Planning a Garden Irrigation System

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Designing a successful irrigation sys tem, whether it’s a sprinkler system, drip irrigation, or a combination of the two, requires knowledge of hydraulics, soil types, plant growth needs, and irrigation equipment. But don’t be discouraged—planning and even in stalling a system is well within the ability of most homeowners. Instructions for installing a sprinkler system are presented later; for information on drip systems, see “Installing an Underground Sprinkler System”. To convert an existing sprinkler system to a drip sys tem, see “Converting Sprinklers to Drip Systems”.

For help with particular problems or for expert advice, look in the phone book or Google “Irrigation Systems and Equipment,” Landscape Contractors,” or “Sprinklers—Garden and Lawn.”

Mapping your property

To design a watering system, you’ll need to prepare a fairly detailed scale drawing of your property, determine the water-retaining characteristics of the soil, and measure water pressure and water flow rate.

Making a scale drawing. Use graph paper to make a scale drawing of the areas to be watered, marking locations and types of plants, any special water needs they have, and whether they’re deep or shallow rooted. Pencil in water sources and structures and such obstacles as fences, walkways, and patios. Also note any slopes and elevation changes, since they can affect water distribution. The more de tailed and accurate your plan, the easier it will be to select the right components when you’re ready to shop.

Now is the time to check with your local building department for any necessary permits. Also, investigate the various products on the market; if you decide on a particular brand, pick up copies of the manufacturer’s literature and workbook.

Determining soil type. Your garden’s soil greatly affects absorption, evaporation, and the lateral movement of water. If you’re not sure of your soil type, wet some and squeeze it into a ball. If it crumbles, your soil is sandy. If some of the ball holds its shape, you have loam. And if it sticks firmly together, it’s clay.

Sandy soil readily accepts water but doesn’t retain it well. Loam accepts and retains water. Clay doesn’t readily accept water, but once it does, it retains it well.

Sprinkler systems work well in sandy soil and in loam. Clay soil holds so much moisture that excessive run off and fungus occur when conventional sprinkler heads are used, so choose low-gallonage heads.

Measuring water pressure. Most sprinklers won’t operate efficiently if the water pressure, measured in pounds per square inch (psi), is too low. To measure your home’s water pressure, borrow or rent a water pressure gauge (see Ill. 4) from a hardware store, a plumber, or a tool rental company. Screw the gauge onto an outdoor faucet and , with all other water outlets turned off, turn the faucet on full. Record psi at each outside faucet location, taking several readings throughout the day, and use the lowest pressure.

As a rule, sprinklers work best with high water pressure. Drip systems need lower pressure, so if pressure is over 75 psi, you’ll need to install a pressure regulator (see Ill. 14).

Measuring water flow rate. Flow rate, the amount of water that moves through pipes in a given period of time, is measured as gallons per hour (gph) or gallons per minute (gpm).

To determine flow rate, place a 1- gallon container under an outdoor faucet and count how many seconds it takes to fill the bucket completely. Then divide the total number of seconds into 60 to determine gpm. Write this figure on your plan; you’ll use it when plot ting circuits.

Note that to use this method of measuring gpm, the outdoor faucet must be the same diameter as your service line.

Both sprinklers and drip irrigation emitters have designated flow rates. Generally, the total output of a circuit of sprinklers or emitters should not exceed 75 percent of your plumbing’s available water flow at the faucet; otherwise, the heads or emitters won’t operate properly, and household water pressure may dip. The solution is to create several separate circuits, each directed by its own control valve.


Ill. 4. To determine water pres sure, screw a water pressure gauge onto an outdoor faucet and turn water on full.

Positioning sprinklers and emitters

Planning your system on paper, whether it’s a sprinkler system or drip irrigation, will help you think the system through, guide you in ordering materials, and serve as a permanent record of where any pipes or tubing is buried.

Locating sprinkler heads, circuits, and pipe runs. The two broad categories of sprinkler heads are spray and rotary. Spray heads operate at relatively low water pressure, from 15 to 30 psi; they’re a good choice for precise, controlled watering of shrubs, irregular landscaping, and fairly small lawns. Rotary heads need more pres sure to operate (from 30 to 70 psi) and throw water substantially further- up to 90 feet; this makes them economical for very large lawns and landscaped areas.

Sprinkler heads also come in many different spray patterns, including full, half, and quarter circles, as well as rectangular shapes. Some heads have adjustable patterns and throw distance. Low-precipitation-rate nozzles reduce runoff, improve spray uniformity, and allow a larger area to be irrigated with a given amount of water.

For open lawn areas where foot traffic and mowing will occur, install pop-up heads that automatically rise when the water goes on and drop down when watering is finished.

On a copy of the scale drawing of your property, detail your proposed sprinkler system (see Ill. 5). Begin by noting where you need to locate sprinkler heads. To establish the spacing and correct water distribution pattern for each head, check the manufacturer’s workbook to find out the radius, or throw, of each type of head. Use a compass to draw the rounded spray coverage patterns, making sure they overlap sufficiently to provide complete coverage.

Next, break your system into separate circuits, or stations, keeping in mind the flow-rate limitations discussed below. Group the heads by sprinkler type and water requirements: don’t place rotary and spray sprinklers, shrub and lawn sprinklers, or low-gallonage and standard sprinklers on the same circuit. Remember that when a circuit goes on, all the sprinklers along its line will deliver water simultaneously.

Starting at the control valve for each circuit, sketch in the piping that will connect the sprinklers on the same circuit. Try to avoid running pipes under paved areas. Note that a T or H- shaped circuit will deliver water more evenly to all heads than a straight-line circuit (the last head in a line typically receives less water pressure).

Choosing and placing emitters. Decide on the correct gallonage and number of emitters for each plant, depending on your soil type (see the chart). In general, use higher-gph emitters for trees and plants in sandy soil, lower ones for shallow-rooted plants and in clay soil. Space emitters closer together for shallow-rooted plants.

Group plants on separate circuits according to their water needs and root depths. If possible, place trees, shrubs, flowers, vegetables, and containers all on different lines so you can schedule watering to suit their individual needs.

For most gardens, plan on using ½-inch polyethylene tubing for lateral lines to shrubs and trees. Be careful not to run the line too long or put too many emitters on it, especially if your system is extensive: the tubing has limits on how much water it can handle efficiently. Be aware that running a line uphill shortens the possible run; running it downhill increases it.


Ill. 5. Planning an irrigation system: Scale drawing of property; Detail of proposed circuits


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Updated: Tuesday, July 31, 2012 22:18