Combination of regional modelling and in-situ data analysis for groundwater assessment in the Mediterranean region Nahed Ben-Salem1, Robert Reinecke2, Alexander Wachholz1, Michael Rode1, Dietrich Borchardt1 and Seifeddine Jomaa1 1. INTRODUCTION AND MOTIVATION 4. METHODOLOGY Models’ outputs will be compared against observations at countries and case studies scales (e.g., Spain, Greece, Crete Island, La Mancha aquifer). Set-up of G3M (v1.0) in the MED (at steady and transient condition). REFERENCE 1 Reinecke, R. et al. 2019. Challenges in developing a global gradient-based groundwater model (G3M v1.0) for the integration into a global hydrological model. Geosci. Model Dev. 12, 2401-2418. 2 de Graaf, I. et al. 2017. A global-scale two-layer transient groundwater model: Development and application to groundwater depletion. Adv. Water. Resour. 102, 53-67. 3 Fan, Y. et al. 2013. Global patterns of groundwater table depth. Science 339, 940-943. 8. FUTURE WORK 7. RESEARCH SIGNIFICANCE and SCIENTIFIC MERIT We argue that large-scale groundwater modelling is crucial tool to confront the challenges of climate change impacts on groundwater availability and to implement science-based adaptation and mitigation measures in the Mediterranean. Description of the selected models Time series from all Spain, Portugal and France as well as some wells in Greece and Tunisia were collected. 2. RESEARCH OBJECTIVE 5. EXPLANATORY VARIABLES 6. PRELIMINARY RESULTS ACKNOWLEDGEMENT: This study is supported by the Sustain-COAST R&I project co-funded under the PRIMA 2018 programme section II. For more information, please visit: https://www.sustain-coast.tuc.gr/en/home ABSTRACT #22  To advance our understanding of large-scale groundwater modeling, which in turn can be an important tool for an improved groundwater resources management by using modelling as means for predicting the effect of climate change and anthropogenic pressures on groundwater levels in data-scarce regions. (EOMAP)  This study aims to evaluate the performance of three global gradient-based groundwater models to represent the steady-state of groundwater levels in the Mediterranean region using ensemble-models (de Graaf et al., 2015, Reinecke et al., 2019 , Fan et al., 2013), Figure 4: Histograms of the three compared models. Figure 2: The MED grid with4 4..4444km km Figure 6: WTD (m b.g.l) Fan et al., 2013.  Three global gradient-based groundwater models are currently available and suitable for regional scale application (de Graaf et al., 2015; Fan et al., 2013; Reinecke et al., 2019). These models are introduced briefly below and Table 1, is presenting the main characteristics of each model (modified from Reinecke et al., 2020). Karstic map of the MED (Derived from the World Karst Aquifers Mapping project WHYMAP (WOKAM, Chen et al., 2017) shows that, 41% of the Mediterranean contains karstifiable rocks and Continuous Carbonate Rocks. Figure 3: Karstic map of the MED (Derived from the World Karst Aquifers Mapping project WHYMAP (WOKAM) (Chen et al., 2017)) The comparison between the three models has been done by aggregation to a rectangular grid that covers the Mediterranean region; with spatial resolution of 4.44 km. 1Helmholtz Centre for Environmental Research‐UFZ, Magdeburg, Germany (nahed.ben-salem@ufz.de, Alexander.wachholz@ufz.de, michael.rode@ufz.de, dietrich.borchardt@ufz.de, seifeddine.jomaa@ufz.de), International Center for Water Resources and Global Change (UNESCO), Koblenz, Germany (Reinecke@bafg.de) 3. MODELS’ DESCRIPTION Figure 1: GW wells distribution in Portugal, Spain, France and Italy. Portugal - 982 wells Spain - 3024 wells France - 4443 wells Italy - 808 wells WTD_Fan et al., 2013 WTD_G3M, 2019 Preliminary results showed that there is a big discrepancy between the three compared model outputs. More specifically, the de Graaf et al. (2017) model presents a deeper water table than Reinecke et al. (2019) and Fan et al. (2013). de Graaf et al. (2017) generally shows