ASSESSING THE CONTROLS ON POST-WILDFIRE DEBRIS-FLOW VOLUME IN THE SOUTHWESTERN UNITED STATES

Debris flows pose a serious threat to downstream lives and infrastructure in the years following wildfire. Debris-flow volume influences the potential downstream impacts of post-wildfire debris flows, as the mobility and inundation potential of these events depend, in part, on flow volume. Therefore, identifying the watershed properties and rainfall characteristics that control post-wildfire debris-flow volume are critical to improving our ability to identify which burned watersheds are most likely to produce impactful debris flows. In this study, we measured the volume of 55 post-wildfire debris-flows across 54 watersheds in Arizona and New Mexico, USA. Using these data, we determined a power-law relationship between debris-flow volume and watershed area that can be used to place first-order constraints on post-wildfire debris-flow volume in Arizona and New Mexico. We also compared the observed volumes against the output of a widely used post-wildfire debris-flow volume model that was developed using data from the Transverse Ranges of southern California, USA. This existing model tended to overestimate observed volumes, often by several orders of magnitude. However, it underestimated debris-flow volume in watersheds that burned primarily at low severity. These discrepancies may be partially attributed to regional differences between southern California and Arizona and New Mexico, including differences in sediment supply and vegetation. Results underscore the importance of collecting post-wildfire debris-flow volume data across a range of geographic settings and disturbance regimes, including in plant communities that tend to burn at lower severity.