Water quality evaluation and characterization of its spatio-temporal variability in the Western Route of South-to-North Water Diversion Project in China

• A comprehensive assessment of water quality in WR- SNWDP were conducted. • Spatio-temporal variability of water quality changes was characterized. • Region-specific key water quality parameters were identified for targeted monitoring. • The driving mechanism of WQI variation and its pathway effects have been clarified. • High nitrogen should be of concern for the planning of WR-SNWDP. As a critical national strategy for water resource allocation currently under planning in China, the implementation of the Western Route of South-to-North Water Diversion Project (WR-SNWDP) prioritize water quality safety as a core consideration. Given the vast areas potentially affected by this project and pronounced spatio-temporal variability in natural and cultural environment, it is imperative to elucidate the evolutionary dynamics of water quality and their driving mechanisms within this area. However, comprehensive water quality assessments for critical regions crosses the two largest river basins of China remain scarce. In this study, we utilized continuous water quality data from 245 national monitoring stations to conduct a multi-faceted assessment of the water quality across the “source area (R1) – downstream of the source area (R2) – transfer area (R3) – intake area (R4)” of the WR-SNWDP. Spatio-temporal evolution of water quality and their driving mechanisms was then evaluated by integrating the multiple statistical analysis methods and Structural Equation Modeling (SEM). Results indicate that the overall water quality in the study area ranged from good to excellent, with a gradual improvement trend during the study period. It exhibited significant annual cyclical variations temporally, primarily driven by precipitation and temperature. Spatially, it was significantly influenced by anthropogenic activities, with moderate water quality predominantly distributed in R4 and excellent water quality concentrated in R2. Total nitrogen (TN) was the primary pollutant in the study area, with concentrations exceeding 2.0 mg/L in all regions except R1. Water quality in each sub-region (R1-R4) was most significantly influenced by TN, Chemical Oxygen Demand by Potassium Permanganate ( C O D M n ), C O D M n , and total phosphorus (TP), respectively. These indicators were identified as key parameters for targeted monitoring. The weighted WQI min model outperform the unweighted version with both models exhibit coefficient of determination exceeding 0.9. These findings contribute to a deep understanding of spatiotemporal water quality pattern and its driving mechanism within the WR-SNWDP, providing evidence-based guidance for optimizing operational plans and adaptive ecological management strategies.