EFFECTS OF RECENT DISTURBANCE HISTORY ON POSTFIRE DEBRIS FLOW ACTIVITY

Fire can substantially increase the likelihood and volume of debris flows in steep landscapes. Fires in the Southwest USA, specifically in Arizona and New Mexico, have been increasing in frequency and severity in recent decades. This motivates the need to understand how responses to future fires can be modulated by recent fire history and the geomorphic response to these recent fires. Here, we address this problem using data collected at four different debris flow monitoring sites that burned twice within a 5-20 year period. We found that postfire debris flow activity following the most recent fire can be affected by recent fire history as well as by the geomorphic response to past fires. For example, the 2021 Telegraph Fire burned more than 1800 km², including areas burned by the 2009 Pioneer Fire and 2017 Pinal Fire. Watersheds that produced debris flows following the 2017 fire did not produce debris flows following the 2021 fire. We attribute the lack of debris flows, in part, to reduced sediment supply in channels. In contrast, we did observe debris flows following the 2021 fire in watersheds that did not produce debris flows following the 2017 fire. Watersheds that burned in both 2009 and 2021 also produced debris flows. Rainfall intensities responsible for triggering debris flows in areas that burned in 2009 and 2021 were similar to the rainfall intensities responsible for initiating debris flows following the 2017 fire. In other words, rainfall intensity-duration thresholds appeared unaffected by two fires that were separated by 12 years. Results demonstrate that the combined effects of multiple fires can alter soil and vegetation properties in ways that make steep landscapes more susceptible to debris flows and also highlight how recent debris-flow history can influence the geomorphic response to subsequent fires by altering sediment supply.