With California’s reservoir levels dropping, just about everyone is wishing the state had gotten more water this year. That doesn’t just depend on the weather, according to a team of scientists. Sierra Nevada forests play a big role in the state’s water supply.

Just like crops, trees consume water. And Sierra Nevada forests are denser than they once were after decades of fire suppression. That could be reducing the amount of runoff coming from the snowpack — runoff that provides water for most of the state.

“We call the Sierra Nevada our water towers for California,” says Roger Bales, a hydrologist with UC Merced. “About 60 percent of our consumable water comes from the Sierra Nevada.”

Bales is working in a pine forest about 20 miles west of Lake Tahoe, to understand the balance between and trees and runoff. His team has installed hundreds of sensors in the American River basin to record snow depth and soil moisture.

“The snowmelt really enters the soil,” he says, “and flows downslope to the nearest stream channel.”

From there, it joins major rivers and goes into reservoirs and canals that reach all the way to cities and farms in the Central Valley, Bay Area and Southern California. “That water travels up the tree trunk and then goes out through the leaves to the atmosphere,” Bales says.

And there are a lot more trees using water today than there once were. Frequent, low-intensity fires once cleared out small trees and maintained spaces in the forest. Decades of suppressing fires has allowed the forest to fill in.
“You go back about 100-to-150 years and the forest data show us there were maybe only half as many trees here,” Bales says.

The snowpack is also less stable in a dense forest. The snow gets stuck in the trees’ branches before reaching the ground and evaporates faster because it’s more susceptible to sun and wind.

Because these changes have happened over millions of acres of forest, Bales says it’s led researchers to a basic question: “If there were half as many trees, would there be more runoff?” he asks.

The research points to yes, he says — potentially a lot more.

http://blogs.kqed.org/science/audio/why-more-trees-in-the-sierra-mean-less-water-for-california/