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Many center pivot irrigation systems in the Delta are 25 years old or more. Based on their experience with these systems, some growers have a perception that center pivots can’t keep up with crop water demand and are converting to surface irrigation. “It’s a disturbing trend,” says Paul Rodrigue, USDA National Resource Conservation Service supervisory engineer at Grenada, Miss. “These conversions reduce irrigation efficiency and increase demand on already stressed groundwater resources.”
DISTRIBUTION of water from an appropriately configured modern pivot system can be more water-efficient than surface irrigation, a NRCS study demonstrates.
With the shift in crops across the region in recent years, and many center pivot irrigation systems 25 years old older, some growers have a perception that center pivots can’t keep up with demand and are converting to surface irrigation.
“It’s a disturbing trend,” says Paul Rodrigue, USDA National Resource Conservation Service supervisory engineer at Grenada, Miss. “These conversions reduce irrigation efficiency and increase demand on already stressed groundwater resources.”
But NRCS studies of center pivot efficiency were last conducted a quarter-century ago, he notes, and those data lacked relevance for today’s systems and crops.
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“The evaluations that were done 25 years ago tended to show that pivots had the capacity for efficient irrigation of that era’s crops,” Rodrigue says. “But the planned application and the actual application tended to be different. The application rate from the system tended to be higher than most high silt Delta soils could absorb, thus resulting in runoff from where it was intended for the water to be soaked into the ground.
“At that time, the recommendation was for application rates less than 3.5 inches per hour, with lower rates being better. But even at rates lower than 3.5 inches per hour, there was runoff. Approximately 30 percent of the systems had application rates higher than 3.5 inches, and nearly 60 percent had rates greater than 2.5 inches per hour.”
Pivots of that era also tended to run at high system pressures, 75 psi or more, and end guns tended to have higher application rates and lower distribution uniformity than the spans, Rodrigue says.
In cooperation with producer Charlie Craig, the NRCS engineering staff and Coahoma County field office staff, working through the Coahoma Soil and Water Conservation District, evaluated a modern medium pressure system on Craig’s farm at Friar’s Point, Miss., with drops and newer model rotator nozzles.
Measurements included irrigation application under the last two segments, travel speed, and application rate. The end gun was not evaluated.
“The system was set to apply 1-inch of water at a speed setting of 15 percent,” Rodrigue says. “Once it was running, in accordance with safety procedures, we checked to insure there were no stray voltages. Then, collection rain guagues were set out in front of the pivot and the other measurements were made as the system walked across the gauges.”
As soon as the application rate measurements were begun, he says, “It was evident that the new model nozzles were providing a much greater wetted diameter than many older nozzles, insuring that application rates would be lower. All the pivot’s drops were directly below the supply pipe. By bringing the nozzles out to the trusses and alternating sides of the pivot for each drop nozzle, the wetted diameter could be increased and the application rate reduced even more.”
While these measurements were being made, the speed of the end tower was also calculated.