New Zealand Journal of Marine and Freshwater Research abstracts
Short communication
Effects of sampling frequency and laboratory detection limits on the
determination of time series water quality trends
BRETT STANSFIELD*
Wellington Regional Council
Wairarapa Division
P. O. Box 41
Masterton, New Zealand
*Present address: Hawke's Bay Regional Council, Private Bag
6001, Napier, New Zealand.
email: brett@hbrc.govt.nz
Abstract Time series trend analysis using the non-parametric
seasonal Kendall Sen Slope estimator and the seasonal Kendall trend test was
conducted on a water-quality data set (raw and flow adjusted) from part of the
Wellington Regional Council's freshwater baseline water quality monitoring
programme. The trend analysis was conducted a number of times to determine what
effects changing the sampling frequency and detection limit may have on the
overall result of the analysis. Results indicate that trends detected using a
low detection limit are often not detected when a higher limit is adopted. This
holds for upward and downward trends. Results also indicate that if the
sampling frequency was changed from monthly to quarterly fewer trends were
detected. The results also show that where the quarterly data shows a trend it
is usually of a slightly different magnitude (slope) compared with the monthly
data. This paper serves to illustrate the importance of considering detection
limits of variables and sampling frequencies for state of the environment
monitoring programmes.
Keywords water quality; state of the environment monitoring;
detection limits
New Zealand Journal of Marine and Freshwater Research, 2001, Vol. 35
M01023
Received 14 March 2001; accepted 5 July 2001
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