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<br />10 <br /> <br />Purpose of Reservoirs <br /> <br />The true function of reservoirs in connection with a munici- <br />pal water supply system is sometimes misunderstood, and there- <br />fore it seems well to discuss this subject before describing in <br />detail the studies that were made incident to the preparation of <br />this report. <br /> <br />Historically the first phose of water development almost <br />always has been by the use of direct flows, the construction of <br />storage reservoirs and the use of the water stored therein follow- <br />ing os 0 later phase, Some ore inclined to place great emphasis <br />on water derived from direct-flow rights, with little emphasis on <br />the role that reservoirs ploy in 0 well balanced water system. <br />Others are inclined to over-emphasize the role of reservoirs under <br />the delusion that reservoirs make water, The true function of <br />storage in connection with municipal and most other uses of water <br />is, over 0 reasonable period of time, to make the water supply <br /> <br />parallel with the requirements by providing seasonal and year- <br />to-year regulation, and to provide some reserves. The relationship <br /> <br />between reservoir capacity and the water supply to be regulated <br />depends upon the degree of variation in water supply during the <br /> <br />season and from year to year, The storage capacity required to <br /> <br />make available the base flow from a given source is much less <br /> <br />than the capacity required to make available the some amount <br />of water from that which occurs in the upper portions of the hydro- <br /> <br />graph representing the water supply source. This situation can be <br />visualized by an inspection of Figure 2 upan which the annual <br /> <br />supply available fram each of Denver's primary sources of water <br />is depicted graphically. <br /> <br />It may be noted from Figure 2 that the yield of the South <br />Platte direct-flow rights is fairly constant because many of the <br />rights hove a senior status, The water supply for Denver's South <br />Platte reservoirs must be derived from occasional flood flows, <br />however, and is therefore quite erratic, all of the dependable <br />base flow having been appropriated by direct-flow users. <br />