The amount of water plants need depends on many different factors, including plant species, plant size, climate, climate region, soil etc. A reasonably accurate estimate of the amount of irrigation water needed can be made using Eto data for our actual zip code. “ETo” is the amount of water needed for irrigation, based on climate conditions (wind, humidity, rainfall, hours of daylight etc). You can find the historic ETo for any zip code in the USA at the website http://www.rainmaster.com/historicET.aspx courtesy of the Rainmaster irrigation controller company, who makes “Smart” irrigation controllers. The ETo value is based on weather stations that manage water needs to keep turf grass healthy, calculated as inches per day. More information on ETo and weather station managed by the State of California is available at California Irrigation Management Information System (CIMIS).
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
Eto: Get this from http://www.rainmaster.com/historicET.aspx . Enter your zip code, and the website will give you the average daily ET value for each month of the year. Below is daily ET data for area code 93023. This value is the daily water requirements based on climate data for a standardized crop (turf grass).
PF: This is the plant factor. Different plants need different amounts of water. Use a value of 1.0 for cool season turf. For water loving shrubs use .80, for average water use shrubs use 0.5, for low water use shrubs use 0.3. More information on plant water requirements is available at Water Use Classification for Landscape Species (WUCOLS).
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 from inches to gallons.
IE: Irrigation efficiency. Some irrigation water never gets used by the plant, this value compensates for that loss of water. I suggest using 0.75 as the value for this. Very well designed sprinkler systems with little run-off that use efficient 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 93023:
start by looking up the Eto for zip code 93023 at the Rainmaster website, which displays a suggested reference value of (.17 for June rounded up to 0.2) .2 inches per day during the month of June.
Now rewrite the formula inserting your values into it:
0.2 (daily Eto value) x 1.0 (grass plant factor value) x 1500 (sq ft) x 0.62 ÷ 0.75 (efficiency factor) = gallons of water per day
Now do the math, just punch the values into a calculator and get your answer:
0.2 x 1.0 x 1500 x 0.62 ÷ 0.75 = ??? 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. For example, Stage 3 drought allocations that mandate water restrictions could reduce the water applied by 30% (multiply by 70%). 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.
To calculate your home sprinklers’ output, the Irrigation Audit Best Management Practice is to conduct a catch can test. Place small cans with straight sides, like pet food or tuna cans, on the lawn in several places and run the sprinklers for 20 minutes. Use a ruler to measure the water in each can and determine the average. Multiply by three to get an hourly irrigation rate. Detailed information about various lawn species’ water needs in different parts of California is outlined in the free UC Division of Agriculture and Natural Resources online publication Lawn Watering Guide for California. The document gives watering guidelines for warm season and cool season grass species growing in 10 climate regions in the state.
Once you know the irrigation systems output and the plant water requirements, the irrigation timer is managed to give the maximum amount of water to the landscaped area in ideal amounts to reduce overwatering and prevent runoff.
To get a get the best value out of your irrigation system, you may need to hire professionals to create an actual sprinkler system design. See the Landscape Sprinkler Irrigation System Design Tutorial .
See websites for estimating water usage including https://www.irrigationtutorials.com/how-to-estimate-water-useage-required-for-an-irrigation-system/ and https://www.irrigationtutorials.com/landscape-sprinkler-system-design-tutorial/