UBC Theses and Dissertations
Influence of turbidity and aeration on the albedo of mountain streams McMahon, Alexander D.
Stream surface albedo plays a key role in the energy balance of rivers and streams that are exposed to direct solar radiation. Most physically based models assume that stream albedo lies between 0.03 and 0.10, based primarily on measurements from low-gradient streams with low suspended sediment concentrations. Albedo should depend upon solar elevation angle, suspended sediment, aeration, and fraction of direct vs diffuse radiation. However, there is no model available for predicting the dependence of albedo on these factors. This study quantified the dependence of albedo of mountain streams on the controlling factors in order to improve the representation of albedo in energy balance studies. Proxy measures for albedo using digital camera imagery were also developed and assessed. Stream surface albedo was measured at nine sites with a variety of gradients and suspended sediment characteristics in the southern Coast Mountains of British Columbia, Canada. As expected, albedo of low-gradient, non-whitewater (flatwater) streams increased with solar zenith angle, suspended sediment concentration, and proportion of diff use to direct solar radiation, ranging between 0.025 during cloudy periods in clear water to 0.25 for turbid water at zenith angles of less than 20 degrees. Albedo varied with discharge in steep reaches or at channel steps and cascades where flow was visibly aerated, with a range of 0.09 to 0.33. In clear weather, albedo exhibited notable diurnal variability at flatwater sampling sites. For example, during late summer, surface albedo typically fluctuated between 0.08 and 0.15 on a diurnal basis at a flatwater site on the highly turbid, glacier-fed Lillooet River. Physically based representations of albedo should be incorporated into energy balance models in order to improve predictions of stream temperature, especially for future scenarios.
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