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Center Pivot End Gun Considerations

Center Pivot End Gun Considerations
By Steve Melvin 
 
End guns have been on center pivots from almost the beginning and the merits of their use have been debated about as long. Some farmers would not have a pivot without an end gun and others would not have a pivot with one. Center pivots are designed to have the lateral pipe just long enough to irrigate as large of a circle or part circle in the field as possible and still not go past the property boundaries. So as you can imagine, the corners of a square field cannot be irrigated.
 
End guns are a large sprinkler that can deliver water up to 120 feet on the outer end of the machine. It is turned on when the pivot goes past the corners to provide a low cost method of irrigating an extra 8-10 acres on a 160 acre field (one half mile square field). The downside is that it is the most inefficient part of the pivot because it does not apply the water very uniformly.
 
Challenges Using End Guns
 
The poor uniformity is caused by multiple factors. First, the application pattern of the sprinkler is not that uniform even in low wind environments when operating as an end gun. Moderate to high wind increases the problem because the water is sprayed a long distance and high into the air resulting in the wind distorting the application pattern and causing high drift and evaporation losses. In addition, the water application depth drops off dramatically towards the outside edge making it difficult to decide how far to plant the crop past the end of the pivot. Plant too far and the crop will suffer drought stress and if not planted far enough will waste water and may stimulate the growth of large weeds. Weed, insect, and plant disease pressure can build up in this area that can move into the rest of the field (Figure 1).
 
Another challenge is keeping the water off the public roadway. Occasionally the shut off device will fail, and water is applied to the road, which must be fixed as soon as possible, but the bigger challenge is deciding at what angle to turn off the end gun. The sprinkler must be turned off well before it gets to the road not only to prevent spraying the road, but also to prevent water from drifting onto the road on windy days (Figure 2). The result is leaving part of the corner dry. The problem can be somewhat alleviated by adding a second smaller end gun with a shorter throw and usually less wind drift potential.
 
Figure 1. The water application depth drops off dramatically towards the outside edge of the end gun pattern making it difficult to decide how far to plant the crop past the end of the pivot. Plant too far and the crop will suffer drought stress and if not planted far enough will waste water and may stimulate the growth of large weeds. (Photo by Steve Melvin)
 
Figure 2. Large end guns cover the most acres but need to be turned off before the entire corner can be irrigated. A second smaller end gun can fill in more of the corners. Many newer pivots have the ability to control two end guns. (Drawing courtesy of Nelson Irrigation.)
 
The extra water the end gun needs must be taken into account when sizing the pump. However, when the end gun is turned off the extra capacity of the pump will generate higher pressure in the system than needed. The extra pressure will not cause any problems with the system, however it will cause the pump to use more energy than is necessary.
 
As you can see, there are several reasons not to use end guns, so how can one come to a good conclusion. The decision of using an end gun should be made based on the availability of water, the cost of pumping the water, and most importantly, will the corner be planted to the same crop as the pivot. If the same crop will be planted into the corners, then the nonuniformity around the outside edge will not result in wasted water.
 
End guns, like every other mechanical device, need to be well designed, maintained, adjusted correctly, and operated at the correct pressure. The end gun is usually designed correctly when the end gun was included as part of the overall sprinkler package design. The sprinkler chart describes how the end gun is to be setup. It includes the needed pressure, the designed part circle arc angles, and the suggested throw radius. If you do not have the sprinkler chart for your system, your dealer can give you a new one.
 
The biggest problem with end guns in operation today and a big part of the reason they get such a bad rap is the operating pressure is not correct. Low pressure is the most common problem resulting in the sprinkler being very aggressive and having extremely poor uniformity. When the pressure is too high, the result is a lot of mist and drift. So, making sure the pressure is at the design rate shown in the sprinkler chart when the end gun is at the highest point in the field is the most important maintenance issue for maintaining the water application efficiency for both the end gun as well as all the other sprinklers.
 
A good method to visually inspect an end gun is to look at the end gun at a right angle from the pivot lateral while it is spraying water. Then determine if the arc is about right based on what is listed in the sprinkler chart. In addition, does the water pattern and the movement of the gun look correct? A typical design could be 100 degrees to the reverse and 70 to the forward for a total of a 170 degree arc (Figure 3). The 100 degrees reverse helps insure good overlap with the sprinklers on the pivot lateral under windy conditions.
 
Figure 3. The sprinkler chart will show the design setup for the end gun. It is important to visually inspect the function of the end gun every few weeks during operation. (Drawing courtesy of Nelson Irrigation.)
 
Conclusion
 
End guns can provide a low-cost method of irrigating more acres with a center pivot. The device should be well designed, maintained, adjusted correctly, and operated at the correct pressure to optimize efficiency. However, they are less efficient than the sprinklers on the lateral pipe because they do not apply the water very uniformly and thus should not be used in every situation. Such as when the corners are planted to a different crop, or when water supplies are short or expensive.
Source : unl.edu