Your answer was oil tanks. people have made the same choice.
It makes a great deal of sense to think of oil tanks and deserts because a great deal of the world's oil has been found in desert regions. However, oil tanks would need to be spaced farther apart then these circles are. Besides, each circle has a diameter of over half a mile! That would be one huge oil tank and they're usually not that big.
The answer is irrigation fields. This picture is from Al Kufrah oasis in southern Libya. Libya is a country located in northern Africa. In this desert area, the only constant suppy of water comes from underground.
To make it possible to grow certain types of plants, the people who live here must irrigate the desert. They use a very long pipe with holes in it which turns around from the center of each field like the big hand of a clock. The water sprays out of the holes and brings life to desert land. Each round field in this image has a diameter of one-half mile and the entire area is about 20 miles across!