Increasing risk of synchronous floods in the Yangtze River basin from the shift in flood timing

Floods are some of the most frequent and severe natural hazards worldwide. In the context of climate change, the risk of extreme floods is expected to increase in the future. While, the trends in flood timing and risk for flood synchronization remain unclear. In this study, the seasonality of flood peaks, annual maximum rainfall, and annual maximum soil moisture in the Yangtze River Basin were examined using observational and reanalysis data from 1949 to 2020. Changes in the timing of extreme events may increase the possibility of concurrent flooding, therefore the risk for synchronous floods were further explored. The results indicate that the seasonality of floods has a strong consistency with that of annual maximum rainfall. In the southern Yangtze River Basin, floods usually occur between early June and early July, with a delayed trend. However, they occur slightly later in the north, generally from late July to early August, with a tendency of advance. Overall, the timing of floods is positively correlated with rainfall and soil moisture peaks, and the correlation is much stronger for annual maximum rainfall. However, for more intense floods or for larger catchments, soil moisture plays an important role in modulating the variations in flood timing. Reverse latitudinal changes in flood timing are expected to result in more synchronous floods. The synchrony frequency exceeded 60 % for most of the stations, and the frequency was increasing for nearly half of the region, especially in the middle reaches, Poyang Lake and south of Dongting Lake. In addition, the flood synchrony scale in the south of the basin showed significant upward trends. These findings would provide important implications for flood risk management and adaptive strategy development.