Evapotranspiration (ET) is driven by radiant energy, e.g., the sun or other heat sources such as pavement and walls.  The role of the plant is for the most part passive, that is, it acts as a conduit much like a soda straw.  It requires the same number of kilocalaries of energy to convert water in the leaf to gaseous water as it does to boil water on a stove.  Evapotranspiration cools the plant the same way that a person feels cool when emerging from a swimming pool.  Although plant stomates can open and close, affecting water exchange, ET is primarily a process of energy transfer.  For many crops, ET is about 70% of open pan evaporation.
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