Cooling Effect of Irrigation About to Evaporate

LIVERMORE, California, August 14, 2007 (ENS) - Expansion of irrigation has in the past masked global warming in California’s Central Valley, but irrigation will not make much of a difference in the future, new research reveals.

"Throughout the major irrigated regions of the world, the cooling influence of irrigation on daytime maximum temperatures will be much smaller in the next 50 years than in the past century, and will likely not continue to curb the effects of greenhouse warming any more," said Celine Bonfils, lead author of the study from Lawrence Livermore National Laboratory and University of California-Merced.

The research team, which includes Bonfils and David Lobell at Livermore Lab, first studied the net impact of widespread irrigation on local and regional climate in California, the top irrigating state in the United States, with 3.3 million hectares under irrigation.

Based on observations of temperature and irrigation trends throughout the state, the authors demonstrated a clear irrigation induced cooling in agricultural areas, and showed that this effect has recently slowed down, according to their study published in the current edition of the "Proceedings of the National Academy of Sciences."

Irrigation cools the land. (Photo by Jack Dykinga courtesy USDA)
In highly irrigated regions of California's San Joaquin Valley, daytime temperatures relative to low irrigated areas have cooled by 1.8 degrees to 3.2 degrees Celsius since irrigation began in 1887, the study shows.

"In comparison, there was no clear effect of irrigation on temperatures over the 1980-2000 period when there was no net growth of irrigation," Lobell said.

"This is not a model result, but something very clearly evident in the data," said Bonfils. "We also looked at other major irrigated regions in the world, and saw a very similar pattern."

"Globally we derive 40 percent of our food from irrigated regions, so we’d like to be able to model future climate changes in these regions," she said.

Irrigation cools the surface of the Earth by increasing the amount of energy used to evaporate water rather than heat the land. The more irrigated the land, the more intense the effect.

"It was quite surprising how well we could distinguish a cooling trend that incrementally increases with the amount of irrigation," Bonfils said.

Other major irrigation regions used in the study include the Aral Sea Basin, Eastern China, Thailand, the Indo-Gangetic Plains of India and Pakistan, and Nebraska, the second most irrigated state in the United States.

In areas where irrigation development has been rapid, including Thailand, the Aral Sea Basin and Nebraska, the research team found the same cooling effect in summer daytime maximum temperatures.

Irrigation of a Nebraska cornfield (Photo courtesy Nebraska Central Telephone Company)
In India, Pakistan and Eastern China, the temperature change due to irrigation is less clear because of the presence of aerosols in the atmosphere that also contribute to the observed cooling by reflecting or absorbing sunlight, they said.

This study also shows that rapid summer nighttime warming observed in Central California since 1915 cannot be explained by irrigation expansion, as other research has claimed.

In 2006, John Christy, director of the Earth System Science Center at The University of Alabama in Huntsville, published a study showing that irrigation of California's Central Valley, which turned it from desert to productive farmland, could be to blame for warmer summer nights that have been recorded in recent years.

"What was once dry, light-colored soil that didn't absorb much solar warmth is now dark and damp and "can absorb heat like a sponge in the day and then, at night, release that heat into the atmosphere," Christy said.

"Our results show that the expansion of irrigation has almost no effect on minimum temperatures and that irrigation cannot be blamed for this rapid warming," Bonfils said.

Lobell said, "An increase in greenhouse gases and urbanization would best explain this trend, which exceeds what is possible from natural climate variability alone."

In California, irrigation expansion is likely to end because of urbanization and an increase in the demand for water to serve the needs of urban populations.

In the United States, irrigation has for the first time decreased by two percent from 1998-2003 and growth in irrigation has already slowed down in many other parts of the world.

A study published in February by the American Geophysical Union's journal "Geophysical Letters" also showed the masking effect that irrigation has on global warming.

Researchers Lisa Sloan, Mark Snyder, and Lara Kueppers from the Climate Change and Impacts Laboratory at the University of California-Santa Cruz, wrote, "Given our results for California and the global importance of irrigated agriculture, past expansion of irrigated land has likely affected observations of surface temperature, potentially masking the full warming signal caused by greenhouse gas increases."

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