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.

中文翻译：

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存在地表流时浮球密度对水库蒸发抑制的影响

摘要 随着人口增长和干旱频发，保护淡水资源成为重要问题，特别是在干旱和半干旱地区。调查表明，蒸发是水库大量损失的原因，因此抑制蒸发可以有效地提高蓄水策略。用模块化浮动元件（例如浮球）覆盖水库，为抑制蒸发损失提供了一种简单而可靠的措施。尽管对浮动覆盖的应用进行了广泛的研究，但尚未系统地研究其在存在地表流的情况下的性能，而覆盖率可能会因覆盖密度而变化。因此，本研究旨在通过实验解决浮球的覆盖率（由改变球的密度和浸入深度引起）对蒸发抑制效率的影响以及对表面流动引起的表面能量平衡变化的潜在影响。为此，一个表面积为 1.13 m2、深度为 0.5 m 的小水盆被直径为 70 mm 和两种不同质量的白色和黑色聚乙烯球完全覆盖，即 90 g 和 127 g（提供 0.83 和 0.77 的表面覆盖）分数，分别）。总体而言，白球显示出最高的蒸发抑制效率。此外，还发现随着球密度的增加，超过一半的体积沉入水中，表面覆盖率和抑制效率会降低。有趣的是，