New Source-Transfer-Sink Analytical Framework for Dynamic Tracking of Nitrogen and Phosphorus Flows and Changes in Watersheds

  Unhealthy nitrogen (N) and phosphorus (P) flows are key drivers of watershed eutrophication and ecosystem degradation, posing significant risks to water quality and environmental sustainability. Existing analytical methods fail to capture the complex interactions of nutrient flows between sources, pathways, and sinks, particularly the interplay between environmental and socio-economic factors. To address these challenges, we developed a novel Source-Transfer-Sink (STS) analytical framework that integrates Substance Flow Analysis (SFA), the SWAT model (Soil and Water Assessment Tool), and high-resolution geospatial technologies. This framework enables comprehensive and dynamic tracking of N and P sources, transfer pathways, and sinks within watersheds, providing critical support for enhanced nutrient management and policy formulation. The STS framework was applied to the Erhai Lake watershed in China, an ecologically sensitive region increasingly threatened by agricultural intensification, urbanization, and eutrophication. The results reveal that over the past 23 years, nutrient dynamics in the watershed have undergone a significant transformation, shifting from a high-input, low-cycling, high-emission model (2000-2013) to a low-input, high-cycling, low-emission model (2014-2022). In 2000-2013, excessive use of chemical fertilizers and improper management of livestock manure resulted in severe environmental losses. By 2022, the implementation of green development policies and strengthened environmental protection efforts increased nitrogen recycling by 64.47% and phosphorus recycling by 63.89%, while overall losses decreased sharply, with nitrogen and phosphorus emissions reduced by 68% and 77%, respectively. Spatial analysis identified nutrient hotspots, primarily concentrated in rural and farmland areas in the northern and western regions (especially Niujie Township and Yinqiao Town), where these areas experience the highest nutrient loss through runoff. By 2022, the nutrient loss intensity in these hotspot areas had significantly decreased and become more homogeneous. Urban areas benefited from advanced wastewater treatment technologies, which reduced nitrogen and phosphorus discharges into surface waters. This study provides a broader and more applicable Source-Transfer-Sink (STS) analytical framework for characterizing nitrogen and phosphorus cycling across watershed systems. It offers methodological support and policy insights for large lakes in rapidly developing regions or countries, enabling a clear presentation of nutrient flow structures and the sustainable management of nitrogen and phosphorus resources.