Analysis of terrain deformation induced by groundwater overexploitation using InSAR, wavelet analysis and hydrogeological data

Abstract

Residents of Kabul, Afghanistan have historically relied on groundwater delivered from unconfined aquifers. However, recent drought events and population growth have resulted in withdrawing groundwater beyond sustainable rates and water level decrease. Groundwater overexploitation might have induced various magnitudes of ground subsidence, however, the issue had been overlooked.  In this study, we assessed the spatial and temporal evolution of ground deformation in Kabul and analysed the main governing processes.Deformation rates were extracted using InSAR SBAS time-series analysis of C-Band Sentinel-1 products from both ascending and descending orbits acquired from 2014 to 2019.  The analysis revealed Line-of-Sight (LOS) mean velocity accounting for -4.08 and -4.3 cm/year for ascending and descending tracks, respectively. High correlation coefficient of 0.75 and the low RMSE of 0.60 cm/year suggest that the two measurements are consistent. To overcome the inherent limitation of InSAR- measurements being made in LOS direction only, multi-geometry data fusion was employed by combining two independent LOS measurements to estimate vertical and horizontal components of displacement.  We delineated four subsidence bowls with highly variable spatial extent and deformation magnitudes in four aquifers present within the city boundaries, the largest of which is detected in Upper Kabul basin area with the vertical component reaching -5.3 cm/year and horizontal motion accounting for 1 cm/year.  The InSAR-derived subsidence time-series results were interpreted in relation to the spatio-temporal variations of hydrological and geotechnical conditions of the study area to identify the triggering factors of subsidence. The trend observed in the deformation time-series and groundwater level change shows a good agreement, suggesting that groundwater drawdown is largely a function of groundwater level change.  Wavelet analysis analysis allowed to distinguish long-term subsidence from recurring clay shrink-swell hazard in the region.  Furthermore, the susceptibility of the aquifers to consolidation was found to be largely caused by the prevalence of fine-grained sediments in the subsurface, such as clay and silt, thus differential consolidation in the area is a function of variation in thickness and types of the lithology of the base sediment.  In the context of climate change and increasing urban sprawl in Kabul, monitoring ground deformation using the satellite InSAR is an invaluable tool that provides new opportunities to inform not only subsidence hazard mitigation plans, but also approaches for sustainable management of groundwater resources and for improved planning of operations such as Managed Artificial Recharge (MAR) of aquifers.