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Explaining the Variability in High-Frequency Nitrate Export Patterns Using Long-Term Hydrological Event Classification
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  • Carolin Winter,
  • Larisa Tarasova,
  • Stefanie Rayana Lutz,
  • Andreas Musolff,
  • Rohini Kumar,
  • Jan Fleckenstein
Carolin Winter
UFZ - Helmholtz Centre for Environmental Research

Corresponding Author:[email protected]

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Larisa Tarasova
Helmholtz Centre for Environmental Research - UFZ
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Stefanie Rayana Lutz
UFZ Helmholtz Centre for Environmental Research
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Andreas Musolff
UFZ - Helmholtz-Centre for Environmental Research
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Rohini Kumar
UFZ-Helmholtz Centre for Environmental Research
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Jan Fleckenstein
Helmholtz Center for Environmental Research - UFZ
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Runoff events play an important role for nitrate export from catchments, but the variability of nutrient 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 soil moisture, snowmelt and the temporal organization of rainfall, in six neighboring mesoscale catchments with contrasting land use types. We related these event characteristics to nitrate export patterns from high-frequency nitrate concentration monitoring (2013 - 2017) using concentration-discharge relationships. Our results show that small rainfall-induced events with dry antecedent conditions exported lowest nitrate concentrations and loads but exhibited highly variable concentration-discharge 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, large 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 homogenous export patterns by high catchment wetness that activated a high number of flow paths. Long-term hydro-meteorological data indicated an increase of events with dry antecedent conditions in summer and decreased snow-influenced events. These trends will likely continue and lead to an increased nitrate concentration variability during low-flow seasons and to changes in the timing of largest nitrate export peaks during high-flow seasons.
Jan 2022Published in Water Resources Research volume 58 issue 1. 10.1029/2021WR030938