Potential evaporation is defined as the amount of
water that could be evaporated were it available. It is a
function of surface and air temperatures, insolation, and wind, all of
which affect water-vapor concentrations immediately above the evaporating
surface. The broad definition of a dryland is a place where
annual potential evaporation exceeds annual rainfall. In the hot deserts
of the Earth, then, potential evaporation is uniformly high. The chart
below shows global potential evaporation, with the hot deserts clearly
indicated as having very high values. While there is nowhere near enough
water available to satisfy potential evaporation in these hot deserts, the
parameter can be thought of as a harshness force, a measure of an
ecosystem forcing function.
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