Explaining the Variability in High‐Frequency Nitrate Export Patterns Using Long‐Term Hydrological Event Classification
Winter, C.; Tarasova, L.; Lutz, S. R.; Musolff, A.; Kumar, R.; Fleckenstein, J. H.
(2022) Water Resources Research, volume 58, issue 1, pp. 1 - 21
(Article)
Abstract
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
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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.
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Keywords: catchment, concentration-discharge relationships, high temporal resolution monitoring, nitrate export, runoff events, water quality, Water Science and Technology
ISSN: 0043-1397
Publisher: American Geophysical Union
Note: Funding Information: Funding for this study was provided by the Helmholtz Research Program, Topic 5 Landscapes of the Future, subtopic 5.2 Water resources and the environment and by the Helmholtz International Research School TRACER (HIRS‐0017). The authors cordially thank the State Office of Flood Protection and Water Quality of Saxony‐Anhalt (LHW) for the provision of discharge data and Germany's National Meteorological Service (Deutscher Wetterdienst, DWD) for the provision of meteorological data sets. Furthermore, the authors thank Michael Rode, Karsten Rinke, Xiangzhen Kong, and Kurt Friese for the provision of high‐frequency and lab data from TERENO observational facilities. Funding Information: Funding for this study was provided by the Helmholtz Research Program, Topic 5 Landscapes of the Future, subtopic 5.2 Water resources and the environment and by the Helmholtz International Research School TRACER (HIRS-0017). The authors cordially thank the State Office of Flood Protection and Water Quality of Saxony-Anhalt (LHW) for the provision of discharge data and Germany's National Meteorological Service (Deutscher Wetterdienst, DWD) for the provision of meteorological data sets. Furthermore, the authors thank Michael Rode, Karsten Rinke, Xiangzhen Kong, and Kurt Friese for the provision of high-frequency and lab data from TERENO observational facilities. Publisher Copyright: © 2022. The Authors.
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