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| Titel |
Effect on nitrate concentration in stream water of agricultural practices in small catchments in Brittany: II. Temporal variations and mixing processes |
| VerfasserIn |
L. Ruiz, S. Abiven, C. Martin, P. Durand, V. Beaujouan, J. Molenat |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1027-5606
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| Digitales Dokument |
URL |
| Erschienen |
In: Hydrology and Earth System Sciences ; 6, no. 3 ; Nr. 6, no. 3, S.507-514 |
| Datensatznummer |
250003567
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| Publikation (Nr.) |
 copernicus.org/hess-6-507-2002.pdf |
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| Zusammenfassung |
| In
catchments with impervious bedrock, the nitrate concentrations in streamwater
often show marked seasonal and small inter-annual variations. The inter-annual
trends are usually attributed to changes in nitrogen inputs, due to changes in
land use or in nitrogen deposition whereas seasonal patterns are explained in
terms of availability of soil nitrate for leaching and of seasonality of
nitrogen biotransformations. The companion paper showed that inter-annual
variations of nitrogen in streamwater are not directly related to the variations
of land use. The aim of this study is to describe nitrate concentration
variations in a set of very small adjacent catchments, and to discuss the origin
of the inter-annual and seasonal trends. Data from four catchments at the
Kerbernez site (South Western Brittany, France) were used in this study. Nitrate
concentrations in streamwater were monitored for eight years (1992 to 1999) at
the outlet of the catchments. They exhibit contrasting inter-annual and seasonal
patterns. An extensive survey of agricultural practices during this period
allowed assessment of the amount of nitrogen available for leaching. The
discharges measured since 1997 show similar specific fluxes but very different
seasonal dynamics between the catchments. A simple, lumped linear store model is
proposed as an initial explanation of the differences in discharge and nitrate
concentration patterns between the catchments. The base flow at the outlet of
each catchment is considered as a mixture of water from two linear reservoirs
with different time constants. Each reservoir comprises two water stores, one
mobile contributing to discharge, the other, immobile, where nitrate moves only
by diffusion. The storm flow, which accounts for less than 10% of the annual
flux, is not considered here. Six parameters were adjusted for each catchment to
fit the observed data: the proportion of deep losses of water, the proportion of
the two reservoirs and the size and initial concentration of the two immobile
stores. The model simulates the discharge and nitrate concentration dynamics
well. It suggests that the groundwater store plays a very important role in the
control of nitrate concentration in streamwater, and that the pattern of the
seasonal variation of nitrate concentration may result from the long term
evolution of nitrogen losses by leaching.
Keywords: nitrate, diffuse pollution, groundwater, seasonal variations,
agricultural catchment, simulation model |
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