(Values are required in all fields, ETo and Rainfall are inches per month)
ETo (Evapo Transpiration)
Evapo Transpiration tells us how much water is required per month to properly account for soil moisture loss via plant growth and plant moisture evaporation.
An excellent ET guide is provided online by California Irrigation Management Systems (CIMIS).
The image above shows a section of an ETo chart for California.
Large images are available at the CIMIS website.
Contact your local garden shop or irrigator to find out the ETo value for your area.
The amount of rainfall per month is subtracted from the ETo to determine the monthly water shortfall (ie how much additional water is required).
The ETo value is an estimate of how much water is used by a well watered, full cover grass surface, 3-6 inches in height; used as a reference point for determining water use by other plant types through the application of a plant coefficient.
Plant Coefficient Plant Type
0.13 Very low water use
0.26 Low water use
0.45 Medium water use
0.65 High water use
Converting inches per month to gallons per month per sq ft
The ETo value represents inches per sq ft required per month, but we are working backwards - we need to know how far a certain amount of water will go for the month.
The multiplication factor is 0.62
(144 cubic inches per sq ft per inch of rain / 231 cubic inches per gallons)
Capillary Irrigation Efficiency
The ETo value is based on 100% irrigation efficiency. With standard spray irrigation, much water is lost to surface evaporation and overspray. Typically only 50% efficiency is achieved, so the calculated ETo value would be doubled to account for this.
Capillary Irrigation (mentioned earlier in this eBook) has a very high efficiency - over 90%.
The ETo rate should be multiplied by 1.1 to account for the 10% inefficiency.
The last step is to calculate the correct garden area that can be irrigated.
G = number of people x graywater per person, per day x 30
ET = Evapo Transpiration rate for your area.
R = Rainfall for the month being considered.
P = Plant coefficient
Irrigation area (sq ft) = G divided by ( (ET-R) x P x 0.62 x 1.1 )
The following shows the calculations made for Tucson, AZ.
ETo & Rainfall values for TUCSON, AZ
Inches required per month (medium water use)
Convert to Gallons per sq ft needed per month
Final value, based on 25 gallons per person per day
A 2 person household, with a graywater diversion and irrigation system, can grow a vigorous 470 sq ft garden space with medium water requirement plants, without any additional water during the hottest / driest months.
By selecting low water requirement plants, the area can be increased to 813 sq ft. Very low water requirement plants increase the area to 1,627 sq ft.
A 4 person household, with a graywater diversion and irrigation system, can grow a vigorous 705 sq ft garden space with medium water requirement plants, without any additional water during the hottest / driest months.
In all cases the garden space can be singular, or a group of smaller garden spaces adding up to the total amount.