基金Supported by the National Natural Science Foundation of China (50809008, 50609001)the Hong Kong Research Council (HKU 7171/06E)+2 种基金the Open Foundation of Hunan Province Key Laboratory of Water & Sediment Science and Water Hazard Prevention (Grant No. 2008SS04)the China Postdoctoral Science Foundation (20060400972)the Dalian Science and Technology Foundation (2007J23JH027)