Wildfire Impacts on Snowpack Phenology in a Changing Climate Within the Western U.S.

Snowpack in the western U.S. is critical for water supply and is threatened by wildfires, which are becoming larger and more common. Numerous studies have examined impacts of wildfire on snow water equivalent (SWE), but many of these studies are limited in the number of observation locations, and they have sometimes produced conflicting results. The objective of this study is to distinguish the net effects of wildfires on snowpack from those of climate. We consider 45 burned sites from the Snow Telemetry network. For each burned site, unburned control sites are identified from the same level III ecoregion. Impacts of climate changes on snowpack are analyzed by comparing pre-fire and post-fire snow water equivalent at the unburned sites. Combined climate and wildfire effects are considered by comparing pre-fire and post-fire SWE at the burned sites. Wildfire impacts are then isolated by taking the difference between the burned and unburned sites. The wildfire-induced changes in SWE are also compared to several possible controlling variables including burn severity, leaf-area index change, dominant pre-fire tree genus, years since the fire, and site elevation. On average, wildfires have advanced melt-out dates 9 days and maximum SWE dates 6 days and reduced annual maximum SWE by 10% across the sites considered. On average, the combined effects of climate and wildfire have advanced melt-out and maximum SWE dates approximately 14 and 10 days, respectively, while decreasing annual maximum SWE by approximately 10%.