13.4: Sprinkler and Nozzle Selection

The center pivot operator should be concerned with the following questions regarding the sprinkler and nozzle package installed on a center pivot.

• What type of sprinklers and nozzles to install on the pivot?
• Are the proper sprinklers and nozzles installed at the correct location along the pivot lateral?
• Are the sprinklers working properly?

Determining the proper nozzle size for each sprinkler along the center pivot lateral is complex. The number of nozzles needed, the size of the nozzles, spacing of sprinklers at the point of concern, the diameter of coverage, pressure loss along the lateral, the use of pressure regulators, and the elevation gain around the field are all issues. In addition, every sprinkler along the pivot lateral is considered individually. The details of this design process will not be considered here. Generally, the nozzle sizes along the lateral will be determined by the center pivot or sprinkler manufacturer. The center pivot owner and operator should obtain a copy of the sprinkler package chart. This chart specifies the type of sprinkler and nozzle sizes to be used at a particular location along the lateral. The operator can use the chart to check the final installation to determine if errors were made in shipping or construction.

An important decision for the center pivot manager is the type of sprinkler device to use. Many choices of sprinklers are available. Early center pivots used only impact sprinklers. These have the same performance characteristics as presented in Chapter 11. Impact sprinklers are appealing because they provide a large diameter of coverage which produces lower application rates and less runoff potential. Recently these sprinklers have been made of plastic leading to sprinkler packages that are price competitive because fewer sprinklers are required with the larger diameter of coverage. The primary disadvantages of impact sprinklers are the higher operating pressures required and the higher potential for evaporation and drift losses. To reduce the evaporation potential low angle impact sprinklers have been developed. The range nozzle on conventional impact sprinklers emits water at an angle 23° above the horizon. The low angle sprinklers discharge water at an angle of about 7°. Low-angle sprinklers can be used with special nozzles for operation at lower pressures than conventional sprinklers.

Special spray head devices that discharge water onto pads have been developed for use on center pivots. The pad could either be stationary or moving. The devices can generally be installed in an upright position or inverted. When the pad devices are inverted they are attached to drop tubes that allow the devices to be positioned below the truss assembly, or even lower to apply water just above the crop canopy. Dropping the devices closer to the crop canopy reduces the potential for evaporation or drift but increases the runoff potential. The advantage of the rotating and wobbling pad devices is the increased diameter of coverage requiring fewer devices while providing lower application rates and better uniformity.

Various types of pads can be used with both the stationary and moving spray heads. The face of the pad can be smooth or grooved. The smooth pad produces smaller droplets. Grooved pads produce small streams of water off of the pad leading to larger drops than the smooth pad. The depth of the groove and the number of grooves on the pad determine the size of the droplets. Pads are also designed for use when the device is upright or inverted. If the device is placed on top of the pivot lateral, a concave pad is used to direct the spray toward the soil. When the device is inverted and dropped below the pivot lateral, a flat or convex pad is used to direct the water horizontally to maximize the diameter of coverage. Two issues are important when selecting the type of pad: drift and energy of impact of the droplet. Small droplets contain less kinetic energy when they reach the soil surface than large droplets. If the soil at the site has low aggregate stability, the large droplets (from pads with large grooves) can cause a seal to form at the soil surface leading to lower infiltration rates. If you irrigate when there is little cover on the soil, then smooth or shallow grooved pads would be desirable. Small drops are affected by wind much more than larger drops. If you irrigate in a windy area and infiltration rates are good, you may choose a deeper grooved pad. In windy areas mounting the spray pad devices below the pivot lateral closer to the crop may be a good idea. In areas with low infiltration rates and/or steep slopes, impact sprinklers may still be preferable because of the smaller runoff potential.