Comparative study of two models to simulate diffuse nitrogen and phosphorus pollution in a medium-sized watershed, southeast China

The aim of this study was to compare and assess two models to calculate diffuse nitrogen and phosphorus emissions in a selected watershed. The GIS-based empirical model and the physically-based AnnAGNPS model were evaluated for comparative purposes. The methodologies were applied for the Jiulong River watershed, covering 14,700 km2, located in southeast China, with intensive agricultural activities. The calculated loadings by AnnAGNPS model was checked by the measured values at the watershed outlet, whereas the calculated nitrogen and phosphorus emission by GIS-based empirical model spatially provided the potential values in terms of sub-watersheds, districts/counties, and land use type. Both models gave similar levels of diffuse total nitrogen emissions, which also fit well with previous estimates made in the Jiulong River watershed. Comparatively, the GIS-based empirical model gave sound results of source apportionment of non-point source pollution (NPS) from the available input data and critical source areas identification of diffuse nitrogen and phosphorus pollution. The AnnAGNPS model predicted reasonable nitrogen loading at the watershed outlet and simulated well for NPS management alternatives under changing land use conditions. The study indicated that the GIS-based empirical model has its advantage in extensive studies as a decisions support tool for preliminary design since it is easily applied to large watersheds with fewer data requirements, while AnnAGNPS has its advantage in detailed emission assessment and scenario development.