Archetypes and Controls of Riverine Nutrient Export Across German Catchments

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Anthropogenic activities such as intensive agriculture and waste water discharge deteriorate the quality of water resources. More specifically, anthropogenic sources increase the aquatic concentration and fluxes of nutrients like nitrogen, phosphorus and organic carbon, leading to eutrophication in rivers, lakes and coastal waters (Carpenter et al., 2011;Schlesinger, 2009). This can pose a threat to water security and downstream aquatic ecosystem health and functioning (Foley et al., 2005). For effective catchment-scale water quality management, knowledge on sources, pathways and reaction processes of critical constituents is needed. Reactive transport at the catchment scale, is however, complex and tends to span a large range of spatial and temporal scales (Gall et al., 2013;Kirchner, 2003;Sivapalan, 2006). This often hinders establishing a unique cause-effect relationship.One way of approaching this complexity involves the analysis of the integrated response of concentration (C) of a constituent and discharge (Q) at a given point in the stream to identify underlying processes and their hierarchy (Basu et al., 2011;Godsey et al., 2009;Musolff et al., 2015). More specifically, characterizing the relationship between concentration and discharge proved valuable to link temporal patterns in the data to dominant processes at the catchment scale (Sivapalan, 2006). This link may however not always be fully

Section: Agricultural Management Defines No 3 -N Event Behaviormentioning

confidence: 55%

Spatial and Temporal Variability in Concentration‐Discharge Relationships at the Event Scale

Musolff

Zhan

Dupas

et al. 2021

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“…Our results show that the average level of exported nitrate concentrations and loads during an event are catchment specific, depending on the land use and related N input. Higher export was observed in catchments with more agricultural land use, which have a higher N input through fertilizer application, whereas lower export was found in the more forested catchments, were N input, stemming from atmospheric deposition and biological fixation, is typically lower (Ebeling et al, 2021). However, we could also show very similar patterns in event-scale nitrate export across all catchments that were strongly dependent on the event magnitude (in regard to runoff) and season (Figure 8).…”

Section: Impact Of Runoff Event Characteristics On Nitrate Exportmentioning

confidence: 61%

“…Yang, Heidbüchel, et al, 2018). These flow paths during the dry period are generally characterized by longer transit times and thus enable more nitrate uptake and removal via denitrification (Ebeling et al, 2021;Ehrhardt et al, 2019;Kumar et al, 2020;Nguyen et al, 2021). As a result from increased biological activity with higher temperatures, nitrate uptake and removal increases, especially in streams and in the riparian zones (Baird et al, 1995;Lutz et al, 2020;Rode, Angelstein et al, 2016), which can lead to reduced nitrate availability compared to colder seasons.…”

Section: Low-magnitude Eventsmentioning

confidence: 99%

Explaining the Variability in High‐Frequency Nitrate Export Patterns Using Long‐Term Hydrological Event Classification

Winter

Tarasova

Lutz

et al. 2022

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Runoff events play an important role in nitrate export from catchments, but the variability of export patterns between events and catchments is high and the dominant drivers remain difficult to disentangle. Here, we rigorously asses if detailed knowledge on runoff event characteristics can help to explain this variability. To this end, we conducted a long‐term (1955–2018) event classification using hydro‐meteorological data, including rainfall characteristics, soil moisture and snowmelt, in six neighboring mesoscale catchments with contrasting land use. We related these event characteristics to nitrate export patterns from high‐frequency nitrate concentration monitoring (2013–2017) using concentration‐discharge (CQ) relationships. Our results show that low‐magnitude rainfall‐induced events with dry antecedent conditions exported lowest nitrate concentrations and loads but exhibited highly variable CQ relationships. We explain this by a low fraction of active flow paths, revealing the spatial heterogeneity of nitrate sources within the catchments and by an increased impact of biogeochemical retention processes. In contrast, high‐magnitude rainfall or snowmelt‐induced events exported highest nitrate concentrations and loads and converged to similar chemostatic export patterns across all catchments, without exhibiting source limitation. We explain these homogeneous export patterns by high catchment wetness that activated a high number of flow paths and by higher nitrate availability during high‐flow seasons. Long‐term hydro‐meteorological data indicated an increased number of events with dry antecedent conditions in summer and a decreased number of snow‐influenced events. These trends will likely continue and cause increased nitrate concentration variability during low‐flow seasons and changes in the timing of nitrate export peaks during high‐flow seasons.

“…This hypothesis echoes the use of EMMA and distinct geochemical signatures for hydrograph separation and source water differentiation (Hooper et al, 1990;Klaus & McDonnell, 2013;Pinder & Jones, 1969), although these approaches do not explicitly emphasize the depths of source waters and the deep and shallow water chemistry contrasts. The shallow and deep hypothesis is supported by extensive CQ data from individual sites and from across scale analysis (Creed et al, 1996;Ebeling et al, 2021;Moatar et al, 2017;Musolff et al, 2017). A recent global data analysis also shows the depth of solute generation, not climate drivers, determines CQ slope b (Botter et al, 2020).…”

Section: Existing Evidence and Implicationsmentioning

confidence: 88%

Vertical Connectivity Regulates Water Transit Time and Chemical Weathering at the Hillslope Scale

Xiao

Brantley

2021

How does hillslope structure (e.g., hillslope shape and permeability variation) regulate its hydro‐geochemical functioning (flow paths, solute export, chemical weathering)? Numerical reactive transport experiments and particle tracking were used to answer this question. Results underscore the first‐order control of permeability variations (with depth) on vertical connectivity (VC), defined as the fraction of water flowing into streams from below the soil zone. Where permeability decreases sharply and VC is low, >95% of water flows through the top 6 m of the subsurface, barely interacting with reactive rock at depth. High VC also elongates mean transit times (MTTs) and weathering rates. VC however is less of an influence under arid climates where long transit times drive weathering to equilibrium. The results lead to three working hypotheses that can be further tested. H1: The permeability variations with depth influence MTTs of stream water more strongly than hillslope shapes; hillslope shapes instead influence the younger fraction of stream water more. H2: High VC arising from high permeability at depths enhances weathering by promoting deeper water penetration and water‐rock interactions; the influence of VC weakens under arid climates and larger hillslopes with longer MTTs. H3: VC regulates chemical contrasts between shallow and deep waters (Cratio) and solute export patterns encapsulated in the power law slope b of concentration‐discharge (CQ) relationships. Higher VC leads to similar shallow versus deep water chemistry (Cratio ∼1) and more chemostatic CQ patterns. Although supporting data already exist, these hypotheses can be further tested with carefully designed, co‐located modeling and measurements of soil, rock, and waters. Broadly, the importance of hillslope subsurface structure (e.g., permeability variation) indicate it is essential in regulating earth surface hydrogeochemical response to changing climate and human activities.