Rain

Here's our first look at the Knights' ballpark:

Removing his cap, Pop rubbed his bald head with his bandaged fingers. "It's been a blasted dry season. No rains at all. The grass is worn scabby in the outfield and the infield is cracking. My heart feels as dry as dirt for the little I have to show for all my years in the game." (2.4)

Of course, there's another kind of drought; the team hasn't won a game in a long, long time. It's been a dry spell for the team as well as the weather. When Roy gets his first home run with Wonderboy, the skies suddenly open:

Somebody then shouted it was raining cats and dogs. The stands emptied like a yawn and the players piled into the dugouts. By the time Roy got in from second he was wading in water ankle deep. […] it rained steadily for three days. (3.98)

That's the end of both droughts. The Knights start winning and the rain turns the ballpark into the field of dreams it should be:

The long rain had turned the grass green and Roy romped in it like a happy calf in its pasture. (3.110)

Roy, the country boy, is right at home on the field. In the novel, rain is what's right with the world; the rain gives life, both literally and figuratively. The water imagery in Roy's first date with Iris by the lake is supposed to let us know that she would be a good person for Roy to have in his life.