We hear a lot of farmer bashing by some segments of the politicized paleo-environmentalist community. One of their standard attacks is about irrigation, claiming that farmers use too much water leaving less for instream flows and downstream cities. We also see bizarre interpretatons of research findings. Some way is found to claim that nearly any finding supports climate change religion. Consider this press release from Livermore which trumpets: "Human activities contribute to California's global warming".

Recent research by scientists at the Lawrence Livermore National Laboratory, the University of California, Merced and the National Center for Atmospheric Research shows that California temperatures have jumped statewide by more than 2.1 degrees Fahrenheit between 1915 and 2000. This warming is likely related to human activities. . .

But all California climate trends during the 20th century aren’t so clear.

For example, less warming is observed in summer. This warming, which mainly occurs at night but not during daytime, is not well explained by historical climate simulations.

“We looked at observations and models and they don’t concur,” said Phillip Duffy, part of the Livermore team and a UC Merced adjunct professor. “One possible reason for this is that most models don’t include factors such as irrigation, which can influence regional climate.”

The team found the lack of a trend in summertime maximum temperatures may be associated with the rapid expansion of large-scale irrigation during the 20th century, an important factor in California that is not accounted for in the models.

“We found empirical evidence that irrigation has a large cooling effect on local summer daytime temperatures but minimal effect on nighttime temperatures,” said Bonfils, who investigated that issue in another Livermore study published in Proceedings of the National Academy of Sciences earlier this year.

Until now, cooling from irrigation may have counteracted the daytime warming from mounting greenhouse gases and urbanization.

This isn't news. The cooling effects of irrigation are well known, and a better title would be something like cool farms or rural heat sinks.

You've heard of urban heat islands. Now researchers have confirmed the existence of their opposite: cool farm patches.

Whereas urban development generates pockets of hot air, irrigated fields tend to cool things down . . .

Bonfils and Lobell compared irrigation and temperature data for California between 1915 and 2000, during which time the area of irrigated land in the Central Valley doubled. They found that maximum daytime temperatures in the area were between 0.9°C and 1.6°C cooler during this period than areas that were only modestly irrigated.

Extrapolating back to when irrigation began in 1887, they calculate that intensively irrigated parts of the Central Valley are 1.8°C to 3.2°C cooler than they would otherwise have been.

That cooling occurs because much of the solar energy that hits irrigated ground during the day goes to evaporate the extra water in the soil and in plants instead of heating the air, explains Lara Kueppers of the University of California at Santa Cruz.

Kueppers had predicted the cool farms effect from climate modelling studies (Geophysical Research Letters, DOI: 10.1029/2006GL028679). The models suggest that the cool air spreads to areas outside the irrigated fields, although the extent of this "spillover" is still unclear.

Bonfils and Lobell say the cool farms effect could explain why minimum and maximum winter temperatures steadily rose in California between 1915 and 2000, whereas maximum summer temperatures did not.

The warmer winter temperatures can only be explained by the greenhouse effect, and the authors speculate that the cool farms effect may have masked the impact of global warming on summer temperatures. Irrigation is mostly carried out during the summer.

Another factor is albedo. The soil is more often exposed in winter, and it absorbs more radiation since it is darker than the plants that cover it during the growing season.

The cool times may not last, however. The amount of irrigation in California has stabilised since 1980, Bonfils and Lobell point out, because expanding urban areas have laid increasing claims on dwindling water supplies. In the US overall, irrigation decreased for the first time - by 2% - between 1998 and 2003.

A rollback of the cooling effect of irrigation in the face of continued global warming could mean that California will be hit by substantial warming, say the researchers. The same is likely to be true of other regions of the world. India, Pakistan and China have become huge irrigators over the past 50 years, but the growth of irrigated areas is slowing down.

This may mean that irrigated regions, which now provide about 40% of global food production, will feel more than their share of warming in the future. In turn, this will inevitably have an impact on food security.

Human civilization has always been hydraulic civilization. Water projects for cities and agriculture are foundational. This isn't something that can slow down or stop without massive consequences. When we recoil from bad systems that drain fossil acquifers or empty lakes we need to maintain balance. Bad systems do not mean that irrigation itself is bad. It has many benefits, not least in these warming times the effects on temperatures.