The influence of river morphology on nitrogen retention at river network scale in the agricultural Bode River, Germany

European Water Framework Directive (WFD) reported that river morphological alteration and diffuse pollution are two dominant pressures of European water bodies at the catchment scale. To achieve good status targeted by WFD, river restoration has received increasing attention. However, less is known about the spatial and temporal effects of stream morphologic characteristics (i.e., meandering, stream order) on instream nitrate retention at the river network scale. The objective of this study is to explore the relationship between in-stream nitrate retention and stream geomorphologic characteristics (sinuosity, width and order) and to assess the effect of natural river conditions on in-stream nitrate retention. Therefore, we implemented a grid-based nitrate catchment model (mHM-Nitrate, Yang et al. 2018) in the Bode catchment (3200 km²) in central Germany, which offers comprehensive long-term and high-frequency data at several water quality gauge stations for model calibration and validation. We evaluated two alternative empirical approaches to quantify in-stream denitrification (based on denitrification velocity and denitrification rate constant, respectively) and conducted scenario analysis on more natural morphological stream conditions by increasing the river sinuosity according to its relationship with stream power.  Results showed that the model well captured the dynamics of daily discharge and nitrate concentration, with Nash-Sutcliffe Efficiency ≥ 0.87 for discharge and Kling-Gupta Efficiency ≥ 0.59 for nitrate concentration from 2015-2018. In-stream retention (including assimilatory uptake and denitrification) by the whole river network accounted for 3.5% and 35.9% of total nitrate loadings in winter and summer, respectively. The summer in-stream denitrification rate was two times higher in the lowland arable area than in the mountain forest area (225.1 and 68.8 mg N m^-2 d^-1, respectively). Similarly, summer in-stream assimilatory uptake was five times higher in the lowland arable area than the mountain forest area (167.9 and 27.2 mg N m^-2 d^-1, respectively). The model scenario representing more natural river network conditions by restoring the river sinuosity can lead to an additional nitrate loading reduction of 20% in 6th order stream network in summer. Our results show that the renaturation of streams can increase nitrate retention in flowing water, with efficiency increasing significantly with decreasing runoff. However, a significant reduction in the nitrate concentration remains limited to the growing season, especially in summer.

Yang, X., Jomaa, S., Zink, M., Fleckenstein, J. H., Borchardt, D., Rode, M. (2018): A New Fully Distributed Model of Nitrate Transport and Removal at Catchment Scale. Water Resources Research, 54 (8) 5856.