Evapotranspiration can be thought of as the amount of water removed from the surface of a gridcell through evaporation (due to temperature, humidity, radiation...) and through transpiration (the process by which plants move water up from their roots and out through their leaves).
Evapotranspiration can be used to estimate the amount of water the local vegetation needs to remain in equilibrium with the ground water supply. It is particularly useful to compare ET to potential ET to determine the overall defeciency of the gridcell's water supply (ET is always less than PET).
Willmott, Cort J., and Matsuura Kenji (2001): Terrestrial Water Budget Data Archive: Monthly Time Series (1950-1999). Available: this address.
(All images are provided at a resolution of 1600x1200 pixels)
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