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# Penman equation

The Penman equation describes evaporation (E) from an open water surface. It is widely regarded as one of the most accurate models, in terms of estimates.

There are numerous versions of the Penman equation, one of which is:

$E=\frac{\Delta R_n + \rho_a c_p C_{at} \delta e}{\lambda_v \left(\Delta + \gamma \right) }$

where:

Δ = Slope of the saturation vapor pressure curve (Pa K-1)
Rn = Net radiation (W m-2)
ρa = density of air (kg m-3)
cp = heat capacity of air (J kg-1)
Cat = atmospheric conductance (m s-1)
δe = vapor pressure deficit (Pa)
λv = latent heat of vaporization (J kg-1)
γ = psychrometric constant (Pa K-1)
Last updated: 10-13-2005 06:32:35
03-10-2013 05:06:04