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Disparate Seasonal Nitrate Export from Nested Heterogeneous Subcatchments Revealed with StorAge Selection Functions
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  • Tam Van Nguyen,
  • Rohini Kumar,
  • Andreas Musolff,
  • Stefanie Rayana Lutz,
  • Fanny Sarrazin,
  • Sabine Attinger,
  • Jan Fleckenstein
Tam Van Nguyen
Helmholtz Centre for Environmental Research

Corresponding Author:[email protected]

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Rohini Kumar
UFZ-Helmholtz Centre for Environmental Research
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Andreas Musolff
UFZ - Helmholtz-Centre for Environmental Research
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Stefanie Rayana Lutz
UFZ Helmholtz Centre for Environmental Research
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Fanny Sarrazin
UFZ
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Sabine Attinger
UFZ Centre for Environmental Research
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Jan Fleckenstein
Helmholtz Center for Environmental Research - UFZ
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Abstract

Understanding catchment controls on catchment solute export is a prerequisite for water quality management. StorAge Selection (SAS) functions encapsulate essential information about catchment functioning in terms of discharge selection preference and solute export dynamics. However, they lack information on the spatial origin of solutes when applied at the catchment scale, thereby limiting our understanding of the internal (subcatchment) functioning. Here, we parameterized SAS functions in a spatially explicit way to understand the internal catchment responses and transport dynamics of reactive dissolved nitrate (N-NO3). The model was applied in a nested mesoscale catchment (457 km²), consisting of a mountainous partly forested, partly agricultural subcatchment, a middle-reach forested subcatchment, and a lowland agricultural subcatchment. The model captured flow and nitrate concentration dynamics not only at the catchment outlet but also at internal gauging stations. Results reveal disparate subsurface mixing dynamics and nitrate export among headwater and lowland subcatchments. The headwater subcatchment has high seasonal variation in subsurface mixing schemes and younger water in discharge, while the lowland subcatchment has less pronounced seasonality in subsurface mixing and much older water in discharge. Consequently, nitrate concentration in discharge from the headwater subcatchment shows strong seasonality, whereas that from the lowland subcatchment is stable in time. The temporally varying responses of headwater and lowland subcatchments alternates the dominant contribution to nitrate export in high and low-flow periods between subcatchments. Overall, our results demonstrate that the spatially explicit SAS modeling provides useful information about internal catchment functioning, helping to develop or evaluate spatial management practices.
Mar 2022Published in Water Resources Research volume 58 issue 3. 10.1029/2021WR030797