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001517 can improve the basis for estimating the probable supply to stock ponds in a particular area. Estimation of water supply from ground water or spring flow can be made with greater accuracy,and with fewer measurements than where surface runoff is the source of supply. The ground water recession curve may be outlined with a few measu rements and the volume of runoff predicted from this recession. Since the principal problem will usually involve sup- ply estimates for periods of drought, the nature of ground water recession in other measured streams during past drought periods in that area should be studied (Blumenstock, 1942). Quality of Water Chemical quality The chemical const ituents of water delivered to the stock pond occasionally affect its usability. Most surface runoff is relatively free of dissolved salts and chemical quality is not a problem. However, where the drainage area of a reservoir encompasses salt deposits, the salt con- centrations in the runoff may become high enough to render the water unusable by livestock. The quality of dissolved salts tolerated by live- stock varies to a considerable extent. In some localities water with con- centrations of 5,000 to 9,000 ppm of sodium chloride are being used, but generally water with concentrations of this salt in excess of 3,000 to 4,000 ppm is cons ide red undes irable. Waters containing salts that are high in magnesium and the sulphate radical are especially undesirable. The chemical quality of water in the stock pond may be influenced by evaporation, particularly where spillage is a minimum. Burningof vegetation in the collecting basin, either accidentally or as a part of the watershed treatment, may also have an effect on the water quality. Seven- fold increases in the dissolved solids in streamflow were reported follow- '\ng a burn in a brush-covered watershed in southern California (from 351 to 2,415 parts per million, Santa Clara Water Conservation District, Santa Paula, 1931-32 to 1934-35). The concentration of solids caused by evaporation from the basin should be carefully estimated as part of the design for adequate water quality for livestock use. Sediment Sediment discharge may be estimated for a local area from measure- ments taken in nearby areas of similar conditons of soils, cover and topog- raphy. Estimates of sediment discharge have been made from simple relations of topography, area, and shape of the watershed (relief ratios) 27 ,/ -0". ,-.>.-;';;'.-'; ~ ~~~ <~'i0' ~ }~:4;<;' r1t.~f}~ il?i~~~ ~3f_~::.~1~~ '," , ii-: ",I