国家基础科学人才培养基金北京师范大学地理学基地项目No.1103403;国家自然科学基金No.41171360;北京师范大学本科生科研基金No.2013-12 Supported by the National Fundamental Fund for Personnel Training Beijing Normal University Geography Base No.1103403,the National Natural Science Foundation of ChinaNo.41171360 and the Undergraduate Research Program of Beijing Normal University No.2013-12