Evaluating Post-Fire Geomorphic Change On Paired Mulched and Unmulched Watersheds Using Repeat Drone Surveys

Sediment redistribution post wildfire can dramatically alter a watershed and pose risks to local infrastructure and water quality. Mulch application is increasingly being used to mitigate post-fire hillslope runoff and erosion, although relatively little is known about its effects on the watershed scale. In this study we use repeat drone surveys to measure erosion and deposition across 6 small (0.5-1.5 km2) watersheds, 3 mulched and 3 unmulched, in the 2020 Colorado Cameron Peak Fire burn scar. Initial drone surveys were gathered in the spring of 2022 shortly after mulching and are to be differenced to surveys done in fall of 2022, capturing the erosional effects of a Colorado monsoon season. The objectives are to (1) quantify sediment volumes and spatial patterns of erosion and deposition on a watershed and hillslope scale over a 1-year period, (2) compare geomorphic change to mulch coverage, precipitation intensity, burn severity, and morphologic metrics, and (3) identify conditions in which mulch may be most appropriate based on findings. Before and after an early summer storm, a sparsely mulched hillslope about 33,000 m2 was UAV surveyed, coregistered in Agisoft Metashape, and differenced to observe 2.72 Mg/ha of eroded sediment deposited down-swale in sediment fences. Sediment yield for the 7 tested swales increased with drainage area and bare ground percentage. Small scale hillslope processes such as rilling were a challenge to detect in the drone surveys due to noise from vegetation. Ground control points were compared between 2 surveys to show relative elevation errors up to 8 cm. Previous studies have shown change detection accuracies down to 2 cm with gcps (Nota et al., 2022) and 10 cm without (Cook & Dietz, 2019) and success in filtering vegetation; thus, our method is being evaluated at different scales to retrieve similar results. The findings of this study can help managers assess sedimentation risks and decide on post-fire treatment.