Runoff-sediment Variation and Its Effect on the Dongting Lake

The water-sediment variation is the key factor for the Dongting Lake evolution and the river-lake relationship adjustment. The variation characteristics of runoff and sediment transport at the important hydrometric stations of the Dongting Lake were analyzed according to the data from 1956 to 2008, with the methods of mathematical statistics, Mann-Kendall, wavelet analysis and accumulating filter. The results show that: 1) The runoff entering the Dongting Lake mainly come from the 4 rivers of Hunan Province (accounting for 59.2% of the total outflow) and the sediment mainly come from the 3 outlets of Jinjiang River (accounting for 80.9% of the total). 2) The runoff and sediment entering Dongting Lake from the 3 outlets decreased significantly, because of the impact of cutting off the Jinjiang River, operating Gezhouba water structure, using the Three Gorges Reservoir and water and soil conservation of the upper reaches of the Yangtze River. The runoff and sediment entering from the three outlets have respectively decreased from 42.6% and 87.7% before cutoff to 21.7% and 59.6%. 3) In recent 50 years, the total sediment deposition has reached up to 52.9xl08t, however, the sediment deposition rate significantly dropped from 73.3% before the Jinjiang cutoff to 34.0% in the initial operation of the Three Gorges Reservoir. The sediment decrease is favorable for the lake maintenance and prolonging the lake life. In the initial operation of the Three Gorges Reservoir, however,the decrease of runoff and sediment from the 3 outlets may also aggravate the shortage of water supply in low water period in the west of Dongting Lake and decrease the water environmental capacity. Meanwhile, the sediment accumulation in the lower reach below Chenglingji station will lead to the rising of flood level in Dongting Lake.