Plugging water loss in center pivots

Your best practice | Fall 2022
To reduce water loss in center pivots, follow the water's journey from source to soil broken into loss points at transmission, air and ground.

As pumping energy costs soar and water resources shrink in many areas of the world, it is becoming more important to minimize water losses in irrigation systems. From a water and energy conservation perspective, a main goal of irrigation is to transfer the largest portion of extracted water from the sources to the root zone with as small an amount of water losses as possible. In the case of center pivots, water losses can be divided into three categories based on where the loss could happen in the journey of water from source to soil: transmission, air and ground.

Transmission losses

Transmission losses occur as water moves from the pump to water applicators or nozzles. These losses are mostly leaks from pipe networks or center pivot components such as joints, couplers and drain valves. In underground pipe networks it is harder to detect leaks, especially if they are not too big. The leaks could remain undetected longer if several pumps are connected to feed one system, a common practice in regions with limited well capacities. Flowmeters at pump outlets and pivot points can be used to determine any losses in buried pipes.

Leaks and losses from center pivot components are easier to identify since they tend to be visible. Examples include water losses caused by wear and tear, corrosion, negligence or malfunctioning of components such as check valves. Figure 1 shows a drain valve that was opened at the end of the season to let the system drain. The operator forgot to close it before starting the system back up, which resulted in a large pool of water near the pivot point. Although losses from center pivot parts occur within the irrigated field, they are considered lost as in most cases the water becomes unavailable to crops due to surface runoff or movement below the root zone.

The amount of transmission losses is highly variable among center pivots and depends largely on the age of the pipe network and center pivot, water quality and the level of maintenance.

Air losses

Air losses are those that take place after water leaves nozzles and before it reaches soil and crop surfaces. The two main air losses are droplet evaporation and droplet drift with wind. Unlike droplet evaporation in air, wind drift may not be a total loss if the drifted droplets land within the field of interest. But even if they end up in the same field, the drift will impact application uniformity and could cause over- and underirrigation in different parts of the field.

Air losses have been shown to be small (only a few percent of the total applied water) for center pivot systems that are properly maintained and operate near recommended pressures, even under hot, dry and windy conditions. The small amount of air loss is especially the case for center pivots that have water applicators closer to the soil surface, within 4 to 5 feet, compared to those that have impact sprinklers on top of the main line.

Overpressurized systems generate droplets that are too fine and can more easily evaporate or get drifted with wind. It is generally recommended that the diameter of droplets should be larger than 0.03 inches. Larger droplet diameters would significantly reduce air losses, but a balance should be maintained as droplets that are too large could break down soil aggregates if they land directly on bare soil. The destruction of soil aggregates can cause surface seals and reduced infiltration rate. Protecting the soil surface with crop residue can help eliminate the negative effects of large droplets on the soil surface. Figure 2 shows a center pivot with dropped nozzles operating on a windy day. Although some drift can be observed, it is retained mainly within the application field.

Ground losses

Ground losses typically occur in two main forms of surface runoff and irrigation water movement below the root zone, referred to as deep percolation. Both forms lead to applied water becoming unavailable to the root system and thus being lost to the crop. Surface runoff is generated when the peak water application rate is larger than the rate at which water can infiltrate the soil. Runoff can also be generated in wheel tracks, where soil is more compacted and thus infiltration rates are significantly smaller than the rest of the field (see fig. 3). Keeping pivot tracks dry by using boombacks or part-circle nozzles would help with reducing runoff as well as preventing deep ruts.

Surface runoff is generated when the peak water application rate is larger than the rate at which water can infiltrate the soil.

The risk of runoff generation is larger for water applicators that have a smaller wetted area or throw distance as they apply a larger rate during the same time to deliver the intended depth of water. In most cases, runoff can be reduced by adjusting a system’s speed to match application rate to soil infiltration rate. If that is not an option, some management practices such as planting circular rows, diking furrows (in case of straight rows, this can assist with rainfall retention too), and increasing surface roughness through crop residue can also help.

Saleh Taghvaeian, PhD, is an extension specialist in water resources and an associate professor in the biosystems and agricultural engineering department at Oklahoma State University.



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