From AMS Glossary
A micrometeorological method to determine the rate of moisture transport (i.e., evaporation rate E or latent heat flux) away from a surface employing the concept of aerodynamic resistance and requiring measurements of net radiation Rn, ground heat flux G, temperature, humidity and wind speed.
The latent heat flux is given by
where λ is the latent heat of vaporization; rc and ra are aerodynamic resistances; Cp and ρ are the specific heat and density of air, respectively; S is the rate of change of saturation vapor pressure with temperature; γ is the psychrometric constant; es is the saturation vapor pressure; Tz is the temperature at a height z above the surface; and e is the vapor pressure. The product λE is the latent heat flux.
Stull, R. B. 1988. An Introduction to Boundary Layer Meteorology. 666 pp.