Exports of organic matter, phosphorus and nitrogen from Sichuan Basin: A critical region regulating water quality of the Upper Yangtze River, China

Abstract

The Chinese Yangtze River is a crucial nexus for nutrient cycling between the Qinghai-Tibet Plateau and the East China Sea, but it faces significant water quality challenges due to enhanced nutrient inputs from its sub-basins. The nutrient exports from the Sichuan Basin, a geographical region with intense human activities in southwest China, are foreseen to have significant impacts on the water quality of the Upper Yangtze River. To investigate the nutrient exports from the Sichuan Basin and their effects, we analysed daily data on Total Nitrogen (TN), Total Phosphorus (TP), and Chemical Oxygen Demand (COD, proxy representing organic matter) collected from 58 monitoring stations on rivers across the Sichuan Basin during 2021–2023. The results indicated that the Sichuan Basin contributed approximately 50% of the increased TN, TP, and COD in the Upper Yangtze River. The Minjiang and Qujiang Rivers rank highest in the Sichuan Basin for TN, TP exports and COD exports, respectively. Hotspots of TN and TP levels were primarily concentrated in the the western basin, while COD hotspots were mainly located in the eastern basin. The spatial analysis identified urbanization and agricultural activities as the primary drivers of nutrient distribution patterns in the Sichuan Basin. These findings underscore the need for targeted policies and strategies to enhance the controlling TN and TP losses from urban and farming areas in the rainy seasons within the catchments of the Minjiang and Tuojiang Rivers is critical for achieving sustainable water quality improvements in the Upper Yangtze River. Moving forward, the implementation of integrated pollution management strategies, supported by real-time monitoring and machine learning-based predictive modeling, is imperative in the Sichuan Basin to address the challenges of water quality deterioration driven by climate change.