Projected Changes in Interannual Variability of Peak Snowpack Amount and Timing in the Western United States

Interannual variability of mountain snowpack has important consequences for ecological and socioeconomic systems, yet changes in variability have not been widely examined under future climates. Physically based snowpack simulations for historical (1970-1999) and high-emission scenario (RCP 8.5) mid-21st century (2050-2079) periods were used to assess changes in the variability of annual maximum snow water equivalent (SWEmax) and SWEmax timing across the western United States. Models show robust declines in the interannual variability of SWEmax in regions where precipitation is projected to increasingly fall as rain. The average frequency of consecutive snow drought years (SWEmax < historical 25th percentile) is projected to increase from 6.6% to 42.2% of years. Models also project increases in the variability of SWEmax timing, suggesting reduced reliability of when SWEmax occurs. Differences in physiography and regional climate create distinct spatial patterns of change in snowpack variability that will require adaptive strategies for environmental resource management. Plain Language Summary A wealth of research has established that warming temperatures associated with climate change in the western United States will generally reduce snowpack accumulation and result in earlier snowmelt timing, with important consequences for water resources and ecosystems. However, changes in the variability of snowpack conditions between years have not been well established. We analyze simulated snowpack data for historical and future climate scenarios and find that changes in variability differ across the western United States. Variability of annual maximum snowpack between years decreases while the timing of peak snow accumulation becomes more variable, particularly in areas transitioning from snow- to rain-dominated precipitation. We also find that consecutive years with very low or early snowpack will become much more frequent. These findings highlight the need to consider changes in snowpack variability in climate change impact assessments and adaptation planning.