Selecting sprinkler packages about more than price (Choice Voice)

(Editor’s note: James Adkins is an irrigation engineer with the University of Delaware, Carvel REC.)

(Jan. 16, 2018) Purchasing the ideal sprinkler for your crop and soil type and irrigation capacities is a critical component of an efficient center pivot system.
Unfortunately, center pivot purchase decisions are often based on price with little regard to whether the type of sprinkler package is appropriate for the operation.
The primary function of an irrigation system is to apply water uniformly in sufficient quantities to meet the crop needs without generating runoff. Uniform application requires that the correct sprinkler be installed at the various positions along the pivot and that the system is operating at the design pressure.
Many center pivot systems were converted in the eighties and nineties from top mounted sprinklers to drop nozzles in an effort to reduce evaporative losses.
This effort was the direct result of research conducted in the Southwest demonstrating a 3 to 6 percent reduction in water lost to evaporation when using 6-foot drop nozzles.
This research was conducted in the high plains of Texas in 20 percent humidity, high winds and extreme heat.
A corn canopy destroys drop nozzle uniformity but this can be overcome by reducing sprinkler spacing. Drop nozzles also add considerable cost to a system, are prone to rust plugging, and require additional maintenance due to goosenecks breaking and leaks.
Once the measured losses are adjusted for the 70-80 percent humidity levels typical in the Mid-Atlantic, the efficiency of a drop nozzle cannot overcome its many disadvantages.
Application Rate
vs. Infiltration Rate
Delmarva sandy soils can have an infiltration rate exceeding 1 inch per hour without runoff.
As the silt and clay content increases, this infiltration rate can be reduced into the 0.4- to 0.6-inch-per-hour range, with slope further reducing the intake rate.
A typical five-tower center pivot with drop nozzles yielding a 5-foot spray pattern will have an instantaneous application rate in excess of 7 inches per hour in a corn canopy while that same nozzle mounted on top of the system can spray 25 feet in diameter with a net application rate of around 1.5 inches per hour.
Likewise, the wobbler, orbiter and rotator sprinkler types mounted on top can yield 40-foot wetted diameters with last span application rates in the 0.7 to 0.9 inch-per-hour range.
In the words of a wise farmer friend of mine, the farther you can throw the water the better the “soak time.”
In order to minimize any off-target water movement in the field, top-mounted nozzles with a large wetted diameter is advantageous.
An ancillary benefit of sprinklers with large wetted diameters is the potential to increase nozzle spacing without sacrificing uniformity thus increasing the orifice diameter and reducing the potential for plugging.
In an ideal scenario where all of the irrigation system components are operating at design specifications, pressure regulators are not necessary (e.g., constant pressure, constant dynamic well level, static corner state, no elevation change).
But, once one of these factors start to degrade pressure, regulators become very important.
All corner systems need pressure regulators installed to compensate for the drastic change in system operating pressure with the corner span closed (and nozzles off).
The larger the orifice, the greater the effect a change in pressure will have on the total volumetric output.
In other words, a change in system pressure will increase the application rate at the end of the pivot more than at the beginning.
Unfortunately, the end nozzles cover the most area further amplifying the negative effects on uniformity. While I recommend pressure regulators on all systems to ensure uniformity, they are required with wobbler/orbiter/rotator sprinklers.
The cost increase going from the bottom of the line sprinkler package to a pressure regulated high tech sprinkler will run roughly $2,000 extra for a 60-acre system or $1.30 per acre per year over the 20-year lifespan of the sprinkler.
A relatively small investment in a quality sprinkler package can make a measurable benefit in application uniformity and likewise improvement in crop yield.