Combining an individual-based dynamic vegetation model with a distributive hydrologic model to improve coupled water-carbon modelling

Abstract. Vegetation and hydrology are tightly coupled: forest structure regulates evapotranspiration and soil moisture, while water availability governs tree growth, mortality, and succession. Yet hydrological and dynamic vegetation models typically maintain a domain-specific focus, limiting their ability to represent the feedbacks between forest dynamics and catchment-scale water dynamics. We address this gap by coupling the distributed hydrological model mHM with the individual-based forest model FORMIND, and apply the coupled framework (FORMIND-mHM) to the Selke river catchment in Central Germany, a mixed deciduous catchment with approximately 40 % forest cover. We examine how explicit vegetation–water interactions affect hydrological flux partitioning, and whether catchment-scale discharge carries exploitable information about stand-scale gross primary production (GPP). The coupled framework achieved discharge performance comparable to stand-alone mHM while substantially altering ET partitioning, shifting the transpiration fraction (E_t/ET) from 0.50 in stand-alone mHM to 0.73 in the coupled framework, closer to observed ranges for temperate forests. Furthermore, a seasonal GPP–discharge relationship emerged consistently across all gauges, demonstrating that river discharge carries information about forest productivity, an analytical pathway accessible only through coupled modelling. FORMIND-mHM is therefore particularly suited to applications where vegetation–water feedbacks are central, including long-term projections of forest dynamics under changing climatic conditions.