Full Description

For many U.S. utilites, the firm yield ratings of surface water supplies are based on the worst 20th century drought, which is typically the drought of the early 1930s. Previous studies of the Newport News Waterworks (Virginia) system reported a firm yield of 57 million gallons per day (mgd) for the most severe drought since 1926. Based on the streamflow period of record, this critical drought was assumed to have a return period of 75 years. In this study, monthly rainfall records extending back to 1836 made it possible to compare 19th and 20th century drought, and to reevaluate the return period associated with a 57 mgdfirm yield for the Newport News Waterworks system. Rainfall record evaluations, based on Standard Precipitation Index (SPI) values, indicated that five 19th century droughts were more severe than any 20th century droughts. Multiple linear regression models were developed and tested to predict 19th and early 20th century monthly streamflows as a function of SPI values. The regression models were used to synthesize monthly streamflows for the period 1836 through 1925. Using a combined 161-year record of synthesized and measured streamflows (1838-1999), firm yield modeling of each drought period showed that the firm yield with a 75-year return period is 44.5 mgd, which is 22 percent lower than the value reported by previous studies of 20th century droughts. The 57-mgd firm yield reported by previous 20th century drought studies corresponds to a 22-year returnperiod. Includes 18 references, tables, figures.

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Edition: Vol. - No. Published: 06/01/2001 Number of Pages: 19File Size: 1 file , 2 MB