Climate: Study suggests that irrigation in California’s Central Valley pumps up western Monsoon rainfall

This NASA satellite image shows California’s Central Valley filled with fog and gives a sense of the size of the valley.

Climate models illustrate a little-known part of the hydrological cycle in the Colorado River Basin

By Bob Berwyn

SUMMIT COUNTY — Research by scientists at the University of California, Irvine suggests a previously unexplored connection between irrigated agriculture in California and summer monsoon rains in the greater Colorado River Basin.

The new study by climate hydrologist Jay Famiglietti shows that huge amounts of the water used for irrigation in California’s Central Valley ends up back in the atmosphere, helping to fuel and intensify summer thunderstorm activity in the interior West.

The moisture, which reaches the atmosphere via evapotranspiration, may add up to as much as 100 billion gallons in additional runoff, about enough water for 3 million people for a year, Famiglietti said.

“Not all of that comes from irrigation … adding water vapor to the atmosphere is adding energy,” he said, explaining that The moisture steaming off the fields help juice the atmospheric pump, intensifying the monsoon cycle and drawing in moisture from the Gulf of Mexico and the Gulf of California.

“We’re able to track where it goes … this shouldn’t be a big surprise to people who look at rainfall,” he said, explaining how the climate models used for the study gave an accurate representation of monsoon precipitation with — and without — the added input of the moisture from irrigation.

According to Famiglietti’s calculations, the moisture from Central Valley irrigation dumps  15 percent more than average summer rain in numerous other states, mostly the Colorado River Basin states. Runoff to the Colorado River increases by 28 percent, and the Four Corners region experiences a 56 percent boost in runoff – June, July August.

The numbers seem to be quite high and may not be reflected by the historic record of Colorado River flows,” said Colorado state climatologist Nolan Doesken.

“I don’t believe there is evidence in the streamflow or in long-term precipitation data in the Colorado River basin to suggest that significant increases occurred following the development of Central Valley irrigation project,” Doesken said. “There is no doubt that a great deal more summer season evapotranspiratio occurs over that area since the vast irrigation began. The question is where does that water vapor end up?” he said.

Famiglietti said the additional water supply can be a good thing, the transport pattern also accelerates the severity of monsoons and other potentially destructive seasonal weather events.

“If we stop irrigating in the Valley, we’ll see a decrease in stream flow in the Colorado River basin,” he said, describing the findings of the paper published this week in the journal Geophysical Research Letters. The data is important in the context of the recent Colorado River Basin study released by the U.S. Bureau of Reclamation. The study showed that the basin faces significant future shortages. All factors including any potential changes in irrigation practices, must be considered in the Colorado River Basin equation, he said.

Famiglietti, an Earth system science professor in the School of Physical Sciences, and colleague Min-Hui Lo, a postdoctoral researcher at the University of California Center for Hydrologic Modeling who is now at National Taiwan University, painstakingly entered regional irrigation levels into global rainfall and weather models and traced the patterns.

“All percent differences in the paper are the differences between applying irrigation to the Central Valley and not applying it,” Famiglietti said. “That’s the point of the study – and the beauty of using computer models. You can isolate the phenomenon that you wish to explore, in this case, irrigation versus no irrigation.”

Famiglietti’s team plans to increase the scope of the work to track how major human water usage elsewhere in the world affects neighboring areas too. A better understanding of irrigation’s impact on the changing climate and water availability could improve resource management in parched or flooded areas.


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