Modelling diffuse nitrogen loadings of ungauged and unmonitored lakes in Denmark: application of an integrated modelling framework

According to the EU Water Framework Directive all freshwater bodies must obtain good ecological status by 2015. In Denmark this means that all lakes with a surface area above 5 ha (∼600 lakes) must be evaluated individually and mitigation measures must be enforced if the ecological status is below "good". In consequence, the nutrient pressures from point and diffuse sources must be assessed based on a quantification of the nutrient loading of each lake. In this study we focus on the loading of nitrogen. Few of the 600 lakes are monitored for runoff and nitrogen loading. Therefore, a national 3D hydrological model covering all major parts of the country was used to estimate runoff from the lake catchments. The diffuse nitrogen input to each lake was simulated with an empirical nitrogen model. Where lakes are located upstream/downstream of each other, a calculation chain involving the nitrogen retention in lakes was created. A case study was selected using a large river basin including 23 lakes in Denmark. The cost-effectiveness of reducing nitrogen loadings from different sub-catchments in the case study river basin to the sea was calculated. The cost-effectiveness associated with reducing the N-loading from the most expensive sub-catchment is shown to be 20 times more expensive than from the cheapest sub-catchment.