Several methods are available for estimating a plant's rate of evapotranspiration. Some involve using specialized devices, such as an evaporation pan or a lysimeter, to measure in the field the components of the overall rate, afterward adding the components together. Others involve calculating the evapotranspiration rate indirectly.
Using one of the indirect methods is the easiest way to proceed, and the result from such a calculation usually is sufficiently accurate for estimating a landscape's irrigation needs.
The most popular indirect approach involves using what's known as the reference evapotranspiration, ET0. The reference evapotranspiration is defined as the rate of evapotranspiration of an extensively studied reference species — one that is well watered, actively growing, and completely shading the soil. Scientists developed the concept of reference ET to avoid having to estimate the evapotranspiration of many different species of plants individually under different sets of conditions.
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