Working Ranch Magazine January/February 2025

pasture management

angle. Often, a booster pump attached to the valve increases end gun pressure beyond what’s needed for the sprinkler package on the main pivot lateral. The efficiency of center pivots relies on uniform application. Uniformity depends on the spacing of the sprinkler devices along the lateral relative to the wetted diameter of the device. If sprin klers are placed too far apart, uniformity (and often crop yield) declines. Optimal sprinkler spacing provides equal oppor tunity to water all crop rows. Adequate overlap ensures that all plants receive approximately the same amount of water. Devices placed into the canopy may require a sprinkler spacing equal to twice the row spacing to ensure ade quate water availability. Other research has shown that the spacing of devices in corn should be, at most, about 7 feet or about three-row widths. Regarding evaporation losses, “pro ducers often overestimate the amount of water that evaporates into the air,” the publication says. “We con ducted studies with the USDA-ARS at Bushland, Texas, just outside Amarillo, using some of the lysimeters in the world to measure water use. Lysimeters are large boxes filled with soil. The lysimeter is weighed frequently with highly accurate scales. The weight change represents the amount of evap oration or transpiration from the sys tem over a (specific) period.” Using a variety of irrigated and non-irrigated plots, evapotranspiration rates from corn plants were measured. Results illustrated that “evaporative loss of water while droplets are in the air is minimal and evaporation from the canopy offsets about half the reduc tion in transpiration,” UNL’s publica tion says. “Our research demonstrates that evaporation of droplets in the air depends strongly on droplet diameter.” The research revealed that evapo ration of small droplets could exceed 20%, but little evaporation occurs when droplet size is greater than 0.040 inches. Advancing technology makes it easier to manage droplet size. “We recommend selection of devices that have medium to large diameter droplets to minimize evaporation and drift losses,” the publication notes.

Diagram courtesy of the “Center Pivot Irrigation Handbook”

“The only concern for large drops is when irrigating soils with little or no residue cover or crop canopy to protect the soil. Large drops strike the soil with significant amounts of energy that can lead to soil structure breakdown, especially for silt loam and fine sandy loam soils.” Loss of soil structure negatively impacts infiltration rates. Reduced till systems that maintain residue cover help protect the soil surface. Irrigation water runoff occurs when the center pivot applies water at a rate exceeding the soil’s ability to infil trate water. Generally, water accu mulates in soil surface depressions. If the amount exceeds surface storage, water begins flowing across the field. The peak application rate is deter mined by sprinkler package design. The duration of irrigation is directly

related to the depth of water applied. “If runoff occurs, the irrigator should consider speeding up the pivot to apply less water and reduce runoff,” the UNL publication recommends. “Long-term solutions include increas ing the surface storage through till age changes and increased amounts of residue. Reduced tillage has proven to improve infiltration over the long term, which is usually advantageous for center-pivot irrigation. “Center pivots have the potential to be uniform and efficient, but they must be properly designed, installed, and maintained to achieve that potential. The basic requirements for center pivot design are presented in this (publication). Additional details for center pivot management are pre sented in the remaining chapters of the handbook.”

36 I JANUARY / FEBRUARY 2025 WORKING RANCH audited readers run 21 million head of beef cattle.