Surprise! Fire can help some forests keep more of their water

Author: Michelle Donahue / Source: Science News for Students

In forests, abundant trees are good and fire is bad, right?

Actually, the reverse can be true — especially in some regions of California’s Sierra Nevada Mountains. That conclusion comes from a new study.

Trees don’t use all the water they absorb. They release some into the air through tiny pores in their leaves. The process is known as evapotranspiration. It’s a big word for how plants shed excess moisture as water vapor. Scientists can measure this released vapor. In forests, they sometimes put sensors on tall towers to get closer to the leaves.

Jim Roche works for the U.S. National Park Service in California at Yosemite (Yo-SEM-ih-tee) National Park. As a hydrologist, he measures the movement of water (in its many forms) on or near Earth’s surface. He is interested in how fires affect water in forests.

Roche and two of his colleagues worked in two river basins in California. A river basin is all the land that supplies water to a stream or river. The scientists compared how fires decreased greenery — a measure of forest cover — in the basins of the Kings and the American rivers from 1990 to 2008. They used satellite images to monitor greenery there from the skies. Next, they calculated water-vapor release in the past based on modern data. Then they compared the water-vapor release from burned and unburned patches of forest during each of the 18 study years.

Evapotranspiration is measured as a depth, in millimeters. That depth is how deep the released moisture — if it were all liquid water — would have covered an area of land. Fires that reduced green cover by 40 to 50 percent saved the forest some 153 to 218 millimeters (6 to 8.6 inches) of water per hectare (almost 2.5 acres) of land, the team found. In burned areas of the American River basin, that added up to roughly 77 billion liters (17 billion gallons) of water per year not being lost to the air.

Without people to suppress them, wildfires usually occur in parts of this…

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