Nitrogen transfers through the Seine drainage network: a budget based on the application of the 'Riverstrahler' model

A new version of the 'Riverstrahler' model has been developed for modelling riverine transfers of nitrogen from diffuse sources within the watershed, using land use data as a forcing function, together with an adjustable ripa rian transfer coefficient. Application of the model to the Seine river syst em (divided into four main sub-basins and one main branch) allows to establ ish a comprehensive budget of nitrogen transfers through the whole drainage network and to quantify the overall retention by riparian wetlands. Accord ing to the sub-basins, the figures indicate that 25-55% of the nitrogen com ing from below the root-zone of agricultural land or from the aquifers is r etained or eliminated before reaching surface water. The lower retention is found in areas where dense agricultural draining has been installed, thus by-passing riparian wetlands. At the scale of the Seine catchment, riparian retention represents 70-110 10(3) tonN y(-1), while in-stream retention ac counts for only 24-32 10(3) tonN y(-1). The data suggest that landscape man agement, like restoration of efficient riparian systems in areas where they have been suppressed or by-passed, might be an efficient alternative or co mplimentary strategy for reducing nitrate contamination of surface water, w ith respect to tertiary treatment of point wastewater discharges.