Automatic irrigation systems save time, water and money -- provided they've been planned and installed properly. Manufacturers and landscapers will prepare designs for potential systems. Regardless of available expertise, make yourself familiar with the concepts used to determine the number of fixtures to place in each zone, or subdivision, of your landscape -- especially if you're planning on installing the system yourself.
Sprinklers, Sprayers, Drip Emitters and Bubblers
Once, automatic irrigation systems consisted of either fixed or oscillating sprinklers, but these provided only one type of overhead watering. Modern fixtures take into account the differences in types of plants, the balance of sun and shade and can be programmed to compensate for type of soil. Sprinkler settings allow narrow arcs of spray to circles of rainlike watering for large areas of lawn. Sprayers provide down-spray in flower or vegetable gardens. Bubblers and spitters -- many with adjustable rate settings -- provide dribbles at the foot of individual plants. Drip irrigation systems snake through vegetable gardens, providing even moisture at a low flow rate that supports growth but helps prevent the spread of disease..
Pressure and Gallons Per Minute
Automatic systems require minimum water pressure, measured in pounds per square inch -- systems typically require between 30 and 40 psi at the source. Volume, or flow rate, is expressed in gallons per minute -- gpm. Given a flow rate of 8 gpm at 36 psi in a given zone, for example, you might install up to five of one model sprinkler that covers a 90-degree slice of lawn, but only one of the same model that rotates a full 360-degree circle. Another set of tables shows that a 180-degree half circle sprinkler set to water a 5-foot radius would need only 0.19 gpm to operate, but one rated to reach a 15-foot radius semi-circle requires a 1.6 gpm flow.
Each fixture decreases water flow, so each zone must be carefully balanced. A rectangular garden plot, for example, might require only a drip hose and a few bubblers for shrubs, while a zone full of sloped, sunny lawn might need six to eight oscillating or fixed sprinklers as well as pressure valves on the downhill slope. Remember, too, that the number and type of fixtures in each zone are limited to the maximum that can function with the water flow rate that enters that specific zone.
The Delicate Balance
Use as many fixtures as you need to cover the zone evenly, but ensure efficient functioning of each fixture by staying within the limit of the available water supply. Although pressure valves control pressure and pipe size steps flow up or down, the draw of fixtures in any one zone must total less than the flow entering the zone. To find your system's gallons per minute rate, fill a 5-gallon bucket with water at the outdoor tap, then divide 5 gallons by the seconds taken to fill. Finally, multiply that figure by 60 to get gallons per minute. Measure water pressure with a simple pressure meter. Armed with these figures, use manufacturer specifications to choose fixtures recommended to cover plantings in each zone.
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