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| Titel |
Antecedent flow conditions and nitrate concentrations in the Mississippi River basin |
| VerfasserIn |
J. C. Murphy, R. M. Hirsch, L. A. Sprague |
| 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 ; 18, no. 3 ; Nr. 18, no. 3 (2014-03-10), S.967-979 |
| Datensatznummer |
250120301
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| Publikation (Nr.) |
 copernicus.org/hess-18-967-2014.pdf |
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| Zusammenfassung |
| The relationship between antecedent flow conditions and nitrate
concentrations was explored at eight sites in the 2.9 million square kilometers
(km2) Mississippi River basin, USA. Antecedent flow conditions were
quantified as the ratio between the mean daily flow of the previous year and
the mean daily flow from the period of record (Qratio), and the Qratio was
statistically related to nitrate anomalies (the unexplained variability in
nitrate concentration after filtering out season, long-term trend, and
contemporaneous flow effects) at each site. Nitrate anomaly and Qratio were
negatively related at three of the four major tributary sites and upstream
in the Mississippi River, indicating that when mean daily streamflow during
the previous year was lower than average, nitrate concentrations were higher
than expected. The strength of these relationships increased when data were
subdivided by contemporaneous flow conditions. Five of the eight sites had
significant negative relationships (p ≤ 0.05) at high or moderately
high contemporaneous flows, suggesting nitrate that accumulates in these
basins during a drought is flushed during subsequent high flows. At half of
the sites, when mean daily flow during the previous year was 50 percent
lower than average, nitrate concentration can be from 9 to 27 percent
higher than nitrate concentrations that follow a year with average mean
daily flow. Conversely, nitrate concentration can be from 8 to 21 percent
lower than expected when flow during the previous year was 50 percent higher
than average. Previously documented for small, relatively homogenous basins,
our results suggest that relationships between antecedent flows and nitrate
concentrations are also observable at a regional scale. Relationships were
not observed (using all contemporaneous flow data together) for basins
larger than 1 million km2, suggesting that above this limit the overall
size and diversity within these basins may necessitate the use of more
complicated statistical approaches or that there may be no discernible
basin-wide relationship with antecedent flow. The relationships between
nitrate concentration and Qratio identified in this study serve as the basis
for future studies that can better define specific hydrologic processes
occurring during and after a drought (or high flow period) which influence
nitrate concentration, such as the duration or magnitude of low flows, and
the timing of low and high flows. |
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