1. College of Environmental Science and Engineering, Hunan University, Changsha 410082, P. R. China
2. Key Laboratory of Water Cycle and Related Land Surface Process, Institute of Geographic Science and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China
3. College of Mathematics and Econometrics, Hunan University, Changsha 410082, P. R. China
4. College of Information Science and Technology, Hunan University, Changsha 410082, P. R. China