Effect of topographic slope on the export of nitrate in humid catchments: a 3D model study

[摘要] Excess export of nitrate to streams affects ecosystemstructure and functions and has been an environmental issue attractingworldwide attention. The dynamics of catchment-scale solute export fromdiffuse nitrogen sources can be explained by the changes of dominant flowpaths, as solute attenuation (including the degradation of nitrate) islinked to the age composition of outflow. Previous data-driven studiessuggested that catchment topographic slope has strong impacts on the agecomposition of streamflow and consequently on in-stream soluteconcentrations. However, the impacts have not been systematically assessedin terms of solute mass fluxes and solute concentration levels, particularlyin humid catchments with strong seasonality in meteorological forcing. Tofill this gap, we modeled the groundwater flow and nitrate transport for asmall agricultural catchment in Central Germany. We used the fully coupledsurface and subsurface numerical simulator HydroGeoSphere (HGS) to modelgroundwater and overland flow and nitrate transport. We computed thewater ages using numerical tracer experiments. To represent varioustopographic slopes, we additionally simulated 10 synthetic catchmentsgenerated by modifying the topographic slope from the real-world scenario.Results suggest a negative correlation between the young streamflow fractionand the topographic slope. This correlation is more pronounced in flatlandscapes with slopes 1 : 60 . Flatter landscapes tend to retainmore N mass in the soil (including mass degraded in soil) and export less Nmass to the stream, due to reduced leaching and increased degradation. Themean in-stream nitrate concentration shows a decreasing trend in response toa decreasing topographic slope, suggesting that a large young streamflowfraction is not sufficient for high in-stream concentrations. Our resultsimprove the understanding of nitrate export in response to topographic slopein a temperate humid climate, with important implications for the managementof stream water quality.

[学科分类] 妇产科学