Abstract
Drought is a natural phenomenon which starts with decreased precipitation and can disrupt the environmental systems by changing the hydrological cycle. This is more conspicuous in hydrological drought. In analysis of hydrological drought, two factors of severity (intensity) and duration play eminent role. These characteristics are highly related and therefore their combined analysis contributes to better understanding of the drought situation. In this research, by using 40-year (1974–2014) daily discharge data of Tajan River, located in Mazandaran province, Iran, and low-flow indices, the best evaluation index of hydrological drought was determined and 10 past hydrological drought events in the region were identified. Then, the best statistical distribution of both drought variables (duration and severity) was selected, based on the goodness-of-fit tests. Five copula functions were fitted to the data. Results showed that Galambos function with the highest maximum log-likelihood (− 8.934) was selected as the best copula function. Results of the bivariate (duration and severity) statistical distribution could be used to analyze the probability of hydrological drought in the region. This bivariate and conditional probability for the worst drought, with duration of 5 months and severity of 0.32, was 6.1 and 28.5%, respectively.
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