A New Explicit Solver for <scp>MODFLOW</scp> Enabling Small Time Step Simulations

Surface water (SW) and groundwater (GW) models, such as MODFLOW and HEC-RAS, have been explored to simulate the complexities of SW-GW interactions. However, individual models are not capable of capturing the full complexity of these interactions. To overcome individual models' shortcomings, researchers introduced the model coupling concept. This concept helps compensate for each individual model's shortcomings and incorporates the models' advantages. However, challenges arise from temporal scale disparities between SW and GW models. To tackle the temporal scale issue, this study introduces the novel explicit solver (EXP1) for MODFLOW 2005, enabling GW modeling using small time steps matching SW models (i.e., 15 min) by reducing runtime and computational burden. The EXP1 solver incorporates an integrated stability criterion to ensure the stability of explicit schemes, and it was systematically evaluated against the Preconditioned Conjugate Gradient (PCG) solver across various scenarios, including a 1-dimensional, 2-dimensional, and a vast 3-dimensional model. Results demonstrated the efficiency and accuracy of EXP1 in predicting groundwater heads and water budget, along with considerably reduced runtimes of up to 33% compared with the PCG solver, with less than 0.4% discrepancy in the water budget. These findings underscore the effectiveness of EXP1 in facilitating groundwater small time step simulations and bridging the temporal scale gap between SW and GW models.