Hydrological drought dynamic using copula functions and drought center migration in the Ganjiang river basin

Understanding the spatiotemporal evolution of hydrological drought is essential for effective drought management. This study investigated hydrological drought in Ganjiang River Basin (GRB), China from 1959 to 2019 using observed and SWAT-simulated runoff data. The Standardized Runoff Index (SRI) and run theory were applied to identify drought duration (D), severity (S), and intensity peak (P). A gravity center model revealed spatial migration patterns, while Copula functions modeled joint distributions among drought characteristics. The findings were as follows: (1) Regions with longer drought durations tended to show greater severity, while shorter events were milder. (2) Drought centers were mainly concentrated in the central GRB, with more pronounced north-south than east-west movement, increasing with intensity. (3) The Gumbel copula can better model the dependence structure between D and P, as well as between D-S and S-P pairs, and for the trivariate relationship among D, S, and P in GRB. (4) Joint return period (T_α) and the co-occurrence return period (T_o) effectively bounded univariate return period (T), with more extreme droughts occurring less frequently. This study provides a quantitative basis for understanding drought dynamic of Ganjiang River Basin and further supports the improvement of regional drought mitigation and water resources planning.