Potential evaporation Play media
This animation shows the projected increase in potential evaporation in North America through the year 2100, relative to 1980, based on the combined results of multiple climate models.

Potential evaporation (PE) or potential evapotranspiration (PET) is defined as the amount of evaporation that would occur if a sufficient water source were available. If the actual evapotranspiration is considered the net result of atmospheric demand for moisture from a surface and the ability of the surface to supply moisture, then PET is a measure of the demand side. Surface and air temperatures, insolation, and wind all affect this. A dryland is a place where annual potential evaporation exceeds annual precipitation.

Estimates of potential evaporation

Thornthwaite equation (1948)

$PET=16\left({\frac {L}{12}}\right)\left({\frac {N}{30}}\right)\left({\frac {10\,T_{d}}{I}}\right)^{\alpha }$ Where

$PET$ is the estimated potential evapotranspiration (mm/month)

$T_{d}$ is the average daily temperature (degrees Celsius; if this is negative, use $0$ ) of the month being calculated

$N$ is the number of days in the month being calculated

$L$ is the average day length (hours) of the month being calculated

$\alpha =(6.75\times 10^{-7})I^{3}-(7.71\times 10^{-5})I^{2}+(1.792\times 10^{-2})I+0.49239$ $I=\sum _{i=1}^{12}\left({\frac {T_{m_{i}}}{5}}\right)^{1.514}$ is a heat index which depends on the 12 monthly mean temperatures $T_{m_{i}}$ .

Somewhat modified forms of this equation appear in later publications (1955 and 1957) by Thornthwaite and Mather.