Aerial photograph-based delineation of artificially drained areas as a basis for water balance and phosphorus modelling in large river basins

Intensive agriculture in lowland river catchments is unthinkable without artificial drainage, but these drainage installations represent also an important pathway for diffuse nutrient inputs to surface waters.Efficient tackling of eutrophication requires distributed nutrient modelling with special emphasis on phosphorus inputs, because of their limitative role in freshwater ecosystems. General decision-making aims at the regional scale, but usually only little information exists about the location of drainage installations within meso- and macro-scale river basins. This lead to the development of an approach, how artificially drained lands can be identified by interpreting aerial photographs, and how these findings can be extrapolated by combining land use and soil information in GIS. The approach results in a map of artificially drained areas, which is presented here for the River Ems basin (12,940 km2), located in north-western Germany. This map provides the basis for distributed modelling of drainage runoff and diffuse phosphorus inputs. From the model results, it can be found, that about 69% of all P-inputs into surface waters are discharged via artificial drainages.Just under 30% of the diffuse P-inputs arise from less than 4% of the catchment area. These subareas consist of drained raised bogs used as grassland. Due to the favourable ratio of input level to area they are now at the focus of mitigation measures. With respect to the implementation of the European Water Framework Directive, the importance of model-supported analyses of nutrient inputs is increasing.