The amount of water needed for irrigation depends on many different factors. A reasonably accurate estimate of the amount of irrigation water needed can be made using Eto data for your actual zip code. “Eto” is the amount of water needed for irrigation, based on scientific research. You can find the historic Eto for any zip code in the USA at the website www.rainmaster.com/historicET.asp courtesy of the Rainmaster irrigation controller company, who makes very good “Smart” irrigation controllers. I use one of their Eagle model controllers on my own home. (Rainmaster get a plug from me as well as a big “thank you” for providing the ETo look up service online.) Unfortunately the Eto value only tells you how many inches per day are needed, which for most folks is a meaningless value. It makes more sense if you think about rainfall which is often also measured in inches. If you find you need 0.20 inches of irrigation, then 0.20 inches of rainfall would provide the required water. But most people in the USA want a value in gallons, which requires you to provide a little more information about your yard. Then you plug the values into a simple formula, and do a little multiplication and division on any calculator.
Formula to calculate the gallons of irrigation water needed per day:
(Eto x PF x SF x 0.62 ) / IE = Gallons of Water per day
Values for the formula:
Eto: Get this from http://www.rainmaster.com/historicET.asp . Enter your zip code, or a nearby zipcode, and the website will give you the average daily ET value for each month of the year. Use the highest value or the “suggested reference value”. Usually they are the same thing.
PF: This is the plant factor. Different plants need different amounts of water. Use a value of 1.0 for lawn. For water loving shrubs use .80, for average water use shrubs use 0.5, for low water use shrubs use 0.3.
SF: This is the area to be irrigated in square feet. So for a 30 foot x 50 foot lawn you would use 1500.
0.62: A constant value used for conversion.
IE: Irrigation efficiency. Some irrigation water never gets used by the plant, this value compensates for that. I suggest using 0.75 as the value for this. Very well designed sprinkler systems with little run-off that using efficent sprinklers can have efficiencies of 80% (use 0.80). Drip irrigation systems typically have efficiencies of 90% (use 0.90).
A 1500 square foot grass lawn in zip code 85232 (Central Arizona)
Start by looking up the Eto for zip code 85232 at the Rainmaster website, which displays a suggested reference value of 0.3 inches per day using June, the driest month of the year in that area.
Now rewrite the formula inserting your values into it:
0.3 (Eto value) x 1.0 (grass value) x 1500 (sq ft) x 0.62 ÷ 0.75 (efficency factor) = gallons of water per day
Now do the math, just punch the values into a calculator and get your answer:
0.3 x 1.0 x 1500 x 0.62 ÷ 0.75 = 372 gallons per day
We could figure out the average daily water use for other months of the year also. Just use the same formula but insert the Eto value from the Rainmaster website for the month you want to get a valve for.
Remember this calculation just gives you an estimated value. There are many other factors that could make this value higher or lower. When planning for how much water a system that has not yet been designed or installed will use, it would be very wise to allow for error by adding 10% or more to the daily water use needed. It is generally better to have too much water, than to have too little! Play it safe!
A common related question is “how much water pressure will my irrigation system need?” The answer depends on a lot of factors, but as a rule of thumb, I would suggest 50 PSI of water pressure as a good starting point for sprinklers, 45 PSI for drip systems. If you have a large yard and want to put the sprinklers farther than 30 feet apart you will need more pressure. For example, if you want your sprinklers 45 feet apart you will probably need 65 PSI of water pressure. To get a real value you will need to create an actual sprinkler system design. See the Landscape Sprinkler Irrigation System Design Tutorial .
Never buy a pump, sprinklers, or any other materials before your sprinkler design is completed!