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.- ,. l" 'l t'...) J;~ solids loads generally occurred in years of largest annual mean stream discharge (fig. 3). However, not all differences in dissolved-solids loads are directly dependent on streamflow. The loads do not increase or decrease in a I: I linear relation with changes in stream discharge. For example, compare the annual loads and annual mean stream discharge for station 09]63500 for 198] and 1984 (table I and fig. 3). In 1984, the stream discharge was about 3.5 times greater than in 198\, but the dissolved-solids load was about]. 7 times greater than in ] 98]. High- flow years on the upper Colorado River are character- ized by larger than average snowpack and spring run- off, which have low dissolved-solids concentrations. Consequently, the increase in dissolved-solids load will not be proportional 10 the increase in stream discharge for high-flow years. Some of the variability in annual dissolved- solids loads (table I) probably is related to errors associated with the computational method. The regression coefficients used in equations ] and 2 were derived from relations of instantaneous loads to instantaneous stream discharge and specific con- ductance. It is assumed these relations also represent mean daily loads. Outliers can have undue influence on regression relations. The SLOAD program has an input-checking routine to flag dissolved-solids, specific-conductance, and stream-discharge data for outliers. Highly suspect data were deleted prior to computation ofregression coefficients. Because specific conductance has a high correla- tion to dissolved-solids concentration in most natural water, the standard error of estimate using equation 2 usually would be less than the standard error of esti- mate associated with equation I, which is based only on streamflow. The standard error of eSlimate associ- ated with each regression relation for each station can be expressed as a percentage for regressions done with logarithm data. For station 09095500, the stan- dard errors of estimate for the 3-year moving regres- sions generally were between 4 and 10 percent, with errors for equation 2 slightly less than for equation I. For station 09] 52500, the standard errors of esti- mate for regressions based on equation I were much higher (17-30 percent) than the errors for the regressions based on equation 2, which were between about 3 and 7 percent. For station 09163500, the stan- dard errors of estimate ranged from 3 to 9 percent for regressions based on equation 2 compared to errors of 1 I to 22 percent for regressions based on equation I. Because the completeness of the daily specific- conductance record varied from year to year for each station, the number of days in which the dissolved- solids load was calculated using equation 2 also varied among stations and years. There might be less uncer- tainty in annual dissolved-solids loads for years with more complete daily specific-conductance records than in annual loads for years with less complete specific- conductance records. The annual Grand Valley dissolved-solids load also is included in table I, and that load is the differ- ence between the annual dissolved-solids load at station 09\63500 and the annual loads for the other three stations. The BOR (1983) reported the annual mean Grand Valley dissolved-solids load (called the Grand Valley salt pickup in that report) as 580,000 tons for 1952-80, compared to 5 I 8,000 tons for 1970-93 as listed in table I. According to the BOR (1986), at least 95 percent of the dissolved-solids load was from shal- low ground-water sources in the Grand Valley, and a large percentage of the ground water was recharged by irrigation. The annual Grand Valley dissolved-solids load ranged from 182,000 to 914,000 tons (table I). It seems unlikely that the dissolved-solids loads from the Grand Valley would vary by such a large amount from year to year because the irrigated acreage and amount of water diverted for irrigation did not change substan- tially on a year-to-year basis. There has been some decrease in irrigated acreage in the Grand Valley from the development of agricultural land for commercial and residential purposes. The magnitude of the Grand Valley dissolved- solids load is affected by errors in the dissolved- solids-load calculations for the four gaging stations. These errors probably vary from year to year, depend- ing on errors associated with the water-quality, stream- flow, and daily specific-conductance data and on errors in the regression equations. There also are year-to-year variations in precipitation runoff, cropping patterns, irrigation practices, and land use that could cause rela- tively small changes in the dissolved-solids load from irrigated areas in the Grand Valley. Another factor that could have an effect on the dissolved-solids load to the Colorado River was the salinity-control projects. DISSOLVED-SOLIDS lOADS IN THE COLORADO AND GUNNISON RIVERS 9