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While this amount of storage may seem large, it is dwarfed by the natural storage provided by <br />Colorado's principal underground aquifers. The Denver Basin aquifers contain approximately <br />150 million acre -feet of recoverable groundwater beneath the six - county metro Denver area and <br />the alluvial and deep aquifers elsewhere within the South Platte and Arkansas Basins contain <br />over 15 million acre -feet. Cities and farms throughout Colorado have recognized the value of <br />groundwater and aquifer storage and have developed strategies to utilize these resources. <br />B. Is Storage the Answer? <br />The traditional purpose for building reservoirs has been to capture excess unappropriated runoff, <br />which usually occurs relatively infrequently and in large volumes. Consequently, traditional <br />reservoirs have had to be fairly large and located directly in a stream channel to be of much <br />value. Apart from their well - documented environmental impacts, such large on- stream <br />reservoirs have other major limitations from a water supply perspective. They are relatively <br />costly to build and cannot be built incrementally in response to gradually growing demands. <br />Rather, they must be fully paid for and constructed "up front," which increases their financial <br />risk and diminishes their economic viability. As a basin becomes over - appropriated, additional <br />storage designed to capture runoff produces ever - diminishing returns in terms of water supply <br />yield, because unappropriated runoff occurs less frequently and storage carry -over periods <br />become longer. This phenomenon is illustrated in Figure V.3 (a graph, based on work by the <br />U.S. Geological Survey, of the storage - annual flow ratio to the yield - annual flow ratio). For <br />example, when storage is equal to average annual flow (1 on the horizontal axis) a little less than <br />40% of the average flow is available for use (0.36 on the vertical axis), whereas doubling the <br />storage (2 on the horizontal axis) only increases availability by about 10% (0.46 on the vertical <br />axis). Evaporation losses compound this problem, becoming a major limiting factor in <br />reservoirs' ability to provide relief over extended drought conditions. Finally, given the <br />Figure V.3. The Regulation -Flow Ratio in Relation to Usable Capacity <br />33 <br />