H150-03
What Is the Fate of Excess Nitrogen in Germany’s Largest National River Basin?

Monday, 14 December 2020: 08:36
Virtual
Fanny Sarrazin1,2, Rohini Kumar3, Kimberly J Van Meter4, Michael Weber2, Andreas Musolff5, Nandita B Basu6 and Sabine Attinger3,7, (1)University of Bristol, Civil Engineering, Bristol, United Kingdom, (2)Helmholtz Centre for Environmental Research - UFZ, Computational Hydrosystems, Leipzig, Germany, (3)Helmholtz Centre for Environmental Research GmbH – UFZ, Leipzig, Germany, Computational Hydrosystems, Leipzig, Germany, (4)University of Illinois at Chicago, Earth and Environmental Sciences, Chicago, IL, United States, (5)Helmholtz Centre for Environmental Research - UFZ, Hydrogeology, Leipzig, Germany, (6)University of Waterloo, Civil and Environmental Engineering, Waterloo, ON, Canada, (7)University of Potsdam, Potsdam, Germany, Institute of Environmental Science and Geography, Potsdam, Germany
Abstract:
Reducing nitrogen (N) levels in European rivers and groundwater bodies is a pressing issue, as evidenced by the recent fines imposed by the European Count Justice on countries such as France, Germany and Greece for exceeding the regulatory limits for nitrate (World Bank report on “Quality Unknown: The Invisible Water Crisis” by Damania et al., 2019). N levels can depend not only on current N inputs to the landscape, but also on the past N inputs that have accumulated through time in the soil root zone and the groundwater in so-called ‘legacy stores’.

This study aims to gain a better understanding of the fate of the soil N-surplus, i.e. whether N is stored in the terrestrial system, denitrified or exported to the stream, and of the impact of N legacies on in-stream nitrate concentration and loading in European river basins. To this end, we apply a parsimonious annual nitrate model, called ELEMENT (Van Meter et al., 2017, Global Biogeochem Cycles), given that available information to constrain the model and test the simulations is limited and has large uncertainties. We examine the model sensitivities and we estimate the model parameters based on the application of ‘soft rules’, to account for the uncertainty in the model inputs and the output measurements. These soft rules are formulated based on the comparison between observed and simulated in-stream nitrate concentration and loading, soil N content and groundwater nitrate concentration.

We present here the results for the case of the Weser river basin which is Germany’s largest national river basin and which discharges into the North Sea. Our results show that the model reproduces well nitrate stream loading, although parameter equifinality arises due to parameter interactions or insensitivity despite the parsimonious structure of the ELEMENT model. We discuss the implications of equifinality for the characterization of the fate of excess N and we determine which additional datasets (e.g. stable water isotopes) could potentially improve parameter identifiability (e.g. travel time) and reduce the simulation uncertainties.

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