1. College of Mathematical Science; Tianjin Normal University; No. 393 Binshuixi Road, Xiqing District Tianjin 300387 P.R. China
2. Tianjin Key Laboratory of Water Resources and Environment; Tianjin Normal University; No. 393 Binshuixi Road, Xiqing District Tianjin 300387 P.R. China
3. College of Urban and Environmental Science; Tianjin Normal University; Tianjin 300387 China
4. Department of Hydrology and Water Resources; The University of Arizona; John Harshbarger Building, 1133 E. North Campus Drive Tucson AZ 85721-0011 USA