Comment on hess-2021-375

doi:10.5194/hess-2021-375-rc2

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.

Highlights

IntroductionNearly 40% of land is used for agricultural activities [Foley et al, 2005], which constitutes the major source of pollution with nutrients such as nitrate (referred as to N-NO3 in this study)
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 is linked to the age composition of outflow
Field observations in central German catchments indicate that in-stream nitrate concentrations (CQ) are generally higher at downstream areas with gentle topography compared to more mountainous upstream areas [Dupas et al, 2017]

Summary

Introduction

Nearly 40% of land is used for agricultural activities [Foley et al, 2005], which constitutes the major source of pollution with nutrients such as nitrate (referred as to N-NO3 in this study). In terms of 60 water age analyses, Jasechko et al [2016] using oxygen isotope data from 254 watersheds worldwide showed significant negative correlation between the young (age < 3 months) streamflow fraction and the mean topographic gradient They stated that young streamflow is more prevalent in flatter catchments as these catchments are characterized by shallow lateral flow, while it is less prevalent in steeper mountainous catchments as these catchments promote deep vertical infiltration. This statistically significant trend is consistent with the common finding that fast 65 shallow flow paths produce young discharge and potentially promote high in-stream solute concentrations [Böhlke et al 2007; Benettin et al 2015; Hrachowitz et al 2016; Blaen et al 2017]. The effect of topographic slope 70 on CQ has not been subject to systematical testing

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