Evaporability

an arbitrary quantity that characterizes the potentially possible evaporation (evaporation not restricted by the water supply) in a given area under existing atmospheric conditions. It is measured with an atmometer from the surface of the water or from the surface of a large natural basin of fresh water or supersaturated soil. Evaporability is expressed in millimeters of the layer of water evaporated. For example, the average annual evaporability in Leningrad is 320 mm; in Moscow, 417 mm; in Odessa, 584 mm; and in Nukus (Middle Asia), 1,718 mm.

Evaporability can differ greatly from the actual evaporation from the surface of the soil and from plants, especially in an arid desert, where actual evaporation is close to zero because of the complete absence of water, but where evaporability reaches 2,000 mm a year and more.