|By Mary Elizabeth Williams-Villano|
When you hear the phrase “low-volume irrigation,” what picture pops into your head? Small spray heads, staked over plantings? Rotating heads, putting out multiple streams of water? Burbling bubblers? Dripline?
If you thought of any of those things, you’d be correct. They’re all part of the low-volume universe; one that includes micro sprays, micro bubblers and micro jets, stationary and rotary stream sprinklers, and drip irrigation. High-efficiency nozzles fit into this category, too.
But there can be some confusion about low-volume irrigation, even amongst contractors; everyone seems to have their own definition of what it is. That is, in part, because there are so many different kinds of products that fall under the low-volume umbrella.
“Generally, low-volume irrigation means any kind of tubing-based, point-source irrigation,” said Orion Goe, product marketing manager, residential and commercial irrigation, for The Toro Company. “It’s very focused, very concentrated and mainly for shrubs and landscape beds.”
When we talk about landscape irrigation, we talk a lot about flows in gallons per minute (gpm). But when we start talking about low-volume, we’re talking about flows in gallons per hour (gph). Technically, any irrigation system that delivers water from 30 gph down to as low as 0.4 or 0.5 gph is considered to be a ‘low-volume’ one. It includes rotary and high-efficiency nozzles with flows as low as 20 gph.
The idea behind low-volume irrigation is to deliver water directly to the root sources of plants. These are thrifty, efficient systems, using much less water to sustain vegetation and wasting virtually none of it. They generally have lower precipitation rates than conventional sprinklers and rotors.
There are many, many benefits to low-volume irrigation systems. The main one, of course, is that they use less water overall. Among the main selling points for residential clients are that such systems lower their water bills, and in areas where watering restrictions apply, eliminate fees or penalties for overuse or runoff.
Commercial clients reap the same benefits, with the addition of gaining eligibility for LEED points. That’s increasingly important to developers of new commercial properties.
Other advantages include decreased water loss from evaporation, wind and runoff; healthier plants with fewer problems with pests and diseases, and greater flexibility in meeting the variable water needs of both new and more established plants. There’s also less soil erosion, particularly when watering on slopes.
One of the biggest pluses is the freedom to irrigate when conventional systems aren’t allowed to.
Municipalities in drought-stricken areas generally restrict overhead sprinkler use, limiting watering to three days, two days, or even one day per week—unless you’re using low-volume irrigation.
When to use what
Thanks to new technology, a contractor designing a low-volume system has many different options from which to choose. Faced with a landscape bed, he asks himself, “What’s the most efficient component or group of components that will deliver just the right amount of water to the plants in this bed? Should I use rotary heads? Bubblers? Micro sprays or micro jets? Inline drip?” These are questions that Samir Shah, global marketing and international sales manager at Azusa, California-based Rain Bird Corporation, has fielded many times in his fifteenyear career there. “The theoretical answer is, ‘It depends’—on the types of plants you’re irrigating and the types of soils they’re in.”
Because he and others have needed to answer these questions so many times, the company produced a low-volume irrigation application guide that can be downloaded free from Rain Bird’s website. It accounts for all the variables involved in selecting a low-volume irrigation solution. Other companies have similar guides.
Let’s take a look at each of the options available.
Irrigation consultant Ken Merboth, CID, CLIA, LEED AP, president of Lincoln, Nebraska-based Water Scape, Inc., designs irrigation systems in the entire western half of the country, mainly for commercial clients.
For turf applications, he prefers rotator-type heads that put out multiple streams of water. One of the main reasons he likes them is that they work well with the types of soils he usually deals with.
“The soils in Nebraska and in most of the Midwest are heavy clay, with a very low absorption rate,” he says. “If you apply water to clay at one-and-a-half to two inches per hour, it’ll start running off in a very short time, and you lose the whole idea of having an even precipitation rate. These low-volume systems put out the water slowly enough for the soil to absorb it.”
The distribution uniformity (DU) of the rotators is so good, he finds he doesn’t have to water as long, or overwater any particular zone to compensate for an uneven distribution of water in certain areas.
Last year, Merboth won a Merit Award from the American Society of Irrigation Consultants for the lowvolume irrigation system he designed at a large estate in Prairie Village, Kansas. The system helped meet the project’s stated goal of a 50-percent reduction in irrigation water use.
“It was a 2.3 acre lot,” he recalls, “and they only had a one-inch service line providing water for the entire spread, which really wasn’t large enough.” However, by using rotating sprinklers on the turf areas, and inline drip in the planting beds, he was able to irrigate very efficiently, with just a few zones.
“The rotators worked really well on the clay soil. If we had used standard rotors, and/or standard sprays, we would have had a third more valves or zones on the property.”
Micro stream spray heads are another option. These operate very similarly to rotators, but without rotating, putting out thin, static fingers of water within short radii, six to 18 inches. The radii are controlled by the amount of water that’s allowed to flow through the adjustable cap.
Micro sprays and micro jets
These sprinklers provide tiny streams of close-in coverage to apply water exactly where it’s needed. Many feature fingertip adjustment of flow and throw. Multiple units can be combined for a ‘blanket’ effect.
Micro sprays and micro jets typically come in 90°, 180°, and 360° patterns, and resemble standard spray nozzles. They’re often used in very dense vegetation or flower beds, as they’re more practical in those applications than point-source drip.
These components allow irrigation of wider swaths with fewer stations. You frequently see them working at resorts or other commercial sites, again in areas where plants are densely packed and/or changed out regularly.
“A micro spray or micro jet is very similar to a traditional spray head or rotating nozzle,” said Goe, “but the volume of water that is coming out of it is far less. You wouldn’t use them for turf. They’re really for flower or shrub beds and things of that nature.”
“If you’re in any type of soil other than hard clay, micro sprays do a fantastic job, because you can cover more of the root zone,” said Sam Thayer, president of Maxijet in Dundee, Florida. “With drip, you’re limited by your disbursement. If you’re not in hard pan, you’ll hardly get any distribution; the water will just go straight down.” Micro sprays tend to have higher trajectories than nozzles. Throwing water at higher angles allows it to get over taller plants a little easier.
“If you have dense shrubs that are going to grow 12 inches in height,” says Shah, “you don’t want something that’ll just spray in front of the plants and not get the water where you want it. You’d be better off using micro sprays on top of 12- or 18- inch-tall risers.”
“If the micro sprays are installed on flex risers, they’re already about a foot above ground,” said Robb Kowalewski, micro irrigation product manager at Hunter Industries in San Marcos, California. “For small areas, they get the job done. Typically, you’re going to install them off of PVC pipes, not spaghetti tubing, so they’re pretty stable systems.”
“We have one micro spray head that goes from zero to 25 gph, and will cover a 14-foot diameter, but it’s still considered ‘micro,’” said Bill Hutcheon, president of Longwood, Florida-based Antelco. “Using something like that means that you won’t have to put a drip emitter in front of each little plant. It’s fully adjustable, so you can put in four, five or six of them, and cover a nice, large area.”
Thayer pointed out another advantage that micro sprays and jets have: when they’re on, they just plain look pretty. “End users like micro sprays because they can see that nice, gentle mist coming out, as opposed to drip, which people complain that they can’t see working.
You can also fertigate through them, and they don’t clog as much as drip emitters.”
One note of caution: if you’re using micro sprays or micro sprinklers in conjunction with drip emitters, it’s not a good idea to force them to participate in a shotgun marriage by inserting them into the dripline.
“People do it, but it’s not a good idea,” said Ron DeWick, regional irrigation advisor, southwest Florida region, for BrightView (formerly ValleyCrest/Brickman). “The inline drip tubing is way too small; it won’t carry the volume of water needed for that other type of component.”
That doesn’t stop people from trying, however. “I’ve seen quite a few instances where someone’s run inline drip, decided they weren’t getting enough volume on a particular plant, and stuck a micro jet on it,” said DeWick. “Then they did it another ten times on that same tubing, and wondered why the drip system didn’t work.”
These components are mainly used for irrigating trees, very dense plantings, containers and hanging baskets. The water simply spills out from under the upper cap. These products can provide up to four gph of water.
Micro bubblers generally screw onto fixed half-inch risers, which are also the pipes through which the water flows into the heads. Flow rates are adjusted by turning the ribbed bubbler bodies. “Envision the water bubbling out or dumping out of the top of this pipe,” said Goe. “A bubbler is really a type of controlled cap on the end of a pipe. You’re not controlling the direction of the water, it just pours down the side of the pipe.”
But you do have control over how much water dumps out of the top via an adjustable gap. Lori Glidden, owner of Taurus Irrigation and Landscape Lighting, Inc., in Austin, Texas, installs nothing but low-volume irrigation systems, consisting mainly of micro bubblers and inline drip. “Those are the two main forms of irrigation in central Texas,” she says, “especially since the drought.”
She uses bubblers on small trees and larger shrubs. “The water bubbles up near the roots of the tree or bush, in a specific diameter. This type of irrigation has been proven to be effective for those types of plant species.”
Merboth uses bubblers in container plantings, but not in landscape beds. “In beds, you end up with water running all over, in different places. They’re not as effective as the micro sprinklers.”
Usually, bubblers are staked above ground; but some contractors bury them. Glidden prefers to keep hers above-grade. “I’ve seen both scenarios,” she said. “There’s no filter along the emission area, so it’s best not to bury them. They really need open air in order to work. Often, they don’t get buried on purpose; they just happen to get covered up by mulch.”
Some contractors will put bubblers inside of valve boxes. “If you do not want them to stick up out of the ground, you’ve got to enclose them,” said Glidden. “They can’t have any dirt around them, because of that lack of filtration.”
We can’t talk about low-volume irrigation without touching on drip. It’s a part of the low-volume world, but it merits its own, separate article.
Drip tubing can be installed at grade and covered with mulch, or subsurface, a few inches below. Some drip tubing has emitters already embedded within it, at 12-, 18- or even 24-inch intervals. There’s also a subsurface drip product that comes in the form of an underground blanket, actually a polyethylene mat with drip tubing and emitters embedded in it.
Other drip emitters can be inserted into blank tubing, wherever an installer wants to put them. “The neat thing about those types of emitters,” says Kowalewski, “is that you can have a zone with different types of plants with different watering needs, and you can balance them out. You can put a couple of one-half gph emitters on a small plant, and a couple of two gph emitters on a bigger one.”
The low-volume universe is extremely retrofit-friendly. Changing over a system is usually just a matter of screwing off the old conventional sprinkler heads and screwing the new ones on. “You can literally unscrew your sprinkler head, screw on a manifold, and run line out to nine different stake assemblies,” said Hutcheon. “We also have little half-inch adapters that you just screw on to the old risers.”
Pressure compensation and filtration
These two things are essential in any kind of low-volume system. Many micro irrigation components are described as ‘pressure compensating.’ This allows the last component on the line, whether it’s a drip emitter, micro spray or bubbler, to perform in the same manner as the one at the beginning of the line.
“Pressure compensation is really important in low-volume,” said Hutcheon. “It saves energy, and allows every component to conform to the same number of gallons.” Uneven pressure means that some components would mist, and others would be starved of water. “Say you have a hill. Without pressure compensation, the bottom of that hill is going to get all of the water. A tree at the bottom would get a lot more water than the tree at the top.”
Filtration is just as important. Micro components have small ports; they need filters to keep large dirt particles from clogging them. Those filters need to be cleaned out occasionally.
Should micro spray or micro jet heads become clogged, they’re ‘super easy’ to clean, according to Thayer. You simply unscrew them and ream out their small orifices with a small paperclip or cleaning pin.
Shah says he’s seen significant growth in the low-volume category, as have other manufacturers. It’s making great strides with contractors, specifiers, municipal entities and end users. “California has led the way because of the drought, but interest is increasing all over the country. People are becoming more conscious of their water use, and are looking for higher-efficiency products. It’s specification-driven.”
This fits with what Merboth has been experiencing. He’s designed low-volume irrigation systems for 160 stores belonging to a major bigbox retail chain, from Washington State to Arizona.
“Four years ago, this company put out a book of irrigation guidelines for all of their stores and warehouse clubs,” he said. “They won’t allow conventional spray heads to even be part of a system anymore, because they’re so inefficient.”
If you want to glimpse the future of landscape irrigation, take a look at a low-volume system. With experts predicting many drier years ahead, irrigating less often but with less waste is going to be essential. These components can help us keep landscapes green, even in the face of dwindling resources.