The effects of fires on runoff and infiltration in the Napa River watershed

During October 2017, the North Bay Wildfires wreaked havoc and tragedy across northern California. The North Bay Wildfires were a series of wildfires that burned over 100,000 acres and killed over 20 civilians within the Mendocino, Napa, Lake, Solano, and Sonoma counties of Northern California. The North Bay Wildfires had a disastrous impact on the natural resources within each county they burned. For engineers, environmentalists, and water managers, determining the impact that a wildfire has on a watershed within a region is crucial as it can cause a greater risk of erosion, mudslides, and flooding. Wildfires have the potential to impact the evaporation, transpiration, infiltration, runoff, water quality, and water supply of a watershed. This study focused on the impact that the North Bay Wildfires had on infiltration and runoff within the Napa River Watershed of Napa County, California. A hydrologic model was prepared to simulate infiltration and runoff within the watershed and their impact from the North Bay Wildfires using United States Army Corps of Engineering (USACE) Hydrologic Engineering Center Hydrologic Modeling System (HEC-HMS). HEC-HMS simulated three scenarios: no fires, post-fire with low severity and post-fire high severity for a 1-year storm event in March 2018. Simulating the no fire condition help demonstrate the impact of the low and high severity post-fires. The results indicate that the North Bay Wildfires increased runoff by 3.4% and could have potentially increased runoff by 12.8% if fires had been high severity. The results also indicate that the North Bay Wildfires decreased infiltration by 2-8% and could have potentially decreased infiltration by 10-35% if fires were high severity. By considering and understanding the impacts that fires have within a watershed, engineers, environmentalists, and water managers can work together to manage and improve the water infrastructure to control the release of water, protect the water supply (i.e. stored in reservoirs and groundwater) and natural resources.