HOW TO: Retrofit to drip
As water becomes more scarce and more expensive for commercial irrigation applications, many maintenance superintendents are recommending the conversion of lawn areas to groundcover and shrub plantings, and the conversion of spray irrigation systems to drip irrigation systems. Thankfully, a number of irrigation manufacturers have come up with adapted technologies that utilize the core of an existing spray irrigation system, so it does not need to be demolished in the process. By utilizing available product information and services, maintenance managers can effectively convert spray systems to drip systems in an easy, convenient and practical manner.
When to spray and when to drip
Before a maintenance manager can make an effective decision as to whether to convert a spray system to a drip system, it is imperative that he or she understand which is most effective for which type of landscape plantings.
Every maintenance manager has his or her own opinions as to when to use spray and when to use drip, so no absolute rules exist to make this determination. However, some generally established guidelines can help you decide. Just remember that the parameters are constantly changing with the development of new irrigation technologies.
There's no dispute that aboveground spray (or rotor) systems are appropriate for turf irrigation. (Subsurface systems are currently on the market and are being improved, but they are not widely used yet.)
In beds, spray irrigation is best for large groundcover beds where the individual groundcover plantings are more than 3 feet apart and planted from 1 gallon or larger in size. Plus, if groundcover is planted from bare root or flats, it is impractical to attempt to drip irrigate each sprig. Other than these two specific uses, however, drip irrigation competes well with spray irrigation in terms of installation cost, long-range maintenance and water-use efficiency.
Drip vs. spray: maintenance issues
Maintenance considerations are important in drip vs. spray decisions. When you grow turf from seed or sprigs, you know that weeds can become a problem during establishment. Until the turf fills in, weeds will find a favorable competitive situation. The same is true of spray systems in beds. Spray irrigation wets large areas of open soil, which favor weeds.
One of the great advantages of drip irrigation is that the water is carefully focused at the base of each plant. This significantly diminishes the weed problems that spray irrigation can promote.
Another important consideration relates to the maintenance supervisor's ability to spot problems. With spray irrigation, if a head breaks or is non-functional, the signs are usually quite apparent. It is not the same with drip irrigation. Unless the maintenance program includes a rigorous monthly review of the system, you may not know a problem exists until a plant is dying. It is the same with subsurface irrigation. If something goes wrong, it is usually indicated by stressed or dying plant material. It is easy enough to overcome, but it does require a rigorous inspection routine, which often falls through the cracks in maintenance programs.
Differences in flow
Spray systems are designed and constructed to provide relatively large volumes of water at higher pressure. Higher volume and higher pressure allow the system to deliver water at greater distances and with good uniformity.
Like drip systems, spray systems usually include a backflow prevention device, and a mainline running from the backflow preventer to the irrigation valves. Beyond that, however, things change. Because spray systems require higher volume and pressure, their lateral lines must be larger in size, and sized properly to reduce friction losses. Lateral lines in a spray irrigation system are generally thin-walled (Class 200 pvc pipe), whereas mainlines, which are under constant pressure, are generally thick-walled (Schedule 40 or Class 315 pvc pipe). These thin-walled lateral lines eventually lead to the irrigation heads.
Contrary to spray irrigation, drip irrigation uses smaller volumes of water at much lower pressures. However, because it is under constant pressure just like a spray system, the mainline for a drip system is also Schedule 40 or Class 315 PVC, and leads to a backflow preventer. Although both types of systems possess electric valves, spray systems have larger valves (to carry more flow). Spray valves are generally sized from a minimum of ¾ inch (if not 1 inch) and up, whereas drip valves generally are sized, depending on the extent of the layout, at 1 inch or smaller. Larger valves cannot effectively sense the low pressure and flow rates of drip systems and will not shut off reliably.
Another important difference between drip irrigation and spray irrigation, relating more directly to flow, is time. Because they are high-flow systems, spray irrigation systems generally require less time than drip to apply the desired amount of water. Depending on the size of the individual irrigation circuit (the valve and its related heads), a commercial spray-irrigation circuit may have flows from 15 gallons to 100 gallons. Because of the volume of flow, less time is required to apply it (depending on soil type and infiltration rates).
In contrast to spray irrigation systems, most drip irrigation systems generally run at a low flow rate of 2 to 3 gallons per minute and, therefore, may require more time to apply an adequate volume of water to plants. This affects the kind of lateral piping needed. Drip systems are generally laid out with polyethylene pipe of smaller diameter than the laterals on spray systems. Drip laterals are generally (again depending on the size of the entire layout) either ½- or ¾-inch polyethylene.
The last major difference between a drip system and a spray system (aside from the sprinklers or emitters) is the pressure regulator and filter for a drip system. Because irrigation sprinklers are designed to throw relatively large volumes of water, they use larger orifices that do not require as fine of filtration. By contrast, the orifices on drip emitters are so small that the water must be carefully filtered so that no minute pieces of sediment can reach the emitter and block it. Drip filters come in different materials and sizes — the individual needs of each circuit will determine which is best.
Drip emitters use smaller apertures and operate at lower pressures so they can effectively drip water onto the soil surface. Even micro-sprayers, with their small apertures, need reduced pressure. Pressure reducers come in several shapes and sizes, all the way from adjustable brass units to fixed-pressure plastic types.
Converting spray to drip
Understanding that the fundamental difference between spray and drip irrigation systems usually is merely the presence or absence of a pressure regulator and a filter, there are two general ways to convert a high-pressure, high-flow spray system to a low-pressure, low-flow drip system:
installation of master filtration and master pressure reduction just before or just after the drip irrigation valves, using standard drip irrigation equipment; or
using specially manufactured equipment at each riser, which reduces pressure and separates the flow of water into individual drip lines.
The first strategy, using standard equipment, requires you to install a master valve, regulator and filter in the mainline upstream of the valves; or one master valve upstream of the valves with a filter and regulator downstream of each individual irrigation valve. On drip systems, you must use a master valve ahead of the irrigation valves because drip irrigation filters do not last long under constant pressure.
I prefer to put a single regulator and filter upstream of the valve manifold because a single larger regulator and filter can be used for all the control valves, rather than the more costly option of putting individual regulators and filters after each control valve.
Why, then, even consider using regulators and filters downstream of control valves? Because you often only need to convert some of the circuits to drip, leaving others as spray zones. If you only convert part of the system, then you must use a regulator and filter downstream of only those individual valves being converted.
The latter option may pose some difficulties. Fortunately, manufacturers produce equipment that offers a third way. With the primary controls from the spray system left in place, you can remove each spray head and replace it with a multi-port drip transfer head. Several types are on the market and are designed to adapt to existing risers. Depending on the brand, you may be able to install them above or below the soil surface, such as in the manufacturer's access box (see photo, page 44). They typically include pressure regulation/compensation and filtration within each transfer head, as well as variable-flow emitters and transfer tubing to deliver the desired amount of water at the required distance from each head. Some models even offer backflow prevention and fertilizer pellet injection capabilities.
Most irrigation designers feel that it is most efficient to design individual irrigation circuits from scratch as a spray or drip circuit. The two types of systems have radically different flow rates and run times, and trying to design a one-size-fits-all system that can meet the needs of either is a real challenge. The same challenge exists in trying to use one individual circuit for both spray and drip.
If the maintenance supervisor or irrigation designer has no choice but to use both drip and spray on the same circuit, it is difficult — but not impossible — to meet the water needs of the drip and spray portions in approximately the same run time. Depending on the square footage of areas receiving spray irrigation vs. the number of plants receiving drip, more than one transfer line from the multi-port head and its related emitter can be used on a plant. For instance, if two plants need water from a drip system, you can direct two individual lines from a multi-port head to each shrub so that they are getting double the amount of drip irrigation they would get from a single line and emitter. Also, if needed, use larger flow emitters, such as 1 to 2 gph, when needed.
Keep the drops dripping
In any irrigation retrofit, the best system will evolve from careful planning and execution. However, the real key to the long-term success of drip irrigation is constant watchfulness. When a drip system fails, you usually can't identify the failure until plant material starts dying, unless you carefully and deliberately monitor the system on a regular basis. Ideally, one individual on the maintenance team should be responsible for this duty. This person should be made completely aware of any nuances of the irrigation system operation and maintenance that may not be visible from a field inspection. These would include any guidelines or restrictions from local water agencies. By carefully planning, installing and maintaining converted drip-irrigation systems, a maintenance supervisor can assure the existing landscape a long, happy life without starting from the “ground up.”
Steve McGuirk is a landscape architect with the Madrone Landscape Group (Soquel, Calif.). You can e-mail him at firstname.lastname@example.org.
STEPS TO SUCCESSFUL DRIP CONVERSION
In an existing landscape and irrigation regime, carefully map those areas that you will convert from spray to drip. Make sure that the spray heads you're leaving will actually cover the plant materials that they're intended to. Remember that most irrigation manufacturers make a wide series of part circle nozzles for each irrigation head — use these nozzles to effectively divide spray zones from drip zones.
As with all irrigation system planning, test the soil to know its physical (textural) qualities. Determine how to effectively adjust the controller (as well as perhaps amend the soil) to get the best irrigation scenario for the individual plantings desired.
Once you've made the decision to separate existing areas into spray and drip, try to work with existing, whole irrigation circuits to separate spray areas from drip areas.
If an existing irrigation circuit must provide both spray and drip, carefully assess the available drip irrigation conversion equipment to make sure that the number of plants you need to irrigate can make do with the amount of water available through the drip emitters. Remember also that plants will require more water as they grow. Will you be able to add more water as the plant matures? If not, look for other options.
When the conversion takes place, make sure the installation contractor or maintenance person carefully marks existing irrigation equipment (valves, mainlines and laterals). Identify and protect these pieces of equipment during construction so that the existing system is not damaged. The installer should also note any changes to the irrigation system on a set of “as-built” drawings.
Make sure that the maintenance program for the landscape includes, at a minimum, monthly inspection of all drip irrigation circuits. The drip system should be inspected under actual operation. Every emitter should be checked during the test. If there are large numbers of drip circuits to check, you may have to spot check or use an inspection rotation, but keep inspections as frequent and complete as possible. As with all irrigation inspections, look for water bubbling out of the ground as an indicator of a leak or line break.
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