The effect of floating balls density on evaporation suppression of water reservoirs in the presence of surface flows

Abstract

The need for protection of freshwater resources with population growth and frequent droughts has become an important issue, especially in arid and semiarid regions. Investigations show that evaporation is the reason of significant losses in water reservoirs and thus suppressing it could effectively enhance water storage strategies. Covering reservoirs with modular floating elements, such as floating balls, offers a simple and reliable measure for suppression of evaporative losses. Despite extensive studies on the application of floating covers, its performance in the presence of surface flows while coverage fraction might change due to the cover density has not yet been studied systematically. The present study thus aims to experimentally address the effect of cover fraction of floating balls (induced by changing balls’ density and thus immersion depth) on the evaporation suppression efficiency and potential impacts on the change in the surface energy balance due to surface flows. For this purpose, a small water basin with 1.13 m2 surface area and 0.5 m depth is fully covered with white and black polyethylene balls with diameter of 70 mm and two different masses, i.e., 90 g and 127 g (providing 0.83 and 0.77 surface cover fraction, respectively). Overall, the white balls showed the highest evaporation suppression efficiency. Also, it is found that with increasing balls’ density so that more than half of the volume sinks into the water, surface coverage and thus suppression efficiency decreases. Interestingly, the results showed that with increasing water surface flow rate, the evaporative loss is first reduced until a certain flow rate (the so-called optimal water flow-rate) and then it increases gradually with associated effects on reservoirs energy balance where cover-water energy exchanges may alter with changing balls density.