Bending of the concentration discharge relationship can inform about in-stream nitrate removal

Nitrate (NO3-) excess in rivers harms aquatic ecosystems and can induce detrimental algae growths in coastal areas. Riverine NO3- uptake is a crucial element of the catchment-scale nitrogen balance and can be measured at small spatiotemporal scales, while at the scale of entire river networks, uptake measurements are rarely available. Concurrent, low-frequency NO3- concentration and streamflow (Q) observations at a basin outlet, however, are commonly monitored and can be analyzed in terms of concentration discharge (C–Q) relationships. Previous studies suggest that steeper positive log (C)–log (Q) slopes under low flow conditions (than under high flows) are linked to biological NO3- uptake, creating a bent rather than linear log (C)–log (Q) relationship. Here we explore if network-scale NO3- uptake creates bent log (C)–log (Q) relationships and when in turn uptake can be quantified from observed low-frequency C–Q data. To this end we apply a parsimonious mass-balance-based river network uptake model in 13 mesoscale German catchments (21–1450 km2) and explore the linkages between log (C)–log (Q) bending and different model parameter combinations. The modeling results show that uptake and transport in the river network can create bent log (C)–log (Q) relationships at the basin outlet from log–log linear C–Q relationships describing the NO3- land-to-stream transfer. We find that within the chosen parameter range the bending is mainly shaped by geomorphological parameters that control the channel reactive surface area rather than by the biological uptake velocity itself. Further we show that in this exploratory modeling environment, bending is positively correlated to percentage of NO3- load removed in the network (Lr.perc) but that network-wide flow velocities should be taken into account when interpreting log (C)–log (Q) bending. Classification trees, finally, can successfully predict classes of low (∼4 %), intermediate (∼32 %) and high (∼68 %) Lr.perc using information on water velocity and log (C)–log (Q) bending. These results can help to identify stream networks that efficiently attenuate NO3- loads based on low-frequency NO3- and Q observations and generally show the importance of the channel geomorphology on the emerging log (C)–log (Q) bending at network scales.

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

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.

Bending of the concentration discharge relationship can inform about in-stream nitrate removal

Nitrate (NO3−) excess in rivers harms aquatic ecosystems and can induce detrimental algae growths in coastal areas. Riverine NO3− uptake is a crucial element of the catchment scale nitrogen balance and can be measured at small spatiotemporal scales while at the scale of entire river networks, uptake measurements are rarely available. Concurrent, low frequency NO3− concentration and stream flow (Q) observations at a basin outlet, however, are commonly monitored and can be analyzed in terms of concentration discharge (C-Q) relationships. Previous studies suggest that more positive log(C)-log(Q) slopes under low flow conditions (than under high flows) are linked to biological NO3− uptake, creating a bent rather than linear log(C)-log(Q) relationship. Here we explore if network scale NO3− uptake creates bent log(C)-log(Q) relationships and when in turn uptake can be quantified from observed low frequency C-Q data. To this end we apply a parsimonious mass balance based river network uptake model in 13 mesoscale German catchments (21–1450 km2) and explore the linkages between log(C)-log(Q) bending and different model-parameter combinations. The modelling results show that uptake and transport in the river network can create bent log(C)-log(Q) relationships at the basin outlet from log-log linear C-Q relationships describing the NO3− land to stream transfer. We find that the bending is mainly shaped by geomorphological parameters that control the channel reactive surface area rather than by the biological uptake velocity itself. Further we show that in this exploratory modelling environment, bending is positively correlated to percentage NO3− load removed in the network (Lr.perc) but that network wide flow velocities should be taken into account when interpreting log(C)-log(Q) bending. Classification trees, finally, can successfully predict classes of low (~ 4 %), intermediate (~ 32 %) and high (~ 68 %) Lr.perc using information on water velocity and log(C)-log(Q) bending. These results can help to identify stream networks that efficiently attenuate NO3− loads based on low frequency NO3− and Q observations and generally show the importance of the channel geomorphology on the emerging log(C)-log(Q) bending at network scales.

Modelling of in-stream nutrient uptake beyond the river reach scale

&lt;p&gt;Nutrient excess in rivers leads to ecosystem harm and can induce detrimental algae growths in coastal areas. In Germany and Europe, the management of riverine systems is complicated by the lack of understanding of nutrient pathways and effectiveness of retention processes from application to export. In this work, we hypothesize that in-stream nitrate uptake effects are linked to the shape or &amp;#8216;bending&amp;#8217; of the concentration discharge (C-Q) relationship. Therefore, the analysis of observational C-Q data may give insight into the dominant controls of the magnitude of in-stream nitrate uptake across different catchment. To explore the concentration discharge (C-Q) behavior in a range of hydrological and biogeochemical conditions, we developed a catchment wide parsimonious (7 parameter) network model framework (spatially explicit at 1x1km&amp;#178;). Here, land-to-stream nutrient transfer was modelled as a power law (C=aQ&lt;sup&gt;b&lt;/sup&gt;), resulting in different nutrient loading according to the contributing area of each grid cell in the network and in-stream load uptake follows Li = Lin*e&lt;sup&gt;-v&lt;sub&gt;f&lt;/sub&gt;*w*L/Q&lt;/sup&gt;, with v&lt;sub&gt;f&lt;/sub&gt; the uptake rate, w and L the width and length of the river section. This approach acknowledges both, spatial variability between river sections (e.g. residence time distributions) as well as at-a-station temporal variability depending on Q. Ten existing stream networks in Germany were evaluated with this model framework in a &amp;#8216;Monte Carlo approach&amp;#8217; for about 1000 predefined parameter combinations and 10 years of discharge data. First results show total nitrate uptakes ranging from almost 0 to 15% and high bending of C-Q curves correlated to high uptake rates. Furthermore, it was found that mean in-stream residence times, more than land-to-stream loading concentrations influence exported nutrient concentrations. The final result of our analysis will allow us to argue if the observed C-Q bending can be indeed related to instream uptake and not to other processes (e.g. denitrification along the subsurface flow path) and to derive the dominant processes shaping the uptake (such as light availability, instream travel time, nutrient stoichiometry, and impact of fine sediments).&lt;/p&gt;