Modelling seasonal N and P loads in three contrasting large river basins using global datasets - Mississippi, Mekong and Rhine River

Nutrients are important components of the global biochemical cycle, and are key controls of the quality of inland and coastal waters. Quantification of the nutrient fluxes from large river basins to the oceans still relies on long-term yearly-load estimates; existing models are essentially empirical budget models that relate total annual basin output to estimates of basin-wide nutrient emissions. In this type of models identification of the source areas, as well as quantification of the riverine loads in the subbasins, is often not feasible. Moreover, due to their empirical nature, existing models are inadequate in predicting future nutrient loads under changed conditions of climate and nutrient emissions. We therefore developed a mechanistic, spatially explicit River NUtrient Flux model, RiNUX, that simulates seasonal N and P loads for large river basins, using globally available datasets. A first version of the model successfully quantified seasonal fluxes of N and P for the Rhine River basin. In this study we demonstrate the application to the Mississippi and Mekong River basins, hence making a step towards global application. Using the model we identified source areas and pathways and we quantified loss rates within the three river basins. Primary results demonstrate that groundwater is the major pathway for N and surface flow for P, and diffuse sources are dominant in all three river basins. When comparing the three river basins, point sources play the most important role in the Rhine River basin making up to 30% of the total riverine nutrient load, which is at least three times more than in the two other basins. The contribution of nutrient supply related to erosion is significantly higher for the Mekong River. The highest loss rates in the river network for N occur in the Mekong, whereas for P the Mississippi shows the highest in-stream loss rate