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. . . .... ..... ~ -.J SPECIAL STUDIES (continued) After all preliminary calculations are made, the model starts at the most upstream point and computes flows at each point, working downstream using the continuity equation: Flow at next sequence point = flow from preceding sequence point + inflow - demand. Where there is not enough water to satisfy a demand, the input file defines which upstream reservoir water can be drawn from to satisfy this demand. Any releases are then routed down through the system to where they are needed. If water cannot be found to meet the demand, a shortage message is printed and calculations proceed. All calculations for reservoir operations are based on the following form of the continuity equation: Storage = inflow - outflow - evaporation + bank storage. A release from a reservoir is determined by either a minimum release rate, a flood control space requirement, demands which draw from the reservoir, power releases to meet generation requirements, or demands which draw from upstream reservoirs and move water through the reservoir. Evaporation) bank storage change, and power production are calculated each time water is moved through a reservoir. Evaporation is determined by a monthly coefficient and the surface area of the lake that month. Change in bank storage is calculated as a percentage of the change in surface storage each month. Th i s percentage ranges from 6.5 perCeO t in Lake Mead to 15 percent in Lake Powell. When calculations have proceeded through the last sequence point In the last reach, the system is checked for a water balance. Salinities are computed next throughout the system by a mass balance accounting procedure. Initial salinities are read in from the hydrology file. Salt pickup is read in from the demand file and is added to the river attached to return flow. The salt is added at the inflow point of the return flow. To route salt through a reservoir, complete mixing is assumed. This assumption may not be true in the larger reservoirs, but for lack of any better method, this assumption is used. This approach, in a simplified form, was verified by Dr. John Hendrick in his PhD dissertation (973), Colorado State University, as applicable to Lake Mead on a monthly basis. The concentration of salt in the outflow is assumed equal to a weighted average of the beginning and ending concentrat ions of the salt in the reservoir. After the salt balance and water is incremented and the procedure begins balance are completed, for the next month. the month 110