Climate Change Evaluation of the Missouri River Basin Mountain Snowpack Accumulation and Runoff

The purpose of this pilot research project was to assess the impact of climate change on mountain snowpack accumulation and runoff in the upper Missouri River Basin Basin. Mountain snowmelt is the leading source of runoff into the Missouri River Basin Mainstem System of reservoirs operated by the U.S. Army Corps of Engineers. During the months of May, June and July (MJJ), about 37 percent of the average annual runoff into the upper Missouri River Basin enters the upper two reservoirs in the Mainstem System during the mountain snowmelt period.

The research questions addressed by this study are: “Are snowpack behaviors changing due to changes in the variability climate drivers as the amount of energy stored in the atmosphere increases? If this is the case, is the upper Basin, specifically the reaches above Fort Peck and Garrison reservoirs, more susceptible to runoff variability during MJJ, based on climate change projections?” The study included a sensitivity analysis of climatological drivers (precipitation, air temperature, relative humidity, evapotranspiration, soil moisture and ENSO/NAO climate patterns) associated with snowpack behaviors (day snowpack begins accumulating, peak SWE, date of peak SWE, melt rate and date of melt-out) of mountain snowpack and subsequent runoff. In addition the pilot research study ascertained the sensitivity of mountain snowpack behaviors and MJJ reservoir runoff to climate variability and projected change based on Community Climate Systems Model Versions 3 and 4 output.

The study results indicate that historical peak mountain snow water equivalents are decreasing but not significantly. MJJ runoff into Garrison Reservoir is decreasing significantly while runoff into Fort Peck is decreasing slightly. With regard to projected snow behaviors using climate projection data, there is a high level of confidence that the date of peak mountain snow water equivalent will be earlier, the date of melt-out will be earlier, and peak SWE will decrease through the end of this century. Projected runoff into Fort Peck and Garrison indicate significant downward trends through the end of this century.