Penman Equation

Penman equation is used approach for estimating evapotranspiration (ET), which is the combined process of evaporation from soil and plant surfaces and transpiration from plants.  It was developed by Howard Penman in 1948 and is especially useful for determining water loss from a reference surface, like crops, using climatic data.  The method combines energy balance and mass transfer principles to calculate evapotranspiration.

 

Penman equation
\( \lambda E \;=\;  [\; m \; ( R_n - G ) +  ( \gamma \; \lambda \; e_a ) \;] \;/\;  (m + \lambda)  \)
Symbol	English	Metric
\( \lambda E \) = Reference Evapotranspiration	-	\(mm\;/\;day\)
\( m \) = Slope of Saturated Vapor Pressure Curve	-	\(kPa\;/\;C\)
\( R_n \) = Net Radiation at the Crop Surface (Incoming and Outgoing Solar Radiation)	-	\(MJ\;/\;m^2\;/\;d\)
\( G \) = Sensible Heat Flux into the Soil	-	\(MJ\;/\;m^2\;/\;d\)
\( \gamma \)   (Greek symbol gamma) = Psychrometric Constant	-	\(kPa\;/\;C\)
\( \lambda_v \)   (Greek symbol lambda) = Latent Heat of Vaporization	-	\(kJ\;/\;kg\)
\( e_a \) = Actual Vapor Pressure	-	\(kPa\)