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| Titel |
Using high-frequency water quality data to assess sampling strategies for the EU Water Framework Directive |
| VerfasserIn |
R. A. Skeffington, S. J. Halliday, A. J. Wade, M. J. Bowes, M. Loewenthal |
| 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 ; 19, no. 5 ; Nr. 19, no. 5 (2015-05-26), S.2491-2504 |
| Datensatznummer |
250120722
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| Publikation (Nr.) |
 copernicus.org/hess-19-2491-2015.pdf |
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| Zusammenfassung |
| The EU Water Framework Directive (WFD) requires that the ecological and
chemical status of water bodies in Europe should be assessed, and action
taken where possible to ensure that at least "good" quality is attained in
each case by 2015. This paper is concerned with the accuracy and precision
with which chemical status in rivers can be measured given certain sampling
strategies, and how this can be improved. High-frequency (hourly) chemical
data from four rivers in southern England were subsampled to simulate
different sampling strategies for four parameters used for WFD
classification: dissolved phosphorus, dissolved oxygen, pH and water
temperature. These data sub-sets were then used to calculate the WFD
classification for each site. Monthly sampling was less precise than weekly
sampling, but the effect on WFD classification depended on the closeness of
the range of concentrations to the class boundaries. In some cases, monthly
sampling for a year could result in the same water body being assigned to
three or four of the WFD classes with 95% confidence, due to random
sampling effects, whereas with weekly sampling this was one or two classes
for the same cases. In the most extreme case, the same water body could have
been assigned to any of the five WFD quality classes. Weekly sampling
considerably reduces the uncertainties compared to monthly sampling. The
width of the weekly sampled confidence intervals was about 33% that of the
monthly for P species and pH, about 50% for dissolved oxygen, and about
67% for water temperature. For water temperature, which is assessed as the
98th percentile in the UK, monthly sampling biases the mean downwards by
about 1 °C compared to the true value, due to problems of assessing
high percentiles with limited data. Low-frequency measurements will generally
be unsuitable for assessing standards expressed as high percentiles.
Confining sampling to the working week compared to all 7 days made little
difference, but a modest improvement in precision could be obtained by
sampling at the same time of day within a 3 h time window, and this is
recommended. For parameters with a strong diel variation, such as dissolved
oxygen, the value obtained, and thus possibly the WFD classification, can
depend markedly on when in the cycle the sample was taken. Specifying this in
the sampling regime would be a straightforward way to improve precision, but
there needs to be agreement about how best to characterise risk in different
types of river. These results suggest that in some cases it will be difficult
to assign accurate WFD chemical classes or to detect likely trends using
current sampling regimes, even for these largely groundwater-fed rivers. A
more critical approach to sampling is needed to ensure that management
actions are appropriate and supported by data. |
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