A simulation model can act as a water budget for a plant surface. Water is added to a budget by precipitation or condensation from dew and lost by evaporation. A surface energy balance model can describe these physical processes as a balance of energies. For example during a dew event, radiant energy is lost from the plant surface and is converted into latent and sensible heat. The latent heat flux effects evaporation and condensation, while sensible heat flux affects the heat losses and gains at a leaf surface. The partition of radiation into latent and sensible heat depends upon the atmospheric conditions and the plant and soil physical properties. Such a physically based understanding of SWD offers the potential to define a theoretical standard for SWD.