In the previous section on capillary action, we have seen that daily irrigation keeps the graywater in the top 3″ to 6″ of soil, where the plants and trees need the water.

However, placing too much graywater in one area will result in graywater leaking into the subsoil, effectively ‘lost’ as it does not benefit the garden. If the topsoil sits on top of a barrier such as heavy clay, or caliche, too much graywater may result in drowning the roots of the garden.

Therefore it is very important to properly determine the correct irrigation area. If you have a small garden, you may not want to re-use all of the graywater generated in your house.

This page gives a detailed explanation of how to calculate the irrigation area for a given amount of graywater.
I will assume that graywater dripperlines are used, as this is the most efficient method of graywater irrigation.
Before getting into the math, let’s do a simple calculation using a Graywater Calculator.

An example calculation

With the following values:
Rainfall                        8
Number of people in the home           0
Graywater gallons per person, per day          4
Plant Coefficient          25
                                        0.45 – Medium water use
The optimal irrigation area is 1,222 square feet
By changing the Plant coefficient to ‘0.26 – Low water use’, the irrigation area now increases to 2,115 square feet.
This is because we still have the same volume of graywater per day, but the plants need less water, so we can grow more plants!

Changing the Soil type to ‘Medium to Heavy Clay’ doesn’t change the irrigation area, just the amount of dripperline required. This is because soil type does not affect the plant’s water requirements.

This is so important I will repeat it again:

This is because soil type does not affect the plant’s water requirements.

No amount of soil or percolation testing alters this irrigation fundamental. If the correct amount of water is applied to the garden, the soil will not be required to carry excess water. 

During winter or heavy rainfall (adding extra water to the soil), the graywater system should simply be turned off.

Irrigation Area Coverage Theory

Potable water irrigation calculations determine the amount of potable water required to irrigate a certain area.
However, with graywater we know how much graywater we have, and need to determine the garden area that will most efficiently use the graywater. This calculator provides the optimum irrigation area based on graywater production.

Throughout this section, it is assumed graywater drip irrigation is used, as it provides over 90% irrigation efficiency.

Other methods such as Branched Drain Systems, and Laundry to Landscape (covered in the later pages are far less efficient (20% to 40%) and therefore will only irrigate an area 1/4 to 1/2 the area irrigated by dripperlines.

The following values are required:

  • ETo (per month)
  • Rainfall (per month)
  • Plant Coefficient
  • Number of people in the house
  • Gallons of graywater per person, per day

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).
This image 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.

Converting inches per month to gallons per month per sqft
The ETo value represents inches per sq ft required per month, but we are working backward – 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)


The amount of rainfall per month is subtracted from the ETo to determine the monthly water shortfall (i.e. how much additional water is required).

Plant Coeffecient

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
Plant Type
Very low water use
Low water use
Medium water use
High water use

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 drip irrigation (mentioned earlier in this guide) has a very high efficiency – over 90%.
The ETo rate is multiplied by 1.1 to account for the 10% inefficiency.

The last step is to calculate the correct garden area that can be irrigated.


number of people x graywater per person, per day x 30
Evapo Transpiration rate for your area.
Rainfall for the month being considered.
Plant coefficient
Irrigation area (sq ft) = G divided by ( (ET-R) x P x 0.62 x 1.1 )

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