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Show are the firm annual yields Ms at each site, and the aggregate groundwater firm yield GR. For the applications with 8 reservoir sites, this results in only 9 decision variables. It was noted from preliminary applications that the Salt Lake County aquifer reaches steady state with uniform annual pumping, after about 11 years. The groundwater applications in this scenario were then simplified to consider the response matrices upto 11 years, and a constant drawdown thereafter upto year 55. This resulted in a considerable decrease in the computation time required for the groundwater applications. The resulting optimal costs of groundwater yield for these 5 applications were first adjusted to correspond to those reported in the literature ( Salt Lake County Water Conservancy District ( 1987), reports an average cost of $ 64.82/ Acre- foot for groundwater, as compared to the $ 17.96/ Acre- foot energy cost determined for the same agency for the historical pumping in Chapter 3) and then related to the annual groundwater yield for Salt Lake City, through regression. It should be noted that this adjustment is probably extreme, since in our study we considered only the most efficient wells for determination of optimal yield, whereas the Salt Lake County Water Conservancy district cost records include all wells operated by them. The actual groundwater costs where the set of candidate wells specified in the Chapter 3 applications are used for groundwater development, should be lower than the numbers used here. The equation that results for optimal annual groundwater costs ( CGRi) as a function of groundwater yield for Salt Lake City is: CGRj = 3.0957* GRj- 5811 ( 7.6) where the cost CGRi is in dollars and the yield GRj is in 1000 Acre- feet/ year. Results from the parametric applications for reservoir/ groundwater screening for Salt Lake City are presented in Tables 7.1 through 7.7, and are summarized in Figure 7.1. Comparison with the results for the surface water applications for the same data ( Chapter 5), a significant reduction in the total and unit costs of water supply is observed. With an upper limit of 40,000 Acre- feet/ year of groundwater yield considered, it is possible to develop 140,000 acre- feet/ year of firm yield for the demand area for approximately the same unit cost as the 80,000 acre- feet/ year development with surface water alone. The order in which the reservoirs enter the solution is about the same as in the Chapter 5 applications. This is to be expected, given the nature of this set of applications. However, the manner in which surface and ground water use is developed is of quite some interest. Since groundwater is generally less expensive than surface water the expectation would be that all the scenarios with low total annual demand ( e. g. 20,000 Acre- feet/ year and 40,000 Acre- feet/ year) would be supplied only with groundwater. This is not the case upon an examination of Tables 7.1 and 7.2. The model provides the maximum yield from the existing reservoir - Mountain Dell, the maximum developable yield without constructing a reservoir from some of the streams with low unit costs ( e. g. City Creek) for water treatment and supplies the balance from groundwater. While this is the manner in which the system is likely to be operated in practice, it was a surprise to see the model coming up with the solution. The validity of the solution is supported by a comparison of Table 7.2 with Table 7.3. When the total demand level is raised to 60,000 Acre- feet/ year ( Table 7.3) the optimal groundwater yield is essentially at its upper bound of 40,000 Acre- feet/ year. The unit cost of groundwater with this yield is $ 40.51/ Acre- foot. The unit cost of the total supply of 40,000 Acre- foot/ year supplied through a combination of Surface and Ground water ( Table 7.2) is $ 27.03/ Acre- foot, i. e. lower. This supply is achieved through 28,341 Acre- feet/ year of groundwater, and yields from City Creek, Little Dell and Little Cottonwood reservoirs. No storage capacity or a small storage capacity for within- year flow equalization is provided at these sites in this solution. The same trend continues as the demand level is increased. Another interesting observation is that the model attempts to 166 |