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REVIEW OF EVAPORATION SUPPRESSION STUDIES E. G. Nielsen, Associate Chief Engineer Bureau of Reclamation U. S. Department of Interior Denver, Colorado The process of evaporation and its importance to the available water supply have been recognized for a long time. A number of methods of various degrees of potential success have been available to the engineer to reduce evaporation losses from reservoirs. This discussion will deal with reduction of evaporation through the use of monomolecular layers, also referred to as monolayers. There is a broad class of polar compounds, which are substances having a mo- lecular structure such that one end has a great affinity for water. When such a substance is in contact with the water, the molecules tend to align themselves always with one end in the water with the remainder of the molecule sticking out of the water. The commonly used household detergents are polar compounds. Not all polar compounds reduce evaporation. - 20 - A suppressing effect on evaporation takes place when the monOmO- lecular layer, of certain compounds, on the water surface is compressed. The molecules in a compressed monomolecular layer can be likened to the safety matches jammed up tightly in their packet. Just why the monolayers of certain saturated straight chain alcohols reduce evaporation is not ex- actly understood, but is considered by physicists to be related somehow to an energy barrier. Although the knowledge of monomolecular layers goes back more than half a century and knowledge of the evaporation reducing effect of the presence of a monolayer goes back to 1925, the consideration of the application of monolayers to reservoir surfaces is very recent. The sub- stance which offered the greatest promise is the l6-carbon straight-chain saturated heavy alcohol known as cetyl alcohol or hexadecanol. It is com- monly used in cosmetics, medicines, foods, and has widespread usage in the chemical industries. In quantities, it sells for around $.45 per pound. The Bureau of Reclamation first gave consideration to evaporation reduction through the use of monolayers in 1952. All freely exposed bodies of water possess some kind of monolayer. Such monolayers come from washout from rain, and from the decomposition products of the organic matter. They are usually very weakly compressed and have no effects upon evaporation reduction. Thus, the application of a monolayer to reservoir surface simply displaces an existing monolayer with one which will be of benefit to us.