calculate potential evaporation from evaporation
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Potential evaporation:-
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.
Estimation of potential evaporation:
Thornthwaite equation (1948) Edit
{\displaystyle PET=16\left({\frac {L}{12}}\right)\left({\frac {N}{30}}\right)\left({\frac {10T_{d}}{I}}\right)^{\alpha }}{\displaystyle PET=16\left({\frac {L}{12}}\right)\left({\frac {N}{30}}\right)\left({\frac {10T_{d}}{I}}\right)^{\alpha }} Where
{\displaystyle PET}PET is the estimated potential evapotranspiration (mm/month)
{\displaystyle T_{d}}T_{d} is the average daily temperature (degrees Celsius; if this is negative, use {\displaystyle 0}{\displaystyle 0}) of the month being calculated
{\displaystyle N}N is the number of days in the month being calculated
{\displaystyle L}L is the average day length (hours) of the month being calculated
{\displaystyle \alpha =(6.75\times 10^{-7})I^{3}-(7.71\times 10^{-5})I^{2}+(1.792\times 10^{-2})I+0.49239}{\displaystyle \alpha =(6.75\times 10^{-7})I^{3}-(7.71\times 10^{-5})I^{2}+(1.792\times 10^{-2})I+0.49239}
{\displaystyle I=\sum _{i=1}^{12}\left({\frac {T_{m_{i}}}{5}}\right)^{1.514}}{\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}}}{\displaystyle T_{m_{i}}}.
Somewhat modified forms of this equation appear in later publications (1955 and 1957) by Thornthwaite and Mather.