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<br />I x = 3.3684, and S = 0.2456. <br /> <br />4.2. RECOMMENDED PROCEDURE FOR ANNUAL MAXIMUM <br />DISCHARGE. The U.S. Water Resources Council (USWRC, 1967, 1976, 1977) <br />recommended the log Pearson type III distribution for annual maximum streamflow <br />frequency studies. This recommendation is followed by USACE. Current guidelines are <br />presented in Bulletin 17B (USWRC, 1981). <br /> <br />The log Pearson type III distribution models the frequency of logarithms of annual <br />maximum discharge. Using Chow's format, the inverse log Pearson type III distribution <br />is <br /> <br />- <br />Xp = X + KS <br /> <br />(10) <br /> <br />in which: <br />Xp = <br />X <br />S <br />K <br /> <br />the logarithm of Op' the desired quantile <br />= mean of logarithms of sample <br />= standard deviation of logarithms of sample <br />= the Pearson frequency factor <br /> <br />X and S are computed with the Equations 8 and 9. <br /> <br />For this distribution, the frequency factor K is a function of the specified <br />probability and of the skew of the logarithms of the sample. The skew, G, is computed <br />with the following equation: <br /> <br />(11 ) <br /> <br />For the values of Table 2, the skew computed with Equation 11 is 0.7300. <br /> <br />The log-Pearson type III frequency factors for selected values of skew and <br />exceedance probability are tabulated in Bulletin 17B and in EM 1110-2-1415. <br />Alternatively, an approximating function can be used. If the skew equals zero, the <br />Pearson frequency factors equal the normal distribution factors. Otherwise, the <br />following approximation suggested by Kite (1977) can be used: <br /> <br />(12) <br /> <br />7-98 <br />