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 c oming from below the root-zone of agricultural land or from the aquifers is retained or eliminated before reaching surface water. The lower retention is found in areas where dense agricultural draining has been installed, thu s by-passing riparian wetlands. At the scale of the Seine catchment, ripari an retention represents 70 - 110 10(3) tonN y(-1), while in-stream retentio n accounts for only 24-32 10(3) tonN y(-1). The data suggest that landscape management, like restoration of efficient riparian systems in areas where they have been suppressed or by-passed, might be an efficient alternative o r complimentary strategy for reducing nitrate contamination of surface wate r, with respect to tertiary treatment of point wastewater discharges.