Analysisof evolution characteristics of compound dry-heat events in Changjiang River Basin based on Copula function

To reveal the spatiotemporal evolution of compound dry-hot events in the Changjiang River Basin, the frequency and extent of the compound dry-hot events were analyzed based on the Standardized Precipitation Index(SPI) and Standardized Temperature Index(STI) using the monthly precipitation and monthly temperature observation data in the Changjiang River Basin from 1961 to 2022.On this basis, a Copula function was used to construct a bivariate joint cumulative probability distribution function for hot (STI) and dry (-SPI), and to calculate the return period of the compound events under different thresholds.The results showed that the middle and lower reaches of the Changjiang River Basin have been the high-incidence areas of compound dry-hot events over the past 62 years.Compared to 1961~1991, the frequency increased during 1992~2022 across all subregions, with a more notable rise in the middle and lower reaches.The growth rates of the extent area for mild and moderate compound dry-hot events were 2%/10a and 1%/10a, respectively.In addition, mild compound dry-hot events occurred frequently in the Changjiang River Basin, with an average return period ranging from 0 to 26 years, while moderate compound events exhibited return periods between 9 and 92 years.The average return period in the upper reaches was slightly higher than that in the middle and lower reaches.The years 2006, 2013, 2019, and 2022 were identified as typical compound dry-hot event years, with joint return periods of 181, 335, 109, and 401 years, respectively.The findings support understanding the evolution pattern of compound dry-hot events in Changjiang River Basin.