Problems Regarding Operating Pressure & Uniformity

Problems Regarding Operating Pressure & Uniformity

Troy Ingram1, Assistant Extension Educator

Derek M. Heeren2, Associate Professor and Water for Food Global Institute Faculty Fellow

Steven R. Melvin3, Extension Educator

Eric Wilkening2, Undergraduate Research Assistant

Aaron Nygren1, Associate Extension Educator

Derrel L. Martin2, Professor and Extension Specialist

Daran R. Rudnick2, Assistant Professor

Chuck Burr4, Extension Educator

Mitiku Mamo1, Assistant Extension EducatorCenter

1 Northeast District, Nebraska Extension, University of Nebraska-Lincoln

2 Department of Biological Systems Engineering, University of Nebraska-Lincoln

3 Southeast District, Nebraska Extension, University of Nebraska-Lincoln

4 West Central Research and Extension Center, University of Nebraska-Lincoln

Pivots operating below or above the designed water pressure can create uniformity issues across fields and/or increase operating expenses. Insufficient pressure can impact application uniformity by preventing adequate water from reaching the far end of the pivot; whereas, excessive pressure may provide good uniformity but inflates energy costs. Decreased water application uniformity is often offset by applying extra water to satisfy needs in under-watered areas, which drives up operational expense and may result in surface runoff and topsoil erosion as well as leaching of plant nutrients in overwatered areas.

Figure 1. Example of a center pivot with a leak at the end of the pivot lateral. An inspection of the pivot early in the season provides time to repair leaks or other problems before irrigation is needed.

Center Pivot Operating Pressure

Checking the pressure regularly (i.e., weekly to bi-weekly) or monitoring continuously can help maintain optimum efficiency and minimizes irrigation costs. Operating pressure can deviate above or below the required pressure for a number of reasons, such as changes in groundwater level overtime, topography, and leaks along the irrigation system. For example, Figures 2 and 3 illustrate the fluctuation of pressure at the end of a pivot overtime and across a field (i.e., pivot rotation degree), respectively.

Figure 2. Example of pressure at the end of a center pivot as a function of time. The red line shows the required pressure regulator inlet pressure.

Figure 3. Example of pressure at the end of a center pivot as a function of pivot angle, which reflects the changing topography in the field. The red line shows the required pressure regulator inlet pressure.

Pressure gauges can be easily installed on existing irrigation systems, and new pivots are often equipped with them. All wireless remote pivot control systems can monitor pressure and send alerts to mobile phones or computers when pressure falls outside a predetermined range. These data can assist managers in maintaining pivot performance. However, it should be noted that the accuracy of gauges may decay over time, but replacing gauges is affordable and simple.

Evaluation of Nebraska Pivots

Thirty-one center pivots equipped with pressure regulators across Nebraska were analyzed. Data were collected by AgSense Field Commander units mounted at the end of the pivots. For this analysis, the ideal range of pressure at the end of the pivots was considered to be 0 to 10 psi above the required regulator inlet pressure; equivalent to 5 to 15 psi above the pressure regulator rating. Approximately 55% of the center pivots evaluated had a pressure below the required regulator inlet pressure for at least 5% of the time (Figure 4). Operating pivots below the required regulator inlet pressure can lower pivot performance (in terms of uniformity) and may result in over- or under-irrigating. It was expected that fields with a greater range in topography would be more likely to have a low pressure (e.g. in a high-elevation portion of the field); however, this was not observed (Figure 5). It is also expected that center pivots with wells where the water table declines significantly through the growing season may be more likely to have low operating pressures.

Figure 4. Comparison of operating to required inlet pressure. Percent of pivots below the required pressure (at least 5% of the time), within the ideal range (between the required pressure and 10 psi above the required pressure), and more than 10 psi above the required pressure (at least 5% of the time).

Figure 5. Percent of pivots below the required pressure (at least 5% of the time) for each of three levels of topography: range of elevation less than 25 ft, between 25 and 60 ft, and greater than 60 ft.

A simulation was performed to predict the effect of low pressure on the uniformity of irrigation application depth along the pivot lateral (Martin et al., 2019). For this example, the design pressure at the pivot point was 41 psi (Figure 6). When the inlet pressure at the pivot point was reduced to 36 psi, 26% of the length of the pivot lateral (representing 45% of the area of the field) had an irrigation application depth less than the intended application depth. When the pressure was reduced to 29 psi, 65% of the length of the pivot lateral (representing 87% of the area of the field) had an irrigation application depth less than the intended application depth.

Figure 6: Ratio of simulated to design nozzle discharge for a simulated center pivot at a location in a field with a 1% slope increasing away from the pivot point. An inlet pressure at the pivot point less than the design pressure for the pivot point results in an uneven distribution of irrigation depth (decreases application uniformity). Figure reprinted from Martin et al. (2019).

Guidelines and Additional Resources

Tips for ensuring that your pivot is operating in an ideal pressure range (from the Center Pivot Irrigation Handbook):

-          Obtain a sprinkler package for your pivot and ensure its proper installation

-          Ensure the pump and pivot are properly matched and ensure that engine and pump speeds are correct for the required voltage and pressure

-          Operate the system when crops are small and inspect for broken or plugged sprinklers and regulators, as well as leaks

-          Buy a good pressure gauge and operate the pivot at design pressure

-          Install an additional pressure gauge at the distal end of the pivot, periodically checking the gauge at the highest elevation. The pressure should be at least the required regulator inlet pressure, which is 5 PSI above the pressure regulator rating.

To learn more about the importance of center pivot operating pressure, see the following references:

-          Martin, D. L., D. M. Heeren, S. R. Melvin, and T. Ingram. 2019. Effect of limited water supplies on center pivot performance. Central Plains Irrigation Association (CPIA) Central Plains Irrigation Conference, Kearney, Nebr. 27 pages.

-          Martin, D. L., W. L. Kranz, T. Smith, S. Irmak, C. A. Burr, and R. Yoder. 2018. Center Pivot Irrigation Handbook. Nebraska Extension: Lincoln, Nebr.


This article was reviewed by Troy Ingram & Aaron Nygren

Sign up for updates from UNL Water

Sign Up Here