L-moments based regional flood frequency analysis of the Firat (Euphrates) and Dicle (Tigris) basins in Türkiye

Floods are a major hydrological hazard in Türkiye, and robust design-flood estimation remains essential for hydraulic infrastructure and risk management in the transboundary Firat (Euphrates) and Dicle (Tigris) basins. This study applies an L-moments based regional flood frequency analysis (RFFA) to annual maximum flow data from 59 gauging stations. Homogeneous regions were identified using L-moment discordancy and heterogeneity measures. Six candidate distributions (GEV, GNO, GPA, PE3, GLO, and WAK) were fitted and evaluated using the L-moment Z-statistic. The Dicle Basin was found to be acceptably homogeneous and was best represented by the generalized normal (GNO) distribution, whereas the Firat Basin was heterogeneous and was subdivided into two homogeneous sub-regions, both best fitted by the generalized extreme value (GEV) distribution. Empirical power-law relationships between drainage area and index flood (mean annual flood) were also developed to support flood estimation at ungauged sites. Monte Carlo simulations were used to quantify uncertainty, indicating reliable performance for moderate return periods and widening uncertainty bounds for very large return periods. These uncertainties should be considered when applying the regional models to high-consequence hydraulic designs.