Effect of floating balls on evaporation inhibition, surface energy balance and biological water quality parameters at different coverage fractions

High evaporation in plain reservoirs in arid regions exacerbates water shortages. Investigations have established that floating elements assembled modular with maximum coverage on the water surface, such as floating balls, can effectively mitigate ineffective water evaporation. Nevertheless, the employment of floating balls to achieve maximum coverage of the water surface may pose a threat to aquatic life and the initial investment is substantial. Thus, this study employs floating balls for partial coverage of the water surface, intending to examine the impact of altering the coverage rate of the floating balls on the efficiency of evaporation inhibition, energy balance, and water quality. To achieve this goal, we established five sets of evaporators on the top of the reservoir dam, each with a surface area of 1.13 m2 and a depth of 0.8 m. These evaporators were covered with varying percentages of black high-density polyethylene floating balls with a diameter of 0.1 m, including 0%, 18.74%, 37.49%, 56.23%, and 74.98% coverage. The results show that the evaporation inhibition rate is less than the coverage rate. The solar radiation flux is decoupled on the evaporation surface, and the change of coverage ratio affects the size, transfer direction, and relative proportion of each component of the energy balance. With the increase of coverage, more radiation flux is converted into sensible heat flux, which increases the Bowen ratio. The water surface covered by the floating balls has a particular effect on improving the water quality. With the increase in coverage rate, the concentration of chlorophyll, nutrient salt, and suspended solids decreased significantly, and the impact on dissolved oxygen in the water was less. Finally, the economic viability of varying coverage ratios of floating balls was also confirmed. Currently, there is a paucity of reservoir-scale studies, and the effect of floating balls on evaporation and energy balance in the flowing water of actual reservoirs needs further investigation.