Meteorological components, such as the thermal exchange between the water body and the environment and discharge play a decisive role for the natural water temperature.
The short wave radiation budget is a result of incoming global radiation (direct and diffuse solar radiation) and the share reflected by the water surface (Albedo). The long wave radiation budget is a result of the thermal radiation of the atmosphere and the emission of the water body due to its own temperature.
With respect to water cooling, evaporation is the most important component of the thermal budget. If the vapour pressure at the water surface is below that of the air layer above, condensation occurs.
Convection is the direct thermal transfer between the water surface and the air. The value depends on the temperature gradient between the water surface and ambient air.
Evaporation and convection are strongly influenced by wind velocity.
Schematic illustration of the thermal exchange processes relevant for streams (according to LAWA 2012)