Preliminary Studies on Variations in Droughts over China During Past 50 Years

After preliminary quality control such as removing missing values, daily precipitation and temperature data of 606 observatory stations across China are selected from the original 731 station dataset to calculate drought index in different regions. Variations in droughts are analyzed based on the calculation results of the compound drought index I_C. The drought index I_C is recommended as an effective operational drought monitoring index from National Standard. I_C has been applied by National Climate Center to monitor drought conditions across the whole country in recent years. The drought areas of different regions and the whole country during 1951-2006 are estimated by transferring the station observations into grid values. In order to discuss changes of drought areas in different regions during the past half century, Kendall'stau method is used to compute the trends of drought conditions.Results indicate that for the country as a whole, there is no obvious increasing or decreasing trend in drought area during past 50 years. Changes in drought areas are not even in different regions. Significant increases in drought area are found in North China and Northeast China with trends of 2.50%/10a and 2.31%/10a respectively. During the period of late 1990s to early of the 21st century, some regions in North China and Northeast China experience unprecedented drought conditions since 1951. There is no evident increasing or decreasing trend in drought areas in the mid-lower Basins of the Yangtze, South China and Southwest China. Decreases are found in Northwest China and the Tibetan Plateau since late 1980s. In China, the longest drought centers mainly occurred in western part of Northeast China, central North China and Yunnan Province etc. These extreme droughts often last for more than 4 months. Statistically most of the longest droughts occur in the period after the year 1980 during past 50 years in Northeast China, North China, eastern of Northwest China, South China and Sichuan Basin etc. Based on the calculations of drought days between the period of 1951-1978 and the period of 1979-2006, it suggests that droughts become much frequent in eastern and southern parts of Northeast China, North China, eastern Northwest China, South China and southern part of Yunnan Province in the later 28 years. While in the mid-lower Basins of the Yangtze and some regions of Southwest China, drought days decrease. No evident increases or decreases of drought days are found in other regions of the country.In addition, the drying trends in China such as North China, Northeast China and some regions of the eastern of Northwest China are mainly detected in the places where temperatures are increasing much more evidently across the country, which indicates that both high temperature and increased drying are produced by regional warming progress.