Process-Based Quantification of the Role of Wildfire in Shaping Flood Frequency

Moderate to high severity wildfire can abruptly alter watershed properties and enhance extreme hydrologic responses such as debris flows and floods. The compounding effects of wildfire on flood hazard, represented here via flood frequency analysis (FFA; e.g., 100-year flood) are of growing importance. Standard statistical FFA approaches are ill-suited to examining this issue because wildfire-affected flood peak 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 watershed in Southern California, which experienced Moderate to high burn during the 2009 Station 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 FFA 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.