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<br />records of the Purgatoire River near Las Animas and Big <br /> <br />Sandy Creek. There have been previous declines in state line <br /> <br />flows during drought periods, for example in 1939-1940 and <br /> <br /> <br />1963-1964, but previous drought periods were broken up by <br /> <br />average or above-average flows after two or three years. <br />The drought cycle in the 1970's lasted for a period of six <br /> <br />years before a return to average and above-average flows. <br /> <br />As in other drought cycles, usable stateline flows tended to <br />become progressively less as the drought persisted." <br />I ,Mass diagram analysis does not provide sufficient <br />information to determine whether the decrease in tributary <br />I inflows during the 1974-1979 period was the sole cause of <br />I the decline in usable state line flows. Conclusions based <br />upon the Big Sandy gage records, without adequate adjust- <br /> <br />ment, are suspect for the reasons discussed below. Further- <br /> <br />more, precipitation records for eastern Colorado indicate <br /> <br />that the drought of the 1970s was no more severe in terms of <br />magnitude or duration than the previous droughts referred to <br /> <br />above. Colorado's contention that the drought 'of the 1970s <br />lasted for a period of six years is apparently based On its <br /> <br />analysis of the Purgatoire River flows. There are many <br /> <br />other factors in addition to the flows of the Purgatoire <br /> <br />River, however, which effect stateline flows. <br /> <br />Conclusion No.5: .Usable state line flows improved <br />beginning in 1930 when precipitation and streamflow started <br /> <br />to recover towards average and above-average conditions <br /> <br />through this basin. The slope of the curves in the 1980- <br />-1984 period on Figure 18 did not recover to the same slope <br /> <br />-19- <br />