Potential evaporation

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Potential evaporation 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.

Video Potential evaporation

Estimates of potential evaporation (mm)

Thornthwaite equation (1948)

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

Where

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

$T_{a}$ 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

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

$I=\sum _{i=1}^{12}\left({\frac {T_{ai}}{5}}\right)^{1.514}$ is a heat index which depends on the 12 monthly mean temperatures $T_{ai}$ .

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

Penman equation (1948)

Penman-Monteith equation (1965)

Maps Potential evaporation

References

Penman, H.L. (1948). "Natural evaporation from open water, bare soil, and grass". Proc. Roy. Soc. London, U.K. A193 (1032): 120-145. Bibcode:1948RSPSA.193..120P. doi:10.1098/rspa.1948.0037.
Brutsaert, W.H. (1982). Evaporation into the Atmosphere: theory, history, and applications. Dordrecht, Holland: D. Reidel. ISBN 90-277-1247-6.
Bonan, Gordon (2002). Ecological Climatology. Cambridge, U.K.: CUP. ISBN 0-521-80476-0.