Water diverted to central California's farmlands boosts rainfall in nearby states and may even exacerbate periodic flooding in some regions, a new study suggests. The phenomenon may also be happening elsewhere in the world.
California's Central Valley—an area almost twice the size of Massachusetts where farmers raise more than 200 different crops, including apricots, asparagus, cotton, and grapes—is one of the largest irrigated regions in the world. Every year, several cubic kilometers of water are supplied to the Central Valley's fields, about 60% of it from river flow diverted into the region and the rest from wells. A significant amount of that liquid evaporates from fields rather than nourishing crops, says James Famiglietti, a hydrologist at the University of California (UC), Irvine. That boosts humidity in the valley, according to previous research, but scientists haven't evaluated its effects farther afield.
So Famiglietti and university colleague Min-Hui Lo employed a global climate model. In one set of the team's simulations, no irrigation occurred. In another set, the researchers added a volume of water equivalent to 350 millimeters of precipitation falling on each square kilometer of the valley's fields between May and October, the time of year when soil moisture typically takes a dive if irrigation isn't provided.
The extra moisture boosted rainfall as far away as western Nebraska and the panhandle of Oklahoma , the team reports in Geophysical Research Letters. Most notably, parts of southern Wyoming and the Four Corners states—Utah, Colorado, Arizona, and New Mexico—received between 4 and 14 millimeters more precipitation each June, July, and August. Overall, that boosts summer rainfall in those areas by 15% above average, which in turn increases runoff into the Colorado River by 28%.
But not all of the enhanced rainfall comes from California moisture, the team notes. As water vapor in the air condenses, it releases prodigious amounts of heat. When that hot air rises, it creates low pressure at ground level in the region surrounding the storms and draws in moist air from surrounding regions, including the Gulf of California and the Gulf of Mexico. "The added moisture really fires up the storm cycle" in the Southwest, Famiglietti says.
The new research "offers one compelling answer to the question of what happens to all of the water evaporated from California's Central Valley farmland," says Lara Kueppers, an ecosystem scientist at UC Merced. The Central Valley is just one of many regions globally that are actively and unsustainably diverting surface water and ground water onto agricultural fields, she notes. "To accurately capture the influence of these regions' on the atmosphere, these massive diversions need to be accounted for."
Yet, climate models typically ignore the effects of moisture from irrigation, Famiglietti says. India, China, and the Great Plains area of the United States are just a few regions where irrigation might significantly humidify the air, and regions downwind are likely receiving increased rainfall as a result, he notes.
Such irrigation doesn't just increase rainfall, adds David Changnon, a climatologist at Northern Illinois University in DeKalb. The enhanced precipitation in Colorado, for example, may be boosting the strength and frequency of local flooding events that commonly occur there in late July and early August.
Also, Changnon says, the findings may provide a glimpse of a future in which the American Southwest becomes increasingly parched. If California ever dials back irrigation because of reduced availability of ground water or reduced flow in rivers now diverted to the Central Valley, precipitation throughout the American Southwest could take a dive. A study of rainfall patterns in the Southwest before the 1940s, when irrigation in the Central Valley became widespread, might provide scientists with a better climatic crystal ball.