Preprints
https://doi.org/10.5194/hess-2021-375
https://doi.org/10.5194/hess-2021-375

  14 Sep 2021

14 Sep 2021

Review status: this preprint is currently under review for the journal HESS.

Effect of topographic slope on the export of nitrate in humid catchments

Jie Yang1, Ingo Heidbüchel2,4, Chunhui Lu1, Yueqing Xie3, Andreas Musolff2, and Jan H. Fleckenstein2,4 Jie Yang et al.
  • 1State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Hohai University, Nanjing, Chin
  • 2UFZ - Helmholtz-Centre for Environmental Research GmbH, Department of Hydrogeology, Leipzig, Germany
  • 3School of Earth Sciences and Engineering, University of Nanjing, Nanjing, China
  • 4Hydrologic Modeling Unit, Bayreuth Center of Ecology and Environmental Research (BayCEER), University of Bayreuth, Bayreuth, Germany

Abstract. Excess export of nitrate to streams affects ecosystem structure and functions and has been an environmental issue attracting world-wide attention. The dynamics of catchment-scale solute export from diffuse nitrate sources can be explained by the activation and deactivation of dominant flow paths, as solute attenuation (including the degradation of nitrate) is linked to the age composition of outflow. Previous data driven studies suggested that catchment topographic slope has strong impacts on the age composition of streamflow and consequently on in-stream solute concentrations. However, the impacts have not been systematically assessed in terms of solute concentration levels and variation, particularly in humid catchments with strong seasonality in meteorological forcing. To fill this gap, we modeled the groundwater flow and nitrate transport for a cross-section of a small agricultural catchment in Central Germany. We used the fully coupled surface and subsurface numerical simulator HydroGeoSphere to model groundwater and overland flow as well as nitrate concentrations. We computed the water ages using numerical tracer experiments. To represent various topographic slopes, we additionally simulated ten synthetic cross-sections generated by modifying the mean slope from the real-world scenario while preserving the land surface micro-topography. Results suggest a three-class response of in-stream nitrate concentrations to topographic slope, from class 1 (slope > 1:60), via class 2 (1:100 < slope < 1:60), to class 3 (slope < 1:100). Flatter landscapes tend to produce higher in-stream nitrate concentrations within class 1 or class 3, however, not within class 2. Young streamflow fractions and nitrate concentrations decrease sharply when flatter landscapes are not able to maintain fast preferential discharge paths (e.g. seepage). The variation of in-stream concentrations, controlled by degradation variability rather than by nitrate source variability, shows a similar three-class response. Our results improve the understanding of nitrate export in response to topographic slope in temperate humid climates, with important implications for the management of stream water quality.

Jie Yang et al.

Status: open (until 09 Nov 2021)

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Jie Yang et al.

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Short summary
We assessed the effect of catchment topographic slopes on the nitrate export dynamics in terms of the concentration level and its seasonal variability using a coupled surface-subsurface model. We identified a threshold-like three-class relation between young streamflow fraction and slope, with distinct mechanical explanations for each class. We emphasized that it can be misleading to assume a straightforward monotonous relationship between any catchment characteristics and young water fraction.