1. School of Mathematics and Statistics , Hubei Normal University , Huangshi , 435002 , P. R. China
2. School of Mathematics and Statistics , Hubei Normal University , Huangshi , 435002 ; and National Cryosphere Desert Data Center, Lanzhou 730000 , P. R. China
3. National Cryosphere Desert Data Center , Lanzhou 730000; and Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences , Lanzhou 730000 , P. R. China
4. Center for Research and Development in Mathematics and Applications (CIDMA) , Department of Mathematics , University of Aveiro , 3810-193 Aveiro , Portugal