By using the expression of precipitation grade in Atlas of the Distribution of Droughts and Floods in China over the Past Five Hundred Years,ranked precipitation data from 2000 to 2008 was completed.Then according to run-length distribution theory,selecting normal year of precipitation as the interception grade level,the precipitation levels which greater than the normal year was defined as the positive run-length,which less than the negative run-length.The positive and negative run-length distribution was ...