by Charlene Burgi
Precipitation rates sound like a rather boring subject, but understanding “PRs” is key to maintaining a healthy landscape and preventing water waste—and they can even tell you about your personality.
Quite simply, the precipitation rate refers to the amount of water—be it rain or irrigation water—that falls in a given period of time. In the irrigation world, PR is measured in inches of water per hour. Your sprinkler nozzles all have a precipitation rate associated with them. By knowing this information you can determine the appropriate runtimes on your controller.
You might ask why I am bringing this up since our Weekly Watering Schedule (WWS) already provides runtimes. It is important to note that the WWS is calculated using nozzles with a PR of two inches per hour. But, some nozzles throw over three inches per hour, while others produce less than one inch per hour. Do you know how much water your sprinklers are putting out in a given time period? With that knowledge, you can assess if you need to double our recommended runtimes or cut them in half.
How do you determine what your PR is? One way is to go to the manufacturer’s catalog and look up the make and model number of your nozzles. Another way is to use the following averages, with the understanding that each manufacturer and nozzle will be different and that PR will also vary depending on the water pressure at each valve:
- Spray nozzles emit around 1.5 – 3 inches per hour;
- Multi-spray nozzles emit around 0.85 – 1 inches per hour;
- Matched precipitation rate rotators emit 0.4 – 0.75 inches per hour;
- Gear-driven rotors emit 0.5 – 0.8 inches per hour;
- Impact heads emit 0.1 – 1.0 inches per hour.
There is a third way to get this information for those of you that like to be more hands-on. You can perform a “catch-can test” by following the instructions here.
This test will help if you have different nozzles on a valve, but let’s hope that you have the same make and model on each station or valve. If your nozzles are different, the PRs will also be different in each part of the irrigated area, leading to poor distribution uniformity (DU).
Are you confused yet? Picture this: Place a handkerchief (does anyone use those anymore?) on a table and place one drop of water on one corner, two drops in another corner, five drops in the third corner and three drops in the fourth corner. The third corner with five drops will be much wetter than the first corner with one drop. Imagine those drops as your nozzles watering your garden. The result is poor distribution uniformity: Parts of the garden will be very wet and other parts will be extremely dry. If the nozzles are mismatched we end up overwatering to get enough water to the dry spots. Meanwhile, water from the wetter areas oozes off the garden and into the streets, gutters and sidewalks. So much for efficiency.
Okay, I said PRs had something to do with personalities and I will confess, I love to watch multi-stream rotators watering the garden. It is a soothing motion of gentle fingers laying water down on the earth. Because the PR is so low, I know the water is slowly percolating into the depths of the soil, helping to develop deep root systems and creating a water-conserving environment for my plants. However, impact heads remind me of a Gatling gun. They shoot water out at a rapid rate, spinning the nozzle back and forth with massive streams of water. They are the “type A” personality of sprinklers. They will water your garden, but it can be more difficult to control the distribution pattern in relationship to what needs water. Gear-driven rotors are used for large expanses of lawn found on golf courses and football fields, and fixed-spray nozzles are, to me, the vanilla of spray nozzles. They can be efficient if installed correctly, but they will emit a lot of water in a short period of time, which can be a problem if watering a slope. What is in your garden and what is your personality preference (if any)?