5. Hidroloji için Uzay Çalıştayı (Hydrospace 2023), Lisbon, Portugal, 27 November - 01 December 2023, pp.1
In many parts of the world, information about the availability of groundwater and the change in storage is limited mainly due to the lack of periodical quantitative monitoring and the reluctance to data sharing. Therefore, the estimation of groundwater storage is a difficult challenge. The objective of this study is to present a comparative analysis of two approaches that can be used in groundwater storage estimation. More specifically, we question if groundwater storage change derived from the Gravity Recovery and Climate Experiment (GRACE) data can be a substitute for storage change estimates obtained from a calibrated numerical flow model.
A MODFLOW-based flow model is constructed for the Alasehir-Sarigol alluvial aquifer that is located in the east of the Gediz river basin, western Turkiye. Due to the over-exploitation of groundwater primarily used for irrigation of vineyards, the area suffers from significant land subsidence. The model is calibrated using 395 hydraulic head observations from twenty-two wells. The calibrated model RMSE value is 5.6 m. The groundwater recharge input of the model is obtained by a remote sensing-supported water balance method. Hydraulic parameters such as hydraulic conductivity and storage coefficient are determined as a result of the calibration of the groundwater flow model. Modeling results for the 2013- 2021 simulation period show that groundwater storage in the aquifer decreases at a rate of 83.9 Mm3/yr. The change in groundwater storage is alternatively determined for the same period from the terrestrial water storage (TWS) mass concentration (mascon) solution of the GRACE remote sensing product. TWS represents the total water content above ground and in the subsurface, therefore data normally needs to be adjusted to obtain an estimate of groundwater storage. However, streams in the region can be ignored as a contributor to the TWS as they are intermittent and have typically low discharges. The soil water content in the unconfined aquifer is determined using ERA-5 data. The discrepancy between both approaches of groundwater storage changes is analyzed in terms of model accuracy and resolution/scale issues of the GRACE data. Furthermore, the possibility of supporting the calibration process of the groundwater flow model through the conjunctive use of the GRACE-derived storage change data is discussed.
Keywords: groundwater, remote sensing, MODFLOW, GRACE, ERA-5
Acknowledgment: This study is funded by the PRIMA program under grant agreement No: 1924, project RESERVOIR (sustainable groundwater RESources managEment by integrating eaRth observation deriVed monitoring and flOw modelIng Results). The PRIMA program is supported by the European Union.