HiPIMS-GWF: A GPU-Accelerated 3D Variably-Saturated Subsurface Solver for Integrated Flood Modeling with Groundwater Components

Groundwater influences flood dynamics by modulating subsurface saturation states and engaging in complex interactions with surface water in multiple pathways. Accurately representing these processes is essential for physically consistent flood prediction, risk assessment, and mitigation strategies. However, groundwater-related processes remain poorly resolved in most existing flood modeling frameworks, which typically employ oversimplified representations of subsurface flow. In this study, we present HiPIMS-GWF, a three-dimensional, variably saturated groundwater flow module integrated into the High-Performance Integrated Hydrodynamic Modelling System (HiPIMS). HiPIMS is a GPU-accelerated, high-performance flood model capable of simulating catchment-scale fluvial flooding driven by extreme rainfall events. The HiPIMS-GWF module provides functionality to solve the three-dimensional Richards equation using both iterative and non-iterative numerical schemes, enabling explicit representation of surface-subsurface water exchanges within a unified modeling framework. Model accuracy is evaluated against a suite of standard numerical benchmark problems, and computational scalability and efficiency are assessed on a multi-GPU computing platform. 

Beyond the acute phase of flooding, we are also interested in investigating the long-term impacts of flood events on groundwater and surface water systems after its recession. Because catchment-scale groundwater dynamics evolve over temporal scales that can be orders of magnitude longer than those of surface flooding, capturing the full hydrological response necessitates extended simulation capabilities beyond the time horizon of flood events. To this end, HiPIMS-GWF introduces a novel modeling flexibility: once floodwater recedes, the high-resolution, physics-based surface hydrodynamic component can be swtiched to a computationally efficient, hydrologic model tailored for long-term watershed simulation. Critically, the spatially distributed fields of water saturation and surface water depth generated by the fully physics-based simulation serve as initial conditions for the long-term mode, ensuring continuity in the representation of system states across timescales. The overall accuracy and robustness of this integrated modeling framework are validated against a real-world flood event.