Analysis of land subsidence caused by groundwater overexploitation in the Gediz River Basin based on Sentinel-1 observations

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Abstract
Land subsidence induced by groundwater withdrawal affects many regions around the world and is considered one of the most extensive phenomena caused by human activity nowadays. The Gediz River Basin is located in the western part of Türkiye, and develops along a regional extension of a horst-graben system. This basin has an agricultural and industrial importance in the region, increasing the water demand and positioning the Gediz Basin as one of the most stressed basins in the country. The main aims of this study are to evaluate the role of tectonics and groundwater withdrawal on land subsidence and on the evolution of faults in the Gediz River Basin. Additionally, other conditioning factors such as soft soil thickness layers are also studied. For this purpose, we processed 123 SAR images in descending orbit and 98 in ascending orbit acquired from Sentinel-1 between 2016 and 2021 by the parallel solution of the Small BAseline Subset (SBAS) algorithm (P-SBAS), allocated in the Geohazard Exploitation Platform (GEP). Secondly, we applied an Independent Component Analysis (ICA) to the InSAR time series in order to separate spatiotemporal patterns of long-term deformation and seasonal variations. P-SBAS results reveal that the maximum subsidence rates measured along the line of sight (-6.40 cm/year) are mainly concentrated in agricultural and urban areas. The results also suggest that there is a direct relationship between InSAR deformation and soft soil thickness, indicating that land subsidence is induced by the compaction of aquitard layers due to the groundwater withdrawal and piezometric head depletion. The analysis of the time series through the ICA shows two types of spatiotemporal deformation trends, one of them corresponds to long term and quasi-linear deformation due to the compaction of the aquitard, and the other represents the long-term deformations with seasonal rebounds produced by the seasonal loading and unloading cycles due to water level fluctuations.   Acknowledgements This research was funded by the PRIMA programme supported by the European Union under grant agreement No 1924, project RESERVOIR and by ESA-MOST China DRAGON-5 project (ref. 59339)