Sustain Our Nation's Forests and Grasslands

Burned forests impact water supplies

NORTH CAROLINA—Healthy forests are important for clean and abundant water supplies. A recent study by Forest Service 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 nationwide study to look at fires' impacts on surface freshwater resources. This comprehensive study, authored by Southern Research Station hydrologist Dennis W. Hallema and other SRS scientists, examined three decades of data regarding fires along with climate and river flow from 168 river basins in the lower 48 states.

Photo: River lined by lush, green forest.
Choccolocco River in Oxford, Miss. Headwater forests supply more than 50 percent of water consumed in the contiguous U.S. via streams and rivers. Forest Service photo.

“The impacts of wildland fires on water resources are extremely variable across the U.S.,” said Ge Sun, SRS hydrologist. “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 Hallema. “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 percent of the water consumed in the lower 48 states,” he said.

Large wildfires increase river flow across the U.S., and the effect can last for years after the fire. The large scale of this study enabled scientists 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. 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.

This research was funded by the Joint Fire Science Program.