1. School of Statistics, Dongbei University of Finance and Economics, Dalian 116025, China
2. MOE Key Laboratory of Western China’s Environmental Systems, Research School of Arid Environment & Climate Change, Lanzhou University, Lanzhou 730000, China
3. School of Mathematics and Statistics, Lanzhou University, Lanzhou 730000, China
4. China Water Resources Beifang Investigation Design and Research Co. Ltd., Tianjin 300222, China