Exactly right on rain being the determining factor. In NE Iowa and SE Minnesota, where I am from, it is one of the most drought-resistant areas of farm land in the nation. No one would waste dollar-one on an irrigation system. (Getting 4 rainless days in a row so that you can bale hay is a bigger issue). As you go west, the influence of the Rocky Mountain rain shadow takes on increasing importance. About half way across SoDak or Nebraska, center-pivot irrigation starts to pencil out.