Less with More

Lowering average application rate expands potential of center pivots

Published online: Jul 03, 2018 Irrigation, New Products Jon Johnston, Irrigation Accessories Co.
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This article appears in the July 2018 issue of Potato Grower.

In 1997, Bradley King and Dennis Kincaid with the University of Idaho published a paper entitled “Optimal Performance from Center Pivot Sprinkler Systems.” In that paper, they stated, “The main disadvantage of center pivot irrigation systems is the high water application rates under their outer spans to compensate for the increased rate of travel.”  

On a typical quarter-mile pivot, the last tower travels seven times faster than the first tower. Sprinkler packages are designed with incrementally increasing nozzle sizes to compensate for the increasing rate of travel and increased area of coverage.

That speed difference translates into 2.35 feet per minute at the first tower and 18.8 feet per minute on the last. In the same amount of time, the first tower is covering just 1.9 acres compared to 24.5 acres on the last tower.

In that same paper, King and Kincaid determined, “Application rates under the outer spans of the standard quarter-mile-long, low-pressure center pivot normally exceed infiltration rate and result in runoff.”

But they did not just identify a problem. They recommended a solution: “The application rate of low-pressure spray sprinklers can be reduced by using offset booms on alternate sides of the center pivot lateral.”

Boombacks, sometimes called offset booms, are add-on accessories for pivots or linears to re-locate or offset the sprinkler 15 feet away from the center line of the span. Spreading out the sprinklers using Boombacks, even with the larger nozzle sizes, can lower average application rate and decrease application intensity.

Howard Neibling, an associate professor and irrigation specialist with the University of Idaho, created a graph depicting infiltration rates using Boombacks. The dotted line represnets the soil’s infiltration rate. That rate will vary according to the type of soil, but all soil types have the same basic curve. The line may be higher up on the graph for tight clay soils and lower down on the graph for looser, sandy soils. The bottom of the graph represents time. As indicated, in the beginning the soil can absorb more water. Over time, the ground becomes saturated and is no longer able to absorb water at the same rate. The curve of the infiltration rate almost flattens out over time. The first arc is from a 45-foot diameter sprinkler. Anything above the dotted infiltration rate line is potential runoff.  The objective is to flatten out the application curve to match the soil infiltration rate.

Adding Boombacks flattens out the application curve by increasing the time of travel. A pivot with Boombacks increasing the wetted footprint takes longer to pass the same point. The amount of water being applied did not change; only the time it took to apply it increased. The area above the dotted infiltration rate line has shrunk dramatically, limiting the amount of water available for potential runoff because of the increased soak time.

Typically, the first thing people do when they see runoff is to speed up their pivot. What happens when you speed up your pivot? You reduce the amount of water being applied. You cannot control the average application rate by adjusting the speed of the pivot. The average application rate is a constant determined by factors other than the speed of the pivot. The average application rate is the same if the pivot is sitting still or moving at maximum speed. The only thing you control with the speed of the pivot is the depth of water being applied. Speeding up the pivot may have eliminated any runoff, but now you are not getting the root penetration or the appropriate amount of water necessary to maximize yields. 

A point under any traditional drop spray pattern will be watered by five or more different sprinklers. All sprinkler manufacturers recommend a 150 to 200 percent overlap for optimal uniformity. The impact to the ground from five or more sprinklers increases application intensity and can lead to soil sealing or crusting. Boombacks can reduce the application intensity significantly by spreading out the sprinklers. 

One large operation wanted to germinate carrots seeds with a pivot. The application rate required was too high for the ground to absorb—especially at the end of the pivot. The grower installed Boombacks on every outlet in opposing directions on the last third of one of his pivots. Over a three-year period, the solution was so effective, he installed several more pivots with Boombacks. After the first three years, an interview with the grower revealed valuable results.

“We have compiled data on all the different ways of irrigating: pivots, pivots with Boombacks, hand lines, wheel lines, drip and flood,” the grower said. “We determined using pivots with Boombacks is more than a solution for carrot germination. We are now putting the water into the ground more efficiently and reaping the benefits.

“The 33 percent water savings is coming from the eliminated runoff. We are now achieving deeper root penetration. All the listed benefits are generated by a more efficient use of water. When compared head-to-head, pivots with Boombacks are by far the most economical way to irrigate.”