Precipitation phase is fundamental to a catchment’s hydrological response to precipitation events in cold regions and is especially variable over time and space in complex topography. Phase is controlled by the microphysics of the falling hydrometeor, but microphysical calculations require detailed atmospheric information that is often unavailable and lacking from hydrological analyses. In hydrology, there have been many methods developed to estimate phase, but most are regionally calibrated and many depend on air temperature (Ta) and use daily time steps. Phase is not only related to Ta, but to other meteorological variables such as humidity. In addition, precipitation events are dynamic, adding uncertainties to the use of daily indices to estimate phase. To better predict precipitation phase with respect to meteorological conditions, the combined mass and energy balance of a falling hydrometeor was calculated and used to develop a model to estimate precipitation phase. Precipitation phase and meteorological data were observed at multiple elevations in a small Canadian Rockies catchment, Marmot Creek Research Basin, at 15-minute intervals over several years to develop and test the model. The mass and energy balance model was compared to other methods over varying time scales, seasons, elevations and topographic exposures. The results indicate that the psychrometric energy balance model performs much better than Ta methods and that this improvement increases as the calculation time interval decreases. The uncertainty that differing phase methods introduce to hydrological process estimation was assessed with the Cold Regions Hydrological Model (CRHM). The rainfall/total precipitation ratio, runoff, discharge and snowpack accumulation were calculated using a single and a double Ta threshold method and the proposed physically based mass and energy balance model. Intercomparison of the hydrological responses of the methods highlighted differences between Ta based and psychrometric approaches. Uncertainty of hydrological processes, as established by simulating a wide range of Ta methods, reached up to 20% for rain ratio, 1.5 mm for mean daily runoff, 0.4 mm for mean daily discharge and 160 mm of peak snow water equivalent. The range of Ta methods showed that snowcover duration, snow free date and peak discharge date could vary by up to 36, 26 and 10 days respectively. The greatest hydrological uncertainty due to precipitation phase methods was found at sub-alpine and sub-arctic headwater basins and the least uncertainty was found at a small prairie basin.

A return-period analysis of annual peak spring break-up and open-water levels for 136 Water Survey of Canada hydrometric stations was used to classify rivers across Canada and to assess the physical controls on peak break-up water-levels. According to the peak water-level river-regime classification and subsequent analysis, 32% of rivers were classified as spring break-up dominated, characterized by low elevations and slopes and large basin sizes while 45% were open-water dominated and associated with alpine environments of high elevations and channel slopes, and smaller basin sizes. The remaining 23% of rivers were classified as a mixed regime. A spatial and temporal analysis (1969-2006) of the river ice break-up season using hydrometric variables of timing and water levels, never before assessed at the northern Canada-wide scale, revealed significant declines in break-up water levels and significant trends towards earlier and prolonged break-up in western and central Canada. The spatial and temporal influence of air temperature on break-up timing was assessed using the spring 0°C isotherm, which revealed a significant positive relationship but no spatial patterns. In the case of major ocean/atmosphere oscillations, significant negative (positive) correlations indicate that break-up occurs earlier (later) during the positive phases of the Pacific North American Pattern (El Niño Southern Oscillation) over most of western Canada. Fewer significant positive correlations show that break-up occurs later during the positive phases of the Arctic Oscillation and North Atlantic Oscillation in eastern Canada. / Graduate