Investigation of changes in the annual maximum flood in the Yellow River basin, China

Flooding in the Yellow River basin (YRB) has changed significantly over the last few decades because of climate change and human activities. Determining how the flooding changed and identifying the main driving factors of flood change is crucial to flood risk assessment and water resources planning. However, few studies have been conducted, especially in the whole YRB. To fill this gap, we investigate the spatial and temporal change of the annual maximum flood (AMAXF) in the YRB using observed data from 32 key hydrological stations (including 15 mainstream stations and 17 tributary stations). The Mann–Kendall test combined with trend index method is used to evaluate the trend in precipitation and AMAXF. The trend results indicate the AMAXF over the whole basin is dominated by decreasing trends: 72% of the stations exhibit significant decreasing trends (at 0.1 significance level) and 22% of the stations show no significant decreasing trends in the AMAXF. Both flood trends and abrupt change time exhibit obvious regional differences: the flood decreases is more pronounced in the midstream basins than in the headwater of the basin; the abrupt changes mainly occurred in the early 1990s for the upper reaches, and in the late 1990s for the middle reaches. To investigate the causes of flood change, the trends of precipitation extremes are analyzed in relation to the trends of peak floods. The analysis reveals that the decreasing precipitation extremes only results in the AMAXF reduction in the upstream basin. The decreasing AMAXF in the midstream and downstream of the basin are mainly attributable to the impacts of human activities (mainly including dam construction and soil conservation practices). In general, anthropogenic impacts play an increasingly important role in the AMAXF changes in the YRB. According to the possible changes in forcing factors, the AMAXF over the whole basin is expected to further decrease in the near future. However, there are still large flood risks in the tributary basins due to the collapse of check dams caused by extreme storms.