The rate that sprinkler heads apply water to the soil or turf
The PR (Precipitation Rate) should be as consistent as possible throughout a zone
Avoid over or underwatering
How to get Matched PR
Use sprinkler heads with the same precipitation rates (matched PR’s)
Use only heads of the same type
Use only heads from one manufacturer
Calculating PR using Total Area Method
Measured in inches of water applied per hour to the irrigated area
Formula for the total area
PR = 96.25 x Total GPM
Divided by Total Area in square feet
96.25 is a constant
Area is 60 ft x 15 ft with the following heads
6 half circle (1.0 GPM nozzles)
4 quarter circles (.5 GPM nozzles)
1. Total GPM = (6 x 1.0) + (4 x .5)= 8.0 GPM
2. Total area = 60 x 15 = 900 sq. ft.
3 PR 96.25 x 8/900 = .86 inches/hour
Calculating PR using Individual Head Method
For square pattern
PR = GPM for full circle sprinkler x 96.25 / spacing 2 (S X L)
This means if you have all half sprinklers double their GPM to get the rate for a full circle
For triangular pattern
PR = GPM of full circle sprinkler x 96.25 / spacing 2 x 0.866
The rows are 25 feet apart and the sprinklers are 30 feet apart in the row.
The heads are arranged in a triangle pattern. The heads use 6 GPM.
(6 x 96.3) / [(25 x 30) x 0.866]
577.8 /  x 0.866
577.8 / 649.5
0.89 inches per hour
What happens if you adjust the radius?
For spray heads you just use the manufacturer's chart.
When you use the radius adjustment on a spray you are simply reducing the water pressure by closing a small valve in the nozzle.
As the pressure drops, so does the radius.
Just look at the manufacturer's chart for the radius you plan to reduce the sprinkler down to. Then read the GPM for that radius!
For example, your designing for 30 PSI. The radius at 30 PSI of the sprinkler you selected is 15 feet with 1.85 GPM according to the manufacturer's chart. But you want the radius to be 14 feet. Looking at the manufacturer's chart you see that the radius of the same sprinkler is 14' at 25 PSI with 1.65 GPM. So the GPM of that sprinkler if you reduce the radius to 14' will be 1.65 GPM. That's because when turn the radius adjustment screw to reduce the radius to 14' what you REALLY did was reduce the pressure to 25 PSI!
Reducing the Radius on a Rotor
For rotor heads the GPM stays the same no matter how much you reduce the radius!
That's because reducing the radius on a rotor doesn't change the amount of water coming out of the nozzle.
To change the radius a small screw extends into the stream of water coming out of the nozzle. The tip of the screw deflects the water which "screws it up" (pun intended) so it doesn't go as far. This creates another problem, however, which is that it really messes up the uniformity of the water.
So when you use the radius adjustment on rotors, you tend to get dry spots. This is one reason I strongly suggest that you use a smaller nozzle if possible rather than using the radius adjustment screw on the sprinkler. The other reason is that when you reduce the radius you really should also reduce the GPM of the sprinkler. Otherwise there will be a lot more water under the sprinkler with the reduced radius. Bottom line- use the radius adjustment screw on rotors only when nothing else will work .
GPM for Spray Heads
It’s easy, just read the manufacturer’s chart
GPM for Rotors
You must select the appropriate nozzle size for each rotor in order to match the precipitation rates.
A simple illustration will help explain
Rotor heads move back and forth across the area to be watered.
The rotation speed is the same regardless of whether the rotor is adjusted to water a 1/4 circle or a full circle.
So the stream from a 1/4 circle head will pass over the same area 4 times in the same amount of time that it takes for a full circle head to make one pass over the area it waters.
With the same size nozzle in both, a 1/4 circle rotor will put down 4 times as much water on the area under the pattern as a full circle rotor will.
(Remember that after every quarter turn the 1/4 circle rotor reverses direction and covers the same area again!)
Rotor reduction in Radius
To match the precipitation rates between these sprinklers, the quarter circle rotor must have a nozzle that puts out 1/4 the amount of water that the full circle nozzle puts out!
A half circle rotor must have a nozzle that puts out 1/2 the water of a full circle.
This is why when you buy a rotor-type sprinkler head they often include a handful of different size nozzles with it.