs subsidence rates, spatiotemporal patterns of micro-seismicity, spatial locations of active faults and aquifer-system compaction in two test sites.