Author: Matthew Lashinsky, Sprays Product Manager for Hunter Industries

If you ask most irrigation professionals how the efficiency of a sprinkler nozzle is determined, most would answer with “distribution uniformity” (DU). DU has been a buzzword for a number of years now for performance of rotors and spray nozzles. As one of the primary marketing points by all manufacturers, it has developed a significant rooting in consumers’ minds. Adopted by several standards committees as a primary metric, and used by professionals at every level of the industry, DU has become the dominant catchall word for efficiency.

Although DU is a good metric for understanding nozzle performance and efficiency, it does not tell the entire story. As the title suggests, DU calculates the evenness of water applied to a given landscape. It is an important metric, as uneven application of water leads to wasted water. However, efficiency in irrigation is far more complex than uniformity alone.

Consider another commonly evaluated metric of rotors and nozzles: precipitation rate. This is typically listed by manufactures for the sake of determining scheduling. But it turns out this is more relevant than punching numbers into the clock. Runoff is the evil nemesis of efficient irrigation. Typical spray nozzles have precipitation rates of more than 2 inches per hour, while most soils have infiltration rates of less than 0.5 inches per hour. In this example, runoff is inevitable. More than half the water is rejected by the soil and runs down the drain, along with the fertilizer, lawn treatments, and homeowners’ hard-earned money.

The solution to this dilemma is to control the precipitation rate. Using a nozzle with a low precipitation rate allows designers and installers a worry-free approach to irrigation. The lower the precipitation rate of the nozzle, the more universal the application for that nozzle. Multi-stream rotary nozzles typically have lower precipitation rates than traditional nozzles due to their design. For instance, the Hunter MP Rotator has the industry’s lowest precipitation rate at 0.4 in/hr.

When determining efficiency ratings, nozzles are tested under ideal conditions in a lab. Water pressure, temperature, and wind are firmly controlled in this ideal setting. In the real world, windy conditions and high system pressures are the norm. This leads to symptoms that are not accounted for by distribution uniformity, especially when tested under ideal lab conditions. Depending on nozzle design, high pressures can cause severe spray atomization. Conventional spray nozzles can produce atomization to a point where the water particle size is light enough to be carried by air and never reach the ground. At 70 psi, it’s estimated that certain nozzles atomize up to 50% of the water exiting the nozzle. This means that maximum possible efficiency under these conditions would be 50%.

Multi-stream rotary nozzles have a unique ability to withstand atomization at higher pressures. This is due to the nature of the design, as pressure-fueled high velocity streams are the desired outcome in a multi-stream rotary nozzle.

When considering the details, it becomes clear that the story of efficiency is deeper than it appears. The one-dimensional view of efficiency determined by DU will change as industry professionals begin to see in multiple dimensions.