Center pivot sprinkler systems really kicked into operation when July and drier weather arrived. Even the pivots on corn haven't had to be used as much as usual. However, it is very likely that the pivot systems will have to make several circles to meet the cotton and soybean water needs during the next two months and maybe into early September.
Pivots can and should be very effective at irrigating if they are managed properly. As with all our irrigation methods, they are not going to be as effective if drainage is limiting. They can be used to water fields that aren't practical for surface irrigation, but the need for good surface drainage is still important.
The combination of poor drainage and rutting can result in down time, extra labor and crop damage when pivots get stuck frequently.
Irrigation timing is important with any irrigation method, but it is especially critical that pivots be started early and run frequently enough to avoid getting behind.
A pivot irrigation will not wet the soil much deeper than about 8 inches in most fields. The soil moisture below this depth should be protected by starting irrigation early before the roots extract the deeper moisture.
I think the concern over causing a shallow root system from watering too early is really not a problem, especially under a pivot. There is a bigger risk of getting behind and not catching up, and if there is still deeper moisture for the roots to get, it will help avoid moisture stress.
Most growers apply about 1 inch of irrigation when they turn a pivot circle if it doesn't cause too much runoff. Unfortunately, some think that doing this every week is an adequate amount of irrigation. The reality is that this application needs to be made about every three to four days, or twice a week if no rainfall is received. This frequency is required to keep up when the crop is using 1.5 to 2 inches of water in a week.
Another way to think about it is that it takes two to three circles of the pivot to equal what is done with one furrow or flood irrigation. In other words, if you furrow irrigate a cotton field four times, then you should probably irrigate your cotton that is under the pivot about 10 times.
Since pivots may make several circles in a season, it is possible that an uneven water application pattern can develop. This can occur because of the difference in application efficiency during day and nighttime hours. If the pivot is started at the same point and at almost the same time on each irrigation, then the part of the field that always gets watered during the daytime will not receive as much water as what gets watered during the night each time.
The cumulative effect of this through the season can result in reduced yield in the part of field that is always watered during the day.
This effect can be reduced by staggering the irrigation starting time by 12 hours on every other irrigation so the day and nighttime irrigated areas are alternated.
Even with breakdowns and sticking problems, pivots usually still require less labor than surface irrigation methods. However, some growers complain that they don't get yields as good under their pivots as they do with other irrigation methods.
Several problems could be contributing to this: inadequate drainage, overdue system and/or sprinkler maintenance, delaying irrigation, not watering often enough, and creation of uneven water application patterns.
Research and Extension studies continue to show yields under center pivots are comparable to the other irrigation methods if the problems are addressed. If your pivot yields aren't what you think they should be, then one or more of these problems may need to be addressed.
This is one of several articles on drainage and irrigation water management. If you have questions or suggestions on topics please contact me: Phil Tacker, 501-671-2267 (office), 501-671-2303 (fax), 501-944-0708 (cell), or firstname.lastname@example.org (e-mail).
Phil Tacker is an Arkansas Extension ag engineer.