Seasonal influence on post-fire debris flow likelihood after the 2020 Lake Fire

Abstract. The increasing severity of wildfires in Western North America is widely hypothesized to lead to an increased likelihood of post-fire debris flows (PFDF), specifically those triggered by high-intensity rain. PFDF likelihoods are highest in the first year and tend to decrease over time. However, it is not well understood how seasonal variation affects the PFDFs initiation in the years following a fire. Here, we monitored the changes in PFDF likelihood of the 2020 Lake Fire in Southern California over a span of four years using field and satellite observations, together with numerical modeling for a subset of drainage basins. We found that unsaturated hydraulic conductivity increased by an order of magnitude during the dry season as compared to the wet season, significantly reducing the PFDF likelihood. Our simulations show that vegetation cover has a smaller impact on PFDF likelihood as compared to hydraulic conductivity or grain size. This study helps clarify the impacts of hydraulic conductivity, grain size, and vegetation on PFDF due to seasonal variation in these parameters for four years after the fire. We suggest that field measurements and modeling approaches should consider how different climatic and seasonal patterns could influence PFDF several years after fires.