What are these circular shape things on the ground when flying over western US?

What you are seing are crops irrigated by center pivot irrigators.

Center pivot irrigation is a form of overhead irrigation consisting of several segments of pipe (usually galvanized steel or aluminum) joined together and supported by trusses, mounted on wheeled towers with sprinklers positioned along its length. The system moves in a circular pattern and is fed with water from the pivot point at the center of the arc. These systems are common in parts of the United States where terrain is flat.

This is how they are planting crops. Because there is not sufficient rainfall to sustain the crops, the crops are getting watered by water that is piped in by humans. Water is run to the center of the circle and can cover a range in all directions. Using the circular shape is the best way to utilize the water.

Some farmers irrigate their fields with a sprinkler system that rotates around a central point...I think they are called pivot irrigation systems. Anyways, they irrigate 360 degrees around a central point and make it look like a big circle of green when you fly overhead. Here are some pictures of what I'm talking about:

http://www.fotosearch.com/photos-images/center-piv...

A giant suburbia destroying the land.

Frisbee's