Does Central Valley irrigation boost the Southwest's rainfall? Irrigation in California's Central Valley pours so much water vapor into the atmosphere that it significantly drives up summer rainfall and runoff in the Southwest, according to a new study. Using a global climate model and estimates of agricultural water use in the Central Valley, UC Irvine scientists concluded that increased evapotranspiration and water vapor export from the valley had a significant effect on the interior Southwest's weather patterns. Average rainfall during the region's summer monsoon season is 15% greater than it would be without the influence of Central Valley irrigation, and the extra precipitation boosts Colorado River flows by 28%, according to the researchers' computer modeling. "If we stop irrigating in the valley, we'll see a decrease in stream flow in the Colorado River basin," said climate hydrologist Jay Famiglietti, senior author of the paper, which appears this week in the journal Geophysical Research Letters. Since Imperial Valley farms and urban Southern California get supplies from the lower Colorado River, the study suggests that some of the water showered on Central Valley fields eventually finds its way to Southern California. Michael Dettinger, a research hydrologist with the U.S. Geological Survey who was not involved in the research, said the study's approach seemed reasonable. But he questioned the results. "I'm surprised to see numbers that high," said Dettinger, referring to the estimates of increased rainfall and runoff. I'm not going to be convinced by a single model. He noted that the Central Valley is only a tiny slice of the huge area swept by winds that blow into the Southwest. "It's really hard to believe that the evapotranspiration and the like" from such a relatively narrow band of land would have such a strong influence, he said. He added that a significant increase in Colorado River runoff has not been evident since the onset of large-scale irrigation in the Central Valley. Dettinger nonetheless called the research interesting and said he would like to see if other models produced similar results.