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<br />Regulation of Flow <br /> <br /> <br />The largest part of the estimated 7.6 million acre-feet <br />per year of surface water available to Colorado users is avail- <br />able only through artificial storage. Storage must be provided <br />to capture the spring runoff for release in late summer, when <br />natural flow is rapidly declining and both municipal and irri- <br />gation demands are near maximum. Reservoirs may also store <br />water from one year to the next, and a few reservoirs in Colorado <br />have sufficient capacity to extend supplies for more than one <br />year of drought. Planned underground storage of surface water <br />in aquifers is practiced on a very limited scale in Colorado. <br />Certain geologic formations could be utilized as reservoirs to <br />store surface water underground efficiently. <br /> <br />storage tends to reduce fluctuations in temperature and <br />suspended-sediment and dissolved-solids concentrations. Reser- <br />voirs provide a safe location for water-system intakes which, <br />when exposed to streamflow, are in continual danger of damage. <br />Evaporation losses are increased by surface storage, but may be <br />minimized by proper location, design, and operation of reservoirs. <br /> <br />Fifty-five cities or towns in colorado own or control more <br />than 3 acre-feet of storage each. The total storage so owned or <br />controlled is about 470,000 acre-feet as compared to a total of <br />about 4,300,000 acre-feet of reservoir capacity in the state. <br />Thirty-nine towns depend on diversion rights alone, and some face <br />almost yearly seasonal shortages, even though there are surplus <br />supplies during most of the year. Additional storage, both <br />surface and underground, is needed to bolster municipal water <br />supplies in many parts of Colorado. <br /> <br />GROUND WATER <br /> <br />Fifty-five percent of the 244 community water systems in <br />the state are supplied exclusively by ground water, but only <br />about 18 percent of the total population is represented. In <br />addition, most of the individual facilities in the 39 communities <br />having no community water system are supplied by ground water <br />from individual small-capacity wells. Thirty-two community water <br />systems are supplied from both ground- and surface-water sources. <br /> <br />Source and occurrence <br /> <br />The original source of ground-water supplies in the state is <br />precipitation. In some places, the water may enter the ground <br />where it falls and percolate directly to the ground-water reser- <br />voir. In other places it may flow overland, a part being absorbed <br />along its course to the streams, lakes, and reservoirs. A part <br />of the streamflow and ponded water also percolates downward to <br />the ground-water reservoir. <br /> <br />16 <br />