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

Authors: RAHMAN, A.T.M. SAKIUR - University Of California, Riverside; SIMUNEK, JIRI - University Of California, Riverside; Bradford, Scott; AJAMI, HOORI - University Of California, Riverside; Meles, Menberu; CHEN, LIN - Chinese Academy Of Sciences; SZYMKIEWICZ, ADAM - Agricultural University Of Poland; PAWLOWICZ, MATEUSZ - Gdansk University; ACERO TRIANA, JUAN - University Of California, Riverside; CASILLAS-TRASVINA, ALBERTO - University Of California, Davis; BEEGUM, SAHILA - University Of Nebraska

Submitted to: Journal of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/4/2025
Publication Date: 7/11/2025
Citation: Rahman, A., Simunek, J., Bradford, S.A., Ajami, H., Meles, M.B., Chen, L., Szymkiewicz, A., Pawlowicz, M., Acero Triana, J.S., Casillas-Trasvina, A., Beegum, S. 2025. A new externally coupled, physically-based multi-model framework for simulating subsurface and overland flow hydrological processes on hillslopes. Journal of Hydrology. 662(A). Article 133842. https://doi.org/10.1016/j.jhydrol.2025.133842.
DOI: https://doi.org/10.1016/j.jhydrol.2025.133842

Interpretive Summary: Mathematical models are needed to better understand and assess interactions between surface runoff water, the unsaturated zone, and groundwater. However, most previous models have separately focused on these domains due to computational demands. An interface has been develop to exchange information between three state-of-the-art models so that interactions can be quantified. This approach is demonstrated to be computationally efficient and accurate for simulating water flow at the hillslope scale. The developed model is of interest to irrigation districts, flood control agencies, groundwater sustainability agencies, and state and federal agencies that are concerned with management of water resources and groundwater sustainability for irrigated agriculture.

Technical 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 like dynamic time-stepping, sequential coupling, 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 reference models H1D-K2 and H1D-MF5, which only represented two simulation domains. The simulation domains include uniform and heterogeneous hillslopes, as well as a complex hillslope with pumping and stream-aquifer interactions. Simulated hydrographs of outflow rates, soil water contents, groundwater heads, and water balance analyses demonstrated high accuracy and consistency with the reference models. These results confirm the reliable exchange of water fluxes between the model components due to the implementation of the boundary condition switching algorithm based on surface ponding, enabling realistic infiltration simulation, and of the vadose zone pressure head adjustments, ensuring accurate recharge flux estimates. The modular design of the H1D-K2-MF5 model allows flexible configuration between surface water, vadose zone, and groundwater components depending on specific study objectives, enhancing its adaptability for diverse hydrological applications. Further developments will include adding reactive solute transport at hillslopes and extending to large-scale watersheds.