Impacts of the Three Gorges Dam on the streamflow fluctuations in the downstream region

Abstract

The impacts of dams on the downstream streamflow is an important issue for the downstream flood control and water resources management. Most of the existing researches only focus on the impacts of dams on the first-order behavior of streamflow, i.e. its magnitude, but ignore the impacts on the second-order behavior of streamflow, i.e. its fluctuation. This study is aimed to study the impacts of the Three Gorges Dam (TGD) on the downstream flow in terms of not only their magnitudes but also fluctuations. The study method includes two steps, i.e. first to use the ARMAX-GARCH model to simulate the conditional mean and conditional variance of the single-site downstream streamflow series, and then to use the DCC-GARCH model to simulate the conditional covariance between streamflow series at different downstream sites. The method is applied to the downstream data series of the TGD for both the pre-dam period and post-dam period. By comparing the results of the two time periods, the conclusions are made as follows: (1) The TGD alleviates the extreme downstream streamflow, and this effect gradually weakens along the river; (2) the memory of single-site streamflow fluctuations in the mainstream of the middle reaches of the Yangtze River has been weakened by the TGD, the memory of streamflow fluctuations in the mainstream of the lower reaches has been enhanced; (3) the single-site streamflow fluctuations of Yichang and Datong have been enhanced by the TGD, and the streamflow fluctuations of the four sites between Yichang and Datong have been weakened; (4) the joint fluctuations (i.e. the covariance) and correlation between the streamflow series at different sites in the middle reaches of the Yangtze River have been weakened by the TGD, the joint fluctuations and correlation at different sites in the lower reaches have been enhanced. The results of this study are helpful for an improved understanding of the impacts of the dam on the downstream streamflow.