1. Prof., Ctr. for Envir. Sci., Peking Univ., Beijing 100871, China; and Key Lab. for Water and Sediment Sci., Ministry of Educ., Beijing 100871, China.
2. Prof., Dept. of Hydr. Engrg., Tsinghua Univ., Beijing 100084, China.
3. Reader, Dept. of Engrg. Sci., Oxford Univ., Parks Rd., Oxford OX1 3PJ, U.K.