University of Birjand-Research Group of Drought and Climate Change Journal of Drought and Climate change Research 3092-6076 Articles in Press 2025 04 17 Frequency analysis of non-stationary hydrological time series using modified reservoir index and copula functions Frequency analysis of non-stationary hydrological time series using modified reservoir index and copula functions 3426 10.22077/jdcr.2025.9142.1132 FA Hamidreza Rahimi University of Birjand Graduate School 0009-0002-1638-446X Abolfazl Akbarpour Department of Civil Engineering, University of Birjand 0000-0002-9693-1998 Mohammad Nazeri Tahroudi Department of Water Engineering, Lorestan University 0000-0002-6871-2771 Journal Article 2025 03 22 In this study, using the modified reservoir index in the Dez catchment basin located in the west of the country, the peak discharge values will be simulated and checked on an annual scale for the Dezful hydrometric station built after the Dez dam reservoir. The annual peak discharge values of the Dezful hydrometric station and the average discharge values of this station have both changed trends and failed in the annual series in 2016. Examining the trend of changes in the reconstructed flow peak values of Dezful hydrometric station in the statistical period of 1398-1357 showed that the Mann-Kendall statistic for the series is -2.48, which does not indicate significant changes. To investigate the best-detailed function according to the evaluation criteria for the peak discharge values of Telezang station with log-normal statistical distribution and the reconstructed peak discharge values of Dezful station with generalized limit statistical distribution in comparison with the best-detailed function for the observed peak values of Telezang and Dezful hydrometry stations with The statistical distribution of both normal log, the detailed function of a type called Galambos was obtained and theta parameter was calculated for the modified peak flow of Dezful and Telezang 9.8841 and for the observed peak flow of Dezful and Telezang 8.6981. In addition to the mentioned cases, in this study, the detailed function was used to estimate the probability of occurrence of the examined data, which led to the presentation of type diagrams for frequency analysis. In this study, using the modified reservoir index in the Dez catchment basin located in the west of the country, the peak discharge values will be simulated and checked on an annual scale for the Dezful hydrometric station built after the Dez dam reservoir. The annual peak discharge values of the Dezful hydrometric station and the average discharge values of this station have both changed trends and failed in the annual series in 2016. Examining the trend of changes in the reconstructed flow peak values of Dezful hydrometric station in the statistical period of 1398-1357 showed that the Mann-Kendall statistic for the series is -2.48, which does not indicate significant changes. To investigate the best-detailed function according to the evaluation criteria for the peak discharge values of Telezang station with log-normal statistical distribution and the reconstructed peak discharge values of Dezful station with generalized limit statistical distribution in comparison with the best-detailed function for the observed peak values of Telezang and Dezful hydrometry stations with The statistical distribution of both normal log, the detailed function of a type called Galambos was obtained and theta parameter was calculated for the modified peak flow of Dezful and Telezang 9.8841 and for the observed peak flow of Dezful and Telezang 8.6981. In addition to the mentioned cases, in this study, the detailed function was used to estimate the probability of occurrence of the examined data, which led to the presentation of type diagrams for frequency analysis.