Every morning in cities across America, devoted joggers step out into the weak light of dawn and set off for their morning exercise. Most of them will pass by dozens of lawns along their route, and many of those lawns will have their sprinklers running in the crisp morning air.

As a result, early morning joggers know better than most which sprinkler systems are watering the pavement, in addition to the grass. Still, not all problems are so obvious. Irrigation systems can be inefficient in ways that are much harder to see, and even some contractors can’t spot the problem just by looking at it.

Sometimes, a person that jury rigs sprinkler heads outside of their intended uses is risking that kind of problem. A mid-range rotor whose nozzle can’t reach any adjacent head may look like it’s working just fine, even while it’s distributing water unevenly across the target area. Just because plants are getting wet doesn’t mean they’re getting watered evenly.

Ensuring that the right amount of water is being spread evenly across a lawn—or in other words, that an irrigation system has a high distribution uniformity (DU)—is no simple task. Whether you’re repairing a bad zone, fine-tuning a good one, or installing a whole new system, you regularly try to achieve a high DU.

A poor choice of head can result in both overwatering and underwatering on the same zone. This harms plants, wastes water and can damage a company’s reputation. How efficiently the target area is watered is ultimately determined by the nozzle size chosen, the operating pressure and the spacing between the individual sprinkler heads.

Of course, knowing a rotor from a rotary nozzle is only the first part of choosing the right sprinkler head for the application. You also have to be aware of the different site conditions, and the challenges they pose. For this reason, sprinklers come in a variety of head types, and each head model usually has a range of nozzle options. Starting from the largest to the smallest, the sprinkler type that offers the biggest radius is the gear-driven rotor.

“In terms of inlet size, there are half-inch rotors, three-quarter-inch rotors and one-inch rotors,” said Orion Goe, product marketing manager of Toro’s Irrigation Business in Riverside, California. “That will, of course, dictate the amount of water that can flow through the sprinkler, and in turn, give you your radius.” For most rotors, that radius falls between 25 and 80 feet.

Another type of head that you may hear referred to as a ‘rotor’ is the impact sprinkler. Impact heads cover a similar area as three-quarter-inch gear-driven rotors, but aren’t as popular these days, for a couple of reasons. “Compared to an impact sprinkler, a gear-driven rotor can usually give you a better radius, better distance and better distribution uniformity,” said Goe.

The impact sprinkler also tends to need higher maintenance, according to Keith Schuemann, irrigation manager with Wolverine Lawn Service in Kalamazoo, Michigan. “The head’s mechanism was exposed as it retracted, so water, leaves and dirt would fall back into the body,” he said. Even so, Schuemann keeps a few impact heads on his trucks, for repair work on older systems.

Whether gear- or impact-driven, rotors are the heavy lifters of turf irrigation. Each of their nozzle options affect the precipitation rate, the pressure and the radius of the head. If none of a rotor’s nozzles quite fit the bill, there is an adjustment screw, but relying on that fudge factor too much can result in an uneven distribution. (Rotor adjustment screws are usually ‘diffusion’ screws and turning them dramatically changes the arc of the water pattern.)

A decade ago, watering an area that was between 16 and 30 feet on a side posed a distinct problem. One way the contractor could handle it was by using a single mid-range rotor and adjusting its radius down, which could cost him in DU.

Alternatively, you could cover the area with a lot of spray heads, which are designed to cover smaller areas. This would improve DU, but it would also increase project costs. Or you could mix the two together. This offers the best of both worlds, but the pressure differential between the head types then becomes a headache.

Fortunately, a new option emerged that made this common, yet awkward, range easier to manage. “A rotary nozzle, or rotating nozzle, is a nozzle that breaks the water up into multiple streams, where a standard spray nozzle sprays the water in a fan-like pattern,” said Goe. Those rotating streams could shoot further, so a spray head retrofitted with a rotary nozzle was then able to cover the intermediate ranges.

Most, but not all, rotating nozzles are adjustable in both radius and arc. Irrigation consultant Ted Sirkin of TM Sirkin in Woodland Hills, California, said, “One company offers fixed-arc rotating nozzles and almost all offer a fixed-arc full-circle model. Properly used, they can achieve impressive DUs, upwards of 75 percent, while covering ranges of 13 to 30 feet.”

For distances even smaller than that, spray heads are the go-to answer. A spray head is the most straightforward design; its cap acts like a thumb on a garden hose to push water out in a fan-like wedge. Because all the water does is push up the riser on its way through and out, there are fewer moving parts that can potentially break.

A spray’s arc can go from 0 to 360 degrees, and various nozzles can be used to specify a radius anywhere from four to 15 feet in length. Often, sprays are used to cover small, rectangular areas like median strips.

All of the major irrigation companies offer a line of side-strip spray nozzles to cover areas such as these, as they are considered essential in longer, narrower areas, including narrow shrub beds, where radius nozzles are simply impractical.

While there is a type of sprinkler head that’s even smaller than a spray, it doesn’t see much use in turf applications. Although there’s no strict delineation between sprays and micro-sprays, the radius of a micro-spray is usually measured in inches. Micro-sprays are more often used for watering flower beds than turf, and are oftentimes pointed at individual plants, rather than covering large areas.

As a landscape contractor, you know better than anyone that there’s no such thing as an average landscape. At the end of the day, the irrigation system you’re working on has to match up with the real world conditions.

One common site condition is the incline of a landscape. “If you’re on a slope or a hill, you have to be careful, because you don’t want the water to run off,” said Kenny Shiflett, owner of Foothill Lawn Service in Earlysville, Virginia. Using a high application rate on a steep slope will waste water. Worse, it could potentially overwater any plant material at the base of a slope—or even erode the slope.

One solution is to only use heads with low application rates when aiming at slopes. Unfortunately, this can be cost prohibitive and may not be enough for the steepest hills. Depending on what else needs watering around the slope, it may also make achieving a satisfactory DU a real problem.

Fortunately, smart irrigation controllers open up a simple mitigating option. “Controllers have a feature called cycle soak,” said Shiflett, “where a set of heads that are on a slope or steep hill will run for a couple of minutes, wait 20 minutes or even two hours, then water again.” By interspersing long periods for absorption with short periods of watering, the slope gets what it needs without creating wasteful runoff.

Cycle soak is also helpful when managing another factor in head choice: soil type. “If you have a clay or compacted soil, a head that puts down a lot of water quickly is just going to generate runoff,” said Frank Luther, irrigation division manager for Piscataqua Landscaping and Tree Service, based in Eliot, Maine.

Clay soils don’t absorb water quickly but their counterparts, sandy soils, have their own problems. “If you have a really sandy, coarse soil, that’s going to wear on the head,” said Luther. “It acts like sandpaper every time the sprinkler head goes up and down.”

On properties with sandy soil, he looks for a head that’s more ruggedly built. The top durability factors Luther looks for are the strength of the retraction spring, and what materials are used in the sprinkler head. Stainless steel is going to be more resilient to abrasion than plastic.

Soils aren’t the only factor when it comes to sprinkler durability. Heads which are placed in areas of higher traffic are also likely to see more wear than average. Right along a driveway is a particularly dangerous zone, because drivers who go off course when backing out can run right over them.

Above ground, wind can be a factor, playing havoc with the careful overlapping patterns of a sprinkler system. Higher pressure heads produce fine droplets, and small, light droplets are carried further by the wind. Heads with longer-distance throws and higher trajectories also have more chance to be affected by wind. The stream from a large rotor might stray by dozens of feet on a blustery day.

When he’s contending with the winds along the coastline, Luther adjusts the angles in his favor. “Where the winds tend to be more frequent and higher strength, I’ll end up using a low-angle nozzle, which keeps the water out of the wind more,” he said.

That adjustment costs him a little in distance, but he finds the smaller distance worth it for the better DU. “However, one rotating nozzle on the market has such a low trajectory that it beats the wind most of the time, regardless of the pressure,” said Sirkin. “And it still pumps the water out from 20 to 28 feet, even under windy conditions.”

Low-angle nozzles aside, adjusting a head’s distance too far outside of its optimal radius can have unfortunate consequences for the DU. “With sprinkler heads, my rule of thumb is that if we have to reduce the radius by 20 percent, we should choose a different head,” said Schuemann.

Sometimes, though, a good distribution for plants isn’t a totally even amount across an entire lawn. A head which points towards a patch of turf that is usually shaded may need a slightly lower precipitation rate than one that is watering a more open area.

Different plant materials can also throw a wrench into the works. Landscaped areas are usually on their own zones, and may even have their own dedicated systems, but that isn’t always the case. Sometimes a sprinkler system needs to water shrubs and flowers alongside grass.

Schuemann relies on spray heads fixed to shrub adapters when he has to water landscape beds along with turf. He makes his own custom risers out of copper, so that he can always be sure that the head will reach over any shrubs or groundcover that might be in the way. Throughout the Sunbelt and Hawaii, above-grade shrub adapters are most often installed on gray Schedule 80 PVC nipples that run from one inch in length up to 36 inches.

With these factors in mind, it can seem like achieving distribution uniformity affordably is an impossible task, but well-designed systems are now achieving DUs of 75 or even 80 percent. Sprinkler systems aren’t going to be as evenhanded as rainfall anytime soon, but they can keep a landscape green and thriving when the supply from the sky is nowhere to be found.

Sprinkler heads are an integral part of most irrigation systems, and knowing their strengths and weaknesses is essential to using them effectively. Property owners may not realize how much a system that isn’t sufficiently optimized affects them.

Educating yourself on sprinkler heads helps you offer that service, and may even open doors when your expertise is plain to see. When it comes to choosing heads, make sure yours is screwed on tight.