Drought changes of critical period of crop growth in Heilongjiang province based on CI index

Based on the daily meteorological data from 1961 to 2008,the meteorological drought comprehensive index(CI)was calculated,and the changes of drought frequency and drought intensity during the critical period of crop growth(from May to August)in Heilongjiang province were analyzed.The results show that the maximum and minimum drought frequencies are 79% and 58% and appear in 1960 and 1980,respectively.The position of drought center seldom changes,and most are located in the southwestern Songnen plains and focus areas move northward.The largest drought intensity occurs in 1960,then in 2000,and the larger areas of drought intensity are steadily located in the southwest Songnen plains and the middle and eastern Sanjiang plains.The drought has been frequent and intensity has increased during the critical periods of crop growth since 2000,especially in the north regions.In the past,drought event is seldom in the Sanjiang plains,but now it occurs from time to time and the intensity is stronger.