Evaporation suppression from open water surface using various floating covers with consideration of water ecology

Abstract

In arid and semi-arid regions, up to 50% of water reservoirs’ capacity is lost by evaporation. Physical floating covers suppress evaporation mainly by intercepting solar radiation. Seasonal comparisons between different physical floating covers in dissimilar environments and their influences on water ecology have not been investigated. Two class-A pan experiments in outdoor settings have been conducted to investigate the influence of various floating covers that vary in color, shape, and coverage percentage (CP) on evaporation suppression efficiency (ESE), with considering water ecology by determining microalgae growth as a bio-indicator. The effect of white, black, and multicolor (mixture) balls, (91% CP) on evaporation rate (ER), ESE, surface water temperature (SWT), and microalgae growth was investigated for nine months, in two locations, i.e., Damanhour and E. Awinat, representing northern and southern Egypt, respectively. Additionally, the performance of white foam-sheet at CP of 91% (F-91%CP) and 100% (F-100%CP), plastic-sheet at CP of 91% (P-91%CP), and 100% (P-100%CP), and white balls (91%CP) were investigated for three weeks. Results showed that floating balls reduced ER (especially in summer months) in both locations with white balls having the greatest effect. In E. Awinat, the 9-month average saved-water depth was 5.17, 4.43, and 4.58 mm d-1 for white, black, and multicolor balls, respectively. The corresponding values in Damanhour were 3.02, 2.63, and 2.73 mm d-1. Using the floating covers, the ESE ranged from 27.0% (P-91%CP) to 96.0% (P-100%-CP and F-100%CP). The SWT of the covered water surface was significantly colder than the control in the morning and mid-day for all covers, except the plastic-sheet. However, by evening, the control was the coldest treatment. The full coverage of the water surface (P-100%CP and F-100%CP) reduced evaporation by 96.0%, but inhibited microalgae growth, while the partial coverage, i.e., white balls and F-91%CP, decreased evaporation by 62.18 and 60.6%, respectively, without detrimental effects on microalgae growth. In conclusion, when water ecology is not a concern, i.e., on-farm irrigation reservoir, then full coverage by plastic or foam sheets attains maximum ESE. However, in multi-purpose reservoirs, ecological considerations are vital, therefore white-balls or F-91%CP are recommended. Further studies in real reservoirs are needed, besides investigating the durability and stability of the used covers.