Constraints on the hydrogeological properties and land subsidence through GNSS and InSAR measurements and well data in Salmas plain, northwest of Urmia Lake, Iran

Global navigation satellite system (GNSS) and interferometric synthetic aperture radar (InSAR) data, along with exploration well data, are used to investigate elastic and inelastic land displacements across the Salmas plain, northwest Iran. GNSS measurements (2006–2012) show that the long-term and seasonal variability of the hydraulic head in the Salmas confined aquifer system (SCAS) control the vertical land deformation. A combination of drought and overextraction from SCAS led to significant changes in the head and land deformation after the 2006–2008 period. It was observed that the amplitude of the seasonal component of the hydraulic head and land-deformation time series increased about 2.5 times after this period. InSAR analysis over period 2014–2018 also revealed that the SCAS is prone to both long-term subsidence (average ~60 ± 3 mm/year in the line-of-sight direction of the satellite) and seasonal elastic vertical deformation (~60 mm). The seasonal elastic components of the vertical land deformations are used to precisely map the boundary of the SCAS. The average skeletal storativity of the SCAS is estimated to be ~0.028 and the time delay between the change in seasonal hydraulic head and its corresponding elastic deformation in the aquifer system is 0–150 days. The total groundwater storage variation of the aquifer system is computed to be ~18 million m³ from 2015 to 2018. Furthermore, the irreversible component of groundwater storage variations associated with the dewatering of compacted aquitards is estimated to be ~2.33 times greater than its recoverable component from aquifer units.