Center Pivots

Center Pivot and Linear Move systems are closely related - they both use a motorized tower/overhead sprinkler concept and use similarly-sized towers and equipment for comparable acreages.The main difference between them is the mode of travel.Some Center Pivots can be ordered with a package to convert them to a Linear Move system.Since both systems are similar, information on center pivots is provided here first to give a basic understanding of the equipment, with additional information and characteristics about linear move systems provided later.

Figure 1. A small, 2-span Center Pivot with end gun

A Center Pivot system does just what the name implies - it "pivots" around a central point known as the "pivot point" or "pivot pad" (Figure 2). Center pivots come in a myriad of sizes, from 1 acre up to 400 acres or more.Pivots are generally economically feasible (relative to other types of irrigation system) when 40 acres or more are irrigated under a single pivot, and are generally the least expensive option when more than 60 acres are irrigated.    

Figure 2. A pivot point or "pad" and pump house

Figure 3. A pivot "span" tower

Irrigation of various field sizes is made possible by using a variety of lengths of "spans" between the supporting "towers" (Figure 3).The length of each individual span can be from 100 to 200 feet or more.Combining several different lengths of spans enables accommodation of almost any size field.

Center pivots generally have an end gun attached to the last tower on an end boom to increase the irrigated area (Figure 4).Since this end gun is supported on an extended boom and does not have a supporting tower directly under it, this addition can greatly increase a pivot's irrigated acreage without requiring additional land clearing.For instance, a 750 foot long pivot installed on a given field will cover 40.5 acres.Add an end gun capable of irrigating another 120 feet and the irrigated acreage is increased to 54.5 acres if the gun is able to operate for the entire circle.The end gun can have an automatic shutoff installed on the pivot point to turn it off when the gun would water non-crop areas such as residence yards, buildings, roads, or forests..

Figure 4. Close-up of a pivot end gun on a boom in operation

The major advantages of a center pivot system are the low labor requirement and the circular travel path.Operating a pivot is a simple job of turning the pivot on and can be done easily by one man (unless the pumping station is a considerable distance away from the pivot).The only other labor required involves regular checks of the pivot to make sure it is operating correctly and has not encountered any unexpected obstructions.The circular travel path means that the pivot will end its irrigation cycle at the same point where it initially started, so the next irrigation cycle will begin at the driest point in the field.

The major limitation of a center pivot is also, oddly enough, its circular travel path.Since the entire machine moves as a single line of spans and towers, the spans at the outer edge of the field must travel considerably faster (and thus cover more area) than those near the pivot point.This means there is a point where applying a given amount of water from these end spans is not possible due to soil characteristics and/or pump and piping limitations.Additionally, care must be used when considering the frequency of watering.If the pivot requires 96 hours to apply a given amount of water to the field and the crop growth stage, climate, and soil type determine that this amount of water is needed every 72 hours, the irrigator will "fall behind" until climate conditions improve.Center Pivot designers have this consideration and others in mind during design of each system, but it is good for the grower to understand this point.

In many instances it is not economically feasible to design the system to provide the maximum water need for the crop.Many center pivots installed for cropping systems including corn are designed to apply a maximum of 2 inches (gross) of water per week, even though corn can use up to 2.5 inches per week during peak water use periods.A system designed to apply 2.5 inches per week apparently costs more than the added corn yields will provide in return.

Another point to consider is the so-called "windshield wiper" pivot installation.In many instances it is less expensive to irrigate a long, narrow field with a single center pivot designed to operate in a half-circle configuration than to install two smaller, full circle pivots.These half-circle pivots have all of the benefits and drawbacks of a normal pivot with two additions - (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, and (2) if the pivot is not returned to its original starting point before the next irrigation cycle is started, the driest area of the field (that was irrigated first during the last cycle) will receive water last.

Center Pivots are available that have a "trailing" tower to irrigate the corners of a field.This trailing tower follows a buried cable or some other guidance system and lags behind the rest of the spans where the pivot "circle" reaches the edge of the field.This trailing span extends to line up with the rest of the pivot as the system approaches a field corner to irrigate that area, then retracts or "lags behind" again as the pivot continues around to prevent contact with obstructions at the field edge.

Many center pivots are also available in a "towable pivot" configuration. This allows the pivot to be moved or "towed" by a tractor from one pivot point to another to increase usefulness and decrease capital costs.Pivots withstand this type of movement well when they are moved once or twice per year, but any attempt to move a pivot weekly is a wasted effort both in terms of crop water management and wear and tear on the pivot structure.Moving a pivot between two pivot points will take between 6 and 12 hours as a rule.Towable pivots are best used when there are two different cropping systems under the two pivot points.The first crop is irrigated until it reaches maturity, then the pivot is moved to the second pivot point to irrigate the second crop with a much later maturity.