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<br />rv natural streamflow values, the mass-balance estimates of dissolved-solids <br />~. discharge were adjusted by the ratio of the two streamflow values. The <br />~ adjusted estimates of mean annual dissolved-solids discharge are listed in <br />~, table 4 and are used as the values of mean annual natural dissolved-solids <br />discharge throughout the remainder of this report. <br /> <br />The accuracy of the natural dissolved-solids discharge estimated by this <br />mass-balance procedure cannot be determined because Iorns and others (1965) <br />provided little information on the uncertainty of their computed and estimated <br />values. Occasionally they gave a range of values for an unmeasured component <br />of the mass balance, but the maximum total range was always within 8 percent <br />of the mean discharge, If the uncertainty in all components had been con- <br />sidered, the range would be greater than 8 percent. An upper limit for the <br />error in natural dissolved-solids discharge can be estimated, because all the <br />study sites are affected by anthropogenic sources of dissolved solids, and the <br />historical discharge must be greater than the natural discharge. The ratios <br />of historical to natural discharge for the study sites ranged from 16 to <br />242 percent, with a median of 60 percent. <br /> <br />13 <br /> <br /> <br />DEVELOPMENT OF A STATISTICAL METHOD FOR ESTIMATION OF <br />NATURAL DISSOLVED-SOLIDS DISCHARGE <br /> <br />Data available for this report were inadequate for estimating monthly <br />natural dissolved-solids discharge by mass balance. An alternative approach <br />was to formulate a statistical model of dissolved solids as a function of <br />historical streamflow and other variables representing upstream water- <br />resources development. The calibrated model then could be used to compute <br />monthly natural dissolved-solids discharge by assuming a value for natural <br />streamflow and assigning all the development variables a value of zero. <br /> <br />Selection of Independent Variables <br /> <br />Monthly data on water-resources development upstream from each site <br />analyzed in this report were provided by the U.S. Bureau of Reclamation <br />(written commun., 1983). The variables related to development were: <br /> <br />1. Total adjustments to streamflow, which included consumptive use, <br />transbasin diversions, and change in reservoir storage. <br />2, Consumptive use, <br />3, Upstream irrigated area. <br /> <br />These available development variables were indicators of two processes: a <br />decrease in dissolved-solids discharge because of diversions, or an increase <br />in discharge because of salt pickup, Within this conceptual framework, re- <br />leases from storage could be considered negative diversions, resulting in an <br />increase in dissolved-solids discharge. <br /> <br />The variables indicative of diversion were total adjustments to stream- <br />flow and consumptive use. Because consumptive use was included in the total <br />adjustments value, these two variables usually were strongly correlated. To <br />decrease the possibility of collinearity among the selected independent <br />variables, two new variables were defined based on the difference between <br />