Identification of temporal trends in annual and seasonal low flows occurring in Canadian rivers: The effect of short- and long-term persistence

In order to investigate trends in time series of hydrological variables, observational records are generally assumed to be independent (IND) or having short-term persistence (STP), as opposed to long-term persistence (LTP). This study systematically investigates the effect of these assumptions for estimating significance of trends using simulated time series of known STP- and LTP-like serial structures and using observational records of annual, winter and summer 30-day low flows occurring in pristine rivers basins of Canada. For the analyses, the Mann–Kendall (MK) test and two modified versions of this test and a block bootstrap resampling based test is used to estimate significance of temporal trends under the assumptions of IND and STP. A semiparametric and a parametric procedure based on the fractional autoregressive integrated moving average modeling approach and a MK scaling test are used to estimate significance of temporal trends under the assumption of LTP. The results of the study suggest that for the majority of the time series of low flows analyzed, the assumption of IND or STP cannot be ruled out. On the contrary, the fluctuating behavior of trends revealed in selected time series of low flows, with longer records, using moving window technique favor the LTP hypothesis. However, only a modest evidence for the presence of LTP in low flow time series using estimates of the Hurst exponent could be revealed, perhaps because of small sample uncertainties. In general, the results of trend investigation suggest that the estimates of trend significance are highly sensitive to IND, STP and LTP assumptions, e.g. adopting IND assumption instead of LTP for a given set of hydrological time series exhibiting LTP, could result in incorrect estimation of trend significance. Also, substituting IND assumption for STP would result in incorrect estimates of trend significance. Therefore, for reliable trend investigation, satisfactory identification of STP- or LTP-like behavior in hydrological time series, which seldom exceed 100 years, is important and challenging and must be given adequate attention in all trend investigation studies.