Modelling the flow of nitrogen and phosphorus in Europe: From loads to coastal seas

A model is described that aims at predicting surface water quality from N- and P-inputs on a European scale. The model combines a GIS-based approach to estimate loads, geohydrological data to define model structure and statistical techniques to estimate parameter values. The model starts with an inventory of diffuse sources of N and P: agriculture and (for N) atmospheric deposition. Nitrogen flows are assumed to follow both surface- and groundwater flows, while for phosphorus only surface water flow is taken into account. In addition to these non-point sources, sewage is an important source of nutrients, in particular for phosphorus. Using physically realistic parameter values, good agreement is found between observed and calculated riverine loads. Results indicate problem areas in two regions: western Europe (Thames, Scheldt, Elbe, Rhine) with its intensive agriculture and high population density and Southern Europe (some rivers in Spain, Italy), where loads may be locally high and dilution is low due to a low water discharge. For the coastal seas of Europe concentrations were calculated by assuming conservative behaviour of N and P. Existing advection-diffusion models were linked and re-calibrated on salinity data. Results indicate that the main problem areas are the Baltic Sea and the Black Sea, with much lower impacts in the North Sea and the Adriatic Sea ; in other coastal waters human impacts are essentially negligible. However, due to seasonal and spatial variability disregarded in this European-scale study, it should be realized that in specific sub-areas, i.c. coastal zone, periods with significant problems still may occur