Regional flood frequency analysis for gauged and ungauged cathments of seyhan river basin in Turkey

Regional flood frequency analysis for the Seyhan Basin in Turkey is done by two different approaches, and generalized growth curves to be used at un-gauged sites are developed by either method. First, the homogeneity of the annual flood peaks series recorded at 11 un-regulated gauging stations in the Seyhan Basin is analyzed by the conventional Cv (coefficient of variation) test; and second, discordance, homogeneity, and goodness-of-fit by the Z tests by the method of L-moments are performed. The Seyhan Basin is subdivided into three homogeneous sub-basins from the aspect of annual flood peaks based on the Q, test, which detects regions having recorded series with variation coefficients close in magnitudes. The probability distributions of Log- Logistic, Pearson-3, log-Pearson-3, Gumbel, and Wakeby, whose parameters are computed by the method of probability-weighted moments, are taken as candidate distributions. The goodness-of-fit tests of Kolmogorov-Smirnov and Crammer Von Misses are applied, and the Wakeby distribution is found to be superior to the others. According to the analyses by the L-moments method, the Seyhan Basin as a whole turned out to be a homogeneous region. By inspection of the L-moment ratio diagrams of L-variation versus L-skewness and L-skewness versus L-kurtosis of those 11 series, and by the Z-statistic analysis of the L-moments method, the Log-Logistic distribution is found to represent the recorded series of annual flood peaks more closely. By regression, an expression relating average peak discharge (Qav) to the catchment area is obtained, and growth curves for the frequency of flood peaks (Qmax.) by both the Cv-based and the L-moments methods are developed using the data of those 11 gauged sites.