Current Status of Time Series Analysis in Hydrological Sciences - Hydrologic Time Series Analysis: Theory and Practice - page 123

tests, viz., the Sen's T test, Spearman's Rho test, Mann-Kendall test and the
Seasonal Kendall test. The linear slopes (change per unit time) of trends were
calculated by using a nonparametric estimator. In addition, the homogeneity
in monthly trends was tested by the Van Belle and Hughes method. The results
of the nonparametric tests indicated that the discharge and sediment
concentration have downward trends, while the temperature, EC and the
concentrations of sodium, potassium, calcium+magnesium, bicarbonate and
chloride have upward trends. In contrast, the concentrations of carbonate, pH,
sulfate, organic matter, and boron have no trends.
Harned and McMohan (1997) examined the monotonically increasing or
decreasing temporal trends in riverine water quality including the suspended
sediment, solids, and nutrients for six stations of the Contentnea Creek Basin
by using the seasonal Kendall test. The variation in water quality, because of
the variation in streamflow, was also accounted for in cases where streamflow
data were available. Nutrient concentrations for Contentnea Creek at Hookerton
have declined since 1980. Total nitrogen, nitrate+nitrite, and nitrate
concentrations have a significant declined trend, with the greatest reductions
occurring from 1980 to 1992. Total ammonia and organic nitrogen
concentrations, which were increasing during the 1980s, have declined since
around 1990. Total phosphorus, dissolved phosphorus and orthophosphorus,
which increased during the 1980, have shown a significant decline since 1988
— the first year of the legislated phosphate detergent ban.
Antonopoulos et al. (2001) analyzed the time series of water quality
parameters and the discharge of Strymon River, Greece for the 1980-1997
period. The nonparametric Spearman's criterion was employed to detect the
existence of trends for the variables: discharge, ECw, DO, SO 4 -2 , Na + , K + and
NO -3 and the evaluation of the best-fitted models were performed by using
F 2 -test and the Kolmogorov-Smirnov test. Furthermore, the relationships
between concentration and loads of constituents both with the discharge were
also examined. According to the correlation coefficient ( r ) values, the relation
between concentrations and discharge is weak ( r < 0.592), whereas the relation
between loads and discharge is very strong ( r > 0.902).
Stansfield (2001) illustrated the importance of considering detection limits
of variables and sampling frequencies by analyzing the trends in water quality
time series (part of the Wellington Regional Council's freshwater baseline
water quality monitoring programme, New Zealand) using the nonparametric
seasonal Kendall test and the Sen Slope estimator. Results indicated that the
trends (upward and downward) obtained using a low detection limit are often
not discernible when a higher limit is adopted. It was also found that if the
sampling frequency was changed from monthly to quarterly, fewer trends
were detected. Moreover, the quarterly data exhibited a trend, which was
usually of different magnitude (slope) compared to that in the monthly data.
Gangyan et al. (2002) examined the temporal and spatial sediment load
characteristics of Asia's longest river, the Yangtze by using the turning point