Trends and sensitivities in late-season snowpack in the Pacific Northwest

Snow that lasts into late spring and summer can be an indispensable source of moisture for high mountain ecosystems and headwater streams, in particular given the dry summers that are characteristic of the Pacific Northwest. A key data need for resource managers in the region is a high-resolution assessment of the factors that drive variations in the persistence of snow cover. Much prior work has been devoted to assessing the trends and sensitivities of spring snowpack. We present research that extends this work by focusing on the sensitivities of late-season snowpack. The analysis is focused on five different forest ecosystems found across the Pacific Northwest: the Olympics, Wenatchee-Okanogan, Willamette, Deschutes, and Malheur forests. We find significant links to North Pacific sea surface temperature (SST) patterns, and furthermore find that regional variations in temperature and precipitation are sufficient to explain 30-50% of the variability in late-season snowpack. In an effort to identify additional controls on snow duration, we analyze separate basins within each of the above forests, using the Distributed Hydrology-Soil-Vegetation Model (DHSVM). The results suggest that the factors governing late-season snowpack vary significantly from region to region. We present several examples in which snow cover is sensitive to a different combination of factors relating to the local landscape, vegetation cover, and climate, and discuss possible generalizations that can be used to approximate snow cover in similar regions. Finally, we discuss application of these findings to NetMaps, a mapping tool used for high-resolution assessment of ecosystem vulnerability and sensitivity to climate change.