基金This study was funded by the Ministry of Science and Technology of China （2001BA611B-01）, SCSMEX Project （1996-2003） and the Ministry of Water Resources of China. A number of investigators have contributed to the results summarized in this paper. They include G. Ren, P. Zhai, G. Tang, L. Zhang, H. Liu, F. Ren, X. Zou, Z. Chu, and more. The authors also thank Ms. J. Zhang and Ms. Y. Song for their great assistance in preparing this paper.