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<br />to <br />Cj <br />(0 <br />~ <br /> <br /> <br />Iorns and others (1965) reported estimates of annual anthropogenic <br />dissolved-solids discharge for selected locations in the Upper Colorado River <br />Basin. These estimates were computed to represent the mean annual dissolved- <br />solids discharge that would be expected if the water-resources developments <br />existing in 1957 had been in operation during the hydrologic conditions that <br />occurred from water years 1914 through 1957, Iorns and others (1965) also <br />estimated the dissolved-solids discharge that could be attributed to irri- <br />gation and municipal and industrial uses, and the dissolved-solids discharge <br />imported or exported in transbasin diversions. <br /> <br /> <br />Geologic and geographic information about the basin is extensive. Geo- <br />logic and topographic maps of the five upper basin States--Arizona,Colorado, <br />New Mexico, Utah, and Wyoming--were used in the study to delineate irrigated <br />areas that have the potential for saline return flows. Iorns and others <br />(1965) provided a detailed description of the geologic conditions in the <br />basin. <br /> <br />Chemical analyses of samples collected from a few locations in water <br />years 1905 and 1906 were reported by Stabler (1911) and summarized on a <br />monthly basis by Iorns and others (1965). These data were not included in the <br />statistical-model calibrations, but were used for a limited verification of <br />the models. <br /> <br />MASS-BALANCE ESTIMATION OF NATURAL DISSOLVED-SOLIDS DISCHARGE <br /> <br />Iorns and others (1965) estimated the mean annual dissolved-solids <br />discharge from natural and anthropogenic sources at several locations in the <br />Upper Colorado River Basin. The purpose of their study was to quantify the <br />effects of .water-resources development on dissolved-solids loading in specific <br />river reaches, The entire discharge entering the upstream end of a reach was <br />considered natural, even though it also may have been affected by development. <br />To determine the natural dissolved-solids discharge required in the present <br />study, certain adjustments had to be made to the estimates of Iorns and others <br />(1965). Generally these adjustments in~olved adding the discharge that had <br />been removed by upstream transbasin diversions and subtracting the discharge <br />that had been contributed by upstream irrigation. Estimates for several <br />reaches had to be combined to produce the natural discharge at some of the <br />sites used in the present study. If Iorns and others (1965) did not provide <br />an estimate for part of the drainage area upstream from a site, the dissolved- <br />solids discharge from that area was determined based on the evaluation by <br />Iorns and others (1965) of areas with similar geology, physiography, and land <br />use. <br /> <br />The mass balance for site 2, the Colorado River near Cameo, Colo., is <br />shown in table 2. The total drainage area is 8,050 square miles and the <br />irrigated area in 1957 was 163,400 acres. Iorns and others (1965) reported <br />the historical mean streamflow to be 2,998,000 acre-feet per year and the <br />dissolved-solids discharge to be 1,578,000 tons per year for hydrologic <br />conditions during water years 1914-57 adjusted to the 1957 level of <br />development, <br /> <br />8 <br />