Climate Change Amplification of Short-Duration, High-Intensity Rainfall may Increase Postfire Debris-Flow Hazard in the Southwestern United States

Postfire debris flows are triggered by short-duration, high-intensity rainfall (oftentimes with recurrence intervals between one and two years) over recently burned areas. Although we have a good grasp on the triggering rainfall conditions for postfire debris flows, the limited spatiotemporal resolution of climate-scale simulations has hindered our ability to understand future trends in the frequency and magnitude of exceedance of these rainfall thresholds. Here, we use dynamically downscaled (~3.75 km), convection-permitting simulations of 15-minute rainfall to evaluate the exceedance of spatially variable rainfall thresholds (based on emergency hazard assessments and precipitation frequency analyses) in the southwestern United States for historical (1990–2005) and late-21^st century (2085–2100) climate epochs under intermediate and aggressive emissions scenarios (Representative Concentration Pathways 4.5 and 8.5). Our results show significant increases in the frequency and magnitude of threshold exceedance in the southwestern United States, including areas dominated by cool-season rainfall (e.g., California) and warm-season rainfall (e.g., Colorado). Simulations indicated greater predicted increases in the frequency of threshold exceedance in some regions where communities may be less accustomed to contending with postfire debris flows (e.g., northern California), compared to areas where the hazard is well known (e.g., southern California). Our predictions of rainfall threshold exceedance frequency and magnitude support proactive measures to mitigate damage and increase resiliency to short-duration, high-intensity rainfall and associated hazards in a changing climate.