Reservoir Evaporation Losses

The steadily expanding reservoir capacity of the United States is not all gain for water supply. The extended water surfaces that have been created have an undesirable accompaniment in loss of impounded water by evaporation. Part of the vaporized water may return as rain, but this will seldom occur in the region from which the water came. The permanent supply of the region thus falls short of its potential. Average gross annual losses through evaporation may be as high as 7 feet of water depth. Under severe drought conditions, evaporation of 10 feet of water in a year may occur. In terms of total volume lost from artificial reservoirs in the United States, over 20 million acre-feet have been estimated to evaporate per year. In the eleven western states, annual evaporation losses exceed 11 million acre-feet …

In recent years, control of evaporation from these open-water surfaces has been seen as a tempting source of water not otherwise available. In achieving such control it has been clear that if the reservoir surface can be protected from the sun, the large evaporation losses which occur in regions of high insolation can be eliminated completely … But the raw water evaporated from artificial lakes, like those formed by dams across canyons and ravines, does not have sufficient value to bear the very large cost of a structural cover …

Although the basic principles of vaporization reduction, through the use of insoluble liquid films on water surfaces, have been known for at least thirty years, it was not until about 1952 that substantial progress was made toward applying them. Certain types of organic chemicals having molecules containing a long hydrocarbon chain terminating in an alcohol group, acid group, or some other radical compatible with water (hydrophilic group), will form an invisible, in-soluble film only one molecule thick on a water surface. The rod-like molecules orient themselves vertically side by side, with the hydrophilic group downward toward the water and the hydrocarbon chain forming the new surface. Because of the extreme thinness of the film, a minute quantity of the added compound will completely cover a large water area. Moreover, these compounds have such low volatility and solubility in water that the film may be remarkably permanent.

—Edward A. Ackerman and George O. G. Lof in RFF's Technology in American Water Development, The Johns Hopkins Press, 1959.