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.

## . . . Potential evaporation . . .

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

Where

${displaystyle PET}$

is the estimated potential evapotranspiration (mm/month)

${displaystyle T_{d}}$

is the average daily temperature (degrees Celsius; if this is negative, use

${displaystyle 0}$

) of the month being calculated

${displaystyle N}$

is the number of days in the month being calculated

${displaystyle L}$

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

${displaystyle alpha =(6.75times 10^{-7})I^{3}-(7.71times 10^{-5})I^{2}+(1.792times 10^{-2})I+0.49239}$

${displaystyle I=sum _{i=1}^{12}left({frac {T_{m_{i}}}{5}}right)^{1.514}}$

is a heat index which depends on the 12 monthly mean temperatures

${displaystyle T_{m_{i}}}$

.[1]

Somewhat modified forms of this equation appear in later publications (1955 and 1957) by Thornthwaite and Mather. [2]