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UBC Theses and Dissertations

Hydrometeorology and streamflow response during rain-on-snow events in a coastal mountain region Trubilowicz, Joel William


Rain-on-snow, in which rainfall occurs upon a previously existing snowpack, complicates runoff response to rain events. In some situations the snowpack absorbs rainfall; in others, runoff is enhanced by significant snowmelt. Rain-on-snow has generated major floods around the world, particularly in coastal, mountainous regions such as southwestern British Columbia, where the rugged topography can cause rapid and varying transitions between rainfall and snowfall within the same watershed, and warm, subtropical storms known as atmospheric rivers can deliver large quantities of precipitation. This thesis sought to further the understanding of rain-on-snow at the regional scale, including its role in runoff response to a wide spectrum of rain-on-snow events. Tools were developed and assessed to help achieve this goal and for use by others. First, the hydrologic utility of output from a regional weather reanalysis model was tested. Results showed air temperature and vapour pressure were likely to be useful, whereas other variables were not accurate enough to be of use. Air temperature, in particular, showed potential ability for more accurate specification of temperature gradients for hydrologic forecasting of rain-on-snow runoff. An analysis of rain-on-snow events across five automated snow pillow sites over 10 years illustrated the importance of understanding the amount of rainfall occurring at high elevations during rain-on-snow, and the relatively consistent enhancement of water available for runoff (WAR) by 25-30% due to snowmelt during large rain-on-snow events. For smaller events, a range of antecedent and meteorological factors influenced WAR generation, particularly the antecedent liquid water content of the snowpack. A probabilistic method for infilling cloud obscured pixels in optical remotely sensed snow cover imagery showed strong performance compared to more standard methods, and illustrated spatial changes in snow cover during the largest flood event within the analysis period. This method was developed to maximize information gain from satellite snowcover imagery while minimizing the transfer of disinformation. Finally, high elevation rainfall during atmospheric river events was found to be the dominant predictor of runoff response in six study catchments in the region. Antecedent snowcover provided only minimal increases in the ability to predict runoff during these events compared to rainfall alone.

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