title: Potential benefits of a spatially targeted regulation based on detailed N-reduction maps to decrease N-load from agriculture in a small groundwater dominated catchment.
abstract: Denmark must further decrease the N-load to coastal waters from agricultural areas to comply with the Baltic Sea Action Plan and the EU Water Framework Directive. A new spatially targeted regulation is under development that focuses on locating N-mitigation measures in areas with low natural reduction of nitrate (N-reduction). A key tool in this respect is N-reduction maps showing how much N is removed by natural reduction processes, i.e. the ratio between the N-load out of the catchment and the N-leaching from the root zone for each spatial unit within the catchment. For the 85 km2groundwater dominated Norsminde catchment in Denmark we have analysed the potential benefits of a spatially targeted regulation and how its efficiency is affected by uncertainty in the N-reduction map. Our results suggest that there are potential benefits of implementing a spatially targeted regulation compared to a spatially uniform regulation. The total N-load at the catchment outlet can be decreased up to 8% by relocating the existing agricultural practice according to the N-reduction map and thus without decrease fertilization inputs. A further decrease in N-load can be obtained by identifying target areas with low N-reduction where N-mitigation measures must be applied. Uncertainty on the N-reduction map is found to lower the efficiency of spatially targeted regulation. This uncertainty can be lowered substantially by using the mean of an ensemble of N-reduction maps. The uncertainty decreases with coarser spatial resolution of the N-reduction map, but this will at the same time decrease the benefit from spatially targeted regulation.
authors.affiliation: Department of Hydrology, Geological Survey of Denmark and Greenland, Øster Voldgade 10, 1350 Copenhagen K, Denmark. Electronic address: alha@geus.dk.
authors.forename: A L
authors.initials: AL
authors.lastname: Hansen
authors.affiliation: Department of Hydrology, Geological Survey of Denmark and Greenland, Øster Voldgade 10, 1350 Copenhagen K, Denmark.
authors.forename: J C
authors.initials: JC
authors.lastname: Refsgaard
authors.affiliation: Department of Agroecology, Aarhus University, Blichers Allé 20, 8830 Tjele, Denmark.
authors.forename: J E
authors.initials: JE
authors.lastname: Olesen
authors.affiliation: Department of Agroecology, Aarhus University, Blichers Allé 20, 8830 Tjele, Denmark.
authors.forename: C D
authors.initials: CD
authors.lastname: Børgesen
keywords: Groundwater; Modelling; Nitrate reduction; Spatially targeted regulation; Uncertainty