3.1 Determination of a baseline period and trend analysis
Meteorological changes are an important driving factor of basin runoff
changes. There are many methods to explore a trend-change in basin
hydrological time series data but each method is based on a different
principle. Due to uncertainty, a single method may cause unreliable
results. The rationality of the results can be improved by analyzing the
hydrological series of the basin in multiple ways. Lyu (2012) studied
the hydrometeorological change trend of the Chabagou River Basin in
China through Mann–Kendall (M–K) test and Spearman (Sp) method. This
study uses the M-K test and Sp method to test the time series trend of
hydrometeorology in the Qinhe River Basin.
Long-series hydrometeorological data are usually divided into multiple
stages for comparative study. The division of stages is generally
accomplished via two methods: 1) the ”manual division method” based on
special years or special events (Miao et al., 2011) and 2) the
”breakpoint division method” based on different statistical methods
(Tabari et al., 2014). The manual division method is simple but lacks
specific division standards. The breakpoint division method is based on
statistical methods to identify mutation years to divide different
periods, which is more scientific. The traditional techniques for
evaluating mutation points include the M-K test (Li et al., 2007),
ordered clustering method (O-C) (Mohamed, 2017), and cumulative anomaly
method (C-A) (Li and Song, 2020). In this study, the baseline period and
change period were comprehensively divided using the above three
methods.