Drought Index in Summer Season of Northwest China

Based on the data from 125 stations during the periods of 1960～2004 in the Northwestern China,the number of rainless day and the total precipitation in summer season from June to August were calculated and found that there is a significantly negative correlation between the number of rainless day and the Precipitation.On the basis of the fact mentioned above,the drought index was defined and the spatial and temporal distribution of drought occurrence was analyzed through the drought index.The results show that the changing trend of drought index in Southern Part of Xinjiang and Southwest Part of Qinghain are reverse compared with other areas,which are two major features of spatial distribution of summer drought index in Northwestern China.According to summer drought index Northwestern China can be classified into five sub-regions: the Eastern Qinghai Plateau,the Hexi corridor,the North Area of Xinjiang,the South Area of Xinjiang and the East Area of Northwest China.Of which,spatial distribution of summer drought index in Eastern Qinghai Plateau over the last 45 years presents continuous increasing tendency and in the South Area of Xinjiang summer drought index during past 45 years presents constant declining tendency;while in the North Area of Xinjiang and the East Area of Northwest China as well the Hexi corridor the changing tendency of summer drought index are reverse with time series before and after 1982.In all five sub-regions,spatial distribution of summer drought index over the past 45 years presents remarkable vibrating change and high frequency vibrating change.