Tracing the sources and transport of the total phosphorus in the upper Yangtze River

Deterioration of water quality is a significant challenge in basin management, posing a serious threat to global water security. In this study, a water quality model named Spatially Referenced Regressions on Watershed Attributes (SPARROW) was applied to quantify the total phosphorus (TP) loads from different sources and investigate the transport processes for the entire upper Yangtze River Basin. The results showed that the phosphate fertilizer use was the main source of TP loads (67.7%), followed by livestock and poultry breeding source (17.6%), point source (10.5%), and rural life source (4.2%), and the phosphorus fertilizer use source dominated the overall spatial distribution pattern of the TP loads. The transport of the TP loads was further traced, and the results showed that 30% of TP loads were retained in the upper reaches of Yangtze River. A total of 7.4% of the river reaches exceeded the water quality management standards, and they could be restored by reducing 60% of phosphate fertilizer use and point sources in their sub-basins according to scenario simulations. Results revealed that controlling TP sources was effective to improve water quality. This study provides a reference for accurately controlling nutrients in other river basins.