The principles of sample entropy algorithm and Lopes-Ruiz complexity are presented in this paper. Integrating sample entropy algorithm into Lopez-Ruiz complexity theory, the authors derive a novel method for studying the hydrological system complexity. The derived method is applied to the monthly mean runoff observation data for Linjiacun, Zhangjiashan and Zhangtou sites in the Weihe watershed area. The obtained results for these sites detect an increase in the complexity of runoff series in early 1970s and 1990s due to the human activities. The appropriate results obtained for the study area lead the authors to conclude that the proposed model provides a new method for analyzing the complexity in hydrological systems and identifying the influence of environmental changes on the runoff series.
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