Spatial and Temporal Variation Characteristics of Drought in Northwest China During 1961-2009with Standardized Precipitation Index

In this paper,monthly precipitation data from 137meteorological stations in Northwest China from 1961to 2009 were used to calculate the standardized precipitation index(SPI).Then the frequencies and area ratios of the light drought,severe drought and extra severe drought,monthly and in the four drought durations(late spring,early summer,summer and autumn),were calculated.Their spatial and temporal characteristics were also analyzed.The results show that(1)northern Xinjiang region,central Qinghai and Hexi Corridor of Gansu are the regions with highest drought frequencies,more than 15months.While in southern Xinjiang region the drought frequency is lower,except for individual months;locations of the drought area has an move forward trend from south to north,from west to east with months;(2)the severe drought frequencies of the most parts of Northeast China are less than 5months,expect for individual areas in Xinjiang,Qinghai and Gansu.The frequency in southern Xinjiang region is still lower.The frequency in summer is higher than that in the other seasons.Northern Xinjiang region and Hexi Corridor of Gansu are places with extra severe drought;(3)drought area ratios of different grades has the similar annual changing characteristics,which generally can be divided into three periods:high drought frequency period(the average drought rate was about 35%) from 1961to 1980,transition period(the drought rate was about 15%)from 1981to 1990and stabilization period(the drought rate less changed,with a slight increasing in recent years)from 1991 to 2009;(4)global warming leads to increase precipitation and glacier meltwater,streamflow and lake water level,causing drought area rate declining since 1987.