How much water to apply, part 1

The volume of water that needs to be applied to your landscape via the irrigation system will help to narrow down the choices. Estimating this amount of water can be done in a variety of ways.

One approach to arriving at a preliminary estimate is as follows:

First, calculate your landscape's need for water, which is indicated fairly well by the water transpired from the landscape's plants plus the water evaporated from the soil. As detailed in another module, the rate of evapotranspiration is yielded by the following equation:

Evapotranspiration rate for landscape = local reference evapotranspiration rate × appropriate plant adjustment factor for landscape

or, in symbols,

ET_L = ET_O × K_L

Next, estimate the irrigation system's efficiency. Every irrigation system is inevitably less than 100% efficient, so more water than the amount lost to evapotranspiration must be applied. The precise amount of this additional water depends on the irrigation system's efficiency.

To estimate the efficiency of a system yet to be installed, consider the past performance of similar systems and anticipate how well the system will be designed and installed. Table 3 may be helpful in this regard.

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