A new externally coupled, physically-based multi-model framework for simulating subsurface and overland flow hydrological processes on hillslopes

Abstract

This study presents an innovative, computationally efficient, and flexible physically-based multi-model framework that couples three well-established state-of-the-art models: HYDRUS-1D (H1D) for vadose zone flow, KINEROS2 (K2) for overland flow, and MODFLOW-2005 (MF5) for groundwater flow. The coupled model, called H1D-K2-MF5, implements advanced techniques, such as dynamic time-stepping, dimensionality reduction, and adaptive pressure head boundary condition switching, to ensure precise simulations of flow processes and interactions among the three modeling domains. The model simulates flow processes at a hillslope scale, including runoff, infiltration, recharge, evapotranspiration, soil moisture redistribution, and groundwater dynamics. Model accuracy and functionality were tested and validated against benchmark simulations and by comparisons with the integrated watershed (IW) model ParFlow. The simulation domains include uniform and heterogeneous hillslopes, as well as a complex hillslope with pumping and stream-aquifer interactions. Simulated time series of outflow rates, soil water contents, groundwater heads, and water balance analyses demonstrated high accuracy and consistency with the IW model. The new model ran 17–20 times faster than ParFlow, demonstrating substantial computational efficiency. Results confirm the reliable exchange of water fluxes between the coupled model components due to the implementation of a boundary condition switching algorithm based on surface ponding, which enables realistic infiltration simulation, and the adjustment of vadose zone pressure heads, ensuring accurate recharge flux estimates. The modular design of the H1D-K2-MF5 model allows for flexible configuration between surface water, vadose zone, and groundwater components depending on specific study objectives, thereby enhancing its adaptability for diverse hydrological applications. Further developments will include the addition of reactive solute transport at hillslopes and extending to large-scale watersheds.