Evaporation Control for Increasing Water Supplies

Abstract

A summary of the leading methods of evaporation control is presented. Eight categories of evaporation control were discussed. The three leading categories of evaporation control discussed were the monolayer, the reduction of the surface-area-to volume method and floating vapor barriers. These methods are less expensive and appear to have a wider range of application than destratification, wind barriers, shading the water and floating reflective barriers. The other method of evaporation control discussed was the use of sand or rock-filled reservoirs. This method was found to be effective but limited to smaller size reservoirs. The use of fatty alcohol to form monolayers will work on larger reservoirs using either the airplane or a pipeline carrying the alcohol in slurry or an emulsified form. An airplane was used to distribute alcohol on the 12,000 hectare Vaal Dam in Africa during an extreme drought. The estimated cost was $0.021 /m3. The method is less cost-effective on smaller reservoirs due to rapid removal of the material by the wind. The reduction of surface-to-volume ratio can be accomplished through the use of proper site selection or if in flat terrain through the use of the compartmented reservoir. The concept of the compartmented reservoir can be used on existing reservoirs but more easily on new ones since it involves the use of construction equipment. The proper use of the compartmented reservoir concept should result in a lower unit cost of water saved than any other presently known evaporation control method. The use of floating vapor barriers of foamed rubber or wax-impregnated expanded polystyrene seemed to have a wider range of use than other floating vapor barriers. The recent development of the wax-impregnated expanded polystyrene for evaporation control is described. The paper describes it to be one of the most promising floating vapor barriers in terms of cost effectiveness and weatherability. Developed at the University of Arizona, this material can be used in large 1.2 x 2.4 m sheets connected together by couplers or rubber straps or as smaller floating squares that would be less prone to vandalism. There is considerable merit to the use of the compartmented system with a floating vapor barrier in the "last" compartment to have water in it. This should increase the dependable water from a compartmented system at a relatively low cost.