Pearson Type III Distribution

The Pearson type (PT) III distribution is the generalized gamma distribution and is one of the most popular distributions for hydrologic frequency analysis. Bobee and Robitaille (1977) compared PT III and log PT III distributions using several long-term records of annual flood flows and found PT III distribution to be preferable, especially when the method of moments (MOM) was applied to observed sample data. Bobee (1973), Chang and Moore (1983), among others, used it for flood frequency analysis. Markovic found practically no difference in fitting of Pearson and lognormal distributions to annual precipitation and runoff data. Matalas (1963) found PT III distribution to be representative of low flows. Obeyesekera and Yevjevich (1985) presented a procedure for generation of samples of an autoregressive scheme that has an exact Pearson type III distribution with given mean, variance and skewness. Harter (1958) prepared tables for percentage points of the PT III distribution. Wilk et al. (1962) described a procedure for preparing probability plots for randon samples from an assumed PT III distribution Haktanir (1991) developed a practical method for computation of PT III frequency factors. Shaligram and Lele (1978) analyzed hydrologic data using PT III distribution and showed that the confidence intervals for this distribution were larger than for the Gumbel distribution.