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Getting to the Root of the Matter07-01-00 | 16
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   Subsurface irrigation distributes water uniformly to the root zone of any trees or plants in the landscape. When applied slowly to the soil at a single point, water moves through the soil in two ways: 1) Downward, pulled by gravity 2) Outward and upward, pulled by capillary action. (Graphic courtesy of Netafim USA)

Nearly every project, whether it is residential or commercial, needs a well-designed irrigation system. At times, you are bound to encounter projects that don't fit into the irrigation norm. You know the ones-narrow median strips, awkward shaped areas, low overhanging roofs, and ground level glass windows. If you were to install a traditional pop-up irrigation system in any of these instances, you would plague your client with wasted water, overspray, spotted windows, and premature asphalt decay. A sub-surface drip irrigation system offers a unique solution to some irrigation quandaries.

Sub-surface drip irrigation is a low volume system sometimes referred to as micro irrigation. This equates to less gpm demand per square foot compared to fixed pop-up spray systems. Therefore, larger areas can be watered at the same time, or the overall water window can be decreased. This means watering can be finished earlier, allowing for earlier play activities on drier ground. This in turn greatly reduces compaction, which is usually caused by excessive soil moisture conditions during use.

Since water conservation and management is a hot-button issue these days, it never hurts to explore a technique that conserves water. Sub-surface minimizes runoff, evaporation, misting, overspray and interruption of spray patterns which are some of the greatest culprits of wasted water. Conservation is achieved by installing dripperline tube 4" to 8" under the soil where precise amounts of water are applied directly to a plant's root zone. Though the plants receive adequate water for optimal growth, water use can be cut by up to 50% by increasing application efficiency. Additionally, when designed and installed as a 'looped grid', the system uniformity becomes very high, 90% - 100%. This is true irrespective of shape or plant material.

Why should you take the time to learn a new irrigation technique? Proficiency in sub-surface irrigation will enable you to bid with confidence on projects that require water saving alternatives. The amount of maintenance required is also minimized with this type of system, which means less time spent repairing and replacing system components. But, you may still have reservations about sub-surface. Maybe you are uncomfortable installing a system that you can't visually inspect. Possibly you are concerned with root intrusion into the emitter tubing that will hinder its watering capability. In this article, LCM will explore the basic design, installation and maintenance of a sub-surface irrigation system. So, read on because with knowledge comes power!

Design: The first step is to determine the type of soil in which the system will be installed. The primary soil types are clay, loam or sand. Clay soil requires wider dripper and lateral spacing whereas sandy, looser soils require closer spacing. What most people regard as "heavy clay" and "sandy" soils are actually mixtures of sand, silt and clay which still allow for adequate water movement and retention between soil particles. Also consider the plant type to be used, the direction of the slope and the available water volume and pressure from the water meter.

The next step is to lay out the laterals on a site plan. Use a manufacturers chart to determine the recommended spacing of emitters and lateral lines for trees, shrubs, or turf. Most sub-surface systems are laid out in a grid with dripperlines at 12, 18, and 24 inches on center. The ends are joined to form a closed 'loop', which ensures even pressures and guards against flow loss from accidental crimping. Dripperline is almost always laid parallel to contours to prevent excessive drain down. Laterals should begin 2" to 4" from any hardscape in order to prevent dry areas along the edges.
In a simple design, a supply manifold and an exhaust manifold are connected to the dripperlines. On smaller systems (below 8 gallons per minute) it is common for the supply and exhaust manifolds to be fashioned from dripperline. Do not exceed 5 gpm in any one portion of the tubing or emitter line. Manufacturers of sub-surface irrigation laterals offer a variety of tubing diameters, flow rates, and emitter spacing. Systems that require long laterals may require the use of pressure-compensating and/or low output emitter line to ensure even flow.
Next, you will need to establish your irrigation zones in relation to your planting zones. Size your lateral zones (an 18" x 18" x 1 gph grid on 1,000 sq. ft. requires approximately 9.1 gpm) in conjunction with the available water supply. North and south exposure and plant types should be taken into consideration. Then calculate dripperline footage and total flow per zone. Each manufacturer provides charts to determine the proper run lengths based on tubing diameter, nominal flow, initial pressure and the spacing between the emitters. The higher the output of the emitter and the closer the spacing, the shorter the allowable lateral length. If you choose to use lower output emitters there is less friction loss and laterals can be much longer. Dennis R. Hansen, ASLA, ASIC, CID, explained that you need to "be very careful about mixing emitter outputs, spacing and line spacing on a site." He suggests that you, "clearly identify the application rates for different zones and never mix outputs within a zone." This, he says, has the same effect as mixing nozzles within a matched precipitation sprinkler zone.
The next step in the process is to locate and size the supply and exhaust manifolds on your plan. Generally, this is installed at the highest point and/or centered on the highest end of the grid. A flush valve is installed in the manifold to flush debris and sediment from the grid during each irrigation cycle. As a general rule of thumb, the number of automatic flush valves needed is one for every 5 to 8 gallons per minute. Automatic flush valves should be located in valve boxes at the low points to help minimize the buildup of possible sediments.
The final step in the design process is deciding the location of the air/vacuum relief valves. They should be placed at the highest points in each zone to minimize the back siphoning of dirt into the emitter line. A 1/2 inch model will generally service up to 9 gallons per minute. Factors that will affect lateral spacing are berms, slopes and flat areas of turf if they exist on the same valve. On berms the laterals should be installed closer togethe. On flat areas, normal spacing (18 - 24 inches) is ideal. Sloped areas should have a greater distance between laterals. The air relief laterals normally run perpendicular to the dripperline laterals. Finally, place valves, filters, and pressure regulators for each zone according to zone flow.
 Subsurface irrigation eliminates overspray and potential water damage by delivering water at the roots, not through the air. This makes the system ideal for irregular shaped median strips, for buildings with glass windows and low overhanging roofs (graphics courtesy of The Toro Company). Inset shows the installation of dripperline in a median strip (photo courtesy of Rain Bird).
Installation: Now that you have taken the steps to design your sub-surface irrigation system, the installation process can be broken down into four basic steps: 1) Hard pipe PVC, 2) Finish grade, 3) Manifolds, and 4) Installation of the dripperline.
The first step is to dig your trenches. This can be done by hand, with a trencher, vibratory plow or insertion shank (for larger projects). Dripperline can also be installed on subgrade with staples and then covered with at least 4 inches of dirt, or on the surface covered with mulch or bark. This type of installation is ideal for annual beds that experience intensive plantings and a lot of gardening activity.
Next, you will want to install the PVC or Polyethylene supply headers. Keep all the laterals clean and prevent soil from entering open ends of header and dripper lines while you are working. Then connect the dripper laterals to the exhaust header or interconnecting laterals. Be sure to thoroughly flush the supply header before connecting the dripperline. Most manufacturers also offer barbed fittings that do not require any additional hardware to achieve a leak proof connection. All you need to do is simply wiggle the tubing over the barbed fitting. After this step you can begin to fill in the trenches.
 The design and installation of a sub-surface system is easy. There are a few key ingredients that are necessary to make your project a success. 1) At the point of connection, you need to install a filter, pressure regulator and control valve. 2) The air relief valve, which normally runs perpendicular to the dripperline laterals, will eliminate any negative pressure that would otherwise draw contaminants into the sub-surface system. 3) Install line flushing valve at the furthest point in the system. This helps to reduce maintenance and increase system performance by cutting down on sediment build-up. (Graphic courtesy of Netafim USA)
Now, install the exhaust header and air/vacuum relief valve. Do not allow the system to become pressurized (all ends closed) before manually flushing the entire system. Install a line flushing valve at the furthest point in the system to help reduce maintenance and increase system performance by cutting down on sediment build-up. Automatic cleaning operation takes place at the start of each irrigation cycle. Finally, record the pressure in each zone and record for future reference. The sub-surface irrigation system should have a minimum of 7-10 PSI.
Maintenance: As with any irrigation system, routine maintenance is the key to a long and healthy life. When you first install a sub-surface system, you should visit the site frequently to check for adequate watering levels. Later you can taper your site visits down to two per year.
Though the irrigation system is not readily visible, there are several different ways to "see" that your system is working. First you should check the flush valve by opening it and observing the flow. A weak flow from the flush end could indicate a break in one of the lines, or a malfunctioning valve, pressure regulator or clogged screen filter. Color or debris in the water could indicate a break in the dripperline grid, but if your pressure is good you may not be getting enough flushing. You can also perform this same test at the air/vacuum relief valves.
Root intrusion into the emitters is an important issue that deserves discussion. There are two approaches to solving this problem: one is by product and the second by management. The Toro Company, based in Riverside, California, manufactures and distributes DL-2000 with Rootguard® technology. During the molding process, the pre-emergent Treflan® is impregnated into each emitter. Once the dripperline is installed, the slow release of Treflan directs roots away from the emitter opening, preventing root invasion. Netafim USA, based in Fresno, California, markets the Techline Filter, which has replaceable impregnated disc filters. Another suggestion to minimize root intrusion is to schedule irrigation with shorter, more frequent cycles. The distribution of moisture also lowers the chance of root intrusion due to stress induced by dry spots.
 With sub-surface irrigation, trees, shrubs, ground cover and turf can be irrigated virtually anywhere that traditional overhead sprinklers are impractical. This unusually shaped landscape is a perfect candidate for sub-surface irrigation. Not only will overspray be kept to a minimum, but the sub-surface system will also eliminate the chances of vandalism and liability issues. (Photo courtesy of Netafim USA)

At right, a Landscape Contractor demonstrates how to connect the dripline laterals to PVC headers. (Photo courtesy of Rain Bird)
The following is a five point maintenance program that will ensure the success of your sub-surface installation.
1) Clean the filters to remove any debris or sediment that may have entered the system during installation. Schedule filter flushing in accordance to your water quality, or if any work has been done on the system. Always open the flush ends and flush the system after any maintenance or repair work has been done.
2) Check each major flush point in the system to make sure that flushing is occurring properly. Automatic flush ends can jam open due to debris (siphon ingestion) or insects.
3) Check for leaks. A leak is easy to locate because the soil in that area is much wetter. During the irrigation cycle, bubbling water is visible from the surface or you may see a stream of water in the air. Use barbed fittings to repair the broken dripperline.
4) Take a pressure gauge reading and compare to the readings that you took when the system was first installed. Larger systems should have a permanent pressure gauge installed at the flush valve.
Proper Scheduling: Though the issue of proper scheduling is not a new issue, it can prove more tricky with sub-surface irrigation because you can't actually "see" when watering is taking place. Watering should take place on a daily or biweekly basis, depending on your soil type, exposure and plant material. This ensures that the soil maintains a certain degree of moistness and minimizes the likelihood that roots will clog the emitters. A moisture sensor is another useful tool that can help regulate the amount of watering that takes place.
Manufacturers have developed formulas to determine the correct application rates. The factors to consider when figuring an application rate yourself are the soil infiltration rate and application rates. With this data you can compare run time with sprinklers and calculate run times from E.T. (evapotranspiration) data.
The formula in Toro's "Sub Surface Irrigation Design" handbook calls for the Landscape Contractor to multiply the gph of the emitter, times a constant of 231.1 and divide that total by the spacing of the emitters along the lateral in inches multiplied by the distance to give you an application rate in inches per hour. So if you have .53 gpm emitters every foot in the tubing and you place your laterals 12 inches apart, then you would multiply .53 x 231.1 and divide that by the sum of 12 x 12. This gives you an application rate of .85 inches per square hour. If you need to apply 1.5 inches per week you would need to run the system nearly two hours every week. Running the system on a daily basis, the run time would be approximately 18 minutes per day. This applies only if the soil moisture content is at field capacity at the beginning of your scheduling. LCM

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