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<br />. <br /> <br />PART VI <br /> <br />EVALUATION Of EXISTING SALISITY CONDITIONS <br /> <br />~ <br /> <br />regression, ..flich is the best fit of the data using Ii straight line, was <br />used to determine the mg/L per year change in T05. The results of the <br />regressions ....ere then tested to see t.41ether they were a statistical <br />aberration or statistically significant. The data ':"ere then analyzed for <br />fluctuations from either a mean, if the regression and significance tests <br />showed no trend (the data could be averaged). or from the regression <br />line. The results of the analysis for fluctuations were the development <br />of confidence intervals. The 95 percent confidence intervals show the <br />range ....ithin which the mean annual TOS fluctuates 95 percent of the time <br />or conversely the range which may be exceeded 5 percent of the time. <br /> <br />..... <br />(..H <br />..... <br /> <br />The limitations of the trend analysis, as described, are based on <br />the inability of the analysis to predict cause and effect relationships. <br />The analysis does not lend itself to predicting future events since there <br />are numerous causes (mechanisms) which combine together to influence the <br />effect (TDS); however, the analysis does allow the simplification of the <br />sometimes confusing variations in the data. <br /> <br />. <br /> <br />The results of the analysis for the period of 1941 to 1980 (see <br />Figure VI-l) for the station belo,", Hoover Dam indicate TDS increased at <br />an average rate of 0.9 mg/L. Statistical tests showed the regression <br />slope was not significantly different than zero. The average annual rDS <br />concentration was found to be 697 mg/L. The 95 percent confidence <br />interval was 697 mgfL :::.. 101 mg/L. <br /> <br />The results of the a.nalysis for the period of 1941-1981 for the <br />!'Itation at Imperial Dam indicate TDS increased at an average rate of <br />3.9 mg/L per year. Statistical tests showed the regression to be sig- <br />nificant (see Figure VI-2). The 95 percent confidence interval was <br />approximately ~ 118 mg/L from the regression line. <br /> <br />Salinity concentrations at Imperial Dam (see figure VI-2) have <br />decreased in an apparent steady decline from 1970 to 1979, dropped <br />notably in 1980, and increased sharply in 1981. Generally the data are <br />showing the buffering effect that over 50 million acre-feet of reservoir <br />storage can have on the sal inity concentration at Imperial Dam. With the <br />Colorado River Reservoir System at such a high storage level, increased <br />flows due to antic ipatory flood control releases in 1980 lowered sal inity <br />concentrations at Imperial Dam temporarily. The next section discusses, <br />in more detail, the factors ,,*,ich combine together to produce the fluctu- <br />ations and trends found in the salinity data. <br /> <br />The 5-year mov ing averages are used to simpl i fy the interpretat ion <br />of the historical data, and to she,"" an approximate simulation of the <br />S to 7 year hydraulic retention time now present in the reservoir storage <br />system. <br /> <br />C. Factors 1n Salinit.... Trend Analvsis <br />. . <br /> <br />. Al though the data vary within the range of expected values, the <br /> recent dec 1 ine of IDS concentrations at some of the !'Itatians on the <br /> Colorado River h., led to the hypothesis that ,ome mechanism( s) mA\' <br /> <br />24 <br />