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MWSI Project Phase II Conjunctive Use Summary Report - DRAFT August 22, 1995 CONJUNCTIVE USE MODEL ANALYSES The conjunctive use model was used to explore the relationships between critical variables including aquifer life, well field pumping and recharge capacity, borrowing and payback rates, peaking storage requirements, etc, The model was also use to examine the potential yield from a conjunctive use arrangement having access to the unused divertible surface supplies available to Denver Water's system operating at a demand level of 335,000 acre-feet per year. (The yield from such a conjunctive use program would partially or wholly used by DW to meet its service area needs; otherwise the amount of unused surface supplies available to a conjunctive use plan from DW's system would be less,) The purpose of these analyses was to initially explore the operational dynamics of a large scale conjunctive use plan and to generally determine whether a conjunctive use approach could be effectively used to develop new water supplies and extend aquifer life, It should be noted that these analyses are preliminary and conceptual in nature, More refined analysis will be required to meaningfully determine the operating rules and yield potential of any specific conjunctive use plan, Input Assumptions A range of input data was established to begin evaluating the potential yield of a conjunctive use plan. The parameters set included the following: . Amount of unused surface supplies available, . Surface system storage level below which borrowing cannot occur, . Surface system storage level below which payback must begin, Payback period, Conjunctive use reservoir size, The available surface supply was assumed to be equal to all of Denver Water's unused divertible surface supplies when the DW system was operating at a demand level of 335,000 acre-feet per year. This demand level represents the safe yield ofthe existing DW system with South Platte effluent exchanges and Blue River substitutions (including Muddy Creek Reservoir) in place, It is recognized that this represents the upper limit of potentially available surface water from the OW system and that DW will use some of this supply in meeting its own future needs, either through participation in conjunctive use plan or otherwise, The minimum storage level in the surface water system below which no borrowing could occur was established at 200,000 acre-feet. The minimum surface storage level below which payback would have to begin was set at 150,000 acre-feet. These levels were estimated by trial and error; the objective was to determine storage levels which would protect the surface system's safe yield without excessively draining its reservoirs. The payback period must occur within the critical period of drought to be useful. Since the major drought in the period of record was 4 52