Alper Elçi

Accurate characterization of key hydro-geomechanical properties, specifically hydraulic conductivity 𝐾 and specific storage 𝑆𝑠, is essential for predicting the responses of aquifer systems to natural and anthropogenic stressors and ensuring sustainable groundwater management. However, such characterization remains particularly challenging in ”ungauged” basins, i.e. where piezometric records, pumping test data, and information about production wells are sparse or unavailable. We propose an iterative framework based on coupling a three-dimensional (3D) groundwater flow (GW) model with a 3D geomechanical (GM) model, with a nested inversion scheme. The workflow begins with calibrating the GW model against piezometric records to acquire an initial estimation of heterogeneous 𝐾 distribution given a homogeneous 𝑆𝑠. The resulting hydraulic head changes are then fused with Interferometric Synthetic Aperture Radar (InSAR) measurements to estimate point-wise 𝑆𝑠 values that are subsequently interpolated across the basin scale. This 𝑆𝑠 field is validated using the GM model and fed back into the GW model for a next iteration of 𝐾 calibration. This iterative cycle continues until the relative change in the estimated parameters falls below a predefined threshold. We apply this approach to the Gediz River Basin (GRB) in western Türkiye, a region experiencing severe land subsidence from groundwater over-exploitation for irrigation. Our results reveal significant heterogeneity in aquifer properties, and the inferred 𝐾 and 𝑆𝑠 distributions allow for the reproduction of both hydraulic head evolution and land subsidence patterns, with a consistent modeling set-up.

Land subsidence poses significant problems in regions facing water management challenges, primarily resulting from unsustainable groundwater abstraction that causes significant drops in piezometric levels. Recognizing the urgent need for effective mitigation measures against land subsidence, this study developed a multicriteria decision-making model on the basis of the analytic hierarchy process (AHP) to evaluate ten mitigation alternatives. Expert opinions were gathered through structured surveys in four Mediterranean areas: the coastal aquifer of Comacchio (Italy), the Alto Guadalentín aquifer (Spain), the Sarıgöl-Alaşehir aquifer in the Gediz River Basin (Türkiye), and the Azraq Wetland Reserve (Jordan). Quantitatively, results of the AHP analysis consistently ranked two alternatives (enhancing water use efficiency and strengthening legal control over groundwater abstraction) as the most preferred to mitigate land subsidence, with statistical tests (Friedman test and Dunn’s post-hoc test) confirming the robustness of these results. The innovative contribution of this work lies in its integration of diverse expert perspectives into a comprehensive, transferable decision-making tool that goes beyond previous approaches by combining technical, economic, environmental, and social dimensions. The results provide valuable, targeted guidance for policymakers and planners, offering sustainable strategies that can be adapted to different regional contexts.

This contribution presents the methodology and results of modeling studies conducted as part of an international research collaboration that aims to develop an innovative approach to characterize from the hydrogeological point of view water-stressed Mediterranean basins experiencing land subsidence due to excessive groundwater extraction. By integrating remote sensing-derived land subsidence rates with an iterative implementation of numerical groundwater flow and geomechanical modeling, we developed an approach to improve the estimation of hydrogeological parameters, mainly hydraulic conductivity (K) and specific storage (Ss), and obtained a flow model that can better match historical groundwater level observations. To improve the characterization of K and Ss, hydraulic head measurements from groundwater monitoring wells and displacement observations from Differential Interferometric Synthetic Aperture Radar (DInSAR) datasets were utilized. This was achieved through a novel procedure using a 3-D groundwater flow simulator (MODFLOW) and a 3D geomechanical simulator (GEPS3D) in an iterative coupled approach, with spatial variations of Ss and K described as stationary Gaussian random fields.This approach was demonstrated in the Alaşehir-Sarıgöl alluvial aquifer located in the Gediz River watershed (Turkiye), where groundwater withdrawal for vineyard irrigation and urban water demand have led to significant land subsidence of up to 10 cm/yr. The simulated hydraulic heads from the flow model were input into the geomechanical model, allowing the calculation of the displacement time series for the subsiding areas, which were then compared with the DInSAR data. The updated distribution of the aquifer system compressibility, as obtained by fitting the simulated to the observed subsidence trends, was iteratively used in the groundwater flow simulator to update the hydrogeological parameter values, thereby improving model performance. Calibrated groundwater flow models are expected to more accurately forecast transient groundwater storage depletion/accumulation and are, therefore, more reliable for water management decision-making. In addition, the outcome of the iterative procedure highlighted considerable heterogeneity in the parameter distribution, underscoring the importance of remote sensing-based land subsidence observations for constraining the parameters of groundwater flow models.Acknowledgments: This study was funded by the PRIMA program under grant agreement No. 1924, project RESERVOIR. The PRIMA program is supported by the European Union.

Water scarcity in the Mediterranean region is increasing due to climate change and anthropogenic pressures. to This situation has intensified drought, reduced streamflow, and decreased soil moisture, putting additional pressure on groundwater to mitigate water stress. Despite the critical importance of groundwater use, there is a lack of centralized and detailed groundwater level data in the Mediterranean region, which is essential for sustainable water resources management. This study aims to establish a comprehensive, accurate and up-to-date groundwater level dataset in the Mediterranean region.The dataset was primarily constructed using available nationwide observation wells from Mediterranean countries and was further enriched with additional research project-based observation wells. It includes groundwater level data with more than 15800 observation wells in Portugal, Spain, France, Italy, Greece, Türkiye, and Tunisia. The historical data (1900-2024) frequency varies from daily to weekly, monthly, bimonthly, and biannually, with the earliest records originating from France. The highest proportion of daily and weekly measurements also comes from wells in France, whereas 90% of the observations in Portugal and Spain were recorded at monthly or bimonthly intervals. The collected groundwater dataset will be presented and discussed. Consistent and detailed monitoring and sharing of groundwater level data are essential for sustainable water management, especially in areas known for water scarcity and rapidly changing climatic conditions.Acknowledgment: This work was supported by the OurMED PRIMA Program project funded by the European Union’s Horizon 2020 research and innovation under grant agreement No. 2222.

In hydrogeological science, it is widely acknowledged that the response of an aquifer to groundwater pumping is predominantly influenced by two key parameters characterizing the aquifer system: saturated hydraulic conductivity Ks and the oedometric bulk compressibility cm. The response must be viewed in terms of changes of pore water pressure p and the deformation of the pore volume which traduces in a movement of the land surface. In a confined aquifer system subjected to groundwater pumping, the variation in groundwater pressure is linearly dependent on Ks, while the deformation of the pore volume is linearly dependent on cm. However, cm also impacts p, particularly the speed of pressure variation over time, as aquifer specific storage Ss is also dependent on cm. The dependency p - cm can be considered “weaker” than that p - Ks. The RESERVOIR Project, funded by the EU-PRIMA Programme, aims to characterize aquifer properties, with a focus on Ss, by optimizing the use of the available measurements. Pressure measurements from piezometers provide fundamental information to quantify Ks through the groundwater flow equation. Additionally, displacement measurements of the land surface provided by InSAR can be optimally used in equilibrium equations to constrain cm (and consequently Ss). This objective is achieved through a novel procedure utilizing a 3D groundwater flow simulator (MODFLOW) and a 3D geomechanical simulator (GEPS3D) in an iterative one way coupled approach. Spatial variations of Ss and Ks are mathematically described as stationary Gaussian random fields. The procedure is applied to characterize the properties of the alluvial aquifer system in the eastern portion of the Gediz River basin, Turkey. In this region, groundwater withdrawal for irrigation has led to a general decline in pore water pressure and land subsidence of up to 10 cm/year over the past decade. The convergence of the procedure was achieved after four iterations, highlighting the presence of considerable heterogeneity in the distribution of parameters. This heterogeneity cannot be effectively constrained without the aid of satellite-based earth observation measurements.

Water resources are under immense stress due to the continuous increase of anthropogenic and natural pressures. Therefore, effective water management backed by advanced tools and methods is essential for the sustainability of water resources. One of these tools is groundwater flow modeling, which can be used to estimate changes in groundwater storage. In this study, we propose an approach to improve groundwater flow modeling by supporting model calibration with remote sensing data. The approach is demonstrated on the Alaşehir-Sarıgöl sub-basin in the east of the Gediz River Basin, a water-stressed basin in western Turkiye. A MODFLOW-2005 based flow model is constructed to determine time series hydraulic head changes and aquifer storage. The model simulation period is from 2013 to 2021. The groundwater recharge input of the model is obtained by a remote sensing-supported water balance method (Batkan et al., 2022). Except for precipitation data measured at meteorological stations, other model parameters are remote sensing products. Evapotranspiration is obtained from the MODIS Global Evapotranspiration product (MOD16A2), and soil water content and runoff are obtained from the ERA-5 Land Model reanalysis dataset. Hydraulic parameters such as hydraulic conductivity and storage coefficient are determined as a result of the calibration of the groundwater flow model. Model performance is improved by using terrestrial water storage (TWS) data from NASA's GRACE mission in the calibration of the storage coefficient. TWS represents the total water content above and below ground in the unconfined aquifer, therefore data needs to be adjusted to obtain an estimate of groundwater storage. 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 calibrated model RMSE value is 7.4 m, which was subsequently improved to lower values after the conjunctive use of the GRACE-derived TWS data.Keywords: groundwater flow modeling, model calibration, remote sensing, GRACE, ERA-5Acknowledgment: This study is funded by the PRIMA program supported by the European Union under grant agreement No: 1924, project RESERVOIR (sustainable groundwater RESources managEment by integrating eaRth observation deriVed monitoring and flOw modelIng Results).