Wildfire Impacts On Annual Flood Peaks Across The Western United States

Wildfires can directly affect public health, the natural environment, and federal and state budgets. The indirect hydrologic impacts of wildfire, such as flooding and debris flows, can also be destructive. Despite the importance of the hydrologic impacts of wildfire, especially for post-fire flood events, their relationship to a wide range of climatological and geophysical settings is poorly understood. This study examines the post-fire streamflow dynamics of a number of watersheds across the western United States (US). In this study, we used US Geological Survey (USGS) annual peak flow and daily mean streamflow; precipitation and snowmelt from the North American Land Data Assimilation System version 2 (NLDAS-2) and NLDAS Variable Infiltration Capacity (NLDAS-VIC) simulations, respectively; and the Monitoring Trends in Burn Severity (MTBS) Project wildfire size and burn severity. We analyzed: 1) the different seasonalities of wildfire and annual peak flows, as well as the time period between a wildfire and the first subsequent flood; 2) changes in post-fire annual peak flows and their corresponding non-exceedance probability; 3) changes in runoff ratios for post-fire flood events; and 4) the relationship between post-fire flood impacts and the climatological and geophysical settings of watersheds. These analyses were applied to 714 watersheds across the western US that experienced at least one wildfire during the 1984-2020 period.