HGS RESEARCH HIGHLIGHT – Effect of topographic slope on the export of nitrate in humid catchments: a 3D model study
Yang, J., Wang, Q., Heidbüchel, I., Lu, C., Xie, Y., Musolff, A., & Fleckenstein, J. H. (2022). Effect of topographic slope on the export of nitrate in humid catchments: a 3D model study. In Hydrology and Earth System Sciences (Vol. 26, Issue 19, pp. 5051–5068). Copernicus GmbH. https://doi.org/10.5194/hess-26-5051-2022
“Additionally, the widely used fully-coupled surface–subsurface technology [HydroGeoSphere] simulates the catchment as an integrated system, providing details of surface water–groundwater exchanges fluxes. These details help to identify paths of rapid discharge to the land surface that can considerably improve the interpretation of nitrate–export patterns.”
Figure 1: (a) Imagery of the catchment “Schäfertal”, (b) synthetic catchments with slopes of 1:20, 1:40, and 1:1000, and (c) measured precipitation and estimate potential evapotranspiration for the representative year (2005)
Figure 6: The simulated (a) Q and ET, (b) spatially-averaged depth of groundwater table from the land surface, and (c) young water fraction in streamflow and evapotranspiration in relation to the topographic slope for the simulated catchments. (d) temporal variations of young water fraction for a steep (1:20) and flat (1:1000) landscape.
Agricultural nutrient runoff refers to the movement of excess nutrients, such as nitrogen and phosphorus, from agricultural lands into water bodies such as rivers, lakes, and oceans. While nutrients are essential for plant growth, excessive runoff can have significant impacts on both human and ecosystem health. When nutrients from agricultural runoff enter water bodies, they can lead to the growth of harmful algal blooms which can produce toxins that contaminate drinking water sources. Furthermore, contact these toxins can pose serious health risks to humans. Nutrient runoff can also disrupt aquatic ecosystems in several ways, potentially leading to eutropheutrophic conditions which disrupt the balance of ecosystems and degrade biodiversity. Addressing agricultural nutrient runoff is crucial for maintaining a healthy environment and protecting human well-being.
This recent study identifies a knowledge gap about the relationship between nitrate runoff and the topographic slope of agricultural catchments. Catchment-scale solute export can be explained by changes in the dominant flow pathways. Solute attenuation is linked to the age composition of outflow and this research is focused on how the topography can affect the age composition. The authors have investigated this relationship by constructing a number of HydroGeoSphere models based on the “Schäfertal” catchment in Central Germany. An interesting approach was used to better understand the impact of slope on nitrate transport; the authors generated 10 synthetic catchments by adjusting the elevations of the land surface and aquifer bottom such that the mean topographic slope ranged between 5% grade (steep) and 0.1% grade (flat). Integrated groundwater-surface water flow and solute transport modelling using HydroGeoSphere were conducted for each synthetic catchment. HGS resolves the spatially explicit details within a catchment and supports transit time distributions through all model domains, allowing the researchers to systematically assess the impact of catchment topography on nitrate mass fluxes and in-stream concentrations.
The results of this study generally indicated that there is a negative correlation between catchment slope and the “young water fraction” (i.e. the fraction of streamflow with an age <3 months), and a positive correlation between mean in-stream nitrate concentrations and catchment slope. In other words, a flatter landscape will retain more nitrate mass in the soil (and result in less nitrate mass flux to streams) since the decreased lateral flow velocity reduces the potential for water to physically transport solute toward the stream, and the increased potential for degradation as subsurface flow becomes a more dominant flow pathway. Overall, this study provides "improved [] understanding of the effects of certain catchment characteristics on nitrate export dynamics with potential implications for the management of stream water quality and agricultural activity, in particular for catchments in temperate humid climate with pronounced seasonality.”
Figure 7: Cross-sectional view of saturation, flow paths and exchange fluxes between the surface and subsurface in the wet season (Feb) for catchments with topographic slope of (a) 1:20, and (b) 1:1000
Abstract:
Excess export of nitrate to streams affects ecosystem structure and functions and has been an environmental issue attracting worldwide attention. The dynamics of catchment-scale solute export from diffuse nitrogen sources can be explained by the changes 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 mass fluxes and solute concentration levels, particularly in humid catchments with strong seasonality in meteorological forcing. To fill this gap, we modeled the groundwater flow and nitrate transport for a small agricultural catchment in Central Germany. We used the fully coupled surface and subsurface numerical simulator HydroGeoSphere (HGS) to model groundwater and overland flow and nitrate transport. We computed the water ages using numerical tracer experiments. To represent various topographic slopes, we additionally simulated 10 synthetic catchments generated by modifying the topographic slope from the real-world scenario. Results suggest a negative correlation between the young streamflow fraction and the topographic slope. This correlation is more pronounced in flat landscapes with slopes < 1 : 60. Flatter landscapes tend to retain more N mass in the soil (including mass degraded in soil) and export less N mass to the stream, due to reduced leaching and increased degradation. The mean in-stream nitrate concentration shows a de- creasing trend in response to a decreasing topographic slope, suggesting that a large young streamflow fraction is not sufficient for high in-stream concentrations. Our results improve the understanding of nitrate export in response to topographic slope in a temperate humid climate, with important implications for the management of stream water quality.
“Overall, this study provided a mechanistic perspective on how catchment topographic slope affects young streamflow fraction and nitrate export patterns. The use of a fully-coupled flow and transport model [HydroGeoSphere] extended the approach to investigate the effects of catchment characteristics beyond the frequently used tracer data-driven analysis.”