In the zone
In order to create the best irrigation design for your site, you need to understand the site's potential as well as its possible limitations. To maximize irrigation system performance, employ sound design strategies that consider site conditions. Customize sprinkler layout and zones that optimize both plant health and growth. Finally, choose a controller that can take full advantage of your irrigation system.
Determine available flow and pressure
Determining both the flow rate and pressure is critical to any sprinkler design. Knowing these values will allow for proper sprinkler selection and spacing. Inspect site elements, such as the water meter, and note the size. Measure the static water pressure or obtain this value from the local water or fire department. Remember to record the pressure during the time of day that the system will be in operation. If you intend to irrigate at night, this is when you need to check the pressure. In new housing developments where construction is ongoing, the water department will be able to provide the intended future static-pressure values. For smaller residential locations, you can obtain flow rates by using a bucket and stop watch. Higher pressures, which are usually present in new construction sites, will give misleading flow values and make it necessary for you to use flow-velocity tables. For larger sites, note the type and size of the service lines and use flow tables to estimate maximum safe flow with velocities no higher than 7.5 feet per second (fps). (Some design manuals suggest velocities ranging from 7 to 9 fps.)
Follow a three-step process to determine available flow at your site:
Make sure that water demand through the sprinklers does not create pressure losses in excess of 10 percent through the water meter.
Be conservative and limit flow through the meter to 75 percent of maximum safe flow.
Do not exceed water velocities of 7.5 fps (or up to 9 fps, depending on how much pressure loss you can afford).
Available flow and pressure loss is determined by using the smallest value from the three-step process above (also see “Example of available flow,” page 32). Friction losses from service lines, backflow devices and shut-off valves will further reduce working pressure. Changes in elevation can increase or decrease available pressure by 0.433 pounds per foot of rise or fall. Good design will limit further losses to no more than 25 percent of the static-pressure value measured at the point of connection (POC). So, reduce the static pressure reading you get by 25 percent to determine the working pressure available in your new site. For example, if you measure 64 psi static at the POC, use sprinklers that operate at or below 48 psi.
Having obtained both flow rate and available working pressure, determine whether these values are adequate for supplying water to your site. You will need to know the acreage, total evapotranspiration (ET) to be replaced during a single irrigation event and the number of hours available in the day during which watering can occur. Because no irrigation system is 100 percent efficient in applying water to the landscape, you should also make allowances for efficiency when determining the “water window” (use 75 percent). (See “Example of available flow,” page 32.) Do not confuse irrigation efficiency with distribution uniformity. I define irrigation efficiency, in this case, as the amount of water lost between the meter and what can be measured in the landscape after an irrigation event. Differences between these two values can be caused by wind draft and misting, among other factors.
Assume a 6- to 8-hour window and 3 days of evaporative losses during the warmest, most demanding period of the year (usually June or July), so you have the option of using a deep, less-frequent irrigation regimen. In the desert Southwest of Tucson, Ariz., this would translate into a minimum need of 35 to 50 gallons per minute (gpm) per irrigated acre, depending on the plant mix. Minimum gpm/acre values will differ based on location in the United States, and whether you intend to irrigate once every 3 days. Values for maximum ET for your location may be available on the Internet or from local Extension Service offices. Many irrigation designs fail to consider this need and are impossible to properly manage once installed. All the sprinklers may work and have adequate pressure and water flow, but if the service line is undersized for the acreage, too many zones will be required. Therefore, there may not be enough hours in the day to completely water a site, making it impossible to properly manage the irrigation system. A 1-inch service line cannot irrigate a 2-acre site. If a site does not have sufficient flow to allow irrigation to occur within a reasonable time window, the site will need more or larger service lines.
Low pressure (~30 psi) can also limit proper management of an irrigation system by forcing you to use spray heads and multiple zones. Having an irrigation system that has few sprinklers on many zones may necessitate long watering windows to adequately irrigate an entire site. You should address low-pressure problems using booster pumps to give yourself more options when you're selecting sprinklers. Properly designed systems allow for good management, while good management is impossible if the sprinkler system was designed poorly to begin with.
The above information focuses mainly on new construction, but may also apply to existing systems in identifying problems. Re-design of inadequate or poorly designed irrigation systems that have already been installed is labor intensive and costly. Doing it right the first time is always a more desirable option, even if there is a slight increase in initial cost over quickly planned and poorly designed systems.
Redesign may be as simple as increasing pressure using booster pumps. Gradual loss in working pressure can occur on an existing site that was properly designed due to a buildup of the surrounding neighborhood. In some cases (if you're lucky), simply changing the number of circuits that operate simultaneously can provide enough pressure increase to avoid having to install a booster pump.
Locate unique needs in the landscape
Dividing the landscape into distinct areas when planning irrigation will make it easier for the irrigation manager to more fully address unique watering needs. Design the system around areas to give plantings in each area custom care as well as maximize irrigation efficiency. Irrigation efficiency is the ratio of water used by the planting to the total quantity of water applied. By separating the landscape into individual areas, you can avoid under- and over-watering. You can also address seasonal watering needs without irrigating plantings that may be dormant. This is especially true in the desert Southwest during winter, where deciduous trees and overseeded turfgrass occupy the same landscape site.
Decide which specific landscape areas have unique needs. If you have a plan-view drawing of the site, use it to identify each area. Draw circles around each location on the plan view that has a particular characteristic. Outline large, open turf areas and indicate the prevailing wind direction. These are areas with no obstructions (such as trees) and are best suited for rotary sprinklers but also can be irrigated using spray heads where available water pressure is low. You can address wind speed and prevailing direction by reducing the spacing between sprinklers and using triangular spacing, if possible. Square areas usually dictate the use of square spacing for optimal uniform application. Use triangle spacing in large open areas to reduce the total number of sprinklers required and to provide greater uniformity in windy locations. Reduce recommended (catalog) spacing between sprinklers whether you use a square or triangle layout to ensure head-to-head coverage under worst-case conditions (i.e., high wind or low pressure).
Method of squares
To consistently provide head-to-head coverage, you may need to employ another technique for improving sprinkler-distribution uniformity in irregularly shaped areas called the “method of squares.” To do this, divide an irregularly shaped area into the largest square space first and use the largest radius sprinklers to fill in this area. Then, divide the remaining areas into progressively smaller and smaller squares, using appropriate radius sprinklers. Finally, any remaining areas will have irregular, usually triangular shapes that you can water with small radius spray heads (see illustration below).
Identify trees or other obstructions, such as lampposts, located within turf areas. Lay out at least three sprinklers around trees or shrubs in a turf setting to ensure adequate coverage. Provide sufficient distance between any sprinkler and tree so that the spray pattern is minimally disrupted. Doing so will minimize injury to tree roots caused by trenching and also lessen damage to the tree trunk and low-hanging branches. To provide good coverage around trees and shrubs, you may need to use sprinklers with a smaller radius than would otherwise be desired based on the size of a given turf area. Tree and shrub placement in the landscape is done with aesthetics in mind, and most of the time it interferes with even, consistent sprinkler spacing. Use separate zones around trees or other obstructions to allow for uniform water application. You can also consider supplying drip irrigation to trees and shrubs within turf areas. This can reduce shallow rooting and minimize competition for available water by the turf. Also, drip irrigation can provide water to trees when the surrounding turf is dormant during the winter months. Having the ability to irrigate evergreen trees without having to water turf is useful where landscape water restrictions exist.
Use separate zones on slopes, hills and mounds. These areas have a greater potential for water runoff and erosion, as well as heat gain or loss, depending on the primary direction of the slope. Having the ability to control these areas separately will allow the irrigation manager greater flexibility in providing for water needs. If it's not possible for you to separate these types of areas, remember that rotary sprinklers usually have a lower precipitation rate than pop-up spray type sprinklers. Because it will take longer to wet a sloped area to the same depth as a flat area, low-precipitation sprinklers are desirable when both conditions occur within a single zone. Using pop-up spray sprinklers is still possible on sloped areas if zoned separately. Multiple watering cycles that allow water to penetrate the soil will reduce any erosion or runoff.
Group areas that have similar plant materials with the same water needs. Separate annuals from perennials, evergreen from deciduous shrubs, and vegetables from potted plants. Also, separate shrubs from trees because rooting depths, and, therefore, irrigation frequencies and water amounts, will be different. Identify areas that remain shaded, such as the north sides of buildings or yards with tall privacy fences or mature trees. These areas can be prone to disease and will usually demand less water than locations that receive more sun. Also, circle areas where wind movement is low or non-existent as these areas are similar in need to shady locations. In contrast, south- or west-facing slopes or plantings adjacent to south- or west-facing walls will require more water if they're in direct sunlight, and should be zoned separately.
Where there are differences in soil type, separate sandy soils from more clayey soils. Sandy soils generally have approximately twice the water-intake rate of clay soils. Also, clay-type soils have a water-holding capacity that can supply water to plants for greater periods of time compared to a sandy soil. Knowing a soil's characteristics will enable proper irrigation management. Finally, identify high-traffic areas such as dog runs or gated entry ways. Traffic compacts the soil and can reduce the infiltration rate. Traffic areas have needs similar to sloped or hilly locations and will benefit from the same irrigation management strategies.
In addition to identifying the unique areas above, be aware of unwanted overspray when deciding where to zone your irrigation system. This is especially important when using treated waste water as an irrigation source. In fact, most city codes require that no overspray occur onto adjacent property. Overspray onto hard surfaces such as sidewalks is acceptable as long as it occurs within your property. But if water runs off onto adjoining property from a sidewalk in your landscape, it's a bad thing. You also may want to avoid overspray onto walls and windows as well. Use low-trajectory sprinklers or a layout that stacks rows of partial circle sprinklers directed away from areas where overspray is unwanted (see illustration, page 32).
Now that you are ready to locate sprinkler heads, select a controller that can accommodate all your irrigation requirements. Remember, the whole idea behind zoning and separating the landscape into different areas is to maximize efficiency and plant health.
Jeffrey Gilbert is a turfgrass research specialist at the University of Arizona (Tucson, Ariz.).
Sources: Hunter Handbook of Technical Irrigation Information; The Irrigation Association Training Manual.
EXAMPLE OF AVAILABLE FLOW
Definition: Available water flow rate during peak demand.
Given: Water meter is 1 inch and service line is 1.25 inches type k copper tubing, and 35 feet from the shut-off valve. Static pressure in the POC is 60 psi.
Pressure loss through meter should be <10 percent or ~6 psi at max flow rate. From pressure loss table a 32 gpm flow is indicated.
75 percent of safe flow through a 1-inch meter is 37.5 gpm (50 gpm × .75).
A 7.5 fps flow through a 1.25-inch type k service line is ~28 gpm.
Use the lowest gpm flow rate from the three steps above. In this example, 28 gpm will be the maximum flow rate of any zone.
Then, subtract friction loss from service line at 26 gpm flow rate:
28 gpm through a 1.25-inch type k copper line has 7.97 psi loss/100 ft. (7.97 psi/100) × 35 ft. = -2.8
Available working pressure at the meter is 60 psi -2.8 or ~57 psi.
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