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<br />(\\)l8'3b <br /> <br />Transient Effect of Initial Salinity <br /> <br />For any dynamic system, the starting or initial conditions strongly <br />influence results during early times. Intervening activities and <br />events eventually mask this effect. Reservoir volumes and salinities <br />are the only initial conditions for the river system. As described <br />in the section on "Run Criteria," each case was run twice to avoid <br />definite trends in projected salinities. The initial run used flow- <br />weighted 1970 salinities while the second or r~cycle run used flow- <br />weighted concentrations from the last year of the initial run. <br /> <br />If the results of the two runs are compared, it is Seen that the two <br />time series converge and eventually coincide. The point of coinci- <br />dence represents the time at which the initial conditions no longer <br />affect results. It is a function of reservoir inflows, capacities, <br />and operating policies as well as demands made on the system. It <br />also depends on the nature of flow through the reservoir and, as <br />such, is a function of the conceptual quality model used. <br /> <br />Theoretically, the two sets of results may not agree exactly within <br />the study period. Practically, when the difference in salinities <br />is less than some arbitrary value, the two may be considered identical. <br />Consequently, a different point in time would be found for arbitrary <br />values of 0.1, 1, or 10 ppm. It is also noted that the magnitude of <br />change between the runs will also affect the time at which the arbi- <br />trary limit is met. <br /> <br />These concepts are illustrated in Figure 9 by salinities at Imperial <br />Dam for Runs No. 33 and 34 involving the initial salinity control <br />projects. Figure 10, for the same runs, presents salinities of Lake <br />Head. These results are typical for moderate developments. The <br />results of Figure 11 at Imperial Dam are typical for extensive devel- <br />opment resulting in a large departure from the 1970 historic condi- <br />tions. These figures are based on a comparison of monthly values. <br /> <br />Figure 12 summarizes the differences in salinity at Imperial Dam <br />between the two runs for every case studied. Values are computed <br />as the recycled less the initial salinities. A positive value denotes <br />an increase and a negative a decrease. This figure is based on dif- <br />ferences in the mean monthly salinity for each year of the study. <br />Consequently, the monthly irregularities depicted in Figures 9-11 are <br />masked. A period of 10 to 14 years is required for differences to be <br />within 5 ppm. <br /> <br />Future Conditions <br /> <br />A number of runs involving future conditions, based on those out- <br />lined in an earlier memorandum, [7] were made to assess the ultimate <br /> <br />26 <br />