Frequency analysis of low flows at gaged points of the Ansung stream

Abstract

The Power and SMEMAX transformation, Gumbel type III, Weibull, log-Pearson, type III, lognormal probability papers based on plotting position formula are considered to estimate 7-day and 30-day low flows for return periods at four gaged points of the Ansung stream in Korea. It is found that the Power transformation is the superior ability to the SMEMAX transformation. A trial is also performed to examine the applicability of the Power transformation by changing the parameters. The Kolmogorov-Smirnov, test (K-S test) and the Chi-square test (x 2 test) are adopted in the trial There may exist a range of acceptable parameters, which implies the high applicability of the Power transformation. The method of moments or the maximum likelihood procedure is adopted to calculate the parameters of the Gumbel type III, the Weibull and the log-Pearson type III distribution. The K-S test is performed to test the goodness of fit for each distribution. The lognormal distribution with the Weibull plotting position consistently performs well and the data separation technique based on the least median of squares (LMS) method improves the coefficients of skewness in the lognormal distribution. The LMS method, easily detects the outliers of the sample and contributes to improve the properties of the normal distribution. The Power transformation, the log-Pearson type III and the lognormal distribution provide good fit to low flows for the Ansung stream. The LMS method may enhance the reliability of low flow analysis. It is also found that the flows at the Ansung stream have been significantly influenced by controlled releases from the upper reservoirs.