Agricultural irrigation in the Central Valley of California doubles the amount of water vapour in the atmosphere, leading to increased rainfall throughout the southwestern United States.
James Famiglietti and Min-Hui Lo of the University of California, Irvine, analysed the impact of irrigation on climate using a global climate model. They found that irrigation adds so much water to the region's hydrological cycle through evaporation and transpiration (whereby plants release water into the atmosphere) that summer precipitation is increased by 15%. As a result, summer run-off is raised by 56% across the region and by 28% in the Colorado River Basin, which is a source of fresh water for southern California.
A better understanding of the impact of irrigation on regional climate and water availability can improve resource management, the authors say.
Geophys. Res. Lett. http://dx.doi.org/10.1002/grl.50108 (2013)