March 14, 2013

The drought of 2012 has many irrigators trying to forget the past. Lack of rainfall and high daily ET rates kept irrigators on the run just trying to keep up. However, this is an excellent time to remember what occurred last summer and make corrections before the irrigation season begins again.

If your field looked those in these photos (Figures 1-3), consider installing a new nozzle package or making other system changes. If you look at a yield map and notice rings of lower yields, you may need to check your system. Remember however, that with 12- to 16-row corn heads, you are harvesting grain for plants watered by multiple nozzles, so lower yielding rings may not show up. The best time to check sprinklers to look for poor application patterns and broken sprinklers is before you plant the crop. When it is time to irrigate, the corn height will likely impair your vision to see obvious problems.

It also takes a year like 2012 to more fully see how some water distribution systems are growing older or underperforming. Little problems that affect water application uniformity can reduce yield. In a “normal” year these likely wouldn't be visible. However, the impact is visible and the yield reduction is likely to be more evident in a year like 2012.

All you had to do is fly over center pivot-irrigated fields to get a birds-eye view of where water application uniformity could be improved. Figure 1 shows a field exhibiting some of these distribution issues. Such problems can be caused by a number of things:

• Sprinklers can blow out of their hole, creating a geyser on the pivot pipeline.
• Sprinklers can wear out and stop rotating or rotate out of control.
• Pumping water level declines so that the system flow rate and pressure no longer match the original design.
• Boots, gaskets, and other seals begin to leak.
• Sprinkler spacing is too wide, leaving gaps of unwatered crop.

Each of these sprinkler issues can result in extra water being applied is some areas and insufficient water being applied in others. In a year like 2012 neither outcome is desirable. In one study, this inconsistency amounted to a yield decrease of 20-40 bu/ac for the affected field areas.

Most of the problems listed here are easy and relatively inexpensive to fix. However, depending on the location of the problem along the pivot pipeline, the economic impact of a single sprinkler or gasket problem can be significant in the amount of yield reduction it causes.

Assessing your irrigation water delivery system now can help you see exactly where the problem is on the pipeline before it affects your crop.

Also remember that problems like those you identified in 2012 likely have been having a subtle impact on grain or forage yield for a number of years. This means that even though the yield impact was more significant in 2012, a small yield loss likely occurs even when rainfall is more normal.

Checklist for Maintaining Your Pivot

Don’t let a poorly maintained system eat into your bottom line. Capitalize on the long irrigation season to assess the water delivery from your pivot, record the problem, and, if possible now, fix the problems and test the solutions. The following checklist covers seven steps to maintaining and managing your pivot for more effective irrigation.

1. Obtain the sprinkler chart for your center pivot and ensure that the package was installed properly. (Check with your dealer for a replacement copy if needed.)
    
2. Determine whether the system capacity is adequate for your location. Check the UNL NebGuide, Minimum Center Pivot Design Capacities in Nebraska (G1851), for more information.
    
3. Ensure that pump and pivot are properly matched. Make sure that the engine and pump speeds are correct for needed voltage or hydraulic pressure and for pressure at the pivot inlet, as well as for engine performance.
    
4. Buy a good pressure gage and operate the center pivot system at the design pressure. It is also a good idea to install a pressure gage at the distal end of the pivot. Periodically check the pressure at the far end of the pivot at its highest elevation. Pressure should be at least 5 psi above the pressure regulator rating.
    
5. Operate the system when crops are small and look for broken or plugged sprinklers or pressure regulators and leaks. If you have questions about the suitability of the existing sprinkler/regulator package, install a new regulator and sprinkler (with the proper nozzle) in the middle of each span and observe any differences between performance of new components and existing devices.
    
6. Observe water application in the outermost span on the steepest portion of the field and the soils with the lowest infiltration rate to see if you have runoff problems. If problems exist:
   
   — Reduce the application depth.
   — Use reduced tillage to enhance surface storage and infiltration.
   — Eventually, evaluate your system and consider whether a different sprinkler package is necessary. Select sprinkler devices that provide at least as much wetted diameter as required in selection procedure.
   — Select devices with large droplet sizes when renozzeling. If you irrigate a significant portion of the year on soils (especially fine sandy loam and silt loam soils) without residue cover, you may want to choose devices that provide medium diameter droplets.
    
7. Routinely maintain mechanical/electrical/hydraulic components.

Chuck Burr,  Water/Cropping Systems Extension Educator
Derrel Martin, Extension Irrigation Specialist
Bill Kranz, Irrigation Specialist, Northeast REC
Gary Zoubek, Extension Educator, York County