Getting In The Zone: Irrigation Zoning Basics

“Figuring out what you need to know isn’t too difficult for commercial jobs,” related Kevin Smith, president, Computerized Design Services, Longwood, Fla. “I take the total gallonage of all of the sprayheads on the system and multiply that number by 12 minutes run time for the sprayheads. Then I do the same thing for the gallonage for all of the rotors and multiply by 40 minutes run time for the rotors. That gives me a total flow for a day for the entire system.”

Smith’s next step is to figure out a gallon-per-minute flow rate by dividing the total daily flow by the available watering time for a day, which is usually eight hours.

“For commercial jobs, the flow rate should be at least 25 gpm, which isn’t difficult to get,” Smith noted. “The next thing to do is figure out the size of your zones based on the flow rate and then separate the plant material based on irrigation needs.”

“Each site should be divided into areas of different watering requirements such as turf, planting beds, ground covers and so on,” noted Larry Keesen, president, Keesen Water Management, Denver, Colo.

“Zones need to then be scheduled so shrubs are irrigated more deeply and not as often, compared to turf,” commented Smith. “And annuals, because of the challenges of getting them established, need their own zone.”

When compared to commercial systems, residential systems present some different challenges to contractors, according to Brian Quill, irrigation and construction manager at Industrial Landscape, San Jose, Calif.

“The first thing to do is determine the static water pressure, the meter size and the mainline size, and all of that information should be available from the site or the water purveyor,” recommended Quill. “Normally, the home will have an existing meter and line that goes up to the house and a contractor can tap into that line at the hose bib and use a pressure gauge to find the static water pressure.”

Quill was quick to point out, however, that the working pressure must also be determined because the static pressure is only a reading in a ‘no flow’ condition.

“Once the water starts flowing, pressure losses occur from the water flowing through the meter, the supply line and the hose bib,” Quill explained. He also commented that many manufacturers provide resource manuals with pre-calculated pressure losses based on various system components.

“Another valuable piece of information is the available flow,” Quill continued. “That can be determined at the hose bib as well with a 5-gallon bucket and a stopwatch.

“It’s a simple test, but it’s important because you might have a reading of 80 PSI for the system but only get 5 gpm of actual water flow,” he noted. “And the flow on residential systems is usually the primary restriction instead of the pressure because most residences, at least in our part of the country, have a 5/8-inch meter, which is small.”

As a result of restricted flow, it’s common for residential systems to be zoned with many valves since the low flow rate requires one valve for three or four heads.

Quill cautioned against using maximum safety capacities taken from pressure loss tables for designing a system because they don’t fit realistically into system designs.

“Using the maximum capacities just creates water hammer by driving flow velocities way up,” he noted. “Even if the system works, the homeowner will probably hear pipes banging in the house every time the system is turned on.”

Smith also recommended designing a residential system based on 80 percent of the maximum gallonage flowing through the water meter to take into account any water uses that take place inside the house while the system is operating.

“Regardless if it’s a residential or commercial system, zones are important so the system doesn’t exceed the maximum capability of the water supply,” Smith noted. “Exceeding the supply will only result in the system operating at a lower pressure, which creates uneven applications and drives irrigation costs up.”

SETUP PRECAUTIONS. Setting up a system according to flow rates and maximum capacities isn’t necessarily enough to guarantee success, however.

As with the attention required for zoning different plant material, the nature of the property can also demand other design alterations that would influence zones.

Snyder said the first thing he checks for on a new site is its orientation to the sun since plant material on the south and west sides of a property are likely to get more sun and need more irrigation.

“When berms or mounds are present, the top should be watered with a separate zone, even if it consists of only one head,” recommended Keesen. “The peak of the berm will dry out much faster than the slopes, and it will require additional water.”

Snyder agreed with Keesen, and added that a separate zone may need to be included for the middle of the slope, depending on how large the hill is.