The flow of one of Southern Californias key sources of water the Colorado River has steadily declined by about 20 percent over the past century, and could continue to do so as the climate changes.

Thats according to a new study from the U.S. Geological Survey, recently published in the journal Science.

It's a worrying sign, since we get about 25 percent of our water from the river.

WHERE'S IT GOING?

The decline can be primarily linked to rising temperatures in the Colorado River Basin, which have increased by roughly 2.5 degrees Fahrenheit since 1913.

The amount of precipitation thats fallen has remained pretty much the same, but much more of it has come down as rain rather than snow during that time. That's a problem for a number of reasons.

For one, snow is a crucial source of long-term water storage, slowly dolling out runoff, and keeping rivers flowing as the weather warms throughout the year.

Second, since snow reflects energy from the sun away from the Earth. When its not there, the Earth heats up faster, the snow melts quicker, and runoff coming down the river evaporates and is taken up by plants at a higher rate.

That means less water for us.

HOW BAD COULD IT GET?

The studys authors estimate a further decrease in runoff of between 1431% by 2050 as the world continues to warm.

In the case where theres more greenhouse gases in the atmosphere, you have a larger decline of the flow than in the case where you have a smaller concentration of gases in the atmosphere, said Chris Milly, senior research scientist with the USGS.

A big caveat: its notoriously difficult to predict precipitation patterns. While the amount of water falling from the sky has stayed relatively consistent over the past 100 years, theres no guarantee itll continue to do so at the same rate as the climate changes.

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