Achieving maximum efficiency from your center pivot starts with limiting runoff and ensuring uniform water application and infiltration. With the early hot, dry start to the season, getting the most from each irrigation will be important in managing input costs.
Runoff
If the water you apply doesn’t soak in uniformly, you are wasting the application. The only real way to know if you have runoff in the field is to get out and look. Look at the last two spans of the pivot on the steepest portion of the field. These spans have the highest water application rate and this field area offers the best chance to spot runoff.
If you have runoff, take steps to reduce or eliminate it. In the short-term, reduce application amounts and/or move sprinklers out of the canopy. Moving sprinklers above the canopy will increase wetted diameter and decrease application rate. In the long-term, you can install a new sprinkler package with a greater wetted diameter and/or use no-till to increase residue on the soil surface and improve the soil infiltration rate. If compaction is reducing infiltration, a deep tillage operation may be needed.
Check Sprinkler Spacing
Figure 1. Wide spacing between pivot irrigaiton sprinkler heads can lead to non-uniform, yield-limiting water application.
One aspect of center pivot system design that can result in non-uniform water application is sprinklers that are spaced too widely apart. Spacing issues can be exacerbated by positioning sprinklers to operate within the crop canopy. Unfortunately, this problem often is not obvious and may only become apparent during extremely dry weather when continued non-uniform water application results in dry areas between sprinklers. In 2004 we saw this issue from aerial photos of pivots showing well-watered and under-watered rings. When sprinklers are placed in the canopy, the corn leaves and stalks interrupt the water pattern. When sprinklers are operated above the canopy, the overlap between patterns is usually adequate, but may become limited once corn grows above the sprinkler height. Figure 1 shows how sprinkler location affected actual applied depth of water.
Research indicates that if sprinklers are located too far apart, generally greater than 7.5 feet, you may have dry rings around the field and their impact may not always be visible. This phenomenon will increase as the pivot makes more and more circles. The wet areas will have normal growth while the dryer areas will be shorter in stature and yield may be affected. In the 2004 study, yields in the dry areas were 20-40 bushels less than in the well-watered rings. This phenomenon likely occurs every year. Normal early season precipitation will mask this effect during vegetative growth, but later in the season it may affect yield. Unless you go out into the field and look, your yield monitor will not pick up the difference in yield as it takes in both the well-watered and under-watered rows together. After an application go into the field and use a soil probe to check the soil water content in several consecutive rows that are perpendicular to the length of the pivot. If one row is extremely wet and the next is almost dry, you have application issues.
Perform an Irrigation System Check-up
Here are some additional steps you can take to ensure your pivot is operating at peak efficiency.
Check for plugged or damaged sprinklers and pressure regulators.
Obtain a sprinkler chart from your dealer and check to be sure your sprinkler package agrees with the printout. You just have to do this once unless sprinklers are replaced with sprinklers you just happen to have around.
Buy a pressure gauge and make sure your system is operated at the designed pressure. Pressure regulators are important to low-pressure sprinkler packages and will not function properly unless the delivery line pressure is at least 5 psi greater than the pressure rating of the regulator. A non-functioning regulator means non-uniform water application.
Chuck Burr Extension Educator, Phelps County Bill Kranz Extension Irrigation Specialist, Haskell Ag Lab, Northeast REC
