Enhancing groundwater representation in SWAT+gwflow with an open-source zonal parameterization and calibration workflow in R

Modeling tools are essential to simulate groundwater-dominated catchments, yet hydrological models such as SWAT + often oversimplify groundwater processes, limiting their performance in hydrogeologically complex systems. This study presents a reproducible R-based workflow for the distributed parametrization and zonal calibration of the gwflow module in SWAT+, improving the representation of surface-groundwater interactions. Two alternative hydrogeological zoning approaches, based on lithology and permeability contrasts, were evaluated in the Tagus River headwaters. Calibration used daily performance metrics, groundwater contribution indices, and monthly reservoir inflows. Both approaches produced robust simulations, but the more detailed zoning yielded improved statistical performance and a slightly better representation of groundwater contribution to streamflow. The results demonstrate the added value of applying zonal calibration of the SWAT + model in conjunction with the gwflow module. The proposed methodology is freely available and transferable, enhancing the applicability of SWAT+ in hydrogeologically heterogeneous basins.