Chapter
Apr 25, 2016
Appendix J
Derivation of the Penman-Monteith Equation
Publication: Evaporation, Evapotranspiration, and Irrigation Water Requirements
Abstract
Deriving the Penman-Monteith (PM) equation is useful in understanding how it functions and some of the assumptions made in its development. Two fundamental approaches to estimating evaporation and evapotranspiration are the surface energy balance equation and the aerodynamic equation. This appendix combines the two approaches. It gives the aerodynamic-based expression for latent heat flux based on the surface-to-air-vapor-pressure gradient. Coincident measurements of relative humidity and air temperature, which are commonly available, or measurement of dewpoint temperature, also commonly available, can yield vapor pressure of air. To create the PM equation, one utilizes the Bowen ratio, which is simply the ratio between latent heat flux and sensible heat exchange from the surface to the air.
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Evaporation, Evapotranspiration, and Irrigation Water Requirements
Pages: 619 - 625
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© 2016 American Society of Civil Engineers.
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Published online: Apr 25, 2016
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Task Committee on Revision of Manual 70
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