Effect of Short- and Long-term Memory on Trend Significancy of Mean Annual Flow by Mann-Kendall Test

Climate variability and change is threatening water resources around the world. One hundred and fourtheen (114) stations from Reference Hydrometric Basin Network (RHBN) around Canada with at least 30 years continuous data (up to 2011) were selected to study the trend in mean annual runoff for different periods of 30 to 100 years in step 10 years by non-parametric Mann-Kendall test. Effect of short term persistant (STP) and long term persistant (LTP) on this test were made through lag 1 serial correlation (r1) and Hurst exponent (H), respectively. r1 for about one third of the total cases considered were negative. H, based on “equivalent Normal deviate” (eNv), was slightly right-skewed with minimum and maximum values of  0.20 and 0.87, respectively. About half of the data sets were anti-persistant (H0.1). The number of positive and negative trends, were nearly the same, though fluctuating for different time spans. p-value after pre-whitening was highly correlated with those of before pre-whitening, for both negatve and positive trends. There were about 16% of cases that pre-whitening decreased the p-values of the Mann-Kendall trend test, where nearly all of them were negatively trended. The effect of LTP on Mann-Kendall trend test was minor, due to inconsistancy of originally significant trend case and significant H of greater than 0.5. For recent 30 years length of record (1982-2011), British Columbia is experiencing positive trends in the west and negative trend in the east. Most parts of the New Brunswick is experincing the positive trend, while negative trend is due to Southeast of Ontario. For the more logest duration of 40 years, trend statistics and geographical pattern were changed. While the significant trends are decreased, more significant negative trends are governed over New Brunswick. There is no positive trend in British Colimbia in the past 50 years (1962-2011) while there are both negative and positive trends in New Brunswick and negative trends are switched to positive trends in south east of Ontario. For long duration of  > 70 years, there are only positive trends in Southeast of Canada (South New Brunswick and South East of Ontario) while centeral and East of Canada have experinced a negative trend.