Effect of continuous and modular floating covers on evaporation losses and microalgal growth

Evaporative losses from the water surface are substantial. Continuous floating covers strikingly suppress evaporation, but adversely affect the water environment. However, modular floating “elements” covers might overcome this issue, but with less evaporation suppression efficiency. We hypothesized that modifying the continuous floating covers by decreasing the coverage fraction (CF) could merge the advantages of both types. Two experiments were conducted to evaluate and compare the influence of modular and continuous floating covers on evaporation losses with emphasizing their impacts on the water environment by determining microalga growth as a bio-indicator. The effect of three modular floating covers, i.e., partially water-filled plastic potable bottles (W-PB), air-filled plastic potable bottles (A-PB), and styrofoam (STF)-disks on the evaporation rate was evaluated. In the second experiment, we investigated the influence of white spheres on evaporation rate in comparison to three continuous floating covers, i.e., styrofoam sheet (STF-sheet), white plastic sheet (WPS), and black plastic sheet (BPS), using three CF, i.e., 0.9, 0.95 and 1.0. The results revealed that modular floating covers decreased evaporation rate for the two months by 53.3, 44.6, 43.0% for STF-disks, W-PB, and A-PB, respectively. In the second experiment, the lowest evaporation rate was obtained using continuous floating covers at CF=1, in which all covers were acting equally, signifying the importance of CF over cover color. However, at CF=0.9, white spheres showed similar evaporation suppression efficiency to white continues floating covers, i.e., STF-sheet and WPS, but higher than the BPS, signifying the role of cover color over cover type when CF<1. Microalgae growth was positively affected by STF-disks and white spheres, and negatively by plastic potable bottles treatments. All continues floating covers at CF=1, inhibited microalgae growth, while at CF<1 continues floating covers boosted microalgae growth, except the BPS. In conclusion, the highest evaporation suppression (~95%) was obtained using continuous floating covers at CF=1, but with potential adverse impacts on the water environment. Modifying the continuous floating covers by decreasing the CF, increased water-saving, and enriched water ecology, relative to modular floating covers.