Downstream Effects Watering fields boosts rainfall

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 12 inGeophysical Research Letters.
This climate link may be crucial to the 40 million people who depend on the Colorado River for drinking water. That number could nearly double in the next 50 years at the same time that droughts are projected to become more common in the Southwest. Since the Central Valley's supply of irrigation water faces an uncertain future, it's important to examine how shortfalls in California might affect climate change in the region, says study coauthor Jay Famiglietti, a hydrologist at the University of California, Irvine.
“We have to understand these connections better to deal with changes in water availability,” he says.
The Central Valley is one of the world's most productive agricultural regions. More than 50,000 square kilometers of the valley are irrigated, equaling one-sixth of all irrigated land in the United States.
A study in 2011 showed that watering the area's crops cools local temperatures and increases humidity. But the work didn't find any larger climate ties outside the region, because it relied on a regional climate simulwation, which has trouble estimating conditions along the boundaries of a study area, Famiglietti says.
To overcome this problem, Famiglietti and Min-Hui Lo, now at the National Taiwan University in Taipei, simulated global climate over a 90-year period. They added in 350 millimeters of water coming from groundwater and surface reservoirs to the Central Valley between May and October each year. The researchers say that's a realistic amount of irrigation based on published agriculture and climate data.
The simulations revealed that evaporation doubles in the Central Valley when there's irrigation. That water vapor circulates to the Southwest during the summer monsoon season, which naturally brings rain to the area. “The monsoon is like a big campfire burning away over the Southwest,” Famiglietti says. “The irrigation acts as fuel on the fire.” In addition to bringing more water to the atmosphere, the water vapor brings more energy. And it changes the regional circulation, drawing in even more water vapor from the Gulf of Mexico.
Together, these changes intensify the monsoon season, resulting in a 15 percent increase in rainfall in Utah, Colorado, New Mexico and Arizona and a 28 percent increase in runoff to the Colorado River compared with simulations lacking irrigation. Some of the water returns to California via the All-American Canal, which brings water from the Colorado River to Southern California, the simulation suggests.
“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.” Many other variables, such as sea surface temperatures, also influence climate in the Southwest. To better estimate the strength of irrigation's effect in the real world, more complex simulations that take these other factors into account are needed, Puma says.
The study also highlights the importance of investigating irrigation's role in climate in other parts of the world, as well as other ways in which people's use of water might have unintended consequences, Famiglietti says.“What we do with water management really has an impact on climate locally, regionally and globally.”