Empirical Models for Postfire Debris-Flow Volume in the Southwest United States

Improving our ability to relate postfire debris-flow volume to rainfall characteristics, terrain attributes, and fire severity is critical for quantifying postfire sediment yields from steep landscapes and predicting changes in debris-flow hazards following fire. This is especially true in the Southwest United States (US) (Arizona and New Mexico), where fire activity has increased in recent decades. In this study, we present a database of 54 postfire debris-flow volumes that we collected across the Southwest between 2010 and 2021. We use these data to develop a multiple linear regression model for postfire debris-flow volume based on peak 30-min rainfall intensity, watershed area greater than 23 degrees, and a soil burn severity variable. We further propose a model that utilizes only rainfall and terrain variables, as well as a model that requires only terrain attribute and fire-severity variables. These models are beneficial in scenarios where there are data limitations. We compare these new models with others developed in the western US to explore differences in the factors that control debris-flow volume across geographic regions. We find that the models introduced here more accurately predict postfire debris-flow volume in the Southwest relative to existing models. We also find that models that include sub-hourly rainfall intensity perform better than those that do not, revealing the benefits of high-resolution rainfall data for constraining postfire debris-flow volume. Results improve our ability to forecast postfire debris-flow volume in the Southwest and provide insights into relationships between rainfall intensity, terrain attributes, fire severity, and debris-flow volume.