Coupling terrestrial and aquatic thermal processes for improving stream temperature modeling at the watershed scale

Water temperature is an important indicator of the health of terrestrial and aquatic ecosystems and influences numerous biological and chemical processes that affect water quality, the health and productivity of biota, and other ecosystem services. In this study, we improved the Soil and Water Assessment Tool (SWAT) model to explicitly consider the thermal processes influencing the temperature of various terrestrial water components (i.e., surface runoff, lateral flow, and base flow) and the heat balance of water in streams. The improved SWAT model with an explicit heat balance description on both terrestrial and aquatic thermal processes (SWAT-HB) was compared with previous versions of the SWAT model that employ only empirical relationships between air temperature and stream temperature (SWAT-ER) or combine empirical terrestrial water temperature estimates with the heat balance of stream water (SWAT-EH) for the Upper Mississippi River Basin (UMRB) and Greensboro Watershed (GW) in the United States. The results show that, even though the SWAT-HB better simulates terrestrial thermal processes, all three models produce comparable simulations of long-term stream temperatures, indicating that stream temperature is mainly influenced by air temperature as water travels along large rivers. Nevertheless, the SWAT-HB and SWAT-EH models achieved better performance than the SWAT-ER model in the winter and spring but exhibited mixed performance in the summer and fall. The better model performance of the SWAT-HB model than the SWAT-EH model can be attributed to the improved estimation of terrestrial water temperature components. Although, empirical methods (e.g., SWAT-ER) are suitable for providing estimation of stream temperature, the coupled terrestrial and aquatic heat balance-based methods (e.g., SWAT-HB) allow for the consideration of land and water management practices (e.g. crop irrigation, land use change, and water use for thermoelectric cooling) effects on stream temperature.