The balance between the net warming or cooling of a volume and all possible sources and sinks of energy. The main sources and sinks of energy typically include the net fluxes of sensible heat, latent heat, and radiant energy. Conservation of energy requires that the energy received by a surface must equal that lost from the surface plus that stored. For water and land surfaces, the main source of energy is net radiation, which equals the sum of short and long waveband radiation downward minus radiation reflected or emitted upward. This energy is normally transferred into the soil (soil heat flux), into the air (sensible heat flux), or into latent heat flux (evapotranspiration or ET). Small amounts of the incoming energy can change the heat content of water or crops at the surface or are converted to other forms of energy (e.g., photosynthesis). Energy balance is often used to estimate evapotranspiration by 1) measuring net radiation, soil heat flux, and sensible heat flux; 2) entering those values into an energy balance equation; and 3) solving for the latent heat flux (ET). Under hot, dry, windy (advection) conditions, heat from the air in addition to net radiation is sometimes available at an underlying cool surface. Advection can potentially increase evaporation rates to higher than the energy available from net radiation alone. See also surface energy balance.