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00152'2. ';'-,., . 0,,,\ " ".,';' Southwest area will justify the cost and effort of using evaporation sup- pressants. Seepage ,<~<-~~:~! \0*k(~ :i>,..~...,~. I The second important res ervoir loss and one that often exceeds evaporation is seepage. Seepage may occur either as flow through or around the dam, or as deep percolation in the stock pond. The former is usually discernible as open flow or as wet swampy areas supporting phreatophyte growth below the dam. Seepage in the pond floor is more difficult to identify and final disposition of the percolated water generally cannot be determined. In most instances it presumably becomes a part of the ground-water supplies where it mayor may not contribute to the base flow of streams or be recovered through wells. Measurements of seepage losses generally have been obtained indirectly by cons idering seepage as the difference between total reces- sion in the reservoir and evaporation. Langbein (1951) has described a precise method of separating the evaporation and seepage losses in the total reservoir recession. The same procedures have been used in sev-, eral studies by the Geological Survey, although most measurements have been made by app~ying an appropriate evaporation rate to the recess ion graph of the reservoir, the difference being regarded as seepage. In most calculations, precipitation on the reservoir surface during the period of measurement is considered althoLigh it has been disregarded by some in calculating losses on the assumption that precipitation i's approximately equal to the losses that would have occurred in the stream reach had the reservoir not been constructed. The measurements and calculations all show that seepage losses can vary greatly depending mainly on geologic conditions within and sur- rounding the reservoir and on depth of water in the, pond; In the measure- ments on reservoir losses in central Arizona reported by Langbein, the seepage varied from 0.05 to O. 3 foot per month which was about one-third of the evaporation. Each of the reservoirs measured was underlain by a thick soil mantle overlaying fine-textured, sedimentary rocks or dense, igneous rocks. Seepage under these conditions should be at a low rate. In a study of stock ponds in the Cheyenne Bas in of Wyoming by Culler (1961), the analysis of the losses at 58 stock ponds showed that annual seepage loss from individual ponds varied from a minimum of one-tenth of the evaporation to a maximum of nearly 18 times the evaporation. The average seepage loss for all ponds was about 1. 5 times the evaporation. Ponds with the low seepage losses were located in areas underlain by dense shales; those with high losses in areas underlain by pervious sandy soils overlaying loosely consolidated sandstone and conglomerate. 32