13.1: Introduction

In 1948 Frank Zybach invented a device he called the “self-propelled irrigator.” This led to the development of center pivot and linear or lateral move irrigation systems (Bittinger and Longenbaugh 1962; Heermann and Hein, 1968). A chapter is devoted to these systems because of the unique management needed to capitalize on the capability of these systems. Additionally, the growth of center pivot irrigation during the last three decades far exceeds the growth of any other method of irrigation. In some areas the amount of land irrigated with surface systems has receded. Many of the fields, previously surface irrigated, have been converted to center pivot irrigation. The growth in center pivot irrigation in one year almost equals the total amount of land irrigated with microirrigation in the U.S. The growth has been very substantial and will likely continue. Three principal reasons drive this growth. First, the systems have the ability to be very efficient. They can apply small depths of water at the time that the crop needs irrigation. Second, the systems require less labor than surface or moved lateral systems. In many areas the scarcity of available labor is a limitation to the amount of land that a farmer can irrigate. Third, the systems have the capability to irrigate crops, soils, and terrains that are infeasible with surface, or periodically-moved sprinkler systems.

The basic components of a center pivot (Martin et al., 2007) are illustrated in Figure 13.1. The pivot lateral is a pipeline with sprinkler outlets. The pivot lateral is supported by a tower assembly. The towers include a structure to support the pipeline plus a motor to propel each tower. Today most pivots are powered by electricity. However, some manufacturers use oil hydraulic motors. The pivot base or pivot point is located at the center of the field. The base can be permanently installed or, for smaller systems that are towed from field to field, the base is mobile. Water is supplied to the inlet pipe on the pivot base. The water is pressurized by a pump upstream of the pivot. Water is carried up the pivot base through a rotating elbow to the inlet of the pivot lateral. Power is supplied continuously to motors installed at each tower using a slip-ring assembly. This device contains contacts that allow the pivot to pick up power from the pivot base while the lateral rotates. A control panel is usually located on the pivot base where the operator can adjust the speed of rotation of the pivot and check on other factors. A road is usually necessary so the operator can conveniently reach the pivot base.

The combination of the pivot lateral, the truss support structure, sprinkler devices, and the tower are called a span. The length of a span can vary from 100 to 200 feet. Installation costs are less with longer spans; however, the maximum length of a span is determined by the diameter of the pipe and the slope and undulation of the terrain. The length of a span can vary along the pivot to adjust to the dimensions of the field. Spans of the pivot can be connected together in either a rigid or flexible fashion. For rolling terrain it is necessary to provide a flexible coupler between spans.

Figure 13.1. Illustration of the components of a center pivot irrigation system.

A pipe called the overhang is often attached beyond the last tower of the pivot (Figure 13.2a). The overhang could be up to 80 feet long. A special sprinkler can be attached to the end of the overhang to increase the amount of area irrigated. This sprinkler is usually called an end gun and is used to water part of the corners not reached by the last sprinkler on the lateral. It only operates when the water from the end gun stays within the field. Since about 1975, there has been a major effort in the center pivot industry to reduce the amount of pressure required to operate center pivots. Systems originally required 75 psi of pressure at the inlet to the pivot. Now, many are designed to operate at 30 psi or less. These pressures are often too low for proper operation of end guns, so a booster pump is installed at the last tower to pressurize the end gun. A valve is used to control when the end gun operates.

Figure 13.2. (a) Example of a center pivot irrigation system (Valmont Industries) with seven spans, an overhang, and an end gun. (b) Example of a center pivot with a corner system (photo b courtesy of Lindsay Corporation).

a) b)

Center pivots can also be equipped with corner watering systems (Figure 13.2b). These systems have a corner lateral that hinges or rotates from the last tower of the main system. The corner system can be guided by GPS or a buried cable that emits a radio signal for the corner tower to follow. Sprinklers on the corner system have special valves that open and close depending on how far the corner lateral has rotated away from the main pivot lateral. Recent developments in center pivot irrigation include remote monitoring and control, high-speed variable frequency drive motors for the towers, low-energy precision application (LEPA) sprinkler packages, variable rate irrigation, and variable frequency drive for the pumping plant (Lamm et al., 2019).

To irrigate rectangular fields, or to irrigate a larger portion of square fields, mechanically moved systems were developed where the lateral moves along a straight line (Martin et al., 2007). These systems are called linear or lateral move systems (Figure 13.3). The spans of these systems are nearly identical to those of center pivots. The unique feature of these systems is how the water is supplied to the lateral. Two types of water delivery systems are common:

• systems that drag a supply hose and
• systems that pump water from a canal that runs parallel to the direction of travel.

Systems supplied by either a hose or buried valves are usually pressurized with the main supply pump. Systems that obtain water from a canal carry a pump along with the system to obtain and pressurized the water. The main supply pump, or surface water supply system, must be hydraulically interfaced with the system so that the water supply is continual but does not exceed the canal capacity or the discharge of the system. The water supply features of these systems affect management.

Figure 13.3. Hose-fed linear or lateral move irrigation systems. (a) A four-span system driven by electric motors (photo courtesy of Lindsay Corporation). (b) A drive unit for a linear move irrigation system driven by oil hydraulic motors (photo courtesy of T-L Irrigation Company).

a)

b)