Ah! I know exactly what you're talking about! The fields are in circles because the irrigation system used rotates around a central pivot-point (where the water line connects into the system). The water is carried over the crops in question through a giant sprinkler on wheels that creeps in a giant circle or semi-circle. If you look a little harder, you'll find those circle fields are only in areas which don't receive adequate rainfall for whatever crop they're trying to grow.

And yes, I seem to have an answer for everything. It's a gift.