Ask The Droughtbusters: Oct. 1998

A. If you are hired to repair an irrigation system that you didn’t install and haven’t maintained, an audit is a good idea. Nothing is as accurate as catch-can measurements and running the data though an auditing or spacing program. Of course, what can you realistically change based on the information from an audit? There are certain items you can correct any problems or inefficiencies quickly.

For the answer, we asked Dr. Ed Norum with the Center for Irrigation Technology at California State University, Fresno, Calif. CIT does much of the uniformity testing for manufacturers and has also developed Hyperspace, a computer program that diagnoses uniformity problems.

GOT A MATCH? Manufacturers offer nozzle sets for their sprinklers so contractors can provide “matched precipitation,” according to Norum. The purpose of these changeable nozzles is to provide the same application rate whether a head is full or part circle. “For example, if your full- circle heads have an 8-gpm nozzle, then the half-circle heads should have a 4-gpm nozzle and quarter-heads should have a 2- gpm nozzle,” advised Norum. “This logic is sound, except when it comes to distance of throw. The smaller nozzles will not provide the same throw and, therefore, your spacing of part-circle heads will be off.”

Since contractors are probably not going to change the spacing of part-circle heads right away, Norum suggested using a nozzle one size larger than matched precipitation would specify for part-circle heads. He also recommended using one nozzle size higher for heads on the upwind side of an irrigated site. “Upwind sides can dry out faster and often need 10 percent to 20 percent more water,” he added.

The remainder of the full-circle heads should all have exactly the same nozzle. Because manufacturers can have different nozzle sets, it is always wise to use the same brand and model of sprinkler heads on turf areas. That way you know you have the correct nozzle.

The whole logic behind matched precipitation is to eliminate the need to mask bad uniformity with longer runtimes. “Our research has shown that poor uniformity is usually discovered around part-circle heads and on upwind sides of turf areas,” Norum remarked. “Many of these problems can be resolved by changing nozzles.”

Another key to efficiency can be selecting the appropriate heads on large turf areas, which usually employ multiple zones. For greatest uniformity, Norum recommended that zones should have close to the same number of matching sprinkler heads so that pressure and flow characteristics will match up.

Adjustable heads offer ways to change the breakup of the stream and the range of rotation. If range of rotation is reduced significantly, contractors may need to change the nozzle for matched precipitation. The purpose of changing the breakup is to divert water closer or farther from the head. This helps part-circle heads apply more water closer to the head where it is frequently needed.