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found that the pH and alkalinity are decreasing, and nitrate increasing, but
these changes were not found in all months.
Van Belle and Hughes (1984) proposed several nonparametric tests for
detecting trends in water quality, because the assumptions of classical
parametric tests are usually not met by water quality data. Also, the additional
peculiarities of data, such as missing values, censored data, and seasonality,
compound the problem of analysis.
Harned and Davenport (1990) analyzed trends in the water quality data of
1945-1988 from major streams flowing into the Albemarle-Pamlico estuarine
system. The nonparametric seasonal Kendall test indicated a change in the
water quality data during the 1945 to 1988 period. The evaluation of water-
quality data and more than 50 basin variables indicated 121 significant
correlations between 11 basin characteristics and 12 water-quality constituents
at 21 estuary locations and seven National Stream Quality Accounting Network
stations.
Yu et al. (1993) analyzed the surface water quality data of the Arkansas,
Verdigris, Neosho, and Walnut River basins, Kansas to examine trends in 17
major constituents by using four different nonparametric methods. The results
indicated that the concentrations of specific conductance, total dissolved solids,
calcium, hardness, sodium, potassium, alkalinity, sulfate, chloride, total
phosphorus, ammonia plus organic nitrogen, and suspended sediment generally
have downward trends. Some of the downward trends were related to the
increase in discharge, while the others were attributed to the decrease in
pollution sources. The homogeneity tests suggested that both the station-wide
trends and basin-wide trends are non-homogeneous.
Robson and Neal (1996) examined the trend of ten years upland stream
and bulk deposition water quality data from Plynlimon, mid-Wales by applying
the seasonal Kendall test. The plotted data on time scale showed long-term
cycles, which relate to the fluctuations in weather patterns at Plynlimon and
thus violate the assumptions of common statistical trend tests. Even though
the seasonal Kendall test was significant for some determinands, the graphs
suggested that many of these trends are unlikely to continue. There was no
indication of changing acid deposition inputs or changing acidity within the
runoff, despite a decline in the UK sulphur dioxide emissions. The streamwater
dissolved organic carbon showed an increase over time, but there was not
corresponding decrease in pH as might be expected from the acidification
theory. There were cyclic variations in bulk precipitation and in streamwater
quality, which indicated that trends cannot be established even with 10 years
of data. Therefore, it was strongly recommended that long-term monitoring
programmes should continue for several decades. It was also emphasized that
graphical analysis greatly enhances data interpretation, and should be considered
as an essential component for trend investigation.
Kalayci and Kahya (1996) detected linear trends in the surface water
quality of rivers in the Susurluk Basin by employing four nonparametric trend
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