By Bill Kranz, PH. D - Associate Professor & Extensive Irrigation Specialist University of Nebraska Lincoln

The goal of all centre pivot irrigation systems is to deliver irrigation water. If the initial field reconnaissance has taken place correctly, the field elevation change, flow rate, pump outlet pressure and pumping water level all combine to match up with sprinkler requirements. Life is good.

But what should be done with systems that have been in the field for several years? I've found systems with sprinklers missing, and systems where tower boots were spewing water down to the wheel track. There are several aspects of this that can have a very detrimental impact on water application uniformity.

1. Leaking systems

If extra water is coming out of the sprinkler coupler or there is a leaky boot, the pressure delivered to every other sprinkler on the system is less than the sprinkler design chart requires. So what looks to be affecting a small area of the field is actually affecting the whole area. Sprinklers cost less than $50, and replacing a boot won't break the bank either.

2. Temporary sprinklers

Another scenario I see involves replacing a sprinkler with one that you have lying around so that the geyser discussed earlier is not visible. Most times the owner has good intentions of replacing the sprinkler with the correct one as soon as a replacement is purchased. But then a week or two goes by, and now he has forgotten to purchase that replacement and he also needs to remember just which one of the sprinklers needs to be replaced.

A field I visited actually had three different types of sprinklers installed on the pivot. the long-term issue with making these temporary fixes is that the sprinkler may not have the appropriate distribution pattern and in most cases, the volume of the water delivered in these areas is not even close to what is on the sprinkler package chart. If the sprinkler in question is near the outside edge of the centre pivot, one sprinkler can cause problems for a number of acres. Again, $30 will buy a replacement that fits the system properly.

3. Sprinkler package design

A third issue is when the current flow rate and pressure does not match the design of the sprinkler package. Consider a situation where the original design of the sprinkler package called for 750 gallons per minute (2839 litres per minute) at 40 pounds per square inch (2.75 bar) at the pump outlet. Now change the situation so that the pump outlet pressure is only 30 psi (2.06 bar). The flow rate at 30 psi (2.06 bar) is close to 660 gpm (2498 litres per minute) and the part of the system most impacted is at the distal end of the pivot, where again, a large number of acres are impacted not only by the reduced flow rate, but the water distribution pattern is altered in a negative way.

The field productivity suffers as a result of the reduced flow rate and pressure. Installing, maintaining and monitoring the pressure being supplied to the centre pivot using a $40 pressure gauge will help identify when this sort of problem is developing.  

4. Sprinkler past life expectancy

Sprinklers have a predetermined lifespan and just cease to function properly. For example, when sprinklers begin having issues, two things normally happen: 1) part of the sprinkler is broken so that it rotates at a much faster rate than normal; and 2) the sprinkler seizes up and stops rotating all together. In both cases, water continues to be distributed by the sprinkler. However, the water distribution uniformity delivered by the sprinkler that seizes up is much lower than it should be because the deflection pad is no longer rotating.  

Remember that sprinklers are designed for an expected lifetime of about 12,000- 13,000 hours of operation. If your sprinklers have been on your system for 15 years, there’s a good chance that some of these problems have already started. The interesting thing about this problem is that the malfunctioning sprinklers are easy to spot from the edge of the field during routine checks throughout the irrigation season.

5. Sprinkler spacing

A problem that has developed in recent years is restricted to those systems that are used to irrigate corn and other tall crops. Several years ago, corn hybrids grew to a final height of between 8 and 9 feet (2.43 and 2.74 m). More recent hybrids grow to a final height of 12 feet (3.6m) or more. Thus, tall corn means that low pressure spray sprinklers installed on drop tubes will be operating in the corn canopy for much of the irrigation season. The corn leaves intercept the water pattern, creating devastating effects on grain yield. Field data collected in Western Nebraska found that corn irrigated halfway between the sprinkler yielded 40 bushels less per acre than corn irrigated directly under the sprinklers mounted on drop tubes.

The issue here was not that the sprinklers were operating in the crop canopy- it was that the sprinkler spacing was too wide for these new corn hybrids. If this system was used to apply fertilizer, both water and fertilizer were not applied very uniformly. Work conducted in Nebraska and Kansas suggest that the maximum spacing between sprinklers should be less than 7.5 feet (2.28m) if the system is used to irrigate corn. Again, it is easy to eliminate this problem.

6. Sprinkler positioning

The final concern is sprinklers that are not installed on the centre pivot in the correct positions. And yes it still happens. One new centre pivot had a mixture of sprinklers installed between the 5th and 6th towers. The grower did not check the system out, but the yield monitor on his combine showed that the corn yield dropped by 80 bushels per acre in the area irrigated by the 5th span, and he was not impressed to say least.

Comparing the sprinkler package design chart with the sprinklers that are installed on the pivot can eliminate these types of problems. Verifying the sprinkler package should only need to happen once in the lifetime of the sprinkler package. The printout gives coupler number, sprinkler nozzle size, and distance from the pivot point. Often the printouts are placed in the pivot control panels. If not, contact the company that installed the sprinkler package sand request a copy of the printout.

Most of the items described are quite easy to remedy. The fix is often less than $100, and does not require very much time. However, failure to keep tabs on your centre pivot can result in reduced yields for a significant impact on your bottom line.