国家自然科学基金青年科学基金51108029;中央高校基本科研业务费专项TD2012-03;水利部公益性行业科研专项201201032Supported by the National Natural Science Foundation of China51108029;by the Fundamental Research Funds for the Central UniversityTD2012-03;by the Non-Profit Industry Financial Program of Ministry of Water Resources201201032