Regional Flood Frequency Analysis Using L Moments for the Buyuk and Kucuk Menderes River Basins of Turkey

The main goal of this study was to determine regional probability distributions for the annual maximum flood data observed at 45 streamflow gauging sites in the Kucuk and Buyuk River Basins in Turkey using index flood L moments. Mann–Kendall trend analysis was previously applied to the streamflow gauging sites’s observed data, and dam effects were sought at all stations. Seven sites were omitted from the analysis because they were not unregulated or trends were present in the data. A regional analysis was performed on the remaining 36 sites, which satisfied the screening criteria of having a minimum record length of $10\text{years}$ , representing unregulated natural flows, and passing the Mann–Kendall test at the 5% significance level. Data from the 36 streamflow gauging sites in the basins were screened using the discordancy measure $\left(D_i\right)$ . Homogeneity of the region was then tested using the L-moments heterogeneity measure by carrying out 500 simulations using the four parameter kappa distribution. Based on these tests, the streamflow gauging sites were found to constitute two homogeneous subregions, namely the upper- and lower-Menderes subregions. Seven comparative regional flood frequency analysis studies were carried out for each subregion using various L-moment-based frequency distributions: the generalized logistic, general extreme value, generalized normal, Pearson Type III, generalized Pareto, kappa, and the five-parameter Wakeby. Based on the L-moment goodness of fit statistic criterion, the generalized normal extreme value distribution has been identified as the best-fit distribution for the upper- and lower-Menderes subregions. A Monte Carlo simulation was used to evaluate the results on the basis of the relative root mean square error relative bias. For estimation of floods of various return periods for ungauged catchments, a regional relationship between mean annual peak flood and catchment area was developed and coupled with the regional flood frequency relationship derived for gauged catchments.