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Researchers look at increasing runoff by thinning forest

When it comes to water, Central Valley farmers and residents could benefit if we axed a few trees, according to a new report by researchers at UC Merced. Now scientists are trying to determine whether that's a good idea.

By "selectively" thinning the Sierra Nevada forest, researchers and a group of environmentalists believe they may be able to simultaneously control wildfires and boost snowpack runoff to local irrigation districts.

A long-term thinning project to increase water runoff is untested. However, cutting down trees in densely forested areas is already a common practice to improve the overall health of the ecosystem and prevent property damage.

"There are certain thinning patterns that have been prescribed to stop wildfire," said Roger Bales, co-author of the study and director of the Sierra Nevada Research Institute at UC Merced. "We also want clearings and spacing for water-based projects. So it's just reconciling those."

What scientists are proposing is employing new thinning patterns that would decrease the size of a forest's canopy, allowing snow that might otherwise be caught in branches to fall to the forest floor. However, if the gaps are too large, the snow will melt too quickly and late-season runoffs will be less than desired.

"You would want to create gaps of certain sizes, but not too big so that winter and early spring sun does not melt the snow," Bales said. "A lot of the water is used in evapotranspiration. If it snows in the canopy it's going to evaporate very fast. You want to get the snow on the ground."

Scientists plan to try out their hypothesis at certain test sites as part of what they are they are calling the Sierra Nevada Watershed Ecosystem Enhancement Project. Researchers plan to measure the impacts of thinning on water runoff and the overall health of the forest ecosystem at these sites before recommending the practice statewide.

While the project could mean more water for residential and industry water users, Bales was cautious not to predict large increases in available resources.

"It could increase or offset possible decreases due to climate warming," he said. "I would be more worried about offsetting losses than trying to get more water."

While cutting down trees might seem counterproductive when it comes to fighting global warming, Bales said carbon is sequestered in the soil as well as in trees. However, he said, more research must be conducted before the overall impact of thinning on climate change can be determined.

In past centuries, the Sierra Nevada forest has been characterized by relatively low forest density, according to the report. Because many parts of the forest are denser than historical levels, the report said thinning can likely be done without hurting the natural ecosystem.

The second phase of the project will begin early next year with testing in the Sierra Nevada's American River basin.

Reporter Joshua Emerson Smith can be reached at (209) 385-2486 or