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Through precipitation, the fields of meteorology and geomorphology are fundamentally linked, thus interdisciplinary efforts are needed to advance understanding and warning of rainfall-driven geohazards. With a focus on recent efforts specific to post-fire debris flows in California, our presentation provides an overview of the benefits and challenges of working in an interdisciplinary team of meteorologists and geomorphologists, as well as results of a recent project demonstrating advancement through the integration of these fields. In this project, we combine high-resolution ensemble precipitation forecasts with post-fire debris-flow models to explore the feasibility and potential value of providing probabilistic post-fire debris-flow hazard information over a burn scar. In sharing these examples, we emphasize the multi-benefit nature of these efforts, and encourage future interdisciplinary efforts that improve warning and mitigation of rainfall-driven geohazards.

Wildfire makes landscapes more vulnerable to debris flows by reducing soil infiltration capacity and decreasing vegetation cover. The extent to which fire affects debris-flow processes depends on the severity of the fire, the climatology of intense rainfall, the pre-fire plant community, and sediment supply, among other factors. As fire expands into new plant communities and geographic regions, there is a corresponding need to expand efforts to document fire-induced changes and their impacts on debris-flow processes. In recent years, several large wildfires have impacted portions of the Sonoran Desertscrub plant community in Arizona, USA, a plant community where fire has been historically infrequent. Following two of these fires, we monitored debris-flow activity at the watershed scale and quantified wildfire-driven changes in soil hydraulic properties using in-situ measurements with mini disk tension infiltrometers. Results indicate that rainfall intensity-duration thresholds for the initiation of post-fire debris flows in recently burned watersheds within the Sonoran Desertscrub plant community are substantially greater than those in nearby areas dominated by other plant communities, such as chaparral. Results provide insight into the impact of fire on debris-flow processes in a plant community where it is likely to be more impactful in the future and help expand existing post-fire debris flow databases into a plant community where there is a paucity of observations.

Runoff-generated debris flows are a common post-fire hazard in the western United States and a growing number of regions around the world. As wildfire continues to emerge across a broader range of geographic regions and plant communities, there is an increasing need for generalizable methods to predict post-fire debris-flow initiation. The prediction of post-fire debris flow during intense rainstorms has traditionally relied upon empirical rainfall thresholds. Rainfall intensity-duration thresholds are often developed based on rainfall data and the hydrologic response to those rainstorms. They are most applicable to the specific regions where data are collected. Here, we present a new predictive approach that utilises processes-based models with fundamental physics and machine learning methods to estimate discharge thresholds for runoff-generated debris-flow initiation in four recently burned areas in the western United States. We assess the performance of the objectively defined discharge threshold-based predictions for post-fire debris-flow initiation from our hybrid framework, which utilises debris-flow timing within rainstorms, physically based numerical simulations of runoff, and the support vector machines method. The proposed thresholds have a good balance between true and false predictions for debris flow and floods. Importantly, our method permits the direct estimation of rainfall intensity-duration thresholds for areas where post-fire debris flow observations are limited.

Debris flows pose a serious threat to human life and infrastructure in downstream areas following wildfire. This underscores the necessity for having a hazard assessment framework in place that can be used to estimate the impacts of post-wildfire debris flows. Current hazard assessments in the western United States (USA) use empirical models to assess the volume of potential post-wildfire debris flows. Volume models provide information regarding the magnitude and potential downstream impacts of debris flows. In this study, we gathered post-wildfire debris-flow volume data from 54 watersheds across the states of Arizona (AZ) and New Mexico (NM), USA, and compared these data to the output of a widely used empirical post-wildfire debris-flow volume model. Results show that the volume model, which was developed using data from the Transverse Ranges of southern California (CA), tends to overestimate observed volumes from AZ and NM, sometimes by several orders of magnitude. This disparity may be explained by regional differences between southern CA and AZ and NM, including differences in sediment supply. However, we found a power- law relationship between debris-flow volume and watershed area that can be used to put first-order constraints on debris-flow volume in AZ and NM.