Why are there so many nozzles to choose from?
The purpose for having many different nozzle sizes (flow rates) is primarily so you can control the precipitation rate, not the radius. The precipitation rate is the amount of water applied to a given area in a fixed period of time, and is usually expressed in inches per hour. (You can calculate the precipitation rate using the first equation given below.) Reducing the nozzle size is a very ineffective way to reduce radius, and it can destroy precipitation rate uniformity if done for this reason. The second equation farther down is probably the most useful one you'll find in this section, as it shows you how to determine the right nozzle flow rate once you know the arc and radius needed for your sprinklers and the precipitation rate for your station.

To calculate the precipitation rate for any regularly spaced (square or triangular) pattern of sprinklers, use this formula:

precipitation rate = 96.25 * 360 / arc for your nozzle * flow rate of your nozzle/ row space in feet / head space in feet.

Where:

• arc for your nozzle = the number of degrees of arc for the sprinkler on which the calculation is based
• flow rate of your nozzle = the number of gallons per minute released by the nozzle at the pressure it will be supplied
• row space in feet = the spacing between rows of sprinklers measured in feet
• head space in feet = the spacing between sprinkler heads in the same row measured in feet

If your sprinklers do not fit a standard triangular or square layout, you'll have to choose a representative value for each term in the equation. If the layout deviates significantly from square or triangular, this equation will be innacurate, and your precipitation rate uniformity will probably suffer as well.

For rotors with the same radius, you should select a smaller (lower flow rate) nozzle for a 90 degree (quarter circle) rotor compared to a 360 degree (full circle) rotor. This is necessary since the 90 degree rotor will be going back and forth four times more frequently over the smaller arc being covered. In this specific case, a 360 degree rotor should have a nozzle that applies four times as much water per minute as the nozzle used in the 90 degree rotor.

It's actually convenient that the radius is insensitive to flow rate, since it allows spacing to be kept constant as the nozzle size changes. Imagine how complicated a layout would be if the rotors in the center of a field (with large nozzles) had to be spaced farther apart than the ones along the edges (with smaller nozzles because they are 180 degree or 90 degree arcs.) If the radius were a strong function of flow rate, you wouldn't be able to maintain an even square or triangular layout pattern across a field up to the edges and corners even in an ideally sized rectangular yard!

If you need to adjust the radius of a rotor, don't use a smaller nozzle, use the radius adjustment screw! Now there's an idea! It's not recommended to use the screw to reduce the radius more than 30% on most sprinklers, since watering uniformity will suffer if you reduce the radius too much. If you need to use a nozzle with a certain flow rate, but need an even smaller radius than you can reach by adjusting the standard nozzle down by 30%, you should go for a low angle nozzle (if available) and then reduce the radius for the low angle nozzle. Since the trajectory of low angle nozzles results in a somewhat shorter radius to start with, you can bring the pattern in even further than the standard nozzles. If a low angle nozzle reduced by 30% is still too big, you should use a smaller rotor to start with. If you're using Hunter PGPs and they seem too large, try Hunter PGMs. If PGMs seem too large, try spray sprinklers. (Always remember, though, you can't match the precipitation rate for rotors and spray heads in most cases, so keep sprays and rotors on separate stations.)

A low angle nozzle is intended to spray water under low overhanging branches, or to keep the spray down close to the ground in windy areas. The reduced radius was not a primary goal of the low angle nozzle, it's just a natural result of a low trajectory. If you need a shorter radius, however, you can take advantage of that "feature".

Sizing rotor nozzles correctly:
The best way to "size", or select rotor nozzles is to define a desired precipitation rate for the station being watered with rotors. We find that 0.5 inch or 0.66 inch per hour is a good starting point so the watering time can be a convenient 2 hr or 1.5 hr period. Some people have a large enough water supply to feed a station with a 1" per hour precipitation rate, for a typical watering time of one hour.

To determine the ideal nozzle flow rate, use this formula:

flow rate of your nozzle = precip rate * arc for your nozzle / 360 * row space in feet * head space in feet / 96.25

In most cases, you can get three to five rotors on a single 12 to 16 gpm station.

If you can't get enough flow on a station to run 0.66 inches per hour precip rate and an adequate number of rotors, use 0.5 inches per hour as the starting point for the calculation. You can run higher than 0.66 inches per hour if you want, but the higher you run, the more critical it is to use multiple start times and short watering periods to give the water time to soak in. You'll also need a much larger supply flow rate, or you'll have to reduce the number of rotors on each station. You can go lower than 0.5 inches per hour, but we don't recommend going lower than 0.33 inches per hour since the evaporation losses would get to be a big part of the water you distribute. It may seem tempting to choose a very low precipitation rate to get your whole yard on an inexpensive four or six station controller. The problem with this is that the evaporation rate is constant, and if you don't water significantly faster than the evaporation rate, you'll waste most of the water you pass through the nozzles. In most cases, you're better off sticking with 0.5 to 0.66 inches per hour precipitation rate for residential jobs.

It's important to select the right nozzle size for your rotors. If you don't, you'll certainly waste a lot of water over the years. By reading and understanding this section, you may be able to optimize both your water bill and the appearance of your lawn.

Now it's time to move on to the rest of the tutorial. Find out how to determine station flow rates.