Analysis of spatio-temporal differences in water quality characteristics and nitrogen and phosphorus forms in typical small watershed

In order to analyze the spatio-temporal differences in water quality characteristics and nitrogen and phosphorus forms of typical small watersheds, identify the constraints of these differences, and provide a basis for scientific and precise pollution control. This study takes the Ximei River in the center of the Sichuan Basin as the research object, analyzing the concentrations of 16 indicators at 24 points during the dry season, normal water season and wet season from 2023 to 2024. By the means of various water quality evaluation methods (single-factor index, comprehensive pollution index and comprehensive water quality identification index) and multivariate statistical analysis methods (Pearson correlation analysis, hierarchical cluster analysis, principal component analysis), the spatio-temporal evolution characteristics and main restrictive factors of water quality were systematically studied, and the evolution and variation laws of the concentrations of major indicators along the course were explored, and the main forms and spatio-temporal differences of nitrogen and phosphorus in water bodies were studied. Results show that the water quality of Ximei River was determined to be Class IV by the single-factor index. Chemical oxygen demand and permanganate index were the main pollutants exceeding the standard, with the over-standard rates being 57.6% and 41.7% respectively. Both the comprehensive pollution index and the comprehensive water quality identification index determine that the water quality grade is Class III. The results of these three water quality evaluation methods have the same spatio-temporal differences, and the water quality in 2023 is worse than that in 2024. The principal component analysis transformed nine water quality indicators into three principal components, with a cumulative variance contribution rate of 82.42%. The oxygen-consuming organic matter and nutrients significantly correlated with the first principal component dominated the water quality changes and showed a strong positive correlation with each other. Meanwhile, the comprehensive score of the principal components presented spatio-temporal characteristics consistent with the water quality evaluation results. The concentrations of chemical oxygen demand, ammonia nitrogen, total phosphorus and permanganate index show similar succession patterns along the course, but their spatio-temporal variations are slightly different. The spatio-temporal differences of nitrogen and phosphorus forms in water bodies are significant. Dissolved phosphorus and dissolved inorganic nitrogen are the main forms of nitrogen and phosphorus in water bodies, and their spatio-temporal variation patterns are consistent. The forms of nitrogen and phosphorus have a strong correlation with water temperature, turbidity and chlorophyll a. Different hydrological conditions, uneven distribution of point sources, differences in atmospheric precipitation and characteristics of agricultural activities are the main factors affecting the spatio-temporal differences in water quality and nitrogen and phosphorus forms of the Ximei River.