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002tn9 calcium concentrations for the Yampa River n.ear Maybell, Colo. (fig. 6). One possible explanation for this involves nonhomogeneity in the watershed. The Yampa River subbasin upstream from the site near Maybe11 is 3,410 mi2 (8,830 km2) and the geology and cl imate vary considerably. Waters originat- ing in different parts of the Yampa River subbasin can contain considerably different proportions of individual cations and anions but still. have rela- tively similar ratios of dissolved sol ids to specific conductance.. Natural seasonal variations in the sources of the water would be expected to affect the relationships between individual chemical constituents and specific conductance more than the relationship between dissolved sol ids and specific conductance. Irrigation return flows, which are most important from May through September, can concentrate some chemical constituents relative to others (Hem, 1970, p. 330) and would be expected to produce a similar effect. The relation between observed and simulated dissolved-sol ids concentra- tions for the Little Snake River near Lily (fig. 7) is not as good as the. relation obtained for the Yampa River near Maybell. .This may be due partly to lesser homogeneity in the Little Snake River subbasin (site-specific relationships) and partly to the fact that only about one-third of the sites sampled by lorns, Hembree, Phoenix, and Oakland (1964) were in the Little Snake River subbasin (regional relationship).. . . Using actual streamflow and water-qual ity records for the sites on the Yampa River near Maybell and the Little Snake River near Lily, time-trend analyses were made for the annual mean values of specific conductance at each site, adjusting for streamflow effects. This procedure is documented by Steele, Gilroy, and Hawkinson (1974) and uses Kendall's Tau nonparametric statistical test on each ranked, annual time series (Conover, 1971). Using data for the 1951-72 water years and a significance level of 0.01, an increase in specific conductance of 14 percent was determined for the Yampa River (Steele and others, 1974, table 9, p. 6?); however, no significant change was determined for the Little Snake River using data for the 1951-69 water years. The trend in increasing specific conductance for the Yampa River is attributed to increasing use of surface water for agricultural and municipal purposes. Analysis of data for the 1951-63 water years showed no significant changes at either of these two sites (Blackman and others, 1973, tabl e III). . Major inorganic constituents were analyzed in samples collected during the August-September 1975 reconnaissance at the 26 sites shown on figure 8, and regional regression equations relating major inorganic constituents to specific conductance were developed from these data (table 8). Graphic depiction of the equations indicates that the regional regression relation- ships calculated from the 26 analyses are similar to those calculated from historic data (table 6). The regional regression relationships allow estima- tion of concentrations of major inorganic constituents throughout the basin from the rather simple measurement of specific conductance. (Information on the precision of major inorganic constituents analyzed during the August- September 1975 reconnaissance is given in table 17 in the Supplemental Information section at the back of this report.) ;'t~ ,"f'~.'~~,::::,:s~'" :~~. ':, :~~:':~;}'i;'i'''~ 31