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California farms water Southwest: central valley irrigation makes Colorado River basin wetter.

Farmers in California help make it rain in the American Southwest, a new computer simulation suggests. Water that evaporates from irrigated fields in California's Central Valley travels to the Four Corners region, where it boosts summer rain and increases runoff to the Colorado River, researchers report online January 29 in Geophysical Research Letters.

This climate link may be crucial to the 40 million people who depend on the Colorado River basin for drinking water. Since the Central Valley's supply of irrigation water faces an uncertain future, it's important to examine how shortfalls in California might affect regional climate, says study coauthor Jay Famiglietti, a hydrologist at the University of California, Irvine.

A study done in 2011 showed that watering the Central Valley's crops cools local temperatures and increases humidity. But the work found no long-distance climate effects because it relied on a regional climate simulation, which had trouble estimating conditions at the boundaries of the study area, Famiglietti says.

To overcome this problem, he and Min-Hui Lo, now at the National Taiwan University in Taipei, simulated global climate over 90 years. They added 350 millimeters of water--coming from groundwater and surface reservoirs--to the Central Valley between May and October each year. The simulations revealed that irrigation doubles evaporation in the Central Valley. That water vapor circulates to the Southwest during the summer monsoon season, which naturally brings rain to the area.

The evaporated water also brings more energy to the atmosphere. And it changes the regional circulation, drawing in even more water vapor from the Gulf of Mexico.

Together, these changes result in a 15 percent increase in summer rainfall in Utah, Colorado, New Mexico and Arizona and a 28 percent increase in runoff to the Colorado River basin compared with simulations lacking irrigation.

"It's a nice first step," says hydrologist Michael Puma of Columbia University. "And it's a link that we need to investigate quite a bit more."


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Title Annotation:Environment
Author:Wayman, Erin
Publication:Science News
Geographic Code:1U9CA
Date:Feb 23, 2013
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