Based on the NCEP/NCAR monthly reanalysis data from 1979 to 2008, this paper has analyzed the variations of the atmospheric water resource over the arid region of Northwest China (ARNC) from the aspects of water vapor transport, precipitable water vapor and water vapor budget, and then has revealed the main causes of the variability. The results show that the annual precipitable water vapor over ARNC has shown increasing trend during the recent 30 years, while the summer water vapor net income has displayed an even more remarkable increasing trend. The westerly water vapor transport in the middle latitude had very important contribution to the variability of the atmospheric water resource over ARNC. The interdecadal descending characters of the regional westerly wind index implies the weakening of the water vapor transport both at the eastern and western boundary of ARNC, the descending trend of the water vapor budget at the eastern boundary, however, was much more significant than that at the western boundary due to the effects of the huge geomorphological structure of ARNC, thereby resulting in a lot of water vapor stranded in the arid region, and the total summer water vapor budget increased.%利用1979-2008年NCEP/NCAR月平均再分析资料,从大气可降水量、水汽输送及收支等方面分析西北干旱区空中水资源的时空变化特征,并揭示其主要原因.研究表明,1979- 2008年西北干旱区整层年水汽含量略呈增加趋势,而夏季水汽总收入呈现显著增加趋势.区域空中水资源的变化与中纬度西风水汽输送密切相关.尽管区域西风指数的年代际减弱反映了西北干旱区东、西边界西风水汽输送的减弱趋势；但受区域特殊地形的影响,区域东边界水汽收支的减弱趋势远强于西边界,使得大量的水汽滞留于西北干旱区,从而导致该区域夏季水汽总收入呈现增加趋势.
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