LOG-LOGISTIC FLOOD FREQUENCY-ANALYSIS

The log-logistic (LLG) distribution is evaluated for flood frequency analysis. Some of its properties and methods of parameter estimation are given, including a new method based on generalised least squares (GLS). The performance of the log-logistic distribution is compared with those of the generalised extreme value (GEV), three parameter log-normal (LN3) and three parameter Pearson (P3) distributions. The results are reported in terms of empirical distribution function (EDF) tests of goodness of fit, on both individual and regional flood series through the application of these distributions to a set of reasonably long annual maximum series for part of Scotland. Some reproductive properties of the LLG and GEV are also compared. In terms of four key properties the LLG performs better than the GEV, LN3 and P3 distributions and is thus commended for further analysis.