‘Atmospheric rivers’ aid the West — and imperil it

High Country News –

When a rainstorm slammed California’s Russian River watershed in December 2012, water rushed into Lake Mendocino, a reservoir north of San Francisco. The cause? An atmospheric river, a ribbon of moisture-laden air that can ferry water thousands of miles across the sky. When the tempest hit, the state was on the brink of an exceptional drought. But instead of storing the surge the storm brought for the dry days to come, the reservoir’s owner, the U.S. Army Corps of Engineers, let it run downstream.

They had to. Army Corps rules say Lake Mendocino must be partly drained during the winter, leaving room for the next deluge to prevent downstream flooding. But in 2013, that space ended up being superfluous, mostly because drought conditions kicked into high gear and the rainy season essentially ended after the December downpour. “We lost that water,” says Shirlee Zane, who serves on the board of the Sonoma County Water Agency, which delivers water from Lake Mendocino to nearby homes. The agency estimates that more than $2 million slipped down the river. It was a loss that might have been prevented if the Army Corps had known they could safely store the water — and one managers would hate to repeat. “We lose millions of dollars of water if we don’t have better forecasting,” Zane says.

Now, scientists are getting closer to solving that problem. Atmospheric rivers can cause dangerous deluges — an atmospheric river contributed to the mudslides that recently killed more than a dozen people in southern California — but they also provide up to half the annual precipitation on the West Coast. To make the most of those benefits while reducing risks, researchers are improving forecasts using computer models, weather balloons and instruments dropped from airplanes. The knowledge they’re gaining could help reservoir managers stockpile more water for dry periods without sacrificing the safety of downstream communities. And as climate change intensifies both floods and shortages in the coming decades, meeting that balance will become even more critical.

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