CONSTRAINING POSTFIRE DEBRIS-FLOW VOLUME IN THE SOUTHWESTERN UNITED STATES

As postfire debris flows become more prevalent across the western United States (US), improved methods are needed to better constrain postfire debris-flow volume. While there are several established methods for predicting volume in southern California, a paucity of data has limited the development of volume models in many other regions of the western US, including the Southwest (Arizona and New Mexico). Here, we present a new dataset of 54 postfire debris-flow volumes collected across Arizona and New Mexico between 2010 and 2021 that we use to develop three multiple linear regression model for predicting postfire debris-flow volume in the Southwest. The best-performing model predicts volume given 30-minute rainfall intensity, watershed area greater than 23 degrees, and a fire severity metric. We also propose a model that considers only rainfall and terrain variables and another that requires only terrain and fire severity variables, both of which can be employed in scenarios where there are data limitations. Finally, we compare the performance of these models against several that were developed elsewhere in the western US to explore how the factors that control postfire debris-flow volume vary across geographic regions. We find that the models introduced here more accurately predict postfire debris-flow volume in the Southwest. The results of this study not only improve our ability to predict postfire debris-flow volume in the Southwest, but also provide valuable insights into the relationships between debris-flow volume, rainfall characteristics, watershed terrain attributes, and fire severity.