Water temperature is a critical physical characteristic of aquatic environments. Besides its direct effects, temperature also influences numerous biological and chemical reactions in water quality models. In fact, virtually all kinetic rates are temperature-dependent. In WTSM, water temperature is calculated using an energy balance approach based on the laws of conservation of energy (heat). This energy balance accounts for heat inputs and outputs from the forcing functions, as well as heat exchange at the water surface and at the sediment–water interface. Schematic of sources and sinks of heat exchange in the multi-layer water column illustrates the heat exchange processes at both the air-water and sediment-water interfaces. The primary sources of heat at the water surface include short-wave solar radiation, long-wave atmospheric radiation, heat conduction from the atmosphere to the water, and direct heat inputs. The main sinks of heat are long-wave radiation emitted by the water, evaporation, and heat conduction from the water to the atmosphere.


Schematic of sources and sinks of heat exchange in the multi-layer water column