Head to Head Coverage
The first thing we need to be aware of is that heads are designed to create even distribution of water only if head to head coverage is achieved.
What does it mean to have head to head coverage? Use the example of heads that throw thirty feet. Heads need to be placed a maximum of thirty feet away from the last head so that their throws overlap. Why do we need the heads so close together? Why can’t the heads meet in the middle? Everything still gets water, right? Yes, everything is getting water, but how much?
When you look at rotors throwing water, it gives the appearance of most of the water falling at the end of the rotor’s throw. The opposite is actually occurring. More water is falling closer in and less and less farther out. The reason we see the opposite is because the water droplet size is larger at the far end so it is easier to see.
Irrigation nozzles are not designed to have an even distribution of water throughout its throw. The further distance from the head, the greater the area of coverage. If we look at five foot increments of a head throwing 30 feet, we see that the total area in the rotor’s last five foot section is more than 10 times larger than the first five foot section. By utilizing head to head coverage, the heavy application of water close-in to a sprinkler combines with the light application from the adjacent heads to provide more even distribution of the water.
Many installers will stretch the spacing to reduce the total number of heads and save money on a project. Stretched head spacing greatly decreases the efficiency of a system causing dry spots during the warm part of the season. It can take twice-as-much water as necessary or more to overcome the lack of efficiency.
It is very important to make sure the heads are operated in the proper pressure range. Heads that are operated with too much pressure produce a misting or fogging spray. Water droplets the size of mist can travel more than fifty feet in a three mile per hour wind. This is usually more of an issue with spray heads that have a maximum operating pressure near thirty pounds per square inch.
The solution to too much pressure is pressure regulation. The best place for pressure regulation is at the heads. Many irrigation manufacturers offer pressure regulation in the stem of spray heads.
Another option would be pressure regulation at the valve. It is usually less expensive because you only need one regulator per zone.
Pressure will still be lost through the piping system to the heads after the pressure regulator so control of the pressure is less precise when regulating at the valve.
Solutions to too-little pressure are a little more difficult and costly. Redesigning the system is often the best solution. Low pressure creates larger water droplets and are often noticed by the “green doughnut” patterns they create.
Matched Precipitation Rates
In the irrigation industry, matched precipitation rate (MPR) is a term that is used to calculate the amount of precipitation in a given area is uniform. In order to be "matched" all sprinkler heads in a given zone must have the same rate of precipitation.
A full circle sprinkler head covers twice the area a half circle sprinkler covers and four times the area a quarter circle sprinkler covers. The full circle sprinkler must throw twice as much volume of water as the half circle sprinkler and four times the volume of water as the quarter circle sprinkler.
Sprinkler zones without matched precipitation rates will have to use approximately 26% more water than properly matched irrigation zones. On a one acre site in the middle of July 2016 would be over 23,000 gallons wasted for the single month.