Variable-rate irrigation could make center pivots much more efficient users of water, especially where there are non-crop areas or variations in soil type or crop mix under the watering circle.
Those conditions are prevalent in south Georgia, noted Calvin Perry, research and Extension engineer, University of Georgia, who helped develop a variable-rate center pivot which is now being offered commercially. Perry spoke about the system during InfoAg Mid-South, in Tunica, Miss.
Variable-rate irrigation has an especially good fit in south Georgia, where there are around 10,000 center pivots, many of which are partial pivots or pivots which overlap with others, according to Perry. With tree lines, neighboring fields and riparian zones often intruding into the circle, “putting in a pivot often is like putting a round peg in a square hole.”
The results can be overwatering in places where pivots overlap and off-target watering where the pivot passes through non-crop areas, noted Perry.
“And there are no uniform fields in the Southeast. We have a lot of soil changes within our fields, and some of our farmers have multiple crops in different stages of development, all being watered by the same pivot.”
End guns close to roadways often create a rural phenomenon Perry jokingly calls “a south Georgia carwash,” which can test the verve, and perhaps the swerve, of an unwary motorist. “About 90 percent of our pivots have end guns, but only about 30 percent have some means of control.”
There is a form of precision irrigation technology currently on the market, said Perry. But the method, which allows the user to slow down or speed up the pivot at certain points in the circle, “only varies water application in wedge shapes.
But the new technology is true variable-rate irrigation. “You can vary the rates based on the water requirements of zones in the field.”
The National Environmentally Sound Production Agricultural Laboratory at the University of Georgia and Farmscan, in western Australia, developed the technology, which uses compressed air, solenoids, valves and a GPS to turn individual sprinkler heads on and off as the pivot moves through the field.
To get started, the farmer uses a combination of yield maps, aerial photos, soil maps, etc., to help rough out zones of variability in a center-pivot irrigated field.
Set up at the computer is simple for the user, according to Perry. When the software program that controls the technology is opened, the center pivot appears as a circle divided into small grids whose size is determined in part by the swath of the sprinkler head. Once the grids have been selected, “you simply select a rate, from zero percent of normal to 200 percent of normal. You just paint it in.”
The normal rate would be what the farmer would apply as his base rate on the field, according to Perry. “So if a farmer wanted to apply a half inch, if he went with 200 percent on part of the field, it would get 1 inch.”
The rate is adjusted through a combination of increasing or decreasing the speed of the pivot and by changing the frequency at which selected sprinklers turn on and off as they move through certain parts of the field. The end gun can also be turned on and off.
The system is very reliable and rugged, “and we've tested it to make sure that it doesn't affect the overall performance of the pivot. But if your field is uniform, it's not going to help. If it's not uniform, it is.”
On one farmer field where the system is being used, there is an overlapping pivot, a sandy ridge and an airplane hanger within the circle. “Variable-rate irrigation is important to the farmer because he waters from a lakebed. He tells us that he can get one additional watering from the water he saves during a variable-rate irrigation.
“In most cases, variable-rate irrigation will save water, improve irrigation management decisions and reduce disease problems in areas that are traditionally overwatered. There is also the potential to reduce costs and increase yields.”
Perry noted that if a farmer “is technology shy, I wouldn't expect him to be able to work with a system like this without a good consultant to hold his hand.”
He added that there is the potential for lightening damage “as there is for anything that's sitting in the middle of a field that is metal. In addition, there are pressure fluctuations when individual sprinklers are turned on and off.”
Work is under way for a variable-rate lateral overhead irrigation system.
Perry noted that a companion project through Cotton Incorporated uses wireless transmission of soil moisture data, “so we can get real-time feedback from the field to help adjust variable-rate control to match the exact, up-to-the minute needs of a field.”
The system was commercialized by Hobbs and Holder Precision Irrigation Technologies and costs run about $15,000 for a 110-acre pivot.
Cost-share money was available for several systems, noted Perry, but only in the Flint River Basin of southwest Georgia. “We had enough money to install 23 systems. The Flint River Soil and Water Conservation District, University of Georgia Precision Team, NRCS and the Nature Conservancy teamed up to run the project.
“The Nature Conservancy funded a full-time person to help us. They want the water in Flint River to remain flowing because of wildlife. They've been great partners.”
Also providing some momentum to variable-rate irrigation is pressure from the Georgia state legislature for farmers to conserve water. “And the state of Florida thinks we are using too much water.
“We anticipate regulatory action in the future that may require use of certain conservation measures for our farmers. Georgia has already passed a metering law where meters are to be placed on every water withdrawal system by 2009. Our farmers know that if you can meter what they're using, the next step is to limit how much that can use.”
More information on the system is available on-line at www.betterpivots.com.