Shift from Snowfall to Rainfall in the Canadian Rockies: Consequences for Snowpacks, Glacier Mass Balance and Streamflow in an Emerging Drought

Abstract:

The winter and spring of 2014-15 brought abnormal warmth to much of Western Canada and a transition from snowfall to rainfall for many winter and spring precipitation events in the Canadian Rocky Mountains where snowfall normally dominates precipitation volumes in these seasons. Spring and summer remained abnormally warm and exceptionally dry. The impact of a warm winter and dry spring and summer resulted in substantial reduction in snowfall and a shift to earlier rainfall in the Canadian Rockies. As a result peak snow accumulation was from 1/3 to 1/2 of long term averages at upper middle elevations and low elevation valley snowpacks ablated shortly after forming in early December. Snowmelt occurred 2 to 6 weeks earlier than average, resulting in earlier than normal spring freshets and exposure of glacier firn and ice. June 1^st snow accumulation was completely ablated or at record low values for most observation stations. The shift from winter and spring snowfall to rainfall and subsequent low summer rainfall resulted in the emergence of exceptionally wide-spread forest fires, rapid glacier melt, low streamflow and severe agricultural drought in Western Canada. By mid-July the seasonal snowpack had largely ablated, discharge rates in the Bow River at Calgary were 40% of average, many mountain streams had dried up and the Athabasca Glacier had experienced 3 m of ice melt. The Cold Regions Hydrological Model was used to simulate the impacts of the snowfall to rainfall transition on the snow redistribution, sublimation and melt processes, runoff and evapotranspiration that control the water balance of selected mountain environments in this period, employing Harder and Pomeroy’s Psychrometric Energy Balance Method to estimate precipitation phase. The results help to diagnose how a “warm drought” impacts the hydrology and glaciology of cold regions environments and suggest the possible impacts of future warmer climates and increased rainfall fraction on this region.