Potential evapotranspiration is the amount of water that would be removed from the surface of a gridcell by evaporation and transpiration, if the amount of water already present in the cell were not a limiting factor. In other words, the potential evapotranspiration over the Sahara desert is very large because the amount of evaporation that could take place there is huge. However, because there isn't any water there to be evaporated, the evapotranspiration that actually takes place is quite small.
Comparing PET to ET helps scientists understand the environmental forces at work on local vegetation. It is particularly useful for farmers looking to determine the volume of water that needs to be applied to their crops for irrigation.
Willmott, Cort J., and Matsuura Kenji (2001): Terrestrial Water Budget Data Archive: Monthly Time Series (1950-1999). Available: this address.
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