So where did the extra moisture in the air come from？Changnon points to farms in the region. As plants grow, they pull moisture from the ground and release it into the air. And among plants, soybeans (大豆) and corn plants release a lot of moisture. Midwestern farms now plant more soybeans and corn than in the past, with 97 percent of farmland today planted with these two crops. In the 1930s, corn and soybeans covered only about 57 percent, Changnon says. He also notes that the plants are planted closer together now than they used to be, so there are more plants per acre than in the past.

　　The second study, like Changnon's, also found an increase in rainfall in the same area. But it points to another possible source for the increased moisture. Alan Robock at Rutgers University was part of the team that produced the second study and presented the group's findings. The team found that irrigation practices in the Great Plains have changed over the years. The researchers studied a vast area in the region. They found that in 1930, farmers in that region irrigated only about 1.8 million acres of farmland. In 1980, however, farmers irrigated nearly 15 million acres. Plants use the water and then release it into the air.

　　These results by Changnon and Robock and their colleagues are the first step toward understanding a change in the weather.

　　20．What does the underlined term “These changes” refer to?

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