The severe sandstorms encroaching on Beijing in the spring of 2000 were the result of drought over Beijing's upstream area.These
make us pay more attentions to the deterioration of the climate environment.In this paper we investigate the possible causes of drought based on the NCEP/NCAR monthly data of volumetric
Soil Water (Sw) Content at the surface layer(0～10 cm depth) in the
central Inner Mongolia region for the period 1950—1999. The results show that SW over the central Inner Mongolia region displays the significant seasonal variation,of a two-wave pattern. High SW occurs in the frozen season (December-March) and rainy season (August-September).Early summer and late fall are the dry season whose SW is very small;May-June is the driest period.The soil in north China has become drying since the middle 1960s with the severest drought period in the 1990s.The interdecadal variability of the period of 20～30a is one of the most important components in SW variations.Warming winters,especially in the 1970s and 1990s may be the most important cause for the drought of surface layer soil in north China.Artificial devastation of surface vegetation expedites the drought course because of increased surface moisture evaporation.Summer precipitation might be the secondary cause for the drought.Our results also demonstrate that the SW can be used as a valuable index for depicting drought.