A new nitrogen retention map for Denmark

The nitrogen load to estuaries have been approximately halved in Denmark since the mid 1980ties, but assessments of the ecological status of marine waterbodies indicates that further reductions in nitrogen loadings are required in some waterbodies to obtain a good ecological status according to the Water Framework Directive. Past and current regulations have primarily relied on a general approach, applying same restrictions for all areas independent on drainage schemes, hydrogeochemical conditions and retention in surface waters. Such a general approach is not cost-effective, as nitrogen retention (primarily as denitrification) varies significantly depending on the physical and biogeochemical conditions. If areas with high and low retention can be identified, regulation can be targeted allowing less strict regulation in some areas and focus stronger regulation and mitigation measures in areas, where nitrate leaching is high and nitrogen retention is low. As a first step in exploring how a differentiated approach can be integrated in national regulation, a national nitrogen model has been developed for Denmark. The model is constructed by linking existing models describing nitrate leaching from the root zone, groundwater transport and reduction as well as surface water retention models. The models are coupled at sub-catchment scale dividing the country into topographic catchments with a mean size of 15 km, which constitutes the computational units in the national model. Model development, calibration and validation have been performed on measurements of nitrogen transport from 340 streams gauging stations covering approximately half of the total area in Denmark. The national model is designed to compute the total nitrogen loads to coastal areas and the transport of total nitrogen within sub-catchments, but can similarly be used to compute national maps displaying the estimated nitrogen retention in groundwater, surface water and the total retention from the field to the sea. Figure 1 Map displaying the total nitrogen retention from the root zone to the marine environment * [email protected], [email protected]: Geological Survey of Denmark and Greenland, Øster Voldgade 10, 1350 Copenhagen K, Denmark ** Dept. of Bioscience, Vejlsøvej 25, P.O. Box 314, 8600 Silkeborg, Denmark *** Dept. of Agroecology, Blichers Allé 20, 8830 Tjele, Denmark