Parametrizing bending of the concentration discharge curve potentially explains dominant in-stream nitrate removal controls
3.12 in Grundwasser-Oberflächenwasser-Interaktionen - vom Flussabschnitt zum Einzugsgebiet
Nutrient excess in rivers leads to ecosystem harm and can induce detrimental algae growths in coastal areas. In Germany and Europe, the management of riverine systems is complicated by the lack of understanding of nutrient pathways from application to export. In this work, we hypothesize that strong uptake effects are likely to be seen in catchments with low average concentrations while high nitrate concentration and loads may mask the uptake effects. To explore the concentration discharge (C-Q) behavior in a realistic setting, the 6 typical catchments were modelled. For this, a parsimonious catchment wide network model is used (1x1km²). Here, the land-to-stream transfer is assumed to be a powerlaw (C=a*Q^b), resulting in different nutrient loading according to the contributing area of each gridcell in the network. In-stream load uptake follows Li = Lin*exp(-vf*w*L/Q), with vf the uptake rate, w and L the width and length of the riversection and Q the discharge. Based on the modeling exercise we can decide on a suitable metric to characterize and potentially quantify instream uptake from observed C-Q relationships across Germany. This metric needs to be explored further by connections to catchment characteristics (such as topography, land-use, potential for stream shading, potential for fine sediments) and biogeochemical conditions (nutrient stoichiometry). This will allow us to argue if the observed C-Q bending can be indeed related to instream uptake and not to other processes (e.g. denitrification along the subsurface flow path) and to derive the dominant processes shaping the uptake (light availability, just instream travel time, nutrient stoichiometry, impact of fine sediments...).