AN OVERVIEW OF POSTFIRE DEBRIS FLOWS IN THE SOUTHWEST US

Wildfires impact vegetation and soil hydrologic properties in ways that increase the likelihood of postfire debris flows in steep, burned watersheds. Over the past 22 years, we have monitored basin responses in 24 burn areas in Arizona and New Mexico. In many instances, we have high-resolution rainfall and/or pressure transducer (PT) data that can be used to constrain debris-flow timing within rainstorms. We have also collected data to quantify debris-flow volumes. Here, we summarize these data to characterize basins that did (B1, n = 135) or did not (B0, n = 112) produce postfire debris flows. For example, basin area ranged from 0.01-13.5 km² with a mean of 0.69 km² for B1 and 0.81 km² for B0. The mean basin slope was 28° for B1 and 26° for B0. The mean basin area burned at moderate to high soil burn severity on slopes ≥23°, which has been identified as a key predictor of debris-flow activity in the western US, was 41% for B1 and 32% for B0. The triggering 15-minute rainfall intensity (I15) across sites with PT data was 45 mm/h, while the peak I15 across all sites was 56 mm/h. Approximately 70% of debris-flow producing storms had return intervals of 2 years or less. Finally, debris-flow volumes ranged from 20 m³ to 14,000 m³ with a mean value of 645 m³. These data can help us quantify important characteristics of burned basins that produce postfire debris flows and can be used to inform selection of future debris-flow monitoring sites.