Irrigation 101

Preventing backflow

The irrigation system supply will require a backflow preventer. The type required will depend on the design of the irrigation system. There are two kinds of backflow, and not all devices protect against both.

  • Back siphonage happens when water wants to travel from an area of high pressure to low pressure. For example, if an irrigation system is running and the street pressure drops due to a nearby fire, the water will be siphoned back toward the supplyside as the pressure in the street becomes lower. All backflow preventers, when properly installed, protect against back siphonage.
  • Back pressure occurs when elevation or other causes result in a higher pressure on the irrigation system than on the supply due. For example, if the system is off and there is a pipe full of water on a slope above the backflow location, it will exert a back pressure on the water tap. The degree of pressure varies with how high above the backflow preventer the water is stacked. Only a properly installed reduced-pressure-type device protects against back pressure.

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There are other considerations in backflow selection, such as the size of the system and degree of potential hazard. The final decision of which device you need is dictated by the water authority that has jurisdiction.

Selecting sprinklers

The most important decisions you must make in designing your system are determining which sprinklers you need and where to install them. Base your selection on the size and shape of the area to be irrigated, and stay within the limits of the available pressure and quantity of water.

Sprinkler spacing should be "head-to-head." That means that if a sprinkler head has a radius-of-throw of 35 feet, it should be spaced 35 feet from other sprinkler heads. A sprinkler that throws 12 feet would be spaced 12 feet from all other sprinklers. Head-to-head spacing is extremely important in small turf systems in order to provide uniform water application.

Uniformity is an important component of any irrigation system design. The more uniform a water application, the less operating time an irrigation system needs to make up for dry spots. The less time the system operates, the less water the system uses. Uniformity is based on the nozzling and spacing of individual sprinklers. Depending on its radius of throw, each sprinkler applies a specific amount of water over a specific area in a specific amount of time. This is the precipitation rate, and it is measured in inches per hour.

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