Evaporation of water requires relatively large amounts of energy, either in the form of radiant energy or sensible heat. Therefore, the evapotranspiration process is governed by energy exchange at the vegetation surface and is limited by the amount of energy available. It is therefore possible to predict the rate of ET given a net balance of energy fluxes.
The primary energy components which supply (or diminish) energy at a vegetation surface are net radiation from the atmosphere (Rn), sensible heat to the air or the equilibrium boundary layer (H), sensible heat to or from the soil (G) and evaporation or evapotranspiration expressed as a flux density of latent heat (XET). Other fluxes or sinks are present, such as energy requirements for photosynthesis, but these are quite small relative to Rn, XET, H, and G. All terms are expressed in units of energy per horizontal area per unit of time. XET is the latent heat required to vaporize one unit of water [MJ Kg-1]. Therefore the energy balance can be written in terms of these four components as
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