Copula-based bivariate drought severity and duration frequency analysis considering spatial-temporal variability in the Ceyhan Basin, Turkey

In recent times, copula families have been mostly employed in bivariate drought duration and severity. Archimedean copulas are mainly used conveniently among various potential families for hydrologic design and water resource management. In this study, drought events are defined by standard precipitation index (SPI3 time series) series for 24-gauge stations in Ceyhan Basin Turkey considering different elevation levels (high, average, and low). Both duration and severity are tested for their dependence with Mann-Kendall and Spearman Rho trend test at a 0.05 significant level. While Gamma and Weibull perform better than the other distributions at average elevation, Log-normal and Weibull distributions are found to fit the DD and DS series better for most stations located in low and high elevations, respectively. The best fit copula is found to construct a joint distribution function. The influence of upper tail dependencies is tested to select the most appropriate copula function. Gumbel and BB1 copulas are often regarded as the best copulas for the DD and DS series, respectively. After acquiring the best fit copula, the joint return periods are modeled for each station. Finally, considering drought risk categories (mild, moderate, severe, and extreme drought), spatial distributions of drought risk-return period are constructed. High risks are observed in the middle part of the basin based on T-DS and T & PRIME;(DS) under mild drought categories. Apart from using univariate frequency analysis, bivariate frequency analysis is performed to model the joint return period based on copula theory which is great of importance for hydrology design and water resource management.