Acknowledgements
Thanks to the research grants from Chinese National Natural
Science Fund (50709038, 50679081), Chinese National 863 High
Technology Research Plan Project (2006AA100203,
2006AA100210), Hong Kong Research Grants Council (HKBU
2465/05M) and Hong Kong University Grants Committee (AoE/
B-07/99). We also wish to thank all the staff of Wuwei Institute
of Water Conservancy for their assistance in fieldwork