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MWSI Project Phase II Conjunctive Use Summary Report - DRAFT August 22, 1995 . Minimwn surface supply storage below which borrowing cannot occur. . Minimum surface supply storage below which surface water payback must occur. The modeling uses as surface supply the OW divertible surface supplies that were shown previously in Figure 16. This data includes the monthly volume of unused surface supplies divertible by the OW system through the period of record from 1947 to 1974, These volumes represent divertible supplies from the South Platte, Blue River, and Mofiat systems which are all asswned deliverable at Strontia Springs directly or through operational schemes. These monthly volumes are the initial input to the conjunctive use model. Figure 16 shows an average annual unused divertible surface supply of 84,500 acre-feet. This is the maximum volume of surface water capture that can be achieved, and is representative of an upper limit of surface supply available from the OW system for conjunctive use assuming that conjunctive use was the "next project added" to the OW system for OW's use, at least in part, To the extent that the conjunctive use plan was not used to meet OW's service area needs, the amount of OW's unused surface supplies available to a conjunctive use plan would be subject to OW's needs. , The next input parameter is the conjunctive use demand. This demand is assumed to be in addition to OW's system demands, The demand is input as an annual volume and the model then detennines monthly volumes based upon a monthly distribution which is input to the model. This monthly distribution is considered typical for the metropolitan area and is presented in Figure 6, The next input parameter is the time period within which payback of borrowed surface water to OW must occur. The payback time is necessary to establish the required groundwater pwnping rate, Payback must be in a short enough time period to meet OW demands during time of drought, and for this study was assumed at 2 years. The maximum borrow limit from the OW surface supply is then input. This limit directly affects the number of wells required and hence project costs. The groundwater pumping rate is determined by the model once the conjunctive use demand, payback time period, and maximum borrow limit are input. The groundwater pwnping rate is equal to the conjunctive use demand, plus the borrow limit divided by the time period for payback. The groundwater pumping rate is presented as a monthly volume, The annual recharge rate is input as a percentage of groundwater pumping rate, This percentage is multiplied by the conjunctive use demand to detennine the monthly recharge volume, The conjunctive use peaking reservoir volume is input directly to the model. The minimum volume is that required to meet the peaking requirements of the groundwater system. 49