Nonstationary risk analysis of drought–flood abrupt alternation events in the Pearl River Basin using the GAMLSS framework: Return period and design values

Study region Pearl River Basin, Southern China. Study focus This study conducted a comprehensive nonstationary risk analysis of Drought-Flood Abrupt Alternation (DFAA) events in the Pearl River Basin (1961–2020). A total of 38 wet-to-dry and 49 dry-to-wet events were identified using the Standardized Weighted Average of Precipitation index. The Generalized Additive Models for Location, Scale, and Shape (GAMLSS) framework was employed to model time-varying parameters for the marginal distributions of intensity and duration. Both stationary and nonstationary bivariate joint distributions were constructed using Clayton and Joe copulas for DFAA events, respectively, with dependence nonstationarity confirmed by Copula Likelihood-Ratio tests. New hydrological insights for the region DFAA properties and dependence structures are nonstationary, and ignoring this leads to systematic underestimation of compound risks. Under nonstationary conditions, 18.4% of wet-to-dry and 4.1% of dry-to-wet events exceeded the 50-year "AND" joint return period. wet-to-dry events were found to be generally more severe than dry-to-wet events. The time-varying copula parameters for the dependency structure decreased by 27% for wet-to-dry events and increased by 43% for dry-to-wet events by 2020 compared to stationary estimates. Multivariate design values substantially diverged from univariate estimates, emphasizing the necessity of multivariate risk assessment. The GAMLSS-time-varying copula framework provides a robust tool for nonstationary frequency analysis, offering critical guidance for adaptive water infrastructure design and climate resilience strategies.