[Spatial and Temporal Characteristics and Controlling Factors of Water Quality of the Dahei River in the Yellow River Basin].

As an important ecological security barrier and an economic core belt for population activities, the Yellow River Basin plays an important role in China's social development and ecological security. To investigate the spatial and temporal changes in the water environmental quality of typical tributaries in the basin and the factors affecting them, this study was based on 12 aspects of water environmental data and month-by-month rainfall and runoff data from 21 monitoring stations in the Dahei River Basin from 2013 to 2022. The entropy-weighted water quality index （EWQI）, stepwise multiple linear regression, hierarchical cluster analysis, and Pearson correlation analysis were used to comprehensively assess the changes in the water quality of the basin and to investigate the factors controlling the water quality. The results showed that： ① The water quality of Dahei River Basin reached the standard of surface water class V and above； the water quality in hilly and mountainous areas was the best； and the overall water quality improved during the study period. ② The water quality in hilly and mountainous areas was better than that in plain areas and built-up areas of cities and towns, and the main controlling factors affecting the water quality after optimization of the indicators in the EWQI model were TN, BOD₅, TP, NH₄⁺-N, and permanganate index, with a coefficient of determination of 0.952. ③ On a spatial scale, the monitoring stations in the Dahei River Basin could be clustered into three categories. The water quality of the S2 station in the L1 category deteriorated after rainfall, whereas water quality improved after rainfall at site S14 in the L2 category and at site S21 in the L3 category, and water quality deteriorated overall at all three categories of sites in the winter. ④ The EWQI of the basin had a positive correlation with runoff and a non-significant correlation with rainfall； the main characteristic pollutants were nutrient salts and organic matter. By revealing the spatial and temporal variations of water quality in typical tributaries of the Yellow River Basin and its response relationship to non-point source pollution triggered by rainfall and runoff events, this study can provide scientific references for the control and management of water quality attainment in typical tributaries of the Yellow River.