Land subsidence and groundwater storage change assessment using InSAR and GRACE in the arid environment of Saudi Arabia

Najran region in Saudi Arabia has been affected by groundwater storage decline and consequent land subsidence for a long period of time. In this study, we utilized Sentinel-1 data for ground surface mapping, to generate interferograms and coherence maps using the Looking into Continents from Space with Synthetic Aperture Radar (LiCSAR) processor to estimate land subsidence rates from 2016 to 2020. Therefore, LiCSBAS-InSAR technique allowed for the identification and study of the changes in surface deformation over a selected period. We validated the observed displacement field by processing the Global Navigation Satellite System (GNSS) station's time series, which spanned from 2017 to 2019. In addition, we also analyzed Gravity Recovery and Climate Experiment (GRACE)-based Terrestrial Water Storage Anomalies observations to estimate groundwater depletion trends in the region. The result of the study indicates the occurrence of subsidence along the valley, as observed using the Small Baseline Subset InSAR technique. The average annual subsidence rate was observed ranging from 1.1 to 5.1 mm per year between 2016 and 2020. Correspondingly, the GRACE analysis showed that groundwater storage depletes at similar to 11.2 mm per year. Moreover, this finding provides valuable information about the excessive use of groundwater for growing vegetation. NDVI analysis has shown a considerable increasing vegetation area change of approximately 34.5 km(2) between 2013 and 2020 with a 0.2 threshold. Overall, the analysis reveals that the time series obtained from InSAR, GRACE, GNSS, and well data exhibits a consistent downward trend over time.