Large images are available at the CIMIS website.

Contact your local nursery or irrigator to find out the ETo value for your area. (The IrriGRAY controller automatically loads most common locations' ET values based on City / Location selection).

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).

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The amount of rainfall per month can be subtracted from the ETo to determine the monthly water shortfall (i.e. how much additional water is required). The IrriGRAY controller automatically determines how much extra potable water is required if not enough graywater is produced, and will suspend adding potable water if rainfall is detected during the day.

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The ETo value is an estimate of how much water is used by a well watered, full cover alfalfa crop  3-6" in height; used as a reference point for determining water use by other plant types through the application of a plant coefficient.

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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

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Important: Plant coefficients are designed for growing crops! Do you or your client really want to grow turf as a crop? This would mean cutting the grass twice a week. Warm climate grasses generally have a plant / crop coefficient of 0.6 

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Recent research by Texas A & M is indicating turf can maintain acceptable 'greenness' with as little as 50% of the published coefficients. Interestingly, this means turf does not need any more water than a low / medium water use landscape if irrigation is managed carefully on a daily basis (as is done by the IrriGRAY controller).

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Capillary Irrigation Efficiency

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The ETo value is based on 100% irrigation efficiency. With standard spray irrigation, much water is lost to surface evaporation, overspray and soil super saturation / gravitational loss. Typically only 50% efficiency is achieved, so the calculated ETo value would be doubled to account for this.

Capillary drip irrigation (covered in section 103.1 Capillary Effect) 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 landscape area that can be irrigated.

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Formula

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G = number of people x graywater per person, per day x 30
ET = Evapotranspiration rate for your area.
R
P = Plant coefficient

Irrigation area (sq ft) = G divided by ( ET x P x 0.62 x 1.1 )