This page provides detailed analysis of greywater irrigation calculations for Tucson Arizona.
The formula: Irrigation area (sq ft) = G divided by ( (ET-R) x P x 0.62 x 1.1 ) requires the following data
ETo & Rainfall values
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 (Tucson AZ)
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.
- By selecting low water requirement plants, the area can be increased to 1,627 sq ft. Very low water requirement plants increase the area to 3,254 sq ft.
In all cases the garden space can be singular, or a group of smaller garden spaces adding up to the total amount.
Actual per person water consumption in Tucson
To calculate daily usage patterns, I am making the following assumptions:
- The average single family residence has 3 residents
- Very little (if any) water is used in the middle of winter for irrigation
- Recoverable graywater is 60% of the total internal water use
The average volume of graywater generated per person per day = 45 gallons.
As a further assumption (of course every house is different, but the following is a typical approach in Tucson),
- 1/3 of the graywater will be used to plant high water use plants
- 1/3 of the graywater will be used to plant low water use plants
- 1/3 of the graywater will be used to plant very low water use plants
Because there are three people in the house, and I am dividing the garden into 3 different styles of planting, it is easiest to just run the calculator 3 times (one for each person, changing the plant type each time).
The predominant soil type in Tucson is caliche, with similar characteristics to medium clay.
The total garden coverage is 2,621 square feet, and requires 600‟ of dripperline.
- Very light water use plants 1,465 sq ft
- Low water use plants 733 sq ft
- Medium water use plants 423 sq ft
On a typical Tucson urban lot, this is enough coverage to plant virtually the entire lot, excluding the building, drive and patio hard scapes.
Compare this with the typical water use in 2004 (from the Tucson Water chart). – although entirely planted, this residence would be saving 23,000 gallons per year.
In addition to saving water, the residence will use the air conditioning less, as the property will be substantially cooler with a vigorous garden (c/w the normal heavy planting of decomposed granite i.e. gravel / rock).
* Utilization of evaporative water cooler graywater, which can be substantial in July / August, has not been included in the calculations above.