Equation for calculating a landscape's ET

When adopting a calculation-based approach, you can estimate the rate of evapotranspiration for a species of interest as follows:

• A plant's rate of evapotranspiration equals the reference evapotranspiration multiplied by the appropriate plant adjustment factor
• Or, expressed with mathematical symbols: ET_C = ET₀ × K_C
• where ET_C is the ET for the plant (or crop) of interest, ET₀ is the reference ET, and K_C is the appropriate plant adjustment factor (crop factor)

Note that in the expression above the ET for the plant of interest bears a subscript "c" (meaning crop). That subscripted "c" is a reminder that this equation works well for agricultural crops, because many species of crops conform relatively well to the definition of an ET₀ reference plant: well watered, actively growing, and completely shading the soil.

This equation is rewritten slightly when applying it to landscape plants:

• A landscape plant's rate of evapotranspiration is equal to the local reference evapotranspiration multiplied by the appropriate plant adjustment factor
• Or: ET_L = ET₀ × K_L
• where ET_L is the ET for the landscape, ET₀ is as defined above, and K_L is the appropriate landscape-plant adjustment factor

Applying this equation for turfgrasses works well. That's because turf landscapes usually consist of a single species grown over large areas, as is the case for agricultural crops.

Landscape plants other than turfgrasses — trees, shrubs, vines, and ground covers — do not conform as well to the definition of an ET reference plant. For example, they often are part of landscapes that consist of various types of plants, forming varied and non-continuous canopies. Also, the goal when growing trees, shrubs, vines, and ground covers is to maintain acceptable appearance and function, not optimal growth and development, as is required for an ET reference plant. Consequently, the methods that work well for agricultural crops and for turfgrasses do not work as well for these other types of plants. As outlined later in this module, a modified version of the aforementioned equation can be used to estimate the evapotranspiration of landscape plants other than turfgrass.

For any type of landscape plant, the sequence of steps necessary to use the previously mentioned equation is as follows:

1. Obtain local reference evapotranspiration (ET₀).
2. Determine an appropriate plant adjustment factor (K_L).
3. Calculate the plant's (or the landscape's) evapotranspiration rate (ET_L).
4. Conduct further calculations if needed to estimate the optimal amount of irrigation water.

You will learn about the particulars of the first three steps as you read through the remaining pages in this module. (The fourth step is the subject of a different module within this Guide.)

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