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Knowledge of water GPM & PSI critical to designing an irrigation system

February 17, 2011  By  Mike Jiggens

By Lorne Haveruk
DH Water Management

Water—ever in demand, ever running short, and ever so important to know
what you have before you begin to design an irrigation system.

Recently, I designed a residential project to find out later that the information provided to me about the water was not accurate. The system took 20 hours to design and was predicated that the site would have a city water supply available providing the standard 12 gallons per minute at 60 pounds per square inch. This was not the case.

The large home was in an older area and the water supply into the majorly renovated home and landscape was to remain as was—a small undersized copper city water supply.


So, what does this mean to an irrigation design? Wrong water size equals a system that will not operate as designed. What does this mean to an irrigation consultant/designer? Somewhere along the way, wrong information was provided or no information was provided and the system now requires a redesign of all the station head groupings, pipe layout, valve allocation, station control wire allocation, resizing irrigation controller to accommodate the additional five or six stations and redoing the water schedule. In other words, about 15 hours work. Who gets to pay? Time will tell.

To add to the issue, the newly proposed irrigation system will not be able to water the entire site within a one-day watering window. Designers’ mandate is to design most sites to be able to be watered in one day. This is critical for southern climates where plants can die if not watered regularly during grow-in period. Not such a big deal in northern climes as our weather tends to be not so harsh, except from mid-July to mid-August.


To prevent this situation from occurring, whoever is responsible for providing the POC (point of connection), GPM (gallons per minute) and PSI (pounds per square inch), needs to be certain the information is correct. For new sites, this information is available from the developer and noted on mechanical drawings. For existing sites, this information needs to be measured. There are two methods to measure water that we utilize to provide as accurate a reading as possible. A very basic method is known as the “Bucket Test.”

To perform the bucket test, mark a one-gallon line in the bucket, grab a stop watch and start the test and time at the same time. When you reach one gallon of water in the bucket, stop the watch and then the water. OK, let’s say the time to collect one gallon of water in the bucket is 10 seconds. There are 60 seconds in a minute and we want to know the GPM. Ten seconds to collect one gpm will end up being (10 sec. x 6 = 60 seconds or one minute), so if you collected one gallon of water, then 1 x 6 = 6 gallons of water would be collected in one minute. This equals 6 GPM. 

Excellent! Now we are getting somewhere. We know we have a water supply of six GPM.  Not a lot for sprays or rotors as they typical require 1.5 gpm to perform, but certainly good enough for residential, low volume systems. 

We now need to know the PSI available. There are two states of PSI—Static and Dynamic or Working Water Pressure. We like to know both and the best way to test this is with a pressure flow meter, available for around $100. You can use a hose bib pressure gauge purchased from any of the irrigation distributors.

If you use a pressure gauge, you will be measuring static water pressure (not moving). You will need to reduce this pressure to compensate for backflow, pipe, valve, fitting, losses that can amount to 15 to 20 psi. Remember to keep pressures high, use one size larger pipe and fittings. To reduce pressure for high pressure situations, decrease pipe and fitting sizes but do not exceed 5 fps water speeds as above this speed you can damage the irrigation system over time.

When designing irrigation systems, use this chart to keep your designs working at optimum performance. Good luck.

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