Winter stream temperature in the rain-on-snow zone of the Pacific Northwest: influences of hillslope runoff and transient snow cover

J A Leach,R D Moore

doi:10.5194/hess-18-819-2014

J A Leach, R D Moore

Open Access

https://doi.org/10.5194/hess-18-819-2014

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Abstract

Abstract. 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 in coastal catchments. 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. Two hypotheses were addressed by this study: (1) winter stream temperatures are primarily controlled by advective fluxes associated with runoff processes and (2) stream temperatures should be depressed during rain-on-snow events, compared to rain-on-bare-ground events, due to the cooling effect of rain passing through the snowpack prior to infiltrating the soil or being delivered to the stream as saturation-excess overland flow. A reach-scale energy budget analysis of two winter seasons revealed that the advective energy input associated with hillslope runoff overwhelms vertical energy exchanges (net radiation, sensible and latent heat fluxes, bed heat conduction, and stream friction) and hyporheic energy fluxes during rain and rain-on-snow events. Historical stream temperature data and modelled snowpack dynamics were used to explore the influence of transient snow cover on stream temperature over 13 winters. When snow was not present, daily stream temperature during winter rain events tended to increase with increasing air temperature. However, when snow was present, stream temperature was capped at about 5 °C, regardless of air temperature. The stream energy budget modelling and historical analysis support both of our hypotheses. A key implication is that climatic warming may generate higher winter stream temperatures in the rain-on-snow zone due to both increased rain temperature and reduced cooling effect of snow cover.

Highlights

Stream temperature influences a variety of biological, chemical and physical in-stream processes, such as dissolved oxygen concentrations and aquatic organism survival and growth rates (Wehrly et al, 2003, 2007; Friberg et al, 2013)
Stream temperature is controlled by different energy exchanges, including vertical fluxes at the stream surface, bed heat conduction, advective fluxes, and heat generated by frictional dissipation of potential energy as water flows downslope
This study addressed two hypotheses: (1) winter stream temperatures in headwater catchments of the rain and rainon-snow zones of the Pacific Northwest of North America (PNW) region are primarily controlled by advective fluxes associated with runoff processes during storm events, and (2) stream temperatures should be depressed during rain-on-snow events, compared to rain-onbare-ground events, due to the cooling effect of rain passing through the snowpack prior to infiltrating the soil or being delivered to the stream as saturation-excess overland flow

Summary

Introduction

Stream temperature influences a variety of biological, chemical and physical in-stream processes, such as dissolved oxygen concentrations and aquatic organism survival and growth rates (Wehrly et al, 2003, 2007; Friberg et al, 2013). Changes in land cover and climate can alter the relative importance and magnitudes of these energy exchange processes, and there are concerns that corresponding temperature responses could be deleterious to existing aquatic communities (Brown et al, 2007; Durance and Ormerod, 2007, 2009), for cold- and cool-water species such as salmonids (Battin et al, 2007). Process-based understanding of the energy exchanges controlling stream thermal regimes at a range of spatial and temporal scales is needed to effectively manage and predict stream temperature. D. Moore: Winter stream temperature in the rain-on-snow zone response to future land cover and climatic changes (Webb et al, 2008; Arismendi et al, 2012)

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