Irrigation and plumbing are frequently lumped together by casual observers. The similarities are clear: water flows from a source through pipes to outlets upon demand. Both are governed by the same laws of physics -- the most important of which relates to friction loss based upon flow.

Friction is energy lost because of resistance. Water forced through any space rubs against the sides of that space, losing energy as a result. By increasing flow through the same space, you also increase the friction loss. In this case, energy is measured as pressure, or pounds per square inch. This pressure was transferred to the water by a pump or by gravity and a change in elevation, such as a water tower. From the pump to the final point of delivery, friction reduces the amount of pressure. Every inch of pipe, every fitting, every valve, and every emitter causes a certain pressure loss. The reason you need to know how much pressure is lost by the irrigation system is each emitter requires a range of pressure to work properly. You also have a number of emitters operating at once.

Your challenge is to move a certain quantity of water from the source to arrive at the outlet at a certain pressure. This is where the difference between plumbing and irrigation starts to show, at least for a residential project. The number of water outlets operating at any one time in plumbing is usually much less than the number of emitters or sprinklers in an irrigation system. The irrigation challenge is more complicated because you have more outlets, each delivering a specified amount of water to a specified location during a specified period of time.

Even if your irrigation system has only one sprinkler head at the end of a pipe, the further the sprinkler is from the source, the less pressure will be available to operate the sprinkler. You can reduce friction loss by using larger pipe. Furthermore, if the sprinkler is located a higher elevation than the source, you lose pressure to gravity. As you add more sprinkler heads, control valves, backflow prevention devices, and flow meters, the amount of pressure at the base of any sprinkler head is lowered.

Let's say, for example, your water source is a one-inch diameter public water line with 75 psi of pressure. You need to choose between sprayheads and rotors or impacts. The sprayheads will pop-up and achieve their designed wetting pattern at 15 to 20 psi. Their effective radius, however, is limited to about 10 feet. The precipitation rate of sprayheads is greater than most rotor heads, so less time would be needed to apply a given amount of water to the plant material.

The rotors/impacts don't operate properly until the pressure at the base of the head reaches 40 psi, however, they can cover a much larger area with a radius more than three times the spray head's. You'd need fewer rotor heads than sprayheads to irrigate the same area. The runtime for the rotors will be longer than for the sprayheads because their precipitation rate is lower because the area is larger.

There are a few other things to consider. The water meter and manual shut off valve cut out ten psi. The backflow prevention device will reduce pressure between 2 to 20 psi. Atmospheric vacuum breakers take the least pressure away, but they are not considered adequate protection in some communities. A pressure vacuum breaker BPD offers better protection for a loss of 5 to 10 psi. The most protection is provided by the reduced pressure BPD which can suck up 20 psi in some instances. So, in many instances, you start with 75 psi and are down to 55 psi before you hit the first control valve.

Each control valve knocks off 2 to 5 psi. Every sprinkler head in a zone increases the flow and friction loss and reduces pressure slightly. The fewer sprinklers you can operate at any one time, the more zones you need to irrigate the area and the more stations your controller will need. There are tradeoffs everywhere you turn.

You control these factors when you design and install an irrigation system. You learn from experience with local water districts and housing developments that, perhaps three rotors on a single zone

works and six doesn't. You might get a fourth rotor in the zone by using 3/4-inch pipe instead of 1/2-inch. If you don't have the pressure available, you reduce the number or switch to sprayheads.

The laws of physics come to play every time you are asked to troubleshoot a customer's irrigation system. You may have to explain why you can't mix sprinkler heads on the same zone and why you can't place too many heads on a single valve. You alert them to the fact that heads in the same zone should be at roughly the same elevation. Then explain why it makes sense to pay for larger pipe in some cases. Understanding these limitations is the difference between the plumber or the do-it-yourselfer and the irrigation contractor.

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