Evaluation of the hydrological drought in the Orantes Basin using multivariate copula theory

Drought is a complex natural phenomenon that varies over time under different climatic conditions. The increasing frequency of drought with climate change requires measures to be taken to mitigate its devastating effects and targeted identification and assessment of drought-prone regions. In this thesis, it is aimed to evaluate the hydrological drought in the Asi River Basin in the Eastern Mediterranean region of Turkey at two different time scales (3 and 6 months) using the Stream Drought Index (SDI) method. Drought severity and classes were analysed by dividing into three different periods as 1954-1976, 1976-1998 and 1998-2020. The transition stages of drought parameters (duration, intensity, peak) were analysed as univariate and bivariate (Duration-Severity and Duration-Peak) using copula functions. In order to jointly evaluate the drought parameters, the most appropriate one among six different copula functions was determined using the Akaike Information Criterion (AIC) and Bayesian Information Criterion (BIC) selection criteria. At four stations, Galambos was found to be the most appropriate copula at both time scales and in almost all (94%) of the bivariate phases. Comparisons were also made between the univariate return periods of drought parameters calculated using the Galambos copula and the bivariate return periods. The results showed that there was a significant increase in all drought parameters such as duration, intensity and peak in the third period compared to the previous periods, and drought intensity showed a particularly significant increase. Comparison of the bivariate return periods, where the duration-severity and duration-peak components were evaluated jointly, E19A007 station located in Demirköprü neighbourhood of Antakya District, Hatay Province was the station with the highest risk among the stations studied for both 3 and 6-month.