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in the year 1956 for the Cache la Poudre record, corresponding to the <br />change observed in the South Platte record. The three year lag time from <br />the time of initial importation to the appearance of the additional flows <br />is probably due to high consumption during the 1954 -56 drought, filling of <br />the reservoirs of the Colorado —Big Thompson Project and the natural lag <br />between application of irrigation waters and the appearance of return <br />flows. Hurr et al. estimate a net gain of imported water to the South <br />Platte River of 145,000 AF per year. <br />The population of the South Platte River basin doubled again in this <br />interval (Figure 3), while the trend line (Figure 1) shows an increasing <br />slope. The Roberts tunnel, supplying Denver with more water imported from <br />the Colorado River basin, was completed in 1964. The average historic <br />diversion through the tunnel has been 43,000 AF /yr and current diversions <br />amount to approximately 80,000 AF /yr. Diversions will continue to increase <br />until the existing system capacity of 100,000 AF /year has been reached. <br />Colorado's Groundwater acts of 1965 and 1969 brought the multitude of <br />irrigation wells in the South Platte alluvium into the priority system, <br />severely curtailing their use and practically preventing the development of <br />new wells. There is another increase in slope of the mass curve in 1969 <br />which is most evident in Figures 2 and 5. <br />1982 — 1986 <br />The extreme change in slope which occurred in the 1982 -86 time period is <br />largely a result'of several abnormally wet years which occurred during this <br />period. <br />