The Integration of Hydrological and Heat Exchange Processes Improves Stream Temperature Simulations in an Ecohydrological Model

ABSTRACT Stream temperature is among the main drivers affecting water quality by influencing chemical reaction rates and biological activity. Due to globally rising air temperatures, increased stream temperatures are equally rising and becoming more relevant for ecosystem health. Stream temperature is influenced by a complex interplay of climate, hydrological processes and catchment characteristics. However, these process interactions are often overlooked when being integrated with hydrological models. This study addresses the limitations of a simplified stream temperature model by using the ecohydrological model Soil and Water Assessment Tool (SWAT+). Our goal was to enhance the process representation in the current stream temperature model by refining the mass transfer processes. We included heat transfer processes improved by a channel shape parameter, and the influence of riparian shading with an improved equation to simplify the modified dew point temperature calculation. The enhanced SWAT+ model was tested at 23 stations in a medium‐sized mountainous catchment with high‐resolution observed stream temperature data. Our results show that the enhanced model significantly improved performance, achieving a mean Kling–Gupta Efficiency (KGE) of 0.8 across all calibration sites. We improved previous advances in stream temperature modelling within this work by focussing on the importance of accurate process representation. A key finding was the impact of runoff component contributions on the stream temperature model performance. The model performed particularly well during spring, autumn and very low to moderate flows. The improved stream temperature representation therefore not only serves as a valuable tool for management decisions and ecological applications but also benefits modelling other water quality variables.