Quantification of the Role of Wildfire in Shaping Flood Frequency for a Southern California Mountainous Watershed

Abstract Submission: Moderate- to high-severity wildfire can abruptly alter watershed properties, which intensifies extreme hydrologic responses such as debris flow and flooding. Understanding the frequency and severity of such compound wildfire-flood hazards is of growing importance, particularly in wildfire-prone regions. However, standard statistical flood frequency analysis approaches are ill-suited to examining this issue because wildfire-affected peak flow observations are limited in number and violate the assumption of independent and identically distributed events. Here, we developed a process-based FFA framework that integrates a stochastic rainfall generator, wildfire simulation, inverse modeling, and a physics-based hydrological model to directly simulate the impacts of wildfire on FFA. We applied this framework in the upper Arroyo Seco (uAS)watershed in Southern California, which experienced M-H burn during the 2009Station Fire. An FFA analysis, performed with simulated peak flows from the first year since fire demonstrates the 100-year flood can be three times larger than simulations that only consider peak flows in non-fire-affected years. On the other hand, coupling process-based FFA with stochastically-simulated wildfire events and watershed’s time-varying hydrologic recovery yields “fire continuum FFA”, a concept introduced here for the first time. Fire continuum accounts for multiple wildfires within very long synthetic time series. Variability in upper tail flood peaks is substantially higher in fire continuum results as compared with pre-wildfire FFA. This result highlights the importance of wildfire inter-arrival time and post-wildfire recovery processes, both of which are expected to change as a result of climatic change and evolving fire management strategies.

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