Spatiotemporal evolution characteristics, driving factors, and risk assessment of water quality in long-distance inter-basin water transfer projects during 2015-2023.

The South-to-North Water Diversion Middle Route (SNWDP-MR), as a large-scale inter-basin water diversion project, has played a significant role in social development and ecological environment protection. However, the spatiotemporal evolution characteristics, seasonal driving factors, and potential risks are still unclear. This study established a methodological framework based on the composite water quality index (WQI-DET), Spearman analysis, Boruta algorithm, positive matrix factorization (PMF), and entropy-analytic hierarchy process (AHP) to conduct system analysis from 2015 to 2023. Parameters including DO, COD_Mn, NH₃-N, SO₄^2-, F.coli, and COD demonstrated an increasing trend from south to north, driven by seasonal dynamics. Except DO, all peaked in summer and dipped in winter. TP remained at low trophic levels. The WQI-DET consistently exceeded 75, but exhibited significant seasonal fluctuations, particularly post-Yellow River Crossing (YRC), decreasing by 5-8 %. Pre-YRC were predominantly governed by TP, NH₃-N, and COD, strongly correlated with temperature, flow velocity, and precipitation. Post-YRC, drivers such as COD_Mn and SO₄^2- intensified, amplifying climate-induced stressors. PMF identified potential sources: Pre-YRC were dominated by endogenous contributions (51.4-64.4 %), with summer algal metabolites. Post-YRC natural input sources surged, up to 40.2 % in autumn, linked to migratory bird excretions and F.coli transmission. Entire canal was "low-risk" (P < 0.4), but sensitive sections (HZ, BPS) in summer and autumn require vigilance against algal blooms. The developed methodological framework demonstrates strong applicability for risk assessment in cement-lined cross-basin water transfer systems, revealing critical water quality patterns in the SNWDP-MR over long-term sequences, providing a robust scientific foundation for future risk prevention strategies.