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:

where:

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

R_n = Net radiation (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)