Coupling WSIMOD and MODFLOW to Enhance the Representation of Groundwater-Surface Water Interactions in Complex Catchments

In catchments characterised by complex hydrological regimes, where streamflow is derived from significant groundwater contributions alongside diverse surface and anthropogenic inputs, accurate process representation is crucial. To address this challenge, this study presents a novel coupling of the Water System Integrated Modelling Framework (WSIMOD), which simulates water quality and quantity across the urban-rural integrated catchment, with the fully distributed groundwater model MODFLOW. The coupled framework is evaluated in the Upper River Lee catchment (UK), specifically focusing on two sub-catchments with distinct hydrological characteristics. To ensure a robust evaluation, the standalone WSIMOD, standalone MODFLOW, and the coupled model were all calibrated automatically using the PEST. While all three model configurations demonstrated satisfactory performance, the coupled model exhibited superior predictive capability, particularly regarding baseflow and groundwater levels. The study highlights the distinct added value of this hybrid approach. Compared to WSIMOD alone, the coupled model captures local spatial variations in groundwater and river dynamics, which is vital for investigating groundwater abstraction impacts. Conversely, relative to standalone MODFLOW, the coupled model provides a more detailed representation of surface water processes, specifically capturing the complex influence of urban infrastructure on the catchment’s hydrology. These results demonstrate that coupling lumped and distributed models is effective for resolving the complex water cycle dynamics of human-impacted catchments.