In the search for tools for evaluating the effects of national action plans
combating diffuse nutrient pollution causing eutrophication of surface wat
ers, a study of the nitrogen (N) flow in 17 Danish agricultural catchments
was carried out. Data on N input and IN harvest for the agricultural year o
f 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 us
ed for separating the hydrograph time-series into three runoff components:
baseflow, interflow and overland flow. Large regional variations in net N i
nput were found ranging from 62 kg N ha(-1) yr(-1) in the loamy eastern par
t of the country dominated by cereal production to 137 kg N ha(-1) yr(-1) i
n the western part characterised by less fertile sandy soils and dominated
by animal husbandry. N leaching from the root zone showed a corresponding v
ariation 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 re
garding net N export, and no relationship could be established between net
IN export and root zone N leaching. This finding was ascribed to a varying
and in some catchments very high (> 80%) N retention during subsurface tran
sport to the stream. The hydrological modelling revealed that loamy catchme
nts had a high proportion of quick flow (overland flow + interflow), wherea
s baseflow dominated the sandy catchments. Further, a highly significant re
lationship between N retention and proportion of quick flow was found empha
sising the importance of understanding the hydrological pathways. This shou
ld be taken into consideration when evaluating the N loading of surface wat
ers resulting from a given agricultural practice and the effects of possibl
e changes in this practice.