Observations and Modelling of Hillslope Throughflow Temperature and Its Influence on Headwater Stream Thermal Regimes in the Rain-on-Snow Zone

Stream temperature controls a variety of biological, chemical and physical in-stream processes. A growing body of research has highlighted the importance of advection associated with surface water and groundwater interactions on stream thermal regimes, and considerable effort has focused on the thermal role of hyporheic exchange and groundwater discharge. However, few studies have focused on advection associated with hillslope throughflow inputs. Current catchment-scale coupled hydrology and stream temperature models (DHSVM, SWAT, HSPF, CEQUEAU, MIKE SHE) use a variety of approaches to estimate throughflow temperatures, but none of these approaches has been evaluated against field measurements of throughflow temperature. We monitored throughflow temperature at fifty locations along a headwater stream located in the rain-on-snow zone of the Pacific Northwest. Current approaches to estimate throughflow temperature were evaluated against field observations and were found to under- or over-predict throughflow temperatures by up to 8 °C, or not be able to represent the influence of transient snow cover. Therefore, we developed a conceptual-parametric hydrology and stream temperature model that simulates hillslope throughflow temperatures. The model successfully predicts throughflow temperatures and highlights the dominant role of throughflow advection and the influence of snow cover on stream thermal regimes during high flow periods and rain-on-snow events.