Assessment of miller fork watershed soil burn severity mapping following the cameron peak fire

Abstract

The Cameron Peak Fire (CPF) burned 84,544 hectares (208,913 acres) within the Arapaho and Roosevelt National Forest in Northern Colorado between August and December 2020. The burn occurred over vast, rugged, and mainly U.S. Forest Service (USFS) land. After starting in the Cache la Poudre Watershed, the fire crossed into the Big Thompson Watershed and became Colorado's largest wildfire on October 14, 2020. Post-fire assessments concluded that the Big Thompson Watershed was at a high risk of sedimentation and other impacts, specifically originating from the remote Miller Fork Watershed and surrounding areas. A key element in post-fire risk assessments is the Burned Area Emergency Response (BAER) process. Just 7-10 days after this portion of the fire was contained, the USFS BAER team generated a soil burn severity map of the area using aerial drone surveys and remote sensing data and did sporadic field spot-checks to verify the mapping of relative burn severity. BAER mapping after the CPF faced unusual difficulties due to late season weather that affected imagery and restricted field validation, potentially reducing accuracy within the Big Thompson portion of the burn. Therefore, to evaluate accuracy of mapping under these challenging conditions, I conducted field work within the burn scar to spot check the BAER soil burn severity map. I focused on the Miller Fork Watershed, which was identified as a watershed of immediate concern post-burn and compared the fall 2020 BAER soil burn severity map to fall 2021 field observations made one year after the burn. I examined 16 areas mapped by the BAER method as unburned, low, moderate, or highly burned, with four plots in each of the four burn severity categories. Using 9 m x 9 m plots with 1 m x 1 m subplots, I evaluated ground condition/cover, canopy cover, vegetative regrowth, burn severity, and signs of erosion. My assessment suggests that the BAER soil burn severity maps are slightly inaccurate for the Miller Fork Watershed and not representative of field conditions. Therefore, utilizing BAER soil burn severity maps for a small area of a burned area need to be ground truthed further to ensure the maps are representative of the field conditions.