Due to high evaporation from open impoundments in Iran, studies on surface temperature reduction in stratified waters have taken place. Although artificial mixing using air-bubble plume has been used for water quality enhancement for a long time (especially in small reservoirs), it is being considered as a new method for evaporation suppression purposes in literature.
By mixing stratified waters, warm epilimnion water gets mixed with hypolimnetic layers (which are 2 or 3 times thicker than epilimnion generally) and its temperature drops. To determine how much the temperature drop in stratified waters can reduce evaporation some models are proposed, whose validity is still under discussion. A major problem of these models is considering secondary effects of destratification. For instance, as much as evaporation rate decreases in stratification period due to the mixing, a significant amount of enthalpy remains in the water, which could be released previously by evaporation’s latent heat. But after all, it is estimated that 20% of evaporation reduction can result from each 5ºc drop in surface temperature.
For reservoirs, the most common destratification device is an air-bubble plume system which consists of a perforated pipe at the bottom of the lake, through which compressed air is pumped. As the air bubbles rise to the surface, they carry the hypolimnetic cold fluid with them. This colder water is ejected from the plume when the density of the air–water mixture is approximately equal to the ambient density or when the surface is reached. Once ejected, and being relatively heavy in comparison to the ambient water, the cold water drops, and mixing occurs, reducing thermal stratification. Should the detrainment process occur before the surface is reached, the bubbles continue to rise, and further water is entrained, forming a new plume and a later detrainment at a higher level. This process may be repeated several times until the air bubbles reach the surface.