1. Xi'an Center of Geological Survey; China Geological Survey; No. 438, Youyidong Road Xi'an 710054 China
2. UNESCO-IHE Institute of Water Education; P.O. Box 3015 Delft DA 2601 The Netherlands
3. School of Environmental Sciences and Engineering; Chang'an University; No. 126, Yanta Road Xi'an 710054 China