Geoscience Reference
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were conducted on three time periods: 1970-2001 (60 stations), 1950-2001
(43 stations), and 1932-2001 (nine stations). The analysis was performed on
the drought event parameters by applying correlation between event parameters
tests, Hurst coefficients and lag-one coefficients, and trend-free pre-whitening
Mann-Kendall (TFPW-MK) tests. The analysis showed that there is no uniform
trend on the streamflow drought in the entire state. However, some trends are
evident for specific regions. Specifically, it is most likely that droughts in the
Republican watershed have become more intense; whereas the drought has
become slightly alleviated in the Missouri and nearby watersheds.
Khaliq et al. (2009a) reviewed usefulness of four methods for identification
of hydrological trends (Mann-Kendall, Spearman rank correlation, Sen's slope
and linear regression tests) in the presence/absence of short-term serial and
cross correlations. The ability of the reviewed tests for detecting trends and
interpreting their collective results is demonstrated by a case study of annual
mean daily flows of Canadian river basins. The results of the case study
indicated that failure to incorporate the effects of serial and cross correlations
in a trend investigation study could result in erroneous conclusions. It is
recommended that the old practice of identifying hydrological trends without
the consideration of serial and cross correlations should be avoided and these
characteristics should be given adequate attention in all studies on temporal
trends.
Khaliq et al. (2009b) investigated trends in annual, summer and winter
time series of 30-day low flows occurring in pristine rivers basins of Canada.
This study investigated effect of independence (IND), short-term persistence
(STP) and long-term persistence (LTP) using the observed low flow data and
simulated time series of known STP- and LTP-like structures. Mann-Kendall
(MK) test along with two modified versions and a block bootstrap resampling
test were used to estimate significance of temporal trends under the assumptions
of IND and STP. A semi-parametric and a parametric procedure based on the
fractional autoregressive integrated moving average modelling approach and
a MK scaling test were used to estimate significance of temporal trends under
the assumption of LTP. The results of the study suggested that for a majority
of the time series of low flows analyzed, the assumption of IND or STP
cannot be ruled out. On the contrary, the fluctuating behaviour of trends
revealed in selected time series of low flows, with longer records, using
moving window technique favour the LTP hypothesis. In general, the results
of trend investigation suggested 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 behaviour in hydrologic time series, which
seldom exceed 100 years, is important and challenging, and must be given
adequate attention in all trend investigation studies.
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