How do Fires Affect Stream Flow in the Lower 48 United States?
Many parts of the United States are experiencing longer wildfire seasons, increased burned area, and higher fire severity associated with forest densification and exposure to drought during a large part of the year. The effects of fire on water supplies are often invisible until the next storm rainfall and vary depending on the geographic region, conditions before the fire, fire severity, and post-fire rainfall distribution. Although forests cover only 24% of the contiguous United States (CONUS), they provide 50% of the nation’s freshwater resources, so researchers must determine the degree that fires impact water supply.
Studies in various parts of the country have indicated that fires enhance stream flow; however, the general pattern for the CONUS is yet unknown. Scientists with the Eastern Threat Center analyzed more than 30 CONUS watersheds having large-scale and long-term (i.e., 30-year) data on climate, burn severity, stream flow, and evapotranspiration. Based on known rainfall-runoff patterns and runoff predicted by the Eastern Threat Center’s Water Supply Stress Index (WaSSI) hydrologic model, the researchers found that post-fire streamflow was higher than expected in 15 watersheds, lower than expected in nine watersheds, and as expected in 12 watersheds. Results to date were presented at the 4th International Conference on Forests and Water in a Changing Environment in Kelowna, British Columbia, and discussed during a live interview on CBC Radio One. Researchers will continue to analyze the over- and under-estimated post-fire stream flow rates relative to expectations to yield new insights into water resource vulnerability to fire impacts.
Right: Water yield after a fire, like this prescribed fire in Duke County, NC, varies depending on fire severity, slope, soil texture, and thickness of the ash layer. Photo by Dennis Hallema, U.S. Forest Service.
Forest Service Partners/Collaborators: Southern Research Station Coweeta Hydrologic Laboratory and Center for Forest Disturbance Science
External Partners/Collaborators: Joint Fire Science Program
Contact: Ge Sun
