High Elevation Post-Fire Landscapes on Snow Melt Trends in Seasonal and Transitional Snow Zones

Marin S. MacDonald1, Molly E. Tedesche2,3, Steven R. Fassnacht1,4,5 1 ESS-Watershed Science, Colorado State University 2 US Army Corps Engineer Research & Development Center, Coastal & Hydraulics Laboratory (ERDC CHL), Cold Regions Research & Engineering Laboratory (ERDC CRREL) 3 University of Alaska Fairbanks 4 CIRA

5 NREL

Abstract Wildfire has become an increasing threat across the Western United States over the past few decades. Although low elevation fires have become more common in Colorado's recent history, larger fires at higher elevation are new, and pose greater threats to Colorado's ecosystems and water availability. Until recently, fires were often caused by summer thunderstorms in lower-elevation ponderosa pine forests. However, 2020 was an unprecedented fire season, with five fires starting in the in July or later at high elevation in the spruce-fir forests of northern Colorado and southern Wyoming, specifically the Cameron Peak, Williams Fork, Middle Fork, Mullen, and East Troublesome fires. Much of the burned area in these five fires is in the seasonal (or persistent) snow zone, which are critical areas for water storage and alpine habitat.

Although research has been done to understand watershed health in post-fire landscapes, looking at how they impact snowpack timing is still relatively new. This study examined the onset of snow melt in relation to burn severity and terrain characteristics, focusing on elevation, aspect, and slope. Satellite imagery from the Sentinel-1 Synthetic Aperture Radar (SAR) was used to estimate the beginning of snow melt. Severity and terrain characteristics were compared for seasonal and transitional snow zones for each burn area, as well as the surround non-burned areas. With changes in tree cover in burn areas, snow pack may be affected in when and how fast melt occurs in the spring months, as well as how much snow accumulates over the winter season.