Scientists have more bad news for drought-stricken California: The climate warming expected in this century is likely to result in even less water flow from the mountains, as trees and plants growing higher on the slopes soak up more of the available precipitation.

This finding should be "of great interest to water managers in California," says Roger C. Bales, a professor of hydrology and environmental engineering at the University of California, Merced, who co-authored the study published Monday in the Proceedings of the National Academy of Sciences. The study foresees "a potential widespread reduction in water supply with warming, with important implications for California's economy and environment."

Bales and Michael L. Goulden of the University of California, Irvine, used direct measurements and remote sensing to examine the impact of vegetation on runoff in the upper King's River basin in California's Sierra Nevada Mountains. They concluded that the warming that is widely projected for the region by 2100—more than 7 degrees Fahrenheit, or 4 degrees Celsius—could decrease the amount of water flowing down rivers by 26 percent.

Most of that water loss would instead be released into the air through the process known as evapotranspiration, by the leaves of the additional plant cover that is spurred by warming temperatures.

Goulden and Bales found that, currently, cold temperatures are the main factor limiting plant growth above 7,875 feet (2,400 meters) in the King's River area. But warming temperatures will allow trees and other plants to move farther upslope, into an area that is currently "disproportionally important for runoff generation."

The Snowpack Is Key

Californians get 75 to 80 percent of their water from the snowpack in the Sierra Nevada. The water serves about 20 million people directly, and it's used to grow about half the fruits, nuts, and vegetables produced in the United States. It's also used to generate hydropower, drive industry, support recreation, and nourish wildlife—all of which are now suffering from the worst drought in the state's recorded history. (See "California Snowpack Measure Shows No End in Sight for Drought.")

"Regardless of what happens with climate change, even in a good year we don't have enough water," says Frank Gehrke, who serves as the chief of snow surveys for California's Department of Water Resources in Sacramento. (Watch video of Gehrke taking a snow survey.)

The Sierra Nevada snowpack serves as a natural reservoir that stores precipitation in winter, when the state gets the most, and surrenders it in summer, when the state needs it the most. Climate warming is expected to diminish that natural reservoir by causing more precipitation to fall as rain rather than snow—but in theory, at least, the state could build more artificial reservoirs to catch the runoff.

The study by Bales and Goulden suggests, however, that a lot of the rain will be snatched first by trees and returned to the atmosphere before it runs off. Although the researchers focused on one part of the Sierra Nevada, "we think the analysis applies basin-wide," says Bales.

Nor is there any reason it shouldn't apply to other mountain ranges in a warming world. Globally, about four billion people rely on water that runs down from mountains, Bales says.

If water managers such as Gehrke are going to take the next step toward an improved understanding of how forest growth affects the availability of water, "they are probably going to need to get more on-the-ground measurements," Bales adds.

Better Forest Management?

The paper's results should be "another call to improve forest management to protect the water system," says Bales. (See "California Drought Launches New Gold Rush.")

For example, he suggests that managers may need to rethink how fire suppression policies impact runoff. "Forests have maybe four times as many trees and two times the biomass as they did a century ago because of fire suppression, and we need to get a better understanding of how that impacts the water balance," says Bales.

By better understanding how vegetation affects runoff, forest managers can plan more targeted thinning campaigns that maximize the amount of water available to downstream users, Bales suggests.

"Managers need to understand what's going to happen to the water runoff from the mountains," says Bales. "This paper is another piece to help inform that and will hopefully get more people's attention."