Assouline, S., Department of Environmental Physics and Irrigation, Institute of Soil, Water and Environmental Sciences, A.R.O.-Volcani Center, Bet Dagan, Israel
Narkis, K., Department of Environmental Physics and Irrigation, Institute of Soil, Water and Environmental Sciences, A.R.O.-Volcani Center, Bet Dagan, Israel
Or, D., Department of Environmental Sciences, Institute of Terrestrial Ecosystems, Soil and Terrestrial Environmental Physics, ETH Zurich, Zurich, Switzerland

Reservoirs enhance availability and temporal stability of water resources, however, water losses due to evaporation may significantly reduce their operational efficiency. Studies have evaluated the efficacy of various types of chemical or physical evaporation barriers often on a qualitative basis. Certain design and maintenance advantages are offered by self-assembling modular floating covers especially for large reservoir surfaces. Evidence suggests that evaporation suppression efficiency of these floating covers is not proportional to the covered surface fraction. Capitalizing on recent experimental results carried out on small vessels, we show that evaporation suppression efficiency of floating covers in much larger reservoirs (3 to 5 order of magnitude in volume) is proportional to the square root of open area. Thermal effects of evaporation from open and partially covered surfaces on surface water temperature are proportional to evaporation rate and thus provide a potential mean for real-time monitoring of cover performance. Floating covers seem to suppress evaporation more effectively than other suspended evaporation barriers. For a fixed covered surface fraction, larger "holes" suppress evaporation more efficiently than many smaller and distributed openings. The concepts and relationships developed in this study provide quantitative means for engineering design and economical evaluation of evaporative barriers for water reservoirs in terms of cover geometry and water saving efficiency. © 2011 by the American Geophysical Union.