Impact of reservoir operation on runoff and sediment load at multi-time scales based on entropy theory

Based on the runoff and sediment load data series obtained at the Guide hydrological station (GHS) in the upper reaches of the Yellow River from 1960 to 2013, the varying quasi-periodic fluctuations of runoff and sediment load existing in different decomposed time scales are revealed by using the complementary ensemble empirical mode decomposition (CEEMD) method pre- and post-reservoir operation. Combined with entropy theory, the concepts of multi-time scale entropy (MTSE) and multi-time scale structural entropy (MTSSE) are proposed to analyze the complex fluctuation characteristics of runoff and sediment load at different time scales. The results show that reservoir operation has a greater influence on sediment load than that on runoff at multi-time scales, and that their correlations change as well. Additionally, reservoir operation can aggravate the complexity of the runoff and sediment load, but with longer fluctuation time scales, this complexity decreases and predictability increases. After reservoir operation, the fluctuation period of high frequency components in runoff and sediment load is elongated, and the information content of runoff and sediment load is focused at the short-middle time scale. Therefore, the available and reasonable monitoring and research periods for runoff and sediment load in the upper reaches of the Yellow River are suggested as 4-7 years and 3-4 years, respectively.