Midwest Cooling From Crop Irrigation

From 1970 through 2009, average high temperatures at the sites in Iowa and Illinois during July and August were between 0.5 and 1.0 degrees F (0.28 and 0.56 degrees C) cooler than they were for the years 1930 through 1969, the researchers found. The amount of precipitation received in the region has changed substantially as well: Average rainfall for July and August from the 1970s through 2009 was about 0.33 inches (0.8 centimeters) higher each month than it was from the 1930s through the 1960s.

[David] Changnon suggested that fewer hot days and more precipitation are linked, because humid air warms more slowly than dry air does. One likely source of the extra moisture is the region’s agriculture. Plants pump vast amounts of water from surface soil into the atmosphere as they grow, and thirsty row crops such as corn and soybeans are much more prevalent in the region these days — about 97 percent of farmland is planted in those crops now, versus about 57 percent in the 1930s, Changnon notes. Also, the plants are spaced more closely now (about 30 inches apart, versus the 40-inch spacing typical in the 1930s), a trend that has boosted the numbers of water-pumping plants per acre by about 60 percent.

From "Crop irrigation could be cooling Midwest" by Sid Perkins in Science News.