watervulnerability

Helena National Forest Watershed Vulnerability Assessment, Northern Region (R1)
Exposure
Many parameters influence the timing and magnitude of runoff for a given watershed. Average winter
precipitation and average maximum winter temperature were initially analyzed to determine watershed
exposure to changes in climate variables.
Precipitation average over the entire forest is projected to increase slightly. All elevations on the forest are
above approximately 3,500 feet. The average elevation is approximately 6,200 feet and the maximum
elevation approximately 9,500 feet. Precipitation is predicted to increase in the winter, spring and fall and
decrease in the summer season.
Projected maximum winter temperatures (Dec-Jan-Feb) for the Helena National Forest for the 2040s time
period are expected to increase. The average temperature across all HUCs went from 0 °C historically to
1.3 °C projected for the 2040s time period. Temperatures are expected to remain relatively cold with the
average maximum winter temperature not exceeding 3 °C for any individual watershed. Temperature is
predicted to continue to increase into the 2080s time period where the average maximum winter
temperature for all watersheds is predicted to be near 3 °C. Hamlet and Lettenamaier, 2007, found
through a series of models of the northwestern United States, that cold river basins, where snow processes
dominate the annual hydrologic cycle (< 6 °C average in midwinter), typically show reductions in flood
risk due to overall reductions in spring snowpack. The Helena National Forest is well below 6 °C average
midwinter and may see reductions in spring runoff flows for this reason.
Since changes in summer and winter temperature are not expected to have a direct effect on infrastructure
and development, change in watershed snowpack (the ratio of April 1 st SWE to Oct-Mar precipitation)
was the only climate factor used to assess exposure. This value has been calculated using downscaled
climate and hydrologic projections for the entire Columbia, upper Missouri and upper Colorado basins.
Figures 7 and 8 show predicted watershed snowpack vulnerability (Littell et al.) watershed for the 2030-
2059 and 2070-2099 time periods, respectively. Both the North Fork of the Blackfoot River and The
Landers Fork watersheds are projected to see the most change in snowpack.
58 Assessing the Vulnerability of Watersheds to Climate Change