SERGHEI-SurEau: Coupling surface–subsurface hydrodynamics with plant hydraulics

Accurately describing forest ecosystem responses to varying climatic conditions—particularly water availability—requires a robust representation of the soil–plant–atmosphere continuum. Recent advances have produced high-fidelity models for both surface–subsurface hydrology and plant hydraulics.  Although these model domains partially overlap in the processes they consider, their implementation is often inadequate compared with their counterparts. To address this limitation, we present a coupled version of two established models: the integrated surface–subsurface flow model SERGHEI-SWE-RE and the plant hydraulics model SurEau-Ecos.SERGHEI-SWE-RE is a performance-portable, high-performance parallel computing model that solves the fully dynamic two-dimensional shallow-water equations for surface flow and the three-dimensional Richards equation for subsurface flow. SurEau-Ecos is a mechanistic, trait-based plant hydraulics model that provides a detailed physiological description of plant water status beyond stomatal closure up to the point of hydraulic failure. The core strengths of both models are coupled through a clean separation at the soil–root interface: SERGHEI-SWE-RE supplies spatially distributed soil water potential fields, while SurEau-Ecos provides the resulting root water uptake. Leveraging the high-performance computing capabilities of SERGHEI-SWE-RE, instances of SurEau-Ecos can be mapped to each node  of the surface mesh and the corresponding vertical soil column. This coupled model—spatially distributed and operating at high temporal resolution—captures hydrodynamic processes in complex geometries, such as lateral subsurface flow, exfiltration, ponding, and deep water reserves, while simultaneously enabling the assessment of forest ecosystem responses, such as drought stress and tree dieback.We present a proof-of-concept version of the coupled model using a transect along an idealised vegetated hillslope, where lateral subsurface fluxes are a key process. The effects of subsurface flow concentration toward the valley on plant-available water are shown and the resulting duration over which trees can sustain drought conditions are analysed.