Regional Trends in Snowpack Cold Content for the Inter-mountain Western United States

Abstract:

Trends for snowpack temperature, density, depth, SWE, and cold content were evaluated for the 1992-2020 winter seasons using the regional Kendall test (RKT) on novel snow-pit data collected from over 50 site locations across the Rocky Mountains, western United States. RKT is a non-parametric test with the proven power of monotonic trend detection in geoscience systems over linear regression by grouping data from certain geographic regions. According to both snow temperature and SWE, the RKT indicates that internal snowpack cold content has shifted toward warming the more massive northern region snowpack and maintaining a colder, less massive snowpack in the southern region. Variations in internal snowpack temperature explained most of the interannual variability in snowpack cold content magnitude in both regions. In the northern region, Thiel Sen’s slope for snowpack temperature was positive indicating the snowpack was warmer in recent years. Thiel Sens’s slope is more suitable for hydrological trend detection than ordinary least squares line fit because it is insensitive to outliers and missing values (which are expected in hydrologic systems). Thiel Sen's slope for SWE trend was generally flat in both regions, indicating little detectable change. High variance around the Thiel Sen’s slope fit for SWE reflects the low correlation that SWE has with cold content interannually. The southern snowpack trend is moderate cooling and the northern snowpack trend is pervasively warmer, which is controlled more by internal snow temperature than SWE. We conclude that northern region snowpack warming promotes less cold content in the future. Measuring and monitoring snow temperature change helps to define the spatial-temporal arrival of spring by observing the snowpack in advance of isothermal conditions. Internal snow temperature and SWE data can be useful in multiple fields, including streamflow forecasting, climate change interpretation, and water resource management, among others.