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MWSI Project Phase II Conjunctive Use Summary Report - DRAFT August 22, 1995 The maximum size relates to the maximum size of a storage facility that can be accommodated by the natural topography in the metropolitan area, although multiple sites can be utilized. The remaining input parameters relate to borrowing and payback triggers, A minimum storage level in the surface water system is set. Below this level, no further borrowing can occur whether or not the debt limit has been reached, At this level, however, the system may still be recharged through surface supplies. Therefore, the model allows another minimum storage level to be established below which payback must occur. Model Operating Rules When surface supply is available, the flow is assumed diverted to first meet conjunctive use demands. The volume of flow is checked against the conjunctive use demand. If the flow is sufficient to meet the demand, the excess is available for groundwater recharge, If the flow meets the demand for groundwater recharge, then any excess is diverted to the conjunctive use reservoir and is credited to storage if volume in the reservoir is available, If volume is not available, the surface supplies are lost from the municipal water supply system. Under the same scenario, if the surface supply is not sufficient (1) to fill the conjunctive use reservoir, and/or (2) to meet the groundwater recharge demand and/or (3) to meet the conjunctive use demand, then water is borrowed from the surface supply system. This borrowing occurs by depleting the surface storage system by the appropriate amount. That borrowed volume is registered as a debit of the conjunctive use system against surface water supplies, That debit would continue to grow as needed until it reaches the specified maximum, In the event that surface supplies are not available and borrowing has reached its limit or surface storage dips below the minimum for payback, then the groundwater system is activated and the entire conjunctive use demand is delivered from the groundwater system. When the groundwater system is activated, it is activated to only meet conjunctive use demands if surface system storage is above the minimum for payback, Otherwise, the system operates at its maximum. The groundwater is first credited against the conjunctive use demand for that month, If the groundwater demand for the month is less than the average montWy demand, the excess is credited to the conjunctive use reservoir. If the groundwater demand for the month is larger than the average monthly demand, the deficit is deducted from the conjunctive use reservoir storage volume. The remaining pumping capacity, that is the difference between pumping capacity and average monthly conjunctive use demands is assumed deliverable to the surface water system and is deducted from the amount owed to the surface water system, when payback is in operation. Once the groundwater system is activated, the system is assumed to operate until payback is complete through pumping or until the surface system refills through capture of storable 50