OBSERVATIONS AND MODELLING OF HILLSLOPE THROUGHFLOW TEMPERATURE IN THE RAIN-ON-SNOW ZONE OF THE PACIFIC NORTHWEST

Stream temperature is a critical water quality parameter. A growing body of research has highlighted the importance of heat advection associated with surface water and groundwater interactions on stream thermal regimes. Current catchment models that predict stream temperature use a variety of approaches to estimate throughflow temperatures, but none has been evaluated against field measurements of throughflow temperature. We monitored throughflow temperature at fifty locations adjacent to a headwater stream located in the rain-on-snow zone of the Pacific Northwest. Current approaches to estimating throughflow temperature under- or over-predicted throughflow temperatures by up to 8 °C, or were unable to represent the influence of transient snow cover. We attempted to apply the SUTRA groundwater model to simulate hillslope thoughflow temperature, but its utility was limited by difficulties in simulating the development of seepage zones during storms. Therefore, we developed a conceptual-parametric model that simulates hillslope throughflow and stream 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.