They are irrigated fields using circular sprinklers. If you look closely at the brown circles, i.e., the ones which are lying fallow, you will see the sprinkler pipe extending from its center attachment point. When the water is turned on, it drives the contraption slowly around and around, thus keeping the entire field watered. I've seen the same kind of arrangement in lots of places in this country, but most pertinently in similar desert terrain in Arizona.
Michael