Effects of post-fire mulch applications on hillslope-scale erosion
Date
2023
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Abstract
Wildfires are increasing in frequency and intensity, greatly altering the landscape and increasing risk of erosion. Mulching is a common restoration technique used after wildfire to enhance protective ground cover and reduce erosion, yet most studies are conducted at the plot-scale. This study applies an experimental approach to evaluate the impact of mulch treatments at the hillslope-scale using varying mulch levels. Similar adjacent hillslopes were chosen to minimize variability in landscape features. The objectives of this research are to 1) examine the effectiveness of post-fire mulching in reducing erosion at the hillslope-scale, and 2) identify landscape features and precipitation factors contributing to the occurrence and magnitude of sediment yield. Sediment fences were installed in convergent swales and planar hillslopes to quantify sediment yields before and after aerial wood mulch application. Rain gauges were installed to compute rainfall amount (mm), duration (hr), and maximum intensities (mm/hr) by storm event. Field observations, coupled with game camera footage, were utilized to evaluate whether each storm produced sediment in the fences. Surface cover surveys were conducted to assess cover changes over the season. Collectively these data were used to 1) identify rainfall intensity thresholds for erosion, 2) examine controls on sediment generation occurrence with a binomial distribution mixed-effects model, 3) examine controls on the magnitude of sediment yield using a gamma distribution mixed-effect model, and 4) assess relative importance of variables relating to sediment yield using random forest models. Threshold rainfall intensities for generating erosion at the study sites were 32-38 mm/hr for MI5, 11-18 mm/hr for MI15, 7-13 mm/hr for MI30, and 5-8 mm/hr for MI60. Across all models of erosion occurrence and magnitude of sediment yield, maximum rainfall intensity and total precipitation were primary drivers of erosion. There was no evidence of a mulch treatment effect on sediment occurrence or magnitude, likely resulting from insufficient initial mulch cover and a high-intensity storm that removed much of the mulch shortly after it was dropped on the hillslopes. Contributing area, slope mean, and slope length showed no influence on sediment yield, likely due to limited variation in these variables between hillslopes. These results highlight the importance of mulch cover that will stay in place under extreme rainfall. Future hillslope-scale studies should consider dropping mulch during a time period that is unlikely to have high intensity rainfall and explore mulch materials and application methods that will better ensure adequate initial cover for reducing hillslope-scale erosion.
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Subject
hillslope
post-fire
mulching
erosion