Irrigation Fundamentals

Part 7, Sprinkler Systems, Center Pivots, and Linears

By Ed Butts, PE, CPI

Of all the methods of irrigation, perhaps none is more apparent from the air than the center pivot. The telltale indication of an irrigated field under a center pivot is obvious from the smattering of perfectly concentric green circles when viewed from 30,000 feet.

This month, as a continuation of this series on irrigation fundamentals, we will outline the basics of center pivot and linear (lateral) irrigation system design.

Center Pivot Basics

Center pivots are one of the most popular and recognizable methods of sprinkler irrigation used in the United States as well as the world. The ability to automatically irrigate up to 320 acres (half section) in one irrigation set by pressing a single button to start the process has inherent and obvious advantages over most of the other and less expensive methods.

Center pivot irrigation uses less labor and water than many other surface irrigation methods, such as furrow or flood irrigation. It also has lower labor costs than ground irrigation techniques that require digging channels and can reduce the amount of soil tillage.

Therefore, it also helps reduce water runoff and soil erosion that can occur with typical methods of ground irrigation. Less tillage also encourages more organic materials and crop residue to decompose back into the soil and reduces soil compaction.

As the term implies, a center pivot operates from a single stationary point situated in the center of a field or designated area—with an assembled length of pipes with attached sprinklers as short as 200 feet and up to 2640 feet (half mile) in total length, rotating in a full or semi-circle around this central point. Flow rates range from 100 to 3000 GPM at pressures between 10 to 75 PSI.

A center pivot making a complete circle irrigates an area equal to the area of the circle with a radius equal to the length of the pivot. Therefore, the irrigated area increases as the system length is squared, making a pivot’s length a major design factor to optimize coverage. This is a primary reason why center pivots are so popular and often occupy most of a field’s width or length.

The stationary or rotational end of the pivot is called the pivot point. As illustrated in Figure 1a, the pivot point is comprised of several elements and forms the beginning of the machine. Since undulating stress is applied to this location, the pivot point must be constructed of four to six steel support legs (1a-A) to resist a full circle rotational stress and anchored to a secure concrete slab. It is connected to the water supply using a J inlet and riser pipe (1a-B and 1a-E) and is where water enters the machine and is distributed to the pivot.

Sealing of water under pressure is accomplished in the pivot swivel joint using a chevron or vee-type gasketed coupling (1a-C). Located at the base of the pivot point, a control panel (1a-D) distributes AC power to the towers for drive and control power. A device referred to as a collector ring (1a-F) is situated at the top of the pivot point and allows continual feed of power to the pivot, using friction slide ring conductors.

Depending on the manufacturer and pipe size, the typical pivot consists of a series of assembled pipes ranging in length from 90 to 250 feet in length between ground support structures. This clear distance between each ground-based support structure is referred to as a span (Figure 1b). Table 1 displays span lengths, pipe sizes, and available materials for one manufacturer: in this case, span lengths from 118 to 213 feet, in pipe sizes from 4 to 10 inches are available.

Available materials for span pipes also vary with the manufacturer. However, typical types include high strength, coated, or poly-lined galvanized or plain steel, aluminum, coated or galvanized chromium-nickel, or stainless steel.

Maintaining proper alignment and integrity between spans is provided using an articulating pipe coupling and alignment system. The method of this joint varies between manufacturers, but usually consists of a vee-gasketed pipe joint and an internal or external hook or ball and receiver to maintain needed joint security as well as provide flexibility for a minimal deflection between spans for undulating or rolling ground.

A short section of flexible rubber hose connects the pipes for hydraulic continuity. Pipes are generally placed around 10 feet above the ground, although heights as low as 5 feet for low profile crops and as high as 20 feet for tall crops are available.

An underpipe bow string truss assembly (Figure 1b) is used to carry and transfer the intermediate loads to A-frame support structures on both span ends. Trusses are spaced every 19 to 22 feet for even load distribution and strength, with the truss rods typically ⅝-inch or ¾-inch in diameter and made of highstrength, low-alloy steel.

The entire assembly is supported at regular intervals on structural steel assemblies called towers (Figure 1b). In addition to supporting the respective pipe and water loads, each tower is equipped with two flotation tires that distribute the load imposed by the full pipes and towers between the span and transfer it to the ground. Each tower is equipped with a power or drive unit. Five types of power units commonly used
to drive the wheels on center pivots include electric motors, water pistons, water spinners and turbines, hydraulic oil motors, and air pistons.

The first pivots were powered by water pistons, but electric motors are most commonly used today because of their speed, reliability, and ability to run backwards and forwards.

The typical electric powered drive unit consists of a control box, small electric motor (approximately ¾ to 1½ HP) attached to a speed reduction gearbox, which together forms a gearmotor (Figure 2a-A). The gearmotor’s dual output shafts attach to a short, splined drive shaft in opposite directions that are thereafter attached to two wheel gearboxes (Figure 2a-B) that drive two large-diameter, vee tread, rubber flotation tires (Figure 2a-C). Steel wheels are offered by some manufacturers. Tire sizes are selected for the crop, ground pressure, terrain, and soil type, but typically run between 11 inches × 22.5 inches to 13.6 inches × 38 inches.

Although most center pivots use air-filled tires, airless solid-material pivot tires are also available for a center pivot irrigation system. Available in a number of types, these tires are typically designed with an aggressive tread for easy rolling to withstand the two hazards that many air-filled tires are defeated by: punctures and repeated exposure to the elements.

Electric power to operate a pivot is typically provided from either a utility source or internal combustion engine or pump-driven generator, while hydraulically driven pivots operate from a closed hydraulic pump system that is often powered from an internal combustion engine.

In many applications, hydrostatic drive systems offer advantages that electrically driven systems cannot. Primarily, continuous pivot movement is provided. This is often critical to uniform water distribution, especially in low-pressure situations. Improved traction also reduces the chances of developing ruts or getting stuck.

A constant-pressure-variable volume hydraulic pump ensures full torque at any speed and an alignment control valve will stop the system if the system gets stuck or seriously misaligned.

Drive units receive continuous power to each wheel from hydraulic motors that are directly coupled to the final drive gear assembly. Specifically designed for low speed, high torque applications, they operate equally well in either direction. For electric systems, a control unit or tower box is situated at each tower to accept control commands and operate the motor. Typically, to lessen the cost of wire, 480 VAC,
3-phase power is used to operate electric motors, although 120 VAC is typically used for control power with 24 VAC or DC occasionally used.

Devices called proximity sensors are installed at each drive unit to keep the assembly in a straight line between the pivot point and drive unit at the end of the machine. The end-drive unit is the principal control that is used to dictate the speed of rotation and thus the amount of applied water.

Water is applied to the land underneath the pipes through regular or irregularly spaced sprinklers that are mounted onto the top or below the pipes. The most common center pivot uses a 6-inch pipe, is a quarter-mile long (1320 feet), and irrigates the circular portion (126 acres) of a quarter section (160 acres), or roughly 79% of the entire field.

Additional acreage that covers the corners can be irrigated if the pivot is equipped with an end-boom or corner gun (see Figure 2b). The use of an end gun can often increase the irrigated acreage between 10 to 14 acres. Figure 3 illustrates a typical 1300-foot-long center pivot with the area irrigated by each 10% of the system length. For the 1300-foot example system shown, the initial 130 feet or 10% of the system length
or span would irrigate 1.2 acres and the last 130 feet would irrigate 23 acres.

This means while the first 130 feet accounts for less than 1%, the last 130 feet of the system accounts for approximately 19% of the total irrigated area of 122 acres under the pivot.

If these systems are planned for use on square sections, some means of irrigating the four corners must be provided, or other uses made of the area not irrigated. In a 160-acre quarter- section subdivision using a full circle pivot, about 34 acres are not irrigated by the center pivot system unless the pivot is provided with a special corner irrigating system or apparatus, which can consist of a swing arm or Big Gun. Thus,
with most corner systems only about 8 out of 160 acres (5%) are left unirrigated.

Unirrigated corners and land beyond the normal reach of the pivot can be covered using a portable or solid-set Big Gun, end boom, or corner gun. See Figure 3.

Sprinkler Packages for Center Pivots

The group of sprinklers installed on the irrigation system is known as the sprinkler package. Sprinklers may be mounted above the pivot pipeline, on the side of the pipeline, or suspended on drop tubes below the pipeline as illustrated in Figure 4. Sprinkler selection for a center pivot is a critical design element. Due to the radial distribution of water and the progressive increase in the number of acres irrigated per each foot of increased length, each additional foot of the pivot must be supplied with a greater amount of water.

Consequently, the water application rate of the center pivot system increases in direct relationship to the distance from the pivot point. The rotating pivot is generally fitted with increasing sizes of impact sprinklers for constant sprinkler spacings (Figure 4A), progressively closer impact sprinkler spacing for variable spacing (Figure 4B), or spinner or spray-nozzle (Figure 4C) sprinklers to equally apply and distribute the water over the circular field.

Currently, there are two types of sprinkler models used for most center pivot and linear equipment: fixed spray and rotating spray (Figure 4C). Both sprinkler types have a wear plate mounted directly below the nozzle, which discharges the water and creates a 360-degree water pattern.

Fixed sprays have no moving parts because the wear plate is fixed. A rotating spray wear plate rotates either fast or slow, depending on the sprinkler model. Rotating sprays produce a large wetted diameter (50 feet to 70 feet) that creates a lowintensity water pattern; this is a major advantage of rotating sprays. This type of sprinkler works well on medium to heavy soil types and reduces water runoff. Fixed sprays have a wetted diameter of 15 feet to 40 feet, which works well on light soils where runoff may not be a big issue.

Fixed spray and rotating spray sprinklers are designed to operate at a low pressure in order to minimize energy (pumping) costs. Design operating pressure for fixed sprays is 6 to 30 PSI and pressure for rotating sprays is 10 to 30 PSI. Most spray heads are placed on drop pipes or tubes that, depending on the crop, lower the heads below the span pipes to within 4 to 5 feet from ground.

For constant-spaced sprinklers, the area to be irrigated by each sprinkler, set at a uniform sprinkler spacing along the pivot, becomes progressively larger from the pivot point outward toward the far moving end. Therefore, to provide uniform application, the sprinklers must be designed for progressively greater discharges, closer spacings, or both from the pivot point outwards toward the far moving end. Thus, the
wetted radius of the sprinkler tends to increase with distance from the pivot point.

Near the pivot point, one spray covers a small area and has a low discharge rate. As the line extends toward the far end, each watering outlet covers a larger amount of acreage. Typically, the application rate near the outer moving end is about 1 inch per hour. However, this exceeds the intake rate of many tighter soils.

With variable spacings, sprinklers are all the same size but the spacing between the sprinklers decreases with distance from the pivot point to account for the change in area irrigated. Thus, the wetted radius is similar for the entire system.

When using spinners or spray heads, different size nozzle openings are needed to compensate for differences in irrigated area but the spacing is narrower along the system due to their lower wetted radius.

To minimize surface ponding and runoff, pivots are usually rotated every 10 to 72 hours depending on the soil’s infiltration characteristics, the system’s capacity, and the maximum desired soil moisture deficit. Self-propelled, center pivot sprinkler systems are suitable for almost all field crops but require fields free from any obstructions above the ground such as telephone lines, electric power poles, buildings, and trees in
the irrigated area.

They are best adapted for use on soils having high intake rates and uniform topography. When used on soils with low intake rate and irregular topography, the resulting runoff often causes erosion and puddles that may interfere with the uniform movement of the pivot around the pivot point.

Most pivot systems are permanently installed in a specific field, but in certain cases, pivots can be designed to be towable and moved to other fields.

The recommended application (irrigation) efficiency to use with center pivot design is 75% (0.75), but with proper system management, application efficiencies with center pivot systems can be as high as 85% to 90% depending on wind speed and direction, sprinkler type, operating pressure, and tillage practices.

Specific advantages and limitations for center pivot systems include:

  • Reduced operating labor, up to 50% less than many other methods
  • High water application uniformity with good water management
  • Light and frequent applications can be made
  • Low pressure systems with nozzle pressures as low as 10 PSI can be used
  • Uniform chemical applications (chemigation) can be performed
  • Pivots can operate as part circle systems because they can operate in either forward or reverse directions.

Some of the limitations are:

  • Corners are not irrigated unless specialized corner systems are used.
  • Groundwater depletion has been observed in regions with long or many pivots and high pumping rates.
  • High application rates at the outer end of a center pivot, up to 1 inch per hour, can cause runoff and erosion where adequate soil surface storage is not provided.
  • Soil surface compaction may increase toward the outer edge of the pivot circle due to the higher application rate.
  • With light applications there is an increase in potential water evaporation losses; therefore, more intense soil moisture management, such as the use of tensiometers, must be performed to prevent soil moisture shortages.
  • Drive wheels may cause ruts in some soils if applied wheel pressure exceeds the safe soil bearing pressure.
  • Topography should be uniform with slopes not exceeding 10% as severely undulating topography produces more runoff and potential drive and tire rutting problems.
  • High initial costs and maintenance costs, especially with corner systems.

Many options are available for center pivot systems. Swing arms that rotate around the normal stationary end of the pivot to cover additional ground. Drop spans to allow dropping off a certain length of the pivot on irregular-shaped fields. Corner systems previously mentioned. Towable systems that allow movement of systems from one field to another.

The cost per acre for a pivot varies directly with its length. Short pivots have a high cost per acre and longer pivots cost much less. On a quarter-mile (1320-foot-long) pivot that irrigates 126 acres, the installed cost may average between $75,000 and $80,000 plus freight. This would place the cost per acre at between $595 and $635 per acre.

Lateral Moves (Linears)

A self-propelled lateral move, also known as a linear, combines the basic structure and guidance system of a center pivot with a traveling water feed system, similar to a soft hose traveling sprinkler. However, instead of rotating through a full circle or part circle from a fixed pivot point, linears automatically travel down a rectangular or square field, similar to a manually moved side roll system.

Lateral move irrigation is far less common, relies on more complex guidance systems, and requires additional management compared to center pivot irrigation, particularly if an engine-driven generator is used for the machine’s power supply.

Electric power needed to operate a linear is generally provided from one of two sources: utility power with a drag cord feeding the machine (Figure 5a) or an onboard engine-generator set (genset) that provides power to the machine.

The genset is typically mounted to the first tower and usually referred to as a power tower. This is illustrated in Figure 5b.

Older versions of linears may use a cable-assist pulling mechanism for the heavier power tower. Guidance is conducted using a furrow and guidance arm, and a global positioning system.

Water is generally fed into the principal drive end (power tower) of the linear and is typically provided from one of two sources: a rubber drag hose or canal feed pumping system. Generally, the same type of 4-inch to 5-inch rubber hose used for soft hose travelers is used for linears.

As with center pivots, linears require fields free from obstructions for efficient and safe operation. Measured water distribution from these systems has shown the highest uniformity coefficients of any system for single irrigation sets under windy conditions, up to 85%.

Systems that pump water from open ditches must be installed on nearly level fields. Even if the system is supplied by a flexible hose, the field must be fairly level in order for the guidance system to work effectively.

A major disadvantage of linear systems as compared to center pivot systems is the problem of bringing the lateral back to the starting position and across both sides of the water supply line. Since a center pivot operates in a circle, it automatically ends each irrigation cycle at the beginning of the next, but because a linear moves from one end of the field to the other, it must be driven or towed back to its starting position.

However, the linear move system can irrigate an entire rectangular field, whereas the center pivot system can irrigate only a circular portion of it unless a corner system is added. Linears travel back and forth across a field, instead of around a central point as a center pivot does. Linears are one of the most efficient forms of farm irrigation, irrigating up to 98% of the field.

Linears can also provide an effective farm management program with chemigation, fertigation, and germination, and decreases leaching that can occur through other irrigation methods. An example of a typical linear system for 100 acres is shown in Figure 6.

Advantages:

  • Up to 98% of the entire field is potentially irrigated
  • Can reduce labor expenses by up to 50% compared with surface, side roll (wheelline), or hand move irrigation
  • High application uniformity because laterals are nearly continuous in travel
  • Can deliver low rates of water, which helps eliminate surface runoff when irrigating on certain tight soils
  • Controlled chemigation can be practiced.

Limitations:

  • High initial and annual operating costs
  • Need to supply water to the moving lateral
  • When the irrigation is complete (pivot reaches the end of the field), the pivot must be rotated back to the starting position or moved endwise to an adjacent field. When moving the pivot endwise, tower wheels must be rotated 90 degrees or be placed on individual tower dollies.

This wraps up this last installment on sprinkler irrigation systems. Next month we will continue our series on irrigation fundamentals with an overview on the most common and oldest method: flood and furrow irrigation.

Until then, work safe and smart.


Ed Butts, PE, CPI, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 40 years of experience in the water well business, specializing in engineering and business management. He can be reached at epbpe@juno.com.