Nonstationary flood frequency analysis and drivers in the middle-lower Yangtze River, China

Study region The middle–lower Yangtze River Basin, China, represented by four hydrological stations: Yichang, Luoshan, Hankou, and Datong. Study focus This study investigates nonstationary flood frequency and climatic and anthropogenic drivers under changing environmental conditions. Generalized Additive Models for Location, Scale, and Shape (GAMLSS) were used to model annual flood peaks to assess the effects of atmospheric and terrestrial covariates, by incorporating antecedent precipitation, flood-season temperature, large-scale climate indices, and reservoir regulation as physically interpretable covariates. New hydrological insights Flood-frequency nonstationarity exhibits pronounced spatial heterogeneity along the middle–lower Yangtze River. Statistically significant nonstationarity is identified at Yichang, Luoshan, and Hankou Stations, whereas Datong Station shows neither a significant monotonic trend nor a significant change point during the study period. An empirically selected 27–28-day antecedent precipitation window, flood-season maximum temperature (TMAX), the modified reservoir index (MRI), and El Niño–Southern Oscillation (ENSO) improve the representation of nonstationary flood-frequency distributions. Reservoir regulation is more strongly associated with flood-frequency variation at Yichang Station, whereas antecedent precipitation and ENSO become increasingly prominent at the lower part of Luoshan and Hankou Stations. Nonstationary models indicate that flood quantiles generally increase with antecedent precipitation and with El Niño conditions in the preceding winter–spring but decrease with stronger reservoir regulation. Seasonally accumulated TMAX shows heterogeneous quantile effects, suggesting possibly different thermal associations between ordinary and extreme flood conditions.