By Nelson Irrigation
Increasing production efficiency is essential for producers to maintain or increase their economic return in an increasingly competitive global market. Irrigation scheduling, irrigation uniformity, and irrigation efficiency maximize production. Center pivot and linear move irrigation systems are capable of attaining uniformities and efficiencies of 90-95 percent with values of 85 percent generally considered the minimum acceptable level before a system needs updating or maintenance. There are a few simple checks an operator can perform to ensure they are getting the most out of their system.
Worn or malfunctioning equipment and improper design and installation are the most common factors that produce cumulative effects on irrigation uniformity. The primary objective in the maintenance of a center pivot or linear move irrigation system is to maintain the design flow rate from each sprinkler. This requires that the design pressure and nozzle size at each sprinkler be maintained. Common problems are worn or plugged sprinkler nozzles, plugged or malfunctioning pressure regulators, and improper installation of the sprinkler package nozzle sizes on center pivots. Of these common problems, only a plugged sprinkler nozzle or regulator is easy to identify when the system is operating. The other common problems are not always readily discernible by visual observation of the system in operation. Malfunctioning pressure regulators can be difficult to identify visually; however, some emit water through the sides of the regulator when they fail structurally. A malfunctioning regulator can result in a sprinkler pressure that will be too high. A sprinkler emitting a fine spray or faster rotation speed relative to adjacent sprinklers may indicate a malfunctioning pressure regulator that is operating above its nominal rating. If a regulator is operating below its nominal rating, sprinklers will produce larger droplets and slower rotation speed. During the irrigation season, the operator should intermittently observe sprinkler performance. This is best done either early or late in the day when the sun is low. Differences between sprinklers are easier to identify in this light. Some growers will install a new sprinkler/regulator assembly on their system and visually inspect for differences between the new and existing components. Care must be taken that pressure ratings and nozzle sizes are the same.
For situations where the sprinkler is readily accessible, flow rate measurements from 10 to 20 randomly selected sprinklers can be used to judge the condition of the sprinkler package. Flow rate measurements can be made using a large graduated container and a stopwatch. The container can be positioned to encompass the sprinkler and capture flow for a specific time measured with the stopwatch. Sprinkler flow rate can then be calculated from these two measurements and compared to the design flow rate value for the nozzle size and pressure from the sprinkler chart. Flow differences greater than ±10 percent indicate a problem.
When one or more sprinklers are found to have flow rates greater than ±10 percent of the nominal flow rate, a second set of 10–20 randomly selected sprinklers should be tested. If more sprinklers are found to have flow rates greater than ±10 percent of the nominal flow rate, the cause should be identified and corrected. Plugging in the nozzle or plugging upstream of the regulator, a “stuck” regulator, or low system operating pressure are common causes of lowered flow. Flows above the nominal flow rate indicate a malfunctioning pressure regulator or worn nozzle. A good strategy against worn nozzles and malfunctioning pressure regulators is to replace the sprinkler package every 5–10 years depending on water quality and hours of use. An accepted industry rule of thumb is to consider replacing sprinkler packages every 10,000 hours.
A pitot tube attached to a pressure gauge can be used to field check pressure regulator operation. Pressure measurements are taken by placing the pitot tube directly into the flow jet exiting the nozzle so that the open end of the tube is perpendicular to the flow jet. The pitot tube should be small to minimize flow interference. Pressures generally should be within 2 psi (14 kPa) of the regulator pressure rating. Measured nozzle pressures should be checked against the sprinkler chart.
Pipeline pressure on the machine should be monitored throughout the year. The best location for a permanent pressure gauge is at the distal end of the center pivot or linear move system. When pressure regulators are used, pipeline pressure should be maintained at least 5 psi (34 kPa) greater than the regulator nominal rating. For example, a machine with 15 psi (103 kPa) regulators should have at least 20 psi (138 kPa) of pressure at the end of the machine at all times. The best time to check the pressure is when the end gun is on and/or the end of the center pivot is at its highest point in the field. This will ensure proper pressure throughout the machine and the rest of the field. If the system is not pressure regulated, the irrigator should verify that the end pressure matches the computer generated sprinkler chart.
A flowmeter can also be a valuable tool for the operation of a center pivot system. Monitoring the flowmeter can ensure that the flow output of sprinklers, regulators, and end guns matches the sprinkler chart. Changing pressures and flow rates during the irrigation season could indicate problems with a sprinkler package and a need for maintenance or replacement.
The height of the sprinkler above the crop canopy, along with the spacing and wetted radius of the sprinkler nozzles, can significantly affect irrigation uniformity. In general, sprinkler height should be approximately 3 ft (1 m) above the plant canopy to ensure good irrigation uniformity. At times this is not possible with taller crops such as corn. Extra attention is needed to ensure proper overlap and runoff control in these cases. In most cases, excellent uniformity can be expected when sprinkler spacing does not exceed 50–75 percent of the wetted radius. It is important to ensure that drops hang straight (vertical) off the pivot pipe. This will allow for maximum coverage for the full rotation of the sprinkler.
For systems frequently irrigating corn and other tall crops in areas with heavy soils, high flowrates, and rougher terrain, up-top sprinklers should be considered. Nelson Irrigation has introduced several new up-top options in the last few years that provide low operating pressures in the 6–20 psi (41-138 kPa) range along with excellent wind-fighting ability that achieve high levels of uniformity and efficiency.
Your sprinkler package is no different than every other piece of machinery on your farm in that it needs consistent monitoring, maintenance and replacement. Without it, production efficiency will be diminished if water and nutrients are not efficiently and uniformly applied to the root zone.
For more information about sprinklers and regulators for mechanized irrigation, visit www.nelsonirrigation.com.