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001519 LOSSES OF WATER Evaporation Evaporation constitutes one of the major losses of water frol;n stock ponds. The drop in the level of stored water due to this cause is equivalent to the annual evaporation less the precipitation on the water surface, and the total evaporation loss in acre-feet from the pond can readily be calculated by applying the net evaporation rate to the average area of the water surface. The average annual evaporation in the Pacific Southwest area, as measured in class A evaporation pan, and as deter- mined from lake surface, has been compiled by the Weather Bureau as previously noted. Experience has shown that because of the shallow depths, evaporation from stock ponds may exceed that in most lakes and at times it probably approaches that in the standard clas s A pan. Mea- surements of evaporation losses from stock ponds by the Geological, Survey (Langbein, 1951; Culler, 1961) suggested a coefficient of O. 85 times pan evaporation. Generally the figure will probably fall somewhere between 0.70 and 0.95. Using a coefficient of O. 85, evaporation from stock ponds in the Pacific Southwest area will vary from a minimum of about 42;inches in the higher elevation northern areas to a maximum of about 85 inches in the lower Gila and Colorado River basins in Arizona and parts of south- eastern Nevada. The summer (May-October) evaporation ranges from 80 percent of the annual evaporation in the Green River Bas in, in parts of the mountainous area of east-central California, (Weather Bureau, 1959) and in the northern Utah and Nevada sections of the Great Basin, to 68 percent in southern Arizona and 62 percent along the coast in southern California. Thus, summer evaporation is highest in those areas where grazing generally is limited to the summer season. Stock ponds should be planned and des igned accordingly. In the southern latitudes, where year-long grazing is practiced, the evaporation is more evenly distributed although a major part again occurs in the summer season. Reduc ing evaporat ion Evaporation losses may be reduced by an appreciable amount through proper design of the stock pond and possibly to some extent by application of monomolecular films. An obvious and desirable design criteria for reducing losses is a low ratio of pond surface area to volume. Unfortunately, most stock pond sites do not permit much leeway in this regard but in selecting sites the possibilities should be recognized and 29 ;'~i.~\*~ ,''',>- ,'~ '- ~"l: -,." ...-.... !~{~~~ ~./.::../} ~f~~l '';'' (1 ,_ ''''';.''