Fractal-based evaluation of the effect of water reservoirs on hydrological processes: the dams in the Yangtze River as a case study

Water reservoirs exercise a considerable influence on hydrological processes and their influence can be treated as one of the influences of human activities on the hydrological cycle at the regional and even global scale. Long daily streamflow series from two gauging stations, Cuntan and Yichang, are analyzed to quantify the effect of the Gezhouba- and the Three Gorges Dams on the Yangtze River flow variations. The Cuntan- and Yichang stations are located up- and downstreams of these two dams, respectively. The quantification entails the employment of conventional multifractal analysis (MFA) and MF-detrended fluctuation analysis (MF-DFA). The streamflow series are divided into six segments based on the time when the Gezhouba- and Three Gorges Dams were constructed. Thus, the effect of these two dams can be compared through MF properties of streamflow before and after the construction of water reservoirs. The effect of the Gezhouba Dam on streamflow downstream may not be reflected by conventional MFA but can be seen from the results of MF-DFA. It should be due to the fact that MF-DFA is on the basis of fluctuations around the dominant trend, reflecting more local information; while the box-counting algorithms investigate the streamflow from the whole view. Particularly, for the inter-station comparison of results obtained by MF-DFA-based analysis, the strongest impact on the streamflow downstream is indicated by the most significant difference in generalized fractal dimension spectrum appearing during the construction of Gezhouba Dam. In addition, after the construction of Gezhouba Dam, the minimal MF dimension at Yichang station start to be less than that at Cuntan station, suggesting that the streamflow becomes less fluctuated, which should be attributed to the filter effect of water reservoir. This study presents a feasible way to evaluate, wholly and locally, the impact of water reservoirs on streamflow in other river basins in the world.