Center pivots are well-engineered structures that effectively deliver water to large circular fields.Each has a main water delivery pipe suspended over the field out of the way of the crops.Sprinklers or spray nozzles can be spaced along that pipe to apply water wherever the pipe is traveling. At each tower, pipe sections are connected with flexible joints that allow the pipe to move through a limited range without twisting or breaking. This flexibility also allows vertical bending that enables pivots to climb moderate hilly slopes.
1. General Center Pivot Movement
The machine moves in a circular pattern, and is fed with water from the pivot point at the centerof the circle. The water is usually pumped from a source such as a well or a river. The pump is connected to the pivot at the pivot point.The outside set of wheels covers the greatest distance and thus sets the master pace for the rotation. The inner sets of wheels are mounted at hubs between two segments and use angle sensors to detect when the bend at the joint exceeds a certain threshold (the wheels should be rotated to keep the segments aligned). Most center pivots irrigate a circular area a quarter-mile (0.4 kilometer) in radius, although some can cover a larger area. Center pivots are typically less than one-third of a mile (0.5 kilometer) in length. Most manufacturers offer a way to adjust the speed of the pivot, andthus the amount of water being applied over a given area, a process called Variable Speed Irrigation (VSI).
Some manufacturers offer packages that not only vary the speed, but also turn on and off valves for groups of sprinklers on the pivot system during operation. This is known as Variable Rate Irrigation (VRI). The Irrigation Delivery Systems report provides more information on VSI and VRI systems.
2. Wiper Pivots
A wiper pivot (or a “half-circle pivot”) runs in a half-circle (or more), reversing its course when a lever hits a stop placed in the field. In many instances, irrigating a long, narrow field with a single center pivot designed to operate in a half-circle configuration is less expensive than irrigating that field with two smaller, full-circle pivots. Wiper pivots exhibit two important differences from center pivots:
(1) Since the pivot does not automatically return to its original starting point, the grower may have to run the pivot “dry” back to that point before irrigating again, which adds wear to the system.
(2) If the grower does not return the pivot to its original starting point before the next irrigation cycle is started, the driest area of the field (the area irrigated first during the last cycle) will receive water last.
3. Challenges with Center Pivots
With all of its advantages, center pivot irrigation does present some challenges. Per the Natural Resources Conservation Service (NRCS) (Shae, Robinson 2009), irrigating in circular patterns is just plain “trickier” than irrigating in rectangles. In a rectangular system, each sprinkler applies water to an identically-sized area. In a circular system, the area increases as the radius increases so that sprinkler applies water to a differently-sized area.
To overcome this irrigated surface area challenge, and to maintain uniform coverage, irrigators calculate and size sprinklers appropriately along the length of the pivot. This maintains uniform water application along the linear length of the center pivot. The greater the radius of the circle, the more the grower must adjust the gallons per minute (GPM) (or liters per minute) to overcome frictional losses and the increased delivery acreage. This is accomplished by adjusting the nozzle diameter to increase flow. Since the outermost spans of the pivot travel farther in a given time period than do the innermost spans, nozzle sizes are smallest at the inner spans and increase with distance from the pivot point. This helps keep a uniform flow of water as the outer wheels set the pace of the rotation.