Assessing nitrogen pressures on European surface water

The European environmental legislation on water, in particular the 2000 Water Framework Directive, requires the evaluation of nutrient pressures and the assessment of mitigation measures at the river basin scale. Models have been identified as tools that can contribute to fulfill these requirements. The objective of this research was the implementation of a modeling approach (Geospatial Regression Equation for European Nutrient losses (GREEN)) to assess the actual nitrogen pressures on surface water quality at medium and large basin scale (European scale) using readily available data. In particular the aim was to estimate diffuse nitrogen emissions into surface waters, contributions by different sources (point and diffuse) to the nitrate load in rivers, and nitrogen retention in river systems. A comprehensive database including nutrient sources and physical watershed characteristics was built at the European scale. The modeling partially or entirely covered some of the larger and more populated European river basins, including the Danube, Rhine, Elbe, Weser, and Ems in Germany, the Seine and Rhone in France, and the Meuse basin shared by France and Belgium. The model calibration was satisfactory for all basins. The source contribution to the in-stream nitrogen load, together with the diffuse nitrogen emissions and river nitrogen retention were estimated and were found to be in the range of values reported in the literature. Finally, the model results were extrapolated to estimate the diffuse nitrogen emission and source apportionment at the European scale.