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| Titel |
Spatial and temporal dynamics of coupled groundwater and nitrogen fluxes through a streambed in an agricultural watershed |
| VerfasserIn |
D. P. Genereux, C. D. Kennedy, H. Mitasova, D. R. Corbett |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250028988
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| Zusammenfassung |
| We studied the spatiotemporal dynamics of the coupled water flux (v) and nitrogen (N) fluxes
(fN = v[N], where [N] is the concentration of a dissolved N species) through a
streambed in an agricultural watershed in North Carolina, USA. Physical and chemical
variables were measured at numerous points in the streambed of a 0.26 km reach:
hydraulic conductivity (K) and hydraulic head gradient (J), and concentrations
of NO3- and other N species in the streambed groundwater, from which water
flux (v= KJ) and N fluxes (e.g., fNO3 = v[NO3-]) through the streambed were
computed, mapped, and integrated over the streambed. The result was a novel set of
streambed maps of the linked variables (K, J, v, N concentrations and fluxes),
showing their spatial variability and how it changed over a year (based on 7 bimonthly
sets of streambed maps). Mean fNO3 during the study year was 154 mmol m-2
day-1; this NO3-flux, together with that of DON (fDON = 17 mmol m-2 day-1)
accounted for >99% of the total dissolved N flux through the streambed (ammonium
was insignificant). Repeat measurements at the same locations on the streambed
show significant temporal variability in fNO3, controlled largely by changes in
v rather than changes in [NO3-]. Changes in v were in turn caused by temporal
variation in both J and K (i.e., streambed hydraulic conductivity, not only head
gradient, changed over time). One of the clearest and most temporally-persistent
aspects of spatial variability was lateral variability across the channel from bank to
bank. Values of K and v were greater in the center of the channel and lower on
the sides. This distribution of K (likely a reflection of sediment dynamics in the
channel) apparently focused groundwater discharge toward the center of the channel,
exactly where nitrate concentration [NO3-] and flux (fNO3) were lowest. CFC
age-dating of groundwater in the streambed confirmed that groundwater beneath
the center of the channel was generally older than that beneath the sides; lower
[NO3-] and fNO3 in the center were most likely the result of greater groundwater age
there and the lower agricultural N use in past decades (not the result of greater
denitrification in the center). fNO3 was characterized by localized zones of high and low
values that changed in size and shape over the study year but remained in basically
the same locations (the same was true of K, J, [NO3-], though less so for v).
70% of NO3- flux occurred through about 38% of the streambed area. Lateral
distributions of the physical hydrologic attributes (K,J,v) were highly symmetrical across
the channel, while those of [NO3-] and fNO3 showed higher values on the left
side of the channel than right, likely a reflection of different agricultural N use on
opposite sides of the stream. Finally, trace gas modeling suggests that about half the
nitrate in the groundwater of this watershed “leaves” the groundwater system via
groundwater discharge into the stream, and the other half by denitrification. The
streambed-based approach taken here offers a number of insights concerning the
spatial and temporal dynamics of linked water and N fluxes, and their controls. |
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