The effects of soil burn severity and slope steepness on post-wildfire hillslope runoff and soil erosion using a portable rainfall simulator in British Columbia, Canada

Wildfires are increasing in frequency and severity in many parts of the world and this trend is expected to continue for the foreseeable future. The objective of this research was to compare how soil burn severity and slope steepness influenced the rate of surface runoff and soil erosion on hillslopes one year after a wildfire. A portable rainfall simulator was used to create high-intensity rainfall in order to instigate surface runoff and soil erosion at various impacted sites across the 2023 North Lucas Lake wildfire in central British Columbia, Canada. Moderate and high soil burn severities were compared to unburnt sites. Hillslopes were concentrated around 20% and 40% steepness, although individual sites ranged from 17% to 45%. Soil burn severity explained increases in runoff and erosion rates more than slope steepness. Compared to the unburnt sites, moderate soil burn severity sites experienced an increase of six times more runoff and 59 times more erosion, while the high soil burn severity sites experienced ~ 10 and 297 times more runoff and erosion, respectively. Less dense, organic-rich and finer-grained sediment was preferentially mobilized and transported following rainfall at the burnt sites. This eroded sediment was enriched in some metals and phosphorus, which in the case of manganese and zinc exceeded Canadian sediment quality guidelines for the protection of freshwater aquatic life. This suggests that high-intensity rainfall events post-wildfire are likely to increase the delivery of carbon, nutrients and contaminants to aquatic ecosystems which are often associated with fine sediment and organic material. A portable rainfall simulator was shown to be an effective tool in determining how different site conditions impact surface runoff and soil erosion following a severe wildfire in central British Columbia. This study highlights some of the field conditions influencing runoff and soil erosion post-wildfire. This information can help provide support for government agencies, local communities and First Nations when making management and recovery decisions for protecting downslope aquatic ecosystems and water resources.