Snowpacks in western United States are projected to decline in size and melt earlier when high emissions climate scenarios are combined with more wildfires, putting an increasing strain on vital water resources.

Research in Science Advances notes that wildfires are increasingly burning in snow-dominated watersheds and are altering the dynamics of snow melt. Snowpacks, a vital water resource that recharges aquifers and sustains streamflow during drier months, are projected to decline in the western United States by between 40 to 75% by 2100 under high emissions climate scenarios.

Snowmelt timing and magnitude are major controls on reservoir storage for municipal water supply, agricultural use, hydroelectric power generation, and flood control. Warmer temperatures and expanding population pressures are only increasing these demands. In the western US, snowmelt-derived runoff from mountainous regions contributes up to 70% of total annual flow.

Burned area in western forest snowpack areas has increased by 9% annually between 1984 and 2017, with areas above 2500 m with historically reliable snowpack seeing the largest relative increase in burned area.

Previous research has claimed that the specific day when all snow has completely melted away is occurring earlier and earlier each year in postfire environments. A primary driver of increased snowmelt is due to the black carbon and other charred woody debris shed by burned trees and vegetation onto the snow surface, decreasing the amount of solar radiation snow can reflect and increasing the amount of energy snow absorbs.

Researchers from the Colorado Schools of Mines and the University of Colorado, however, said that previous research has focused on only one to three fires in specific areas, rather than adopting a larger-scale approach. Those researchers have since published their own larger-scale study, using two statistical models and satellite imagery to provide a western US–wide analysis on snowpack in burned areas. They found in virtually every region that has snowpack, snow melts earlier in the first year after a fire than in non-burned areas.

“The Pacific Northwest and northern Sierra Nevada are most vulnerable to snow disappearing up to two weeks earlier after a fire,” the study said. “Additionally, California and Oregon have the largest additional change in the effect of fire on [the snow disappearance date] under +2°C of warming, while the Intermountain West (Colorado, Utah, and Wyoming) experiences small additional earlier melt. In warmer environments, snow disappearance due to postfire energy balance changes can even occur in midwinter, disrupting the anticipated timing of snowmelt runoff.”

Click here to read the full research paper.