Post-Wildfire Impacts on Snow Accumulation and Melt: Hydrologic Implications for Headwater Catchments

Reduced snowpacks and earlier snowmelt have been shown to affect fire frequency and severity in the western United States. Wildfire disturbance also affects patterns of snow accumulation and ablation by reducing canopy interception, increasing turbulent fluxes, and modifying the surface radiation balance. Recent work documenting snow-vegetation interactions in burned and unburned forests show that burned forests experience increased snow accumulation but earlier and more rapid melt. This investigation explores the albedo effect of wildfire on subsequent years' snowpack. Traditional conceptions of snow-vegetation interactions are based on studies focused on forest harvesting, not on fire-affected watersheds and no study to date has examined the albedo effect. At the local-to-watershed scale, wildfire-derived black carbon sloughing from burned trees onto the snowpack has been suggested as an important forcing of earlier melt and anecdotal evidence suggests that snowpacks in the wildfire areas appear to be experiencing this forcing. Our study area is a mountainous, dense forest at an elevation of about 1750 m in the Oregon Cascades that was burned in August 2011. Here we compare measurements of spectral albedo of snow that were acquired along transects through burned and unburned areas of the study site. Results show that snow albedo in the burned forest is about 50% lower than snow albedo in the nearby, unburned forest. Soot particles and burnt woody debris from charred trees accumulate at the snowpack surface darkening the snowpack and absorbing more radiant energy in the burned forest than in the unburned forest. Mass of debris accumulated at the snowpack surface is an order of magnitude greater in the high severity burned forest as compared to the nearby, unburned forest. We measured surface energy balance using data acquired at meteorological stations within the burned and unburned areas. Even with increased snow accumulation in the burned forest the snow disappeared several weeks earlier than in the unburned forest, mainly due to the substantial decrease in snow albedo. We discuss the hydrologic implications for headwater catchments in the West.