Comparisons of Montane Snow Water Equivalent Projections: Calculating Total Snow Mass in Regions with Projection Agreement and Divergence in the Western United States

Abstract Montane snowpack is a vital source of water in the western United States. Here, we use a large-ensemble approach to evaluate the agreement across 124 snow water equivalent (SWE) projections with statistically downscaled forcing between end-of-century (2076–95) and early twenty-first century (2106–35) periods. Comparisons were performed on dates corresponding with the end of winter (15 April) and midspring snowmelt (15 May) in five western U.S. domains. Using 1) the percent change to end-of-century SWE across different ensembles of snow projections and 2) the shift between early twenty-first century and end-of-century SWE distributions for each snow projection, we identified relationships between projections that were consistent across each domain. In low to midelevations, end-of-century SWE decreases were 48% and larger on 15 April. These regions had projected changes to SWE that were both high confidence and in relative agreement across projections. Despite this, the majority of 15 April SWE volume existed in higher elevations where the magnitude and direction (positive or negative) of SWE changes were most uncertain. The results of this study show that large-ensemble approaches can be used to measure coherence between snow projections and identify 1) the highest confidence changes to future snow water resources and 2) the locations and periods where and when improvements to snow projections would most benefit estimates of future snow water resources. Significance Statement A majority of spring and summer runoff in the western United States originates from mountain snowpack. Yet, future projections of snow in these regions are uncertain. We identify where snow projections agree and the proportion of snow that falls within regions where end-of-century projections of snow disagree as a result of disparate snow modeling approaches. Results show that a majority of the area of the Rocky Mountain and Cascade Mountain ranges have projected changes to snow that agree in relative magnitude, but most of the annual snow water supply exists in higher elevations where projections of snow diverge. These results highlight where further research may most improve our understanding of future snow water supplies and the associated downstream water resources.