Solar radiation is the sole energy input of the Earth-atmosphere system. Its partitioning into surface fluxes of latent and sensible heat is determined by the physical properties and availability of surface water, the “evaporative demand” of the atmosphere, and the nature of the surface. Evaporation is the connecting link between the system’s energy and water balances. The quantification of a system’s energy balance requires a definition of its boundary conditions. They consist of the spatial and temporal dimensions of the system and its exchange surfaces, their physical transport properties, the energy states across the system boundaries, and possible modes of energy transfer.