Potential evapotranspiration

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.

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.

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 $T_{m_{i}}$ .^[1]

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

Penman equation (1948)

The Penman equation describes evaporation (E) from an open water surface, and was developed by Howard Penman in 1948. Penman's equation requires daily mean temperature, wind speed, air pressure, and solar radiation to predict E. Simpler Hydrometeorological equations continue to be used where obtaining such data is impractical, to give comparable results within specific contexts, e.g. humid vs arid climates.

Penman–Monteith equation (1965)

References

^ 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. Bibcode:2007JAWRA..43.1604B. 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. A193 (1032). London, U.K.: 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.

External links

ag.arizona.edu Global map of potential evaporation.

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[thornthwaite48-1] Thornthwaite, C. W. (1948). "An approach toward a rational classification of climate". Geographical Review. 38 (1): 55–94. doi:10.2307/210739.

[black07-2] Black, Peter E. (2007). "Revisiting the Thornthwaite and Mather water balance". Journal of the American Water Resources Association. 43 (6): 1604–1605. Bibcode:2007JAWRA..43.1604B. doi:10.1111/j.1752-1688.2007.00132.x.

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