Now you're ready to calculate the total amount of water that needs to be applied to a landscape. The equation is as follows:

Total water applied = Landscape's rate of evapotranspiration ÷ Irrigation system's efficiency

Or, in symbols,

TWA = ET_{L} ÷ IE

Example: You determine that a Los Angeles office park has a plant adjustment factor, or K_{L}, of 0.24 and discover that the local reference evapotranspiration rate for Los Angeles, or its ET_{O}, is 6.5 inches for the month of July. You subsequently multiply the two to arrive at an evapotranspiration rate, or ET_{L} , of 1.56 inches for that park. You estimate that the efficiency of the park's irrigation system (IE) will be about 70% or 0.70, so you proceed to divide 1.56 by 0.70 to obtain the total water to be applied (TWA), which equals 2.22 inches for the month of July.

To convert from inches of water applied to the landscape to total gallons, you can use a conversion factor of 0.62 and the area to be irrigated in square feet:

Estimated water in gallons = estimated water in inches × area to be irrigated in square feet × 0.62

This initial estimate provides one significant criterion to use for selecting an appropriate type of irrigation system: the system needs to be capable of applying the total amount of water that you've calculated is needed.

Once you've selected an irrigation system, it's advisable to check periodically the actual volume of water applied: the amount of water applied during a given span of time is the difference in water-meter readings at the beginning and end of that time. Adjustments can then be made as needed to further reduce any wastage of irrigation water. Bear in mind that the overall efficiency of an irrigation system can only be improved if it's less than 85% to 95%.

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