SCALING UP A HILLSLOPE SCALE COUPLED SURFACE-SUBSURFACE HYDROLOGICAL MODEL TO WATERSHED SCALE: A COMPUTATIONALLY EFFICIENT MODELING FRAMEWORK

Numerical modeling of watershed functions comprises computation of flow processes, partitioning, movement, storage, and redistribution of water fluxes in space and time. However, the integrated modeling of these processes is challenging due to computational burden, extensive data requirements, and/or reliance on simplifying assumptions. This study introduces a novel and computationally efficient modeling framework by leveraging two state-of-the-art process-based models: HYDRUS-1D (H1D) for unsaturated flow and KINEROS2 (K2) for overland flow. The framework extends a hillslope-scale coupled H1D-K2 model to simulate watershed-scale processes, where H1D replaces the three-parameter Parlange's infiltration equation in the event-based K2 model and a boundary condition switching accounts for surface ponding and water exchange between the two model domains. The structure of the coupled watershed-scale H1D-K2 model comprises cascades of connected rectangular planes, channel elements, and 1D soil profiles to simulate 1D overland flow, infiltration, unsaturated zone flow, and recharge. The computational efficiency is further enhanced by implementing the dynamic time-stepping approach and dimensionality reduction. Here, we present the performance of the watershed model using benchmark simulations and comparison with observations, demonstrating its efficacy in efficiently representing watershed-scale hydrological processes, including distributed recharge estimation for accurate groundwater modeling.