Modelling of lake mixing induced by air-bubble plumes and the effects on evaporation

One of the main concerns regarding water storage in Australia, and other semi-arid countries, is the high rate of evaporation that inevitably leads to significant water loss. In this paper, the use of air-bubble plume systems to reduce evaporation from large reservoirs is assessed. A destratification system was designed for a large dam based on its depth and stratification strength with the intention of destratifying the reservoir in a short time period. The model DYRESM was then used to simulate the water dynamics under destratification conditions. Different strategies for the operation of the aeration system were assessed, from 10-days operation periods at times of high evaporation rates to continuous operation over longer time spans. The modelled water column temperatures and evaporation rates were analysed and it was found that artificial destratification was only effective in reducing evaporation in spring. In summer, heat is added to the water at a rapid rate, and artificial destratification only helps reduce evaporation in the initial days of operation. The effect of artificial destratification in reducing evaporation in autumn depends on the operation of the system during summer. If operated in summer, the rates of evaporation in autumn will increase due to the additional heat added to the water during the summer. In winter, overturn takes place and artificial destratification has no influence on water temperatures and evaporation. It was concluded that aeration by air-bubble plumes would only be effective in reducing evaporation if the hypolimnetic water does not become warm when mixing takes place. This is an ideal situation, but is unlikely to happen in practice.