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.
Estimates of potential evaporation (mm)
Thornthwaite equation (1948)
is the estimated potential evapotranspiration (mm/month)
is the average daily temperature (degrees Celsius; if this is negative, use ) of the month being calculated
is the number of days in the month being calculated
is the average day length (hours) of the month being calculated
is a heat index which depends on the 12 monthly mean temperatures .
Somewhat modified forms of this equation appear in later publications (1955 and 1957) by Thornthwaite and Mather. 
Penman equation (1948)
- Thornthwaite, C. W. (1948). "An approach toward a rational classification of climate". Geographical Review 38 (1): 55–94. doi:10.2307/210739
- Black, Peter E. (2007). "Revisiting the Thornthwaite and Mather water balance". Journal of the American Water Resources Association 43 (6): 1604–1605. doi:10.1111/j.1752-1688.2007.00132.x
- 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.
- ag.arizona.edu Global map of potential evaporation.
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