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Show Tables 7.8 and 7.9 report the firm yield from groundwater in each case. The corresponding entries for groundwater cost represent the average annual groundwater cost, for the firm groundwater yield as well as the pumping required in the surface water failure years. Tables 7.8 compares well with Table 7.3 ( same annual yield, with surface water and groundwater as competing, rather than conjunctive resources), as far as the reservoir sites selected and their storage capacity. The increased surface water yield, with failures considered reduces the steady yield solved for from groundwater. The total and unit costs of supplying 60,000 acre- feet/ year are about 8% higher in the case where both ground and surface water are used for supply but without conjunctive use of the resource. The configuration of reservoirs with demand at 120,000 Acre- feet/ year and conjunctive use of surface and groundwater ( Table 7.9) is quite different from the corresponding solution where conjunctive use is not considered ( Table 7.6). Storage capacity at every site is developed when conjunctive use is not considered. The Mill Creek, Emigration Creek and Red Butte Creek sites are not developed when conjunctive use is considered. As expected, the largest failure of yield in any year is larger when the demand increases from 60,000 Acre- feet/ year to 120,000 Acre- feet/ year. It is of interest to note that the ratio of the degree of failure (. 41/. 19 or 2.1) is approximately the same as the ratio of the yields ( 120,000/ 60,000 or 2). The year by year failure pattern is also basically similar for the two levels of demand. The peak failures occur in the years with the lowest annual flows, and higher failures of surface water yield occur in the years preceding the worst year, than in the year following the worst year. The reservoirs selected by the model with the annual demand set at 120,000 Acre- feet/ year have roughly the same size as the applications for this demand level, with conjunctive use not considered. The reservoir yields are however higher in the conjunctive use case, since failures of yield are allowed. The total and unit costs for the case where conjunctive use of surface and groundwater is not considered are almost 60% higher. Considerable savings result from considering conjunctive use as the demand level increases. For both application, the constraints on minimum allowable head in the aquifer were active, with the minimum heads occurring in the years of increased pumpage, due to surface water yield failures. The dual values associated with the minimum head constraints at the critical nodes were very large, suggesting that this constraint was a limiting factor to groundwater development. The increased reservoir yield with yield failure considered, and the constraint on minimum allowable head, likely contribute to the reduced levels of groundwater usage in the conjunctive use applications, relative to the applications in section 7.1. 7.3 Summary Two optimization models in the framework of yield models were formulated to analyze water resource development considering groundwater and potential surface water reservoir sites. The first model developed considered the development of reservoirs as an alternative to groundwater development. The reservoir - groundwater system was not operated conjunctively. The economics of reservoir development versus well development were analyzed. Groundwater yield was specified as a lumped parameter in this model. Groundwater economics were exogenously specified to the model from a parametric analysis of an optimization model for groundwater management that was cognizant of water rights, water quality maintenance and drawdown constraints. The second model formulated considered reservoir development in conjunction with groundwater use. Reservoir yield 175 |