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0016~)S Salt is modeled as total dissolved solids only. Precipitation and ion exchange are not modeled. Figure 10.1 shows example computations of the salt routing process. At a diversion point, CIR is computed by taking the current flow in the river multiplied by the current CIR, subtracting the flow in the diver- sion multiplied by the concentration of the diversion, and dividing by the new flow in the river. The CIR at an inflow point is computed simi- larly except the terms are added instead of subtracted. The concentra- tion of a return flow is equal to the concentration of the diversion multiplied by the ratio of the diversion flow to the return flow. 10.3 Reservoir Mixing CRSM assumes that the reservoirs are of uniform sal inity, and completely mixed at all times. While this is not completely accurate, the dif- ference between this assumption and correlating inflow to outflow through a lag function is small. This was verified as applicable to Lake Mead on a monthly basis by Dr. John Hendrick in his Ph.D. disser- tation (1973), Colorado State University. The concentration in a reservoir is based on the volume of the inflow, the volume of the release, the concentration of the inflow, and the con- centration in the reservoir at the beginning of each month. The model has an algorithm which determines the concentration of the reservoir release by weighting the current month's and previous month's reservoir concentrations. The qual ity of the release which resul.ts is that which would occur from a constantly mixed reservoir with a uniform inflow rate. I i Any flows to bank storage occur at the monthly reservoir concentration. These flows are mi xed into the bank storage "tank" and saved for future use. During the period of falling reservoir elevation, flows come out of bank storage at the average concentration of the bank storage "tank." A diversion from a reservoir is made at the same concentration as the concentration in the reservoir. 10.4 Flow-Weighted Averages Average salinities at specific locations are often output from the CRSS as flow-weighted averages. A flow-weighted average is an annual average equal to the sum of tons of salt for each month divided by the sum of the monthly flows. This is in contrast to a time-weighted average, which is the average of each month's concentration. The existing stand- ards in the Colorado River Basin are flow-weighted, and flow-weighting gives a better representation of the overall salt load at a station, especially one which has under90ne upstream re9ulation. 48