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<br />MWSI Project <br />Phase II Conjunctive Use Summary Report - DRAFT <br /> <br />August 22, 1995 <br /> <br />Groundwaier Recharge <br /> <br />Groundwater recharge effectively lets the underground aquifer system be utilized as a <br />storage reservoir, The significant benefits of groundwater storage include no evaporation losses, <br />no inundation impacts, and a tremendous volume of available underground storage volume, The <br />disadvantage of underground storage is the additional infrastructure required to both add water to <br />and remove water from the underground storage system, <br /> <br />Groundwater recharge can reduce the depletion of the aquifer supplies and hence can <br />extend the life of the aquifer. The potential exists to recharge to the extent that aquifer levels can <br />be maintained at equilibrium over the long term thereby allowing conjunctive use yields to be <br />sustained indefinitely, assuming that groundwater recharge could be economically achieved at <br />sufficiently high rates for long periods of time, and that groundwater depletions from non- <br />participating areas would not be an interfering factor. Groundwater recharge to any significant <br />extent would reduce the long term costs of well infrastructure required to maintain production as <br />aquifer levels decline, <br /> <br />On-Stream Surface Storage <br />On-stream surface storage can playa major role in conjunctive use through the <br />mechanism of "borrowing". Borrowing from existing surface water storage can greatly expand <br />the yield of a conjunctive use system. Typically, surface water suppliers utilize their surface <br />storage for protection from multi-year drought events. To meet that goal, the operators attempt <br />to maintain storage at levels where runoff can be guaranteed to produce full reservoirs in most <br />years. If reservoirs are maintained at these relatively high levels, there are many years when the <br />spring runoff not only fills the reservoir but produces storable flow which spills from the <br />reservoir and is lost from the municipal water supply system, If the reservoir operator allows <br />further drawdown of the reservoir system, then the risk is that storage would not be replenished <br />and the system can fall short of the supply required to meet demands in times of drought. <br /> <br />However, under a conjunctive use plan, the operator can allow greater drawdowns in the <br />surface supply system with the assurance that withdrawals can be made from the underground <br />storage reservoir (Denver Basin aquifers) to supplement water supplies if reservoirs do not refill. <br />By allowing this additional drawdown each year, more storable surface flows are captured. <br />Through this arrangement the role of "drought protection" is partially shifted from surface <br />reservoirs to aquifer storage, thereby allowing the surface reservoirs to be used more fully for <br />runoff capture, In this manner, a conjunctive use plan can increase the yield of surface supply <br />storage, <br /> <br />Operational Planning <br /> <br />In conjunctive use arrangements which include borrowing from surface storage, an <br />operational plan would be required which would allow for borrowing while protecting the yield <br />of the surface system and minimizing any environmental or recreational impacts that may result <br />from changes in surface system operations. The amount of borrowing would need to be <br /> <br />33 <br />