[A-AS03] Extreme Events and Mesoscale Weather: Observations and Modeling

[AAS03-P06] Effects of wildfire burned scars on downstream peak discharge scenarios in the California watersheds.

*Wasitha Randeepa Ranga Munipurage¹, Yusuke Hiraga¹ (1.Tohoku University, Japan)

Keywords:Curve Number, flood risk assessment, peak discharge, SWAT model, wildfire

Extreme precipitation and wildfires have become increasingly prevalent due to global climate change. Fire-induced land cover changes have been predicted to increase the risk of floods. Thus, understanding how wildfires affect severe runoff scenarios in the context of climate change is critical. California's watersheds face increasing challenges due to the compounding effects of wildfires and extreme precipitation events, which significantly impact downstream discharge scenarios and water management strategies. This study investigates the effects of wildfire-burned scars on downstream peak discharge in 30 California watersheds using the Soil and Water Assessment Tool (SWAT). The SWAT model was used to simulate daily discharge over a 20-year period, demonstrating strong model performance with R² values ranging from 0.67 to 0.86 and Nash-Sutcliffe Efficiency (NSE) values between 0.65 and 0.86. The difference between observed flood discharge volume and simulated unburned scenarios was analyzed during post-fire periods, accounting for model errors. From the analysis, significant post-fire discharge increases were identified, with an average rise of 17.1% in 83.3% of watersheds in the first year following a fire. Statistically significant positive correlations (p < 0.01) were also found between enhanced discharge volume and the percentage of burned watershed area. Wildfire impacts were quantified using Curve Numbers (CN) by modifying CN in the SWAT model, with increments varying from 16.5% to 30% based on burn severity and land use type. A linear relationship between wildfire area burned and CN increment was established, with an estimated slope of 0.39, offering new insights into post-fire flood simulation. The study also revealed that incorporating historical wildfire activity substantially increased the Probable Maximum Flood, with discharge volume rising between 3.74% and 25.9%. These wildfire-induced increases are comparable to California's climate change projections (10-50%), highlighting the importance of considering wildfire effects in flood risk assessments and water management strategies.