Analysis on Probability of Extreme Drought Period from May to September over Northwest China
西北地区5-9月极端干期长度的概率特征分析

The extreme drought period from May to September was counted form the daily precipitation data of 131 stations in Northwest China from 1961 to 2000.Further,we have studied the distribution of extreme drought period over Northwest China and have analyzed its correlation with total precipitation.The distribution of extreme drought period over Northwest China was simulated by the Gumbel Distribution Model by adopting Probability Weighted Method(PWM) to estimated the parameters of distribution function.The results showed that the spatial distribution of mean extreme drought period and the distribution of annual precipitation in recent 40 years were generally reverse over the Northwest China,but the region with the shortest extreme drought period didn't superpose the most precipitation region in south Shaanxi;the shortest extreme drought period located in south Qinghai and south Gansu plateau,with high sea level elevation.The relationship between the extreme drought period and the total precipitation from May to September is negative.The distribution of extreme drought period once in 50 years and the distribution of probability more than 20-day extreme drought period is similar.