UBC Theses and Dissertations
Winter stream temperature in the rain-on-snow zone of the Pacific Northwest Leach, Jason Andrew
Stream temperature dynamics during winter are less well studied than summer thermal regimes, but the winter season thermal regime can be critical for fish growth and development. The winter thermal regimes of Pacific Northwest headwater streams, which provide vital winter habitat for salmonids and their food sources, may be particularly sensitive to changes in climate because they can remain ice-free throughout the year and are often located in rain-on-snow zones. This study examined winter stream temperature patterns and controls in small headwater catchments within the rain-on-snow zone at the Malcolm Knapp Research Forest, near Vancouver, British Columbia, Canada. A diagnostic energy budget analysis highlighted that advective fluxes associated with hillslope throughflow inputs were a dominant control on the winter stream thermal regime. In addition, stream temperatures during rain-on-snow events were generally lower than during rain-on-ground events after controlling for air temperature. Methods for estimating throughflow temperatures embedded in stream temperature models were evaluated against field observations, and were found either not to account for the role of snow or to under- or over-predict throughflow temperatures by up to 5 °C. Therefore, a conceptual-parametric hillslope throughflow temperature model that coupled hydrologic and thermal processes, and accounted for the role of snow was developed and evaluated against field observations of throughflow temperatures. The hillslope throughflow temperature model was linked to stream energy exchange processes in order to predict stream temperature. The stream temperature model accurately predicted streamflow and winter stream temperature at three study catchments. The model also simulated lower throughflow temperatures during rain-on-snow versus rain-on-ground events, although the magnitude of cooling was less than suggested by empirical results. A key implication of this research is that climatic warming may generate higher winter stream temperatures in the rain-on-snow zone due to both increased temperature of throughflow inputs and reduced cooling effect of snow cover.
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Attribution-NonCommercial-NoDerivs 2.5 Canada