News Release

Burned Forests Impact Water Supplies

April 16, 2018

Asheville, NC — Healthy forests are important for clean and abundant water supplies. A recent study by USDA Forest Service (FS) scientists looks at how wildland fires including mega fires and prescribed burns affect annual river flow.

“Burned forests impact water supplies” was published in Nature Communications and is the first nation-wide study to look at fires impacts on surface freshwater resources. It was authored by the FS’s Southern Research Station (SRS) scientists Dennis W. Hallema, Ge Sun, Peter Caldwell, Steven Norman, Erika Cohen, Yongqiang Liu and Steven McNulty, and Oregon State University professor Kevin Bladon. This comprehensive study examined three decades of data regarding fires along with climate and river flow from 168 river basins in the lower 48 states.

“The impacts of wildland fires on water resources are extremely variable across the U.S.,” said Ge Sun, Hydrologist, SRS. “Our study is to assist with mitigation strategies that can be designed locally to suit local climate, watershed characteristics, and wildland fire conditions.

Recent wildland fire seasons are now longer due to recurring drought, more ignition sources and more fuels. “Our findings show that climatic variability and fire characteristics both affect river flow, and therefore regional water management strategies need to be flexible and adaptable,” said Dennis Hallema, Hydrologist, SRS. “The challenge for the near future is to determine where the increased river flow can be treated economically as a source of water, and used to reduce the impact of droughts. “Forests are key in this discussion, because they provide 50% of the water consumed in the lower 48 states,” he said.

“Large wildfires increase river flow across the U.S., and this effect can last for years after the fire, said Hallema. “River flow increased the most in drier parts of the Lower Colorado Basin, in the Pacific Northwest and in California. The large scale of this study enabled us to determine that the annual river flow changed, and in most cases increased, when a fifth of the basin or more was burned by wildland fire.”

Increases in water flow can be good and bad. “The good news is that an increase in flow can reduce water supply stress in some areas that are experiencing long term drought,” said Ge Sun, Hydrologist SRS. “The bad news is that burned forests can cause water quality problems from soil erosion and sediment during flooding immediately, or long after the fires have occurred. This is especially problematic in watersheds designed for drinking water supply down streams.”

Not all fires affected annual river flow. “In the subtropical Southeast, there was no appreciable change in river flow after prescribed burns,” said Hallema “This is because prescribed burns cover a smaller area. While severe wildfires can reduce the ability of the soil to absorb water when it rains because of the intense heat, prescribed burns are not nearly as hot by design,” he said. “We did not find evidence that prescribed burns affect flow in larger basins.”

This research was funded by the Joint Fire Science Program.