Regional evaluation of groundwater-surface water interactions using a coupled geohydrological model (SWAT+gwflow)

Study region: The research is conducted for the Scheldt river basin, where seven major watersheds located in Belgium and partly in France are included in the analysis.Study focus: A proper representation of groundwater-surface water interactions with (geo)hydrological models is possible via a coupled model. However, such models have disadvantages, such as complex code modifications and new tunings, and are computationally expensive. Therefore, their application on large spatial scales is limited. A newly developed model, SWAT+gwflow integrates the Soil Water Assessment Tool (SWAT+) with the groundwater module gwflow and has the potential to overcome these limitations. However, this coupled model has not yet been evaluated at a regional scale; hence, we present the evaluation of this model for regional studies using a global aquifer data over the seven watersheds in the Scheldt basin. Furthermore, we have investigated and quantified water balance components within the basin, with a focus on groundwater-surface water exchange. New Hydrological Insights for theRegion: From the results (Nash-Sutcliffe efficiency (NSE) of 0.8-0.9 for all catchments based on monthly average streamflow during calibration and validation periods), we consider the model to be a good simulator of hydrology in the basin. In addition, the simulated groundwater head shows good agreement with observed well data (with a mean absolute error of less than 0.42 m). Also, the rivers in five of the seven watersheds are found to be strongly dependent on groundwater discharge to the streams. We conclude that (1) the SWAT+gwflow model is capable of accurately modeling hydrological processes and state variables in the seven watersheds using global aquifer data and limited computational time, (2) the climategridded dataset can successfully be used for (geo)hydrological studies, and (3) the groundwater surface water interaction increases over the years (from 1975 to 2021) with a strong increment found in the Grote Nete (3.7 fold) and upper Scheldt (2.3 fold) watersheds. These results are, moreover, promising for data-scarce regions where geohydrological modeling relies on the use of global datasets, but the mere success of this modeling application does not guarantee the accuracy of the dataset for other locations, hence, further verification is required. Furthermore, although in this study, the gwflow module is integrated into the SWAT+ model, it could also be integrated into other surface water models for other studies.