2.3 Study Steps
By using CEEMDAN method, the time series of rainfall, runoff and sediment in the source area of the Yellow River are decomposed to obtain IMF component sequences at different time scales. Furthermore, the co-integration theory is used to construct the ECM for the original time series (ECM-OTS) and the CEEMDAN component sequences (ECM-CEEMDAN), and then the runoff is forecasted by ECM-OTS and ECM-CEEMDAN respectively. Finally, the runoff forecasted value of each IMF component is reconstructed to get the runoff forecasted value of ECM-CEEMDAN, and the fitting value and forecast accuracy of these two ECM models are compared to draw a conclusion. The flow chart of study steps is shown in Fig. 1.
Fig. 1 Flow Chart of Study Steps
  1. Results and Conclusion
  2. Data Source
The source area of the Yellow River refers to the area above the Tangnaihai hydrological station, which is located in the northeast of the Qinghai Tibet Plateau of China. The geographic coordinates are between 95°50 ’~103°30’ E and 32°10 ’~36°05’ N, the basin area is 122,000 km2, and the average annual runoff is 20.37 billion m3. The water source is mainly supplied by rainfall, followed by glacial snow melting water and groundwater. The change of runoff in the source area of the Yellow River has a vital influence on the change of water resources in the whole Yellow River Basin.
The measured rainfall, runoff and sediment time series from 1966 to 2013 at Tangnaihai hydrological station are shown as in Fig. 2.
Fig. 2 Time series of rainfall, runoff and sediment at Tangnaihai station in the source area of the Yellow River
  1. CEEMDAN Decomposition
The CEEMDAN method is used to decompose the time series of rainfall, runoff and sediment in the source area of the Yellow River for multi-time scales. The decomposition results are shown in Fig. 3-5.
With the CEEMDAN method, the annual runoff, rainfall and sediment data seires at Tangnaihai hydrological station from 1966 to 2013 are decomposed into a fifth-order mode, including four IMF components and one residual. It reflects the multi-time scale evolution characteristics of rainfall, runoff, and sediment in the source area of the Yellow River. The IMF1 component of each variable has the shortest period and the highest frequency, and the period of other components gradually gets longer and their frequency gradually decreases. The periodic changes of the component time series are shown in Table 1.
Fig. 3 Time series of rainfall components in the source area of the Yellow River
Fig. 4 Time series of runoff components in the source area of the Yellow River
Fig. 5 Time series of sediment components in the source area of the Yellow River
Table 1 Periodic changes of rainfall, runoff and sediment component time series