Effects of Large Wildfire Burned Scars on Downstream Flood Discharge in California Watersheds; A Hydrological Modeling Study

Abstract Submission: Extreme precipitation and wildfires are becoming increasingly prevalent due to global climate change. Fire-induced land cover changes are predicted to increase the risk of floods. Understanding how wildfires may affect severe runoff scenarios in the context of climate change is critical. 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). Pre- and post-fire discharge scenarios were simulated, with an error term calculated to account for model uncertainty. Post-fire peak discharge increments were compared with basin-scale fire characteristics, including burn severity, area burned, land use, and wildfire location. The Curve Number (CN) was adjusted to quantify wildfire impacts on peak discharge, and the required CN increase was compared across watersheds and fire characteristics. Probable Maximum Precipitation (PMP) scenarios were also explored to assess post-fire peak discharge increments. Results show high pre-fire SWAT calibration performance (R² and NSE > 0.65) and consistently positive post-fire annual maximum daily discharge offsets. Positive correlations were observed between discharge variations and burned area percentages. CN increments positively correlated with watershed area burned, with the most significant increases in mixed forests and downstream fire locations. This study reveals that Probable Maximum Flood (PMF) under fire influence can be up to 108.4% larger than without fire impact. These findings underscore the importance of considering wildfire impacts in flood risk management and water resource planning, providing valuable insights for developing predictive relationships and management strategies.

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