Detection of Trends in Low Flows

Low-flow index of 7-day from 57 hydrometric stations was extracted and examined to detect trends and changes in timing of summer and winter 7-day low-flows. Man- Kendall (MK) nonparametric trend test was applied to the time series to investigate the existence of any trend at a 95% confidence level. In order to account for serial correlation, the variance of the S statistic was modified if its absolute value exceeded a certain confidence interval. Significant upward trends were observed in the Northern Canada where almost all of the stations located above latitude 60o showed an increasing trend in 7-day low-flow quantities. A significant downward trend dominated Atlantic Provinces and southern British Columbia. No evidence of significant trends in Prairies was found. It was observed that 7-day summer lowflow in southwest significantly shifted to arrive earlier, but later in the west and northwest of Canada. In 88% of detected trends, the winter 7-day low-flow shifted to arrive earlier; however, both winter and summer low flow was experiencing a shift toward earlier dates in the eastern part of the country. Furthermore, in western Canada winter trends were downward while summer trends were upwards.

Keywords:

7-day Low-streamflows , Trends , Serial correlation , Site significance