Disparate Seasonal Nitrate Export from Nested Heterogeneous
Subcatchments Revealed with StorAge Selection Functions
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.