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Science & Environment

How Too Many Trees Contribute To California's Drought

STEVE INSKEEP, HOST:

Here's a breathtaking fact about California's drought - human beings are now competing for water against trees, and the trees are winning. The trees are in the Sierra Nevada, the mountains above San Francisco. And we have a report from Lauren Sommer of member station KQED.

LAUREN SOMMER, BYLINE: OK, at face value, it's a pretty strange idea. Why would turning on a shower in Los Angeles be related to pine forests more than 300 miles away?

ROGER BALES: We call the Sierra Nevada our water towers for California.

SOMMER: Roger Bales is a hydrologist with the University of California Merced, and we're in a forest about 20 miles west of Lake Tahoe. There's usually 10 feet of snow here in the winter, snow that becomes California's drinking water.

BALES: About 60 percent of our consumable water comes from the Sierra Nevada. The snow melt really enters the soil and flows down slope to the nearest stream channel.

SOMMER: It joins rivers and goes into reservoirs, canals, reaching cities and farms in the bay area all the way to Southern California. Bales is working to measure that runoff with hundreds of sensors here in the forest.

BALES: Let's take this mast down and...

SOMMER: His team is checking one of them on a 15-foot pole. It records snow depth and soil moisture.

BALES: OK, good, got it.

SOMMER: Bales says we aren't the only ones using this water. Pine trees consume huge amounts of water, which means it doesn't run off and end up in reservoirs.

BALES: That water travels up the tree trunk and then goes out through the leaves to the atmosphere.

SOMMER: The more trees, the more water they use. And there are a lot more trees in the Sierra Nevada than there used to be. Decades of Smokey Bear and putting out fires has allowed forests to grow denser - twice as dense in some places. Bales says that leads to a basic question.

BALES: If there were half as many trees, would there be more runoff?

SOMMER: The answer is yes, he says, potentially a lot more.

BALES: Is it 20 percent, 30 percent or 40 percent? We're sort of in that range, but that's a hypothesis.

SOMMER: That amount of water could make a big difference in a record drought like this one.

SCOTT STEPHENS: I think the water piece is really huge. I think it's underappreciated, but it's massive.

SOMMER: Scott Stephens is a professor of fire science at the University of California Berkeley. He says one way to help forests look like they once did - with fewer trees - is to let more naturally caused fires burn. But there are homes and communities to consider.

STEPHENS: Letting fire work in those lands is risky. Sometimes it's going to go as expected, and once in a while, it goes wrong.

SOMMER: Meaning the fire gets out of control. Timber companies could cut small trees, thinning the forest, but that's expensive in remote areas and often faces environmental opposition. Stephens says there's no easy answer here. Both options have downsides, but both will be needed. And climate change isn't helping, he says.

STEPHENS: If we don't act today, our grandkids' grandkids are going to have so few options.

SOMMER: A recent study from the University of California 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.

STEPHENS: 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.

SOMMER: Stephens says the good news is that California water districts are joining the conversation about how to manage forests. The connection between the drought, the state's water supply and trees is becoming hard to ignore. For NPR News, I'm Lauren Sommer. Transcript provided by NPR, Copyright NPR.