Abstract
Design stream flows are frequently used in water pollution control programs to provide adequate protection against pollutant exposure periods of a given duration. An analysis of the effect that simple stream flow dilution has on the x–day average concentration of a pollutant demonstrates that the x–day harmonic mean flow is a more meaningful statistic to use in computing design flows than is the customary arithmetic mean flow. The significance of this result was examined by computing design flows for 60 rivers throughout the country. Substantial differences were found for the mean daily flow—a design flow suggested for water quality criteria based on protecting human health against lifetime exposures. The harmonic mean daily flow is typically 20–60% of the arithmetic mean daily flow. Data from the 60 rivers showed very good agreement with a theoretical relationship between the arithmetic, harmonic, and geometric means of a lognormal distribution.
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