Changes to Snowpack Energy State from Spring Storm Events, Columbia River Headwaters, Montana

Abstract The generation and release of meltwater during the spring snowmelt season can be delayed because of spring storm episodes with snow accumulation and/or sustained subfreezing temperatures. The delayed release of snowmelt often extends beyond the particular storm event because of changes to the internal state of energy in the snowpack that prevents transmission of meltwater. Following a storm, two energy deficits internal to the snowpack must be overcome before surface melt can drain and exit the snowpack: 1) cold content created by heat lost during the episode must be removed and 2) dry pore space must be filled with liquid water to residual saturation. This study investigates the role of these two processes in spring snowmelt following past storm episodes in western Montana. The analysis addresses ~10 yr of historical snowpack and air temperature data from 33 stations in the Columbia River headwaters. Results indicate that the addition of pore space has a greater impact on delaying snowmelt than does the addition of cold content, with snow accumulation events responsible for 86% of the collective energy deficit imposed on the snowpack during storm episodes. Nearly all refreezing events occur within one month of peak snowpack, but accumulation events are common up to 50 days later. Under standardized conditions representing clear weather during the spring season, these energy deficits could all be overcome in a matter of hours.