Abstract:

In recent years, the hydrological droughts frequently happened in the middlelower Yangtze River. The Three Gorges Reservoir with just finished the pilot of impoundment is thus involved in the debates of these drought events. The objective of this paper is to quantify the impact of water regulation of Three Gorges Reservoir (TGR) on water levels in the middlelower Yangtze River after flood season and to elucidate the real effect of the TGR on the lowering of water levels during this period. To separate the contribution component of the TGR regulation to the water levels from various influencing factors, a coupled hydrodynamic analysis model for the middlelower Yangtze River was developed and validated using the measurements of water levels and discharges from 2006 to 2011 in three controlling hydrological stations. We quantified the changes of water levels in the recent six years from 2006 to 2011 when the TGR carried out pilot impoundment, based on the simulation results from two scenarios with and without the TGR. Quantitative contributions purely induced by TGR regulation were quantified accordingly. The results showed that TGR regulations would have a significant effect on the water levels after flood season in the middlelower Yangtze River. The TGR regulation can make the water levels quickly decrease after flood seasons. And the TGR regulation can double the drawdown speed of the lake. It made the low water level occurred early about half a month in the middlelower Yangtze River in the recent six years. The lowering of water level induced by TGR regulation was 070 m at Luoshan, 060 m at Hankou, and 041m at Datong averagely from September 10 to October 31. Among these months, the most significant lowering of water levels was found in October and the decreases of water levels were all over 1 meter at the three stations. Apart from the magnitude of water levels, obvious changes were also observed in the frequency distributions of water levels at that time at three stations. The high water levels decreased in probability, while the probability of low water levels rised. The TGR regulation must decline water supply for the middlelower Yangtze River after flood season because of its normal operation. Thus, it is worth noting that the changing tendency of water levels might affect the use of water resources and wetland ecosystems significantly. Meanwhile, we should also note that the clear water release should possibly make a longdistance bed scouring in the main stream of the middle Yangtze River. It may induce a decrease of water levels further