Sierra Nevada forests are denser than they once were, potentially reducing the amount of runoff that reaches California’s reservoirs. (Dan Brekke/KQED)

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.

UC Merced’s Roger Bales and Ziran Zhang work on a snow sensor tower in the Tahoe National Forest. (Lauren Sommer/KQED)

When trees use water through the process of evapotranspiration, it doesn’t run off into rivers and reservoirs.

“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.

“Is it 20 percent, 30 percent or 40 percent?” Bales says. “We’re sort of in that range. But that’s a hypothesis. Our back-of-the-envelope calculations suggest that you could get anywhere from half a million to a million acre-feet additional water out of the Sierra Nevada.”

A million acre-feet of water is enough to supply two million households in California for a year — an amount that could make a big difference during a drought.

Managing Overgrown Forests

“I think the water piece is really huge,” says Scott Stephens, professor of fire science at UC Berkeley. “I think it’s under-appreciated but it’s massive.”

Stephens has found similar results in the Illilouette Creek basin in Yosemite National Park. About 40 fires have been allowed to burn there over several decades, reducing the number of trees per acre.

“It looks like there’s 20 percent more surface water leaving the streams in that area since the fire program began in the mid-1970s,” he says.

The widely spaced trees also make the forest more resistant to high-severity fire.

But leaving naturally caused fires to burn over large areas of the Sierra Nevada is tricky, he says, especially near houses and communities.

“Letting fire work in those lands is risky,” Stephens says. “Sometimes it’s going to go as expected and once in a while it goes wrong.”

Another option is to allow timber companies to cut small trees, thinning the forest. It’s commonly done where roads already exist, but can be prohibitively expensive in remote areas and often faces environmental opposition.

Climate change could make the problem even worse. A recent study from UC Irvine found California’s forests will be using even more water by the end of the century, because warming temperatures will make the growing season longer. Runoff could drop by as much as 26 percent.

“If we don’t act today, our grandkids’ grandkids are going to have so few options,” Stephens says. “It’s going to be warmer. It’s going to be more difficult to do this work and they’re going to be basically chasing their tails.”

Stephens says the good news is that California water districts are joining the conversation about how to manage forests. While it didn’t used to be on their radar, the connection between trees and our drinking water is becoming hard to ignore.