Key Role of the Lakes in Runoff Supplement in the Mid-lower Reaches of the Yangtze River During Typical Drought Years

To examine the abnormal runoff characteristics and their influencing factors along the mid-lower reaches of the Yangtze River (MLRYR) during the different extreme drought hydrological years of 1978 and 2006, history records of water levels and discharges were collected at Yichang, Hankou, Datong, Chenglingji, and Hukou stations. The Dongting and Poyang Lakes in the middle part play important roles in supplement runoff of Yangtze River during extreme drought years. In these years, the widening gap of the water level at Chenglingji and Hukou stations, gathered the outflow speed of lakes and supplied more water to the Yangtze River. In 1978, before the building of the Three Gorges Dam (TGD), the water supplied by the two lakes was 1.946×1011 m3 and 8.95×1010 m3, accounting for 30% and 14% of the runoff at Datong station at the same time, respectively. The lowest discharge at Datong station reaches 7220 m3/s. After the Three Gorges Dam´s building up, discharges in the whole year at Datong station were more than 10000 m3/s by the adjustment of lakes (reservoir) in 2006 and the annual average was 20975 m3/s. In the spring of 2007, TGD had a positive impact on the discharge at Yichang station, the discharge was more than 4000m3/s. The water supplied by Dongting and Poyang Lakes was 1.853×1011 m3 and 1.444×1011 m3 during 2006, accounting for 28% and 22% of the runoff at Datong station at the same time. This study results showed that the lakes had played important roles in supplement of water along MLRYR in the extreme drought years.