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A Hotter Climate Is Shrinking The Water Supply In The Western U.S.

AILSA CHANG, HOST:

Drought is intensifying across the West. Almost half the country's population is facing dry conditions, but it's more than just one bad year. A hotter climate is also shrinking water supplies, and that is posing a fundamental threat to water systems that millions of people rely on. NPR's Lauren Sommer has more.

LAUREN SOMMER, BYLINE: A few weeks ago, Arizona water officials held a public meeting that everyone was hoping wouldn't happen.

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TED COOKE: The reason we're meeting today is that Lake Mead is 38% full.

SOMMER: Lake Mead, the largest reservoir on the Colorado River - and in the country, for that matter - has been plummeting. This summer, it's expected to drop to its lowest point since it was first filled. So Ted Cooke, who manages the Central Arizona Project, which supplies Colorado River water in the state, was trying to prepare everyone.

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COOKE: So this is a painful reduction.

SOMMER: Their water supply would be cut by about 30%, which would be partially made up for with groundwater and conservation. But the announcement wasn't a surprise.

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COOKE: This is a day we knew would come at some point, meaning...

SOMMER: The writing has been on the wall because of decisions that go back a century.

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UNIDENTIFIED PERSON: Stained with the soil of five states, the river cuts deep into the desert.

SOMMER: There was no way for cities from Denver to Los Angeles to grow without a reliable water supply. The Colorado River was it.

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UNIDENTIFIED PERSON: Farmers and ranchers in Colorado, Wyoming, Utah and New Mexico wait for irrigation projects.

SOMMER: So in the 1920s, the states, in intense negotiations, divided up the river's water.

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UNIDENTIFIED PERSON: The Colorado River felt the reins of man's control.

SOMMER: But there was a problem. To figure out how much water there was, state leaders looked at river flows from the previous 20 years. And it was unusually rainy, says Eric Kuhn, who managed Colorado's water policy for several decades.

ERIC KUHN: They ended up appropriating more water than the river could actually produce.

SOMMER: Now climate change is shrinking that water supply even more. When it's hotter, more water evaporates from plants and soils, which means less makes it into reservoirs. And drier soils are like a sponge. They soak up rainfall, which also reduces how much runs off into rivers. Hotter temperatures have caused half of the reduction in flow in the Colorado River over the last 20 years, according to one study. And it adds up to even bigger shortages going forward.

KUHN: So we can really no longer look at the past and say the amount of water we've had in the last hundred years is what we can expect in the future. That is no longer true because of climate change.

SOMMER: Climate change is also hitting another key water source in California - its mountain snowpack.

ANDREW SCHWARZ: We're getting a lot less stream flow coming off the same amount of snowpack.

SOMMER: Andrew Schwarz is climate action coordinator with California's Department of Water Resources.

SCHWARZ: We're not seeing as much water showing up in our rivers as we would have expected with the same amount of rain or snow that we had gotten historically.

SOMMER: Schwarz says even if it rains the same amount in California, 2 degrees Celsius of warming will reduce the water supply for 25 million people by about 15%. California also grows most of the country's fruits and nuts. But the way that water is divided up through a system of water rights isn't very flexible, says Alvar Escriva-Bou, a research fellow at the Public Policy Institute of California.

ALVAR ESCRIVA-BOU: We have to start thinking in a new climate, new water availability, which actually makes our water rights difficult to make.

SOMMER: Those are tough conversations. Any attempt to change water rights spawns lawsuits. On the Colorado River, states have negotiated a temporary agreement to make cutbacks, but that only lasts till 2026. In California, some regions are still grappling with the idea that from here on out, drought isn't a temporary thing.

ESCRIVA-BOU: What we are seeing is that we have been using more water than we have, so the reality here is that we have to make a reduction of water use over the long term.

SOMMER: The good news, Escriva-Bou says, is that investing in water conservation makes a difference, whether it's irrigation on farms or replacing lawns at people's houses. Make those changes now, and they can save water for decades to come.

Lauren Sommer, NPR News. Transcript provided by NPR, Copyright NPR.