3.1 Multi-temporal analysis
CEEMDAN was adopted for the multi-temporal decomposition of the annual
runoff and sediment discharge series measured at the Tangnaihai station
during 1960–2013. Finally, five layers were obtained. There are four
were IMF components and one was a trend term (residual (RES) component).
The decomposition results are shown in Figure 3.
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Figure 3
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Figure 3 reveals that the annual runoff and sediment discharge series
measured at Tangnaihai station in the source region of the Yellow River
showed complex multi-period variations and fluctuations. Although they
have different amplitude fluctuations, the runoff and sediment discharge
components of the same frequency vary simultaneously. This indicates
that the runoff and sediment discharge in the source region are
reasonably correlated with each other on both the macroscopic and
microscopic scales. The IMF1 component shows the greatest amplitude, the
shortest period, and the highest frequency. The RES component reflects
the macroscopic variations in the runoff and sediment discharge and it
demonstrates that both variables decline gradually with time.
For all the decomposition series, their periods and amplitude variation
were analyzed. The results are summarized in Table 1.
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Table 1
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According to Table 1, the quasi-periods of the IMF runoff components are
the same as those of the corresponding IMF sediment discharge
components. The IMF1 components for both the runoff and sediment
discharge have high frequencies and short quasi-periods. The IMF2
components have medium frequencies and quasi-periods. Furthermore, IMF3
and IMF4 are low-frequency components and they have medium/long and long
periods, respectively. Overall, the minimum, maximum, and average
amplitudes of the IMF components decreased progressively. As the
decomposition scale increased, the amplitude fluctuations of the
components declined and the periods lengthened. Lastly, the annual
runoff and sediment discharge declined gradually macroscopically.