water Article Bivariate Frequency of Meteorological Drought in the Upper Minjiang River Based on Copula Function Fangling Qin 1, Tianqi Ao 1,2,* and Ting Chen 1,3,* 1 College of Water Resources & Hydropower, Sichuan University, Chengdu 610065, China;
[email protected] 2 State Key Laboratory of Hydraulic and Mountain River Engineering, Sichuan University, Chengdu 610065, China 3 Heavy Rain and Drought-Flood Disasters in Plateau and Basin Key Laboratory of Sichuan Province, Chengdu 610072, China * Correspondence:
[email protected] (T.A.);
[email protected] (T.C.); Tel.: +86-028-8540-1149 (T.A.); +86-028-6028-0028 (T.C.) Abstract: Based on the Standardized Precipitation Index (SPI) and copula function, this study analyzed the meteorological drought in the upper Minjiang River basin. The Tyson polygon method is used to divide the research area into four regions based on four meteorological stations. The monthly precipitation data of four meteorological stations from 1966 to 2016 were used for the calculation of SPI. The change trend of SPI1, SPI3 and SPI12 showed the historical dry-wet evolution phenomenon of short-term humidification and long-term aridification in the study area. The major drought events in each region are counted based on SPI3. The results show that the drought lasted the longest in Maoxian region, the occurrence of minor drought events was more frequent than the other regions. Nine distribution functions are used to fit the marginal distribution of drought duration (D), severity (S) and peak (P) estimated based on SPI3, the best marginal distribution is Citation: Qin, F.; Ao, T.; Chen, T.