Snowpack Change from 1982-2016 over Conterminous United States

Snow water equivalent (SWE) variability and its drivers over different regions remain uncertain due to lack of representativeness of point measurements and deficiencies of existing coarse-resolution SWE products. Based on our recently developed daily 4 km SWE and snow depth data from 1982-2016 over conterminous US (ConUS) (Broxton et al. 2016a,b; Dawson et al. 2017), here we quantify and understand the snowpack change at 4 km pixels for the first time (Zeng et al. 2018, under review). The median peak snow area is 4.2e6 km², covering 54.3% of ConUS, while the peak total snow mass of 278 gigatons (of water) is about 18 times the average annual volume of Colorado River.

Annual maximum SWE decreased significantly (p < 0.05) by 41% on average for 13% of snowy pixels over western ConUS (8.5e4 km², or the size of South Carolina) from 1982-2016. In contrast, only 2% of the pixels over eastern ConUS showed significant but small trends. Over ConUS, snow season was shortened by 34 days on average (significant at p < 0.05) for 9% of the snowy pixels (1.1e5 km², or the size of Virginia), primarily caused by the earlier ending and later arrival of the season over western and eastern ConUS, respectively. While Sierra Nevada and western ConUS have 12% and 8% of the snowy pixels with significant trends of 1 April SWE, the corresponding trends using all snowy pixels are not significant. This demonstrates the importance of using our gridded data in trend analysis and that significant trends at some 4 km pixels do not necessarily translate to a significant trend over a region.

There are large interannual and interdecadal variabilities of snowpack and precipitation but a relatively steady increase of temperature during this period. October-March mean temperature and accumulated precipitation largely explain the temporal variability of 1 April SWE over western US, and considering temperature alone would exaggerate the warming effect on SWE decrease. In contrast, temperature plays the primary role in the 1 April SWE variability over eastern US. Conclusions remain the same for 1 March SWE.