Irrigation 101

Irrigation systems consist of many parts and pieces, but you can describe them basically as a collection of sprinklers turned on and off with an automatic controller. Other important pieces-pipe, fittings, wire and valves-are used within the system to connect the sprinklers and controller.

All irrigation systems require a design-even if it might be only in someone's head. Then you need the correct components to carry out that design and the know-how to install the system properly. All three needs-design, components and installation-no matter how large the system, will result in an efficient irrigation system that will work for many years.

PART 1: IRRIGATION DESIGN

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To properly design an irrigation system, you first need to determine the available pressure and flow from the water source. Use a pressure gauge to measure the pressure for an existing facility. A 0-to-160 psi pressure gauge works well. With a silcock adapter you can determine pressure by attaching it to the water supply of a house or building.

Read the gauge with no water running (static pressure) and then again with a faucet on, toilet flushed or other water flowing to determine dynamic pressure. As a designer, you are more interested in dynamic pressure than static pressure. You want to determine if there is enough pressure to operate the irrigation system or if there is too much pressure. Either way, your design will have to account for available pressure.

Irrigation-system pressure depends on flow, so large differences between static and dynamic pressures indicate large friction losses in the water-supply system that you should investigate. For new homes or buildings, the local water department, fire department or developer can give you an estimate of the static pressure.

Measuring flow is more difficult. There are several procedures: using a chart, measuring flow with a bucket or using the "three-rule" method. Many small irrigation contractors use a chart.

  • Based on the size of the service line into a building and the size of the water meter, a chart can conservatively estimate the amount of water available. These charts are usually based on minimum pressures of 50 psi. For example, a house with a 3/4-inch service line and a 5/8-inch meter will supply 10 gpm. A 1-inch service line and 1-inch meter will supply 22 gpm.
  • You also can fill up a 5-gallon bucket from the silcock and, using the second hand on your watch, determine the flow. For example, if a 5-gallon bucket fills in 30 seconds, you have 10 gpm. This may be conservative; the silcock is usually far from the water meter and plumbed off 1/2-inch pipe, which restricts the flow.
  • The three-rule method is useful on larger systems. The rules:
    1. The friction loss through the water meter should not be more than 10 percent of the static pressure.
    2. The actual flow through the meter should not exceed 75 percent of the safe flow through the meter.
    3. The velocity through the service line should not exceed 5 to 7.5 feet per second (fps) depending on the service-line material.

You will need a friction-loss chart for the service line material and the water meter to make the calculations. The calculations will provide three numbers-the lowest is the available flow from the facility. For example: if rule 1 gives a result of 22 gpm, rule 2 yields 20 gpm and rule 3 yields 18 gpm, the available flow would be 18 gpm.

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