Regional Differences in High Elevation Snowpack Decline Along the North American Rocky Mountains

ABSTRACT The Rocky Mountains (RM) provide the ‘water towers’ for western North America, with deep winter snowpack accumulations that melt to contribute flows for the extensively utilised Columbia, Colorado, Saskatchewan, Missouri and Rio Grande River systems. With climate change, winter and spring warming are increasing seasonal and elevational rain versus snow proportions and altering the annual patterns of snowpack accumulation and melt. Prior studies have reported declines in snowpack extent or water content, especially on an index date, April 1. These declines could reflect reductions in the total annual snowpacks or earlier transitions to snowmelt. To resolve these influences, we assessed daily snowpack patterns at 314 snow pillow stations in the higher elevations along the 2500 km transboundary RM corridor, over three decades from 1991 to 2020. We found regional differentiation, with little change in the maximum snow water equivalent (SWE max ) or its timing (Day max ) in the most‐northerly, Canadian RM region (BC, AB); slight declines in the Northern US (ID, MT, WY) and Central US (UT, CO); and major declines in the Southern US (AZ, NM; average ΔSWE max : −2%/yr; ΔDay max : −0.75%/yr). With compound influences of declining SWE max and earlier Day max , the April 1 SWE (SWE Apr1 ) was more responsive, with progressive decline at some Northern US and Central US stations, and steep decline in the Southern US region (ΔSWE Apr1 : −6.5%/yr). Due to these compound influences, we recommend that future analyses include snowpack maxima and seasonality as well as April 1 measures, since that precedes the peak snowpack for higher elevation and northern sites, but follows the peak for lower and southern sites, confounding trend comparisons. Thus, higher elevation RM snowpacks are declining but with considerable latitudinal variation, displaying slight change in magnitude and seasonality in the northern regions, and greater change southward. These patterns contrast with some other climate change patterns that display increasing responsivity with higher latitude.