Analysis on the Runoff Characteristics in and Out Dongting Lake in Recent 60 Years

In order to explore the runoff change characteristics in and out of the Dongting Lake Basin,using the annual runoff time series data,time series analysis was performed for nine stream flow gauging stations located at the main rivers of three outlets and Four Rivers(Xiang River,Zi River,Yuan River and Li River) in the Dongting Lake. The concentration degree and concentration period,Mann-Kendall trend test method,variation coefficient method and the runoff anomaly percentage were performed to analysis the annual runoff series of the main control hydrological station in Dongting Lake Area. The results showed that: ① The runoff concentration period of Dongting Lake Area mainly concentrated in June-July of each year,and the maximum runoff time is the end of June and early July. The flood occurred mainly in mid to late July. Runoff period synthetic vector directions are ranged from 103.2° to 190.2°,which can reflect the maximum runoff of the rivers in and out of the lake. The concentration period was in consistent with the actual runoff during the year distribution statistics. ② There is great variation of yearly runoff,with variation coefficient ranging from 0.194 to 0.761. It tells that the runoff stability is poorer. The ratios of extreme values are all above 0.6,and river runoff attenuation is more obvious. ③ Judging from the alternation regularity of runoff,frequency of enormously wet and dry year of Four Rivers Basin is lower than that of normal hydrological year. Namely,the frequency of normal hydrological year is relatively low in Four Rivers Basin. Frequency of enormously wet and dry year of Ouchikou is relatively high,accounted for 32.79% and 57.38% respectively. The interannual variation of runoff is relatively severe,which is not conducive to the rational use of water resources. It also implies that Dongting Lake Area is more frequent to encounter drought and flood disasters. Above conclusions show that the flood of the Dongting Lake is the emergence of flood the three outlets and Four Rivers Basin. But the flood is also controlled by the flood from the upper and middle reaches of Changjiang River. The long-term trend changes of stream runoff are controlled by the variation of climate changes,and impact caused by human activities and water conservancy project construction.