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  How to Calculate Landscape Water Needs
The species factor (k s ) is one of three factors used to determine landscape water needs. A microclimate factor (k mc ) and planting density factor (k d ) also need to be included in water needs estimates. The three factors are used in the following relationship to generate a landscape coefficient (K L ):

Landscape coefficient = species factor x microclimate factor x density factor

K L = k s x k mc x k d

The derivation of this equation and numeric values for k mc and k d are described in UC Leaflet 21493.

The landscape coefficient is a functional equivalent of the crop coefficient used for estimating water needs for turfgrass and agricultural crops.

Using species factor ranges from the WUCOLS list , an estimate of water needs can be made. For example, selecting a groundcover species from the moderate category (e.g., Pelargonium peltatum ), the k s range would be 0.4 to 0.6. If no adjustments are needed for density or microclimate (k d = 1.0 and k mc = 1.0), then the landscape coefficient (K L ) is caluclated as follows:

K L = k s x k d x k mc
Using a midrange k s value of 0.5,
K L = 0.5 x 1.0 x 1.0
K L = 0.5

Now, to determine how much irrigation water is needed, the relationship ET L = K L x ET o is used, where ET L is the estimated water need of the planting and ET o is reference evapotranspiration. If the planting is in San Diego, for instance, and the water needs for the month of July are sought, then the following calculations are made:

ET o for July in San Diego = 4.6 inches (approx.)
K L = 0.5
ET L = K L x ET o
ET L = 0.5 X 4.6 inches = 2.3 inches

Therefore, it is estimated that 2.3 inches of irrigation water are needed to maintain this groundcover in good condition in San Diego for the month of July.
To determine the total gallons needed for the planting, then the area of the planting (square feet) and a conversion factor * are included as follows:

For a 1,000 ft 2 planting:

Gallons of water needed =
ET L x area x 0.62* gallons/ft 2 - in.
= 2.3 inches x 1,000 ft 2 x 0.62 gallons/ft 2 - in.
= 1,426 gallons

If a k s of 0.4 was used, then the following amount of water would be needed,
K L = k s x k d x k mc
K L = 0.4 x 1.0 x 1.0
K L = 0.4

 ET L = K L x ET _
        = 0.4 x 4.6 inches
        = 1.84 inches
Gallons needed for 1,000 ft 2 = 1.84 x 1,000 x 0.62 = 1,140 gallons

Irrigation managers will need to determine whether the planting needs the lesser (1,140 gals) or the greater (1,426 gals) amount of water in July to maintain good health and appearance. Perhaps even a higher ks value (0.6) is appropriate for the planting, generating a higher estimate of water needs. Regardless of the ks value selected, it needs to be emphasized that these are estimates of water needs. They are not absolute determinations of water needs. They are estimates intended to give irrigation managers an approximation of the amount of water to apply. Adjustments to estimated amounts will likely need to be made in the field. NOTE: For species typically planted only in the shade, use a "high range" ks value. For example, for a "moderate" species (ks range of 0.4 to 0.6) use the 0.6 value. This will compensate for the microclimate factor (kmc) adjustment used for shaded areas when calculating the landscape coefficient (KL). Otherwise, shade will be compensated for twice, in the ks factor and in the kmc factor.

In addition to water needed to meet plant needs, water also needs to be applied to account for system efficiency losses. This amount is added to the estimated needs of the planting. The following formula can be used to determine total water needs:

Total water applied (TWA) = plant needs (ET L ) / irrigation efficiency(IE)

For example, if the plant needs are estimated to be 2.0 inches, and the irrigation efficiency is 75%, then the total water applied is:

TWA = ET L  / IE
TWA = 2.0 inches /  0.75
TWA = 2.66 inches

System efficiency may vary considerably. Efficiency evaluations should be made to determine the appropriate IE value to use.

extracted from:

A Guide to the Water Needs of Landscape Plants
Revised 4/1/94 by L. R. Costello, University Of California Cooperative Extension