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Center pivot irrigation
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Satellite image of circular fields characteristic of center pivot irrigationCenter pivot irrigation (sometimes called central pivot irrigation) is a method of crop irrigation in which equipment rotates around a pivot. A circular area centred on the pivot is irrigated, often creating a circular pattern in crops when viewed from above.


The hub of a center-pivot irrigation systemCentral pivot irrigation is a form of overhead (sprinkler) irrigation consisting of several segments of pipe (usually galvanized steel or aluminium) 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 circle. These systems are common in parts of the United States where terrain is flat. Most center pivot systems now have drops hanging from a u-shaped pipe called a gooseneck attached at the top of the pipe with sprinkler heads that are positioned a few feet (at most) above the crop, thus limiting evaporative losses. Drops can also be used with drag hoses or bubblers that deposit the water directly on the ground between crops. The crops are sometimes planted in a circle to conform to the center pivot. This type of system is known as LEPA (Low Energy Precision Application).

Originally, most center pivots were water-powered. These were replaced by hydraulic systems (T-L) and electric motor-driven systems (Lindsay, Reinke, Valley). Most systems today are driven by an electric motor mounted at each tower.

Center pivot equipment can also be configured to move in a straight line, where the water is pulled from a central ditch. In this scenario, the system is called a linear move irrigation system.
http://en.wikipedia.org/wiki/Center_pivot_irrigation