The term ‘potential evaportranspiration’ refers to “idealized conditions in which there would be enough rainfall to provide sufficient moisture for all possible evapotranspiration in an area”.
image source: nelson.wisc.edu/sage/data-and-models/atlas/maps/pevapotrans/atl_pevapotrans.jpg
In order to determine the potential evapotranspiration for any place or area, several factors like temperature, latitude vegetation, and permeability and water retention capacity of soil are taken into consideration.
Such places as have surplus of precipitation over evapotranspiration are marked by surplus of water for underground storage. Even large or small water bodies may have surplus water in them which may be used for canal construction or other purposes.
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But in areas where evapotranspiration is in excess of precipitation, no water is available for storage. Even if certain amount of water is stored during the rainy season, it is quickly evaporated into the dry and warm air. Under such conditions soils become dry and the vegetation gets parched up.
Irrigation engineers as well as farmers are equally interested in having knowledge of potential evapotranspiration, so that they can make a correct assessment of their needs and the availability of water supply.