Analysis of the Hydrology and Flow of Nitrogen in 17 Danish Catchments

In the search for tools for evaluating the effects of national action plans combating diffuse nutrient pollution causing eutrophication of surface waters, a study of the nitrogen (N) flow in 17 Danish agricultural catchments was carried out. Data on N input and N harvest for the agricultural year of 1993/1994 was obtained from questionnaire surveys facilitating the set up of N balances. Net export of N from the catchments measured at the outlet was obtained from time-series of stream water chemistry and discharge from 1993-1997. N leaching from the root zone of each field was calculated using an empirical model. A physically based lumped rainfall-runoff model was used for separating the hydrograph time-series into three runoff components: baseflow, interflow and overland flow. Large regional variations in net N input were found ranging from 62 kg N ha^-1 yr^-1 in the loamy eastern part of the country dominated by cereal production to 137 kg N ha^-1 yr^-1 in the western part characterised by less fertile sandy soils and dominated by animal husbandry. N leaching from the root zone showed a corresponding variation with regional averages ranging from 34.5 kg N ha^-1 yr^-1 to 90.9 kg N ha^-1 yr^-1. No similar regional pattern could however be found regarding net N export, and no relationship could be established between net N export and root zone N leaching. This finding was ascribed to a varying and in some catchments very high (>80%) N retention during subsurface transport to the stream. The hydrological modelling revealed that loamy catchments had a high proportion of quick flow (overland flow + interflow), whereas baseflow dominated the sandy catchments. Further, a highly significant relationship between N retention and proportion of quick flow was found emphasising the importance of understanding the hydrological pathways. This should be taken into consideration when evaluating the N loading of surface waters resulting from a given agricultural practice and the effects of possible changes in this practice.