Proper sprinkler selection is vital to irrigation system performance. Sprinklers are the gravy on the meat of your system -- sound hydraulics. Moreover, sprinklers are the most visible and vulnerable part of your system.
Different spacing options with the same sprinkler/nozzle combination.
A single layout with different nozzle sets. Images courtesy of Aqua Engineering (Watermark IDEA software)
By Luke Frank
Proper sprinkler selection is vital to irrigation system performance. Sprinklers are the gravy on the meat of your system — sound hydraulics. Moreover, sprinklers are the most visible and vulnerable part of your system. They’re the end of the line — and when poorly configured or maintained, everybody knows it.
“Once you’ve determined water quality and pressure, sprinkler selection is probably the most important decision in creating a high-performance irrigation system,” said professional irrigation consultant Gaylon Coates, ASIC, with Coates Irrigation Consultants, Inc., Gilbert, Ariz.
For example, when working in sandy soils, a sprinkler’s wiper seal absolutely has to perform to allow the head to retract into the ground completely after every irrigation cycle. That’s pretty straightforward. But some perform better than others. Heads that don’t retract are kicked, mowed, chewed, swiped, plowed and generally brutalized. All of your hydraulic precision is meaningless to the client and the public when your heads fail.
Get your heads in the game
Today’s irrigation systems are more complex. Simple matters, like which heads better retract below grade when idle, are no-brainers for seasoned irrigation design professionals. With such knowledge and experience planted firmly in their cerebrum, they look beyond mechanical trifles to real performance issues, of which there are plenty.
Because site and system parameters are so varied, we’ll look past the fundamental choice of spray heads versus small or large rotors properly spaced according to plant material, size and shape of the irrigated area. There are at least three key factors that play into effective sprinkler selection before you even get to the site — matched precipitation rates, pressure compensation, and, never to be ignored, local service.
One hard and fast sprinkler performance rule is matching application rates. “Fortunately, the major sprinkler manufacturers have vastly improved their product lines over the past couple of decades,” said Brendan Lynch, ASIC, professional irrigation consultant and principal with Lynch & Associates, Ltd., Annapolis, Md. “When properly specified, sprinkler precipitation rates pretty closely match within product lines; low-pressure heads last longer and improve uniformity; sprinkler arcs and radii adjustments are easier; and generally sprinkler heads last longer.”
Ensuring that sprinklers are throwing the same amount of water evenly over a zone is one of the holy grails of irrigation design. Achieving 80-percent distribution uniformity in the field is magnificent, even with stellar technology. “Manufacturers are committed to developing ‘families’ of sprinklers that offer matched precipitation rates anywhere from five to 30 feet, but the level of control on small versus large nozzles really varies,” said Doug Macdonald, ASIC, professional irrigation consultant and principal with Aqua Engineering, in Ft. Collins, Colo. “That puts a lot more responsibility on the contractor at installation, and the client to effectively manage the system.” The role of each of these groups in enduring sprinkler performance cannot be overstated.
Certainly, properly zoning your system helps. Separately valving full- and part-circle heads equalizes distribution, as does zoning spray and rotor heads separately. But Coates agrees, “Once you get into the intermediate heads that throw 30-50 feet, it’s tough to match precip rates. It’s not an issue that manufacturers ignore. They’re working on it in controlled environments — indoor and outdoor laboratories — but there are a lot of variables in the field that affect uniformity.”
Pressure regulation needs more traction in the design, installation and management stages of system development. Stored and alternative water sources call for much more pumping, and pressure regulation should be used to keep from damaging piping and sprinkler heads. “In the Southwest, we’ve attributed 60 to 70 percent of the sites evaluated to pressure-regulation problems that stretch sprinkler products beyond their specification,” said Coates. “Sometimes pressure regulators are specified, but not installed; other times, the field crews don’t even know they’re there or that they’re adjustable.”
Sprinklers atomize water optimally at specific pressures. Excessive pressure whacks uniformity, sending a fine mist downwind away from the target. Inadequate pressure creates donut patterns around the heads. Either way, the management team has to deliver sufficient water to the driest areas. Basement irrigation designers don’t understand this because they look at results on paper.
“Consequently, there’s a lot of wasted water, because too many designs are performed by unqualified staff, who never observe sprinklers operating in the field,” Coates added. “Pressure regulation is a big reason why products don’t perform, or fail altogether. But there are some products with a higher margin of error and sheer mechanical tolerance. Our experience is that intermediate heads that throw from 20-40 feet seem to be most vulnerable to pressure vagaries and uneven distribution.”
Another key factor in proper sprinkler selection stated firmly by Macdonald, Lynch and Coates is one’s choice of manufacturer and local distributor support. “The most successful projects include strong support by local manufacturer and distributor representatives who stand behind their products,” said Macdonald. “We rely on those who are enthusiastic about assisting the designer, contractor and manager in selecting and supporting the best sprinkler for the project. They provide the performance data necessary for sprinkler selection during the design process; and the field support during installation and system management to ensure long-term success for the project.”
Lynch takes it a step further. “We won’t select products without market familiarity,” he said. “Lesser-known product lines might be popular in some regions, but in the Mid-Atlantic region, the major manufacturers dominate the market.”
From the podium to the trenches
With a better understanding of minimum sprinkler performance requirements, let’s step onto the site — another important, but too often neglected, process in optimal sprinkler selection. “Anyone designing an irrigation system from plans alone is really doing their client a disservice,” said Lynch. “A keen observation of soils, slopes, sun and wind exposures, hardscape features, foot and vehicle traffic, public accessibility and other site-specific conditions is imperative.” Even interviewing onsite water managers and maintenance foremen can shed tremendous light on product selection and long-term system performance.
Determining how water will travel on the surface and through the soil profile reveals where heads will be placed and how much water they’ll throw in a given cycle. “Soils and site topography are fundamental in choosing the right sprinklers,” Macdonald explains. “Clearly, we can’t apply water faster than it can percolate through the profile, but we have to ensure that a full irrigation set in peak season falls within the watering window that is established by the client.”
Simply stated, tight clay soils need lower-precipitation-rate sprinklers (half an inch per hour or less), while course, sandy soils can take more water in a shorter window. “But trying to achieve a consistent soil profile can be tough,” said Lynch. “Not only do soil types vary from one area of the site to another, but public projects often have all manner of trash, concrete and asphalt slough, wood scraps and who knows what else through the top 12 inches of soil. Sure, installation contractors pick out what they can, but it can be a real issue.”
Hill and dale, berms and swales all make for enchanting landscape textures, but pose real irrigation design and management challenges. “Higher-application-rate sprinklers used in large, sloped areas can cause significant problems, like landscape slides and ponding,” said Macdonald. “On projects where low application rates aren’t a practical solution, we have to specify higher-application-rate sprinklers, and address the soil infiltration rate at the control system through cycle-and-soak programming schedules.”
Start with the basics: zone the head and toe of the slope separately, which enables longer runtimes at the top and shorter at the bottom. Pressure regulation and check valves are standard sprinkler specifications under these conditions, Lynch added.
Proper hydrozoning — dividing your site into similar soils, plant material and sun and wind exposures — plays a critical role in sprinkler selection throughout the site. Low-trajectory heads and adjustable-arc nozzles can add to your precision and irrigation efficiency. And, of course, proficient scheduling completes the process. All of these functions have to be assessed onsite and observed once the system is planted and operating.
Every site, and client, is different. There’s a cavalcade of irrigation technology at your fingertips to address in great detail sprinkler head performance, but Coates cautions against designing an overly complicated system. “In our high-tech world, logic is our best common denominator,” he said. “Technology is great, but friendly, durable systems are what we need. There’s really no magic to our profession. In this business, loads of observation, good science and common sense equal success.”
Luke Frank, communications director for the American Society of Irrigation Consultants (ASIC), is a writer, editor and publisher in the green industry focusing on water resource development, management and conservation. ASIC is an organization of professionals within the irrigation industry that provides a forum wherein irrigation design professionals can meet to exchange information and advance skills and techniques in irrigation design, installation and product application. For more information, visit www.asic.org.