GIS-BASED MODELING OF SOLUTE FLUXES AT THE CATCHMENT SCALE - A CASE-STUDY OF THE AGRICULTURAL CONTRIBUTION TO THE RIVERINE NITROGEN LOADINGIN THE VEJLE FJORD CATCHMENT, DENMARK

A model that predicts nitrogen loading is developed and applied on an agricultural catchment draining to a Danish Fjord. Increases in nitrog en loadings of the Danish coastal waters have been recorded and the tr ansformation in the Danish agricultural sector is established to be th e main cause. The development in Danish agriculture in the last 40 yea rs has resulted in significant changes in the agricultural production structure which are of major significance to the water quality. Theref ore, there is a need for methods which can relate the consequences of changes in the agricultural production structure to the nitrogen loadi ngs of coastal waters. In this study riverine nitrogen loads are model led by combining spatially distributed data for: (1) non-point source nitrogen leaching; (2) point source nitrogen emissions; (3) soils; and (4) elevation data of the terrain and of the ground water head, with a nitrate decay process. The decay process is expressed as a first-ord er decay with respect to distance, having two different decay constant s according to different flow velocities. N-leaching was partitioned i nto a component with high velocity (mainly tile drainage) and a compon ent with low velocity (mainly baseflow) based on soil type/flow-path c orrelations. The nitrogen loading model was implemented into GIS to ca lculate nitrogen loads for sub-catchments in the Vejle Fjord catchment , Denmark. The two nitrate decay-coefficients were estimated by a tria l and error method. Discrepancies between model calculated and measure d N-loads vary from -16 to 17%. A source apportionment of the agricult ural contribution to the riverine nitrogen loading revealed that lives tock production is responsible for about 60% of the total non-point so urce loading while part lime farming contributes with approximately 23 % of the total non-point source load. The remainder of the N-loadings originates from plant production and background sources. (C) 1998 Else vier Science B.V. All rights reserved.