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. It was developed by Michael Penman from Kelso in his laboratory.

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

$E=\backslash frac\{m\; R\_n\; +\; \backslash rho\_a\; c\_p\; C\_\{at\}\; \backslash delta\; e\}\{\backslash lambda\_v\; \backslash left(m\; +\; \backslash gamma\; \backslash right)\; \}$

where:

m = Slope of the saturation vapor pressure curve (Pa K^-1)

R_n = Net irradiance (W m^-2)

ρ_a = density of air (kg m^-3)

c_p = heat capacity of air (J kg^-1)

C_at = atmospheric conductance (m s^-1)

δe = vapor pressure deficit (Pa)

λ_v = latent heat of vaporization (J kg^-1)

γ = psychrometric constant (Pa K^-1)

which (if the SI units in parentheses are used) will give the evaporation E in units of kg/(m²·s), kilograms of water evaporated every second for each square meter of area.

Equations | Hydrology