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