1. School of Mathematics, Sun Yat-sen University, Guangzhou, 510275, China.
2. Department of Biostatistics, Columbia University, New York, NY 10032 USA.
3. Corresponding author. School of Mathematics, Sun Yat-sen University, Guangzhou, 510275, China.
4. U.S. Geological Survey, Wetland and Aquatic Research Center, Gainesville, FL 32653 USA.