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<br />I <br />I <br />I <br />, I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />In planning for water supply systems, either the driest two-year or <br />five-year period is usually selected as the critical design period. Using <br />the same data, the driest two- and five-year periods were identified and <br />recurrence intervals estimated. The results are as follows: <br /> <br />Period Recurrence Interval <br />(Two Year) <br />1976-1977 44 Years <br />1934-1935 40 Years <br />1954-1955 30 Years <br />1966-1967 30 Years <br />(Five Year) <br />1963-1967 42 Years <br />1933-1937 28 Years <br />1934-1938 16 Years <br />1959-1963 15 Years <br /> <br />Although 1977 was the driest year on record, it was preceded and followed by <br />good water years and is not included in any of the four driest five-year <br />periods. There was concern that including th~ 1977 dry year in the <br />hydrologic period would impose too severe a condition upon the project. <br />Previous USBR studies of the Yellow Jacket Project used the hydrologic <br />periods of 1947-1965 and 1952-1965 which did not include the two driest <br />years of record. <br /> <br />For project planning purposes, the critical hydrologic condition should have <br />a recurrence interval between one and two times the project life. In this <br />study, a project life of 50 years has been assumed, thus making the l60-year <br />interval of a 1977 year be in excess of three times the project life. With <br />a major portion of the potential water from the Yellow Jacket Project <br />designated for municipal and industrial use, the reliability of the water <br />supply is of prime importance. For this reason, the year 1977 was included <br />in the hydrologic study period. <br /> <br />11-7 <br />