Choosing a sprinkler: The ABCs of DRCs

Ideally, irrigation systems apply water like rain--uniformly. But a typical sprinkler's basic throw pattern shows that it applies more water close to the head and less water near the outer edge of its radius. This is why we space sprinklers so carefully--to make up for these differences.

Knowledge of a system's application patterns can help you make sound decisions that improve your system's application efficiency and thereby save water. We call this pattern or curve the single-leg distribution rate curve (DRC).

The term single leg refers to the pattern of a single ray originating from the center of a circle drawn around the sprinkler. On rotor sprinklers, all legs or rays exhibit the same DRC when operated in non-windy conditions.

The single-leg DRC isn't as valid for pop-up sprays because of variability between rays. Research suggests that you truly need a three-dimensional depiction of a pop-up spray to be able to compare pop-up sprinkler performance.

Figure 1 (opposite page) depicts the effect of overlapping and complementing sprinklers. Designers often space sprinklers at the effective radius recommended by the manufacturer. The figure shows each sprinkler's DRC as a smooth curve. The figure also shows that the actual depth of applied irrigation water varies due to the combined effect of multiple sprinklers. You can see how the desired minimum application is a flat horizontal line indicating the minimum amount of water you must apply to keep the turf healthy and aesthetically appealing.

To put Figure 1 into perspective, understand that the low spots of the actual application must be at or above the desired minimum application. This is because we assume that any turf receiving less irrigation than that minimum likely will show stress. Of course, this figure is idealized and probably too simplistic in some ways. After all, the shape of a real-world DRC is never so smooth, and the actual depth of application varies much more dramatically.

With that in mind, imagine--in general--what happens if you stretch the sprinkler spacing. Why would anyone stretch sprinkler spacings and exceed the manufacturer's recommended maximum? So one sprinkler contractor can underbid competitors by using fewer sprinklers. Fewer sprinklers translates to fewer laterals, less piping, less wiring, fewer controller stations, etc.

When a low-ball sprinkler contractor stretches the sprinkler spacings, the DRC remains the same, but the sprinklers no longer can throw to their complementing sprinklers. Imagine if you sketch this new scenario out into an illustration similar to that in Figure 1. You'll find that a dip forms in the middle area between the sprinklers. The dip represents less applied water and, consequently, inadequately irrigated turf.

In this situation, to achieve the minimum depth of application between sprinklers--where even less water now is applied--you will begin wasting large amounts of water near the sprinklers. An astute buyer recognizes the inherent problem in a bid showing stretched spacings. He or she then contracts with a different installer who presents a sound and water-efficient design. Unfortunately, however, some buyers take the low-cost system, regardless. They are ultimately saddled with poor uniformity and higher annual water bills.

Differences in DRCs Laboratory researchers determine sprinkler DRCs, so wind doesn't distort the sprinkler patterns. For these tests, researchers operate a sprinkler with a given nozzle at a known pressure. They space catch cans evenly away from the sprinkler and then measure the depth of water in each receptacle, recording it after completing the test.

To thoroughly understand what spacing between sprinklers is best, you must use the actual DRC for the sprinkler and nozzle in question, operating at a specified pressure. Figure 2 (above right) shows several representative DRCs.

DRC "A" is ideal--and non-existent in real life. Its shape is that of a wedge. Imagine another sprinkler, with a similar wedge-shaped DRC, located at the tip of the wedge. The two wedges would complement each other perfectly, and the resulting application from both sprinklers would be perfectly flat, resulting in perfectly irrigated turf. As you well know, no one can achieve this ideal DRC.

Thus, consider DRC "B," an actual DRC for an actual sprinkler/pressure/nozzle combination. If you were considering an irrigation system, would you choose this sprinkler if you knew its DRC looked like this? Of course not. The irregularities in this system's DRC would make it virtually impossible to achieve a uniform, efficient irrigation application.

Now consider DRC "C." This is also a real DRC for a real sprinkler/pressure/nozzle combination. You can see that the shape of this DRC is much more uniform and more predictable than that of DRC "B." You could easily design an efficient irrigation system using this sprinkler, pressure and nozzle.

Conceivably, DRC "B" and DRC "C" could result from the same sprinkler and nozzle operated at different pressures. Therefore, it's important not to discount a sprinkler or nozzle solely because it has an unacceptable DRC for one operating circumstance. Instead, pick a sprinkler then evaluate which nozzle and pressure combinations provide an acceptable DRC.

You can obtain DRCs from manufacturers as well as from independent facilities. For example, the Center for Irrigation Technology (CIT) in Fresno, Calif., operates an independent testing lab to determine and publish DRC data. The data from all of CIT's tests are available through its publications or its database, which is integrated with analysis software.

In the past, some irrigation manufacturers were reluctant to offer DRC data. However, irrigation designers and scrutinizing end-users have begun demanding it. The design community has learned to properly evaluate a DRC and use it for both the designer's and the manufacturer's advantage. Manufacturers' catalogs may one day publish DRC data, along with other performance data. As a result, manufacturers inevitably will place increased emphasis on developing nozzles that offer better DRCs, improved performance and better sprinkler uniformity.

Along those lines, some manufacturers are using new technologies to improve nozzle-development programs while decreasing research and development costs. For example, stereo lithography is a technique that allows manufacturers to quickly and inexpensively fabricate an irregular shape, such as a sprinkler nozzle, from epoxy. They then can use these plastic-like, three-dimensionally-correct prototype nozzles for testing. Stereo lithography dramatically decreases nozzle development and testing time. More importantly, the technology makes it easier for design engineers to conceptualize and develop prototypes of nozzles with unique shapes and characteristics. Another process, called selective laser-centering, results in a similar, but flexible, end product also suitable for tests.

Using comparative metrics You can evaluate sprinkler uniformity from a theoretical basis by using the sprinkler DRC and an assumed sprinkler spacing and pattern. Irrigation designers use three uniformity parameters or metrics to do so. Christiansen's Coefficient of Uniformity (CCU) is the oldest parameter, having been developed for agricultural sprinkler systems in the 1940s. Another is the distribution uniformity (DU) parameter. It is influenced by under-irrigated areas in the turf and is more widely used than CCU. In recent years, many have recognized still another--the scheduling coefficient (SC)--as the preferred parameter for turf irrigation because it is sensitive to under-watered areas. CCU compares the average difference in a measured depth from a catch can to the mean depth. DU corrects for some of the turf-related shortcoming and deficiencies of CCU by evaluating the average precipitation rate of the driest 25 percent of the catch cans divided by the mean precipitation rate. Although DU is a considerable improvement over CCU, it may still cause you to lose perspective of unwatered areas.

Thus, as mentioned, many have accepted SC as the metric for turf-sprinkler evaluation. CIT developed SC with manufacturer input. Put simply, it is the average precipitation rate divided by the least precipitation rate extracted from the catch cans.

A perfect--and non-existent--SC is 1.0. Good sprinkler nozzles will have an SC of 1.15 to 1.5. SC is named from the fact it can be used as a run-time multiplier. For example, assume the SC is equal to 1.2. If a runtime of 30 minutes is adequate based strictly on the turf water requirement and the sprinkler precipitation rate, then the actual run time would be 30 minutes multiplied by 1.2 or 36 minutes. This is the operating time necessary to provide adequate (minimum) water to the relatively dry areas in the landscape to ensure optimal turf appearance.

Obviously, simply purchasing a sprinkler without understanding aspects such as DRCs can be risky. Look at a variety of factors, consider DRCs, and then you can be assured you've purchased the most appropriate sprinkler for your site.

Stephen W. Smith is president of Aqua Engineering Inc. (Fort Collins, Colo.) He recently published Landscape Irrigation: Design and Management.

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