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In this study, we monitored the surface soil moisture at 15 locations within an area of 100 km x 100 km in the Mongolian steppe region. A principal component analysis was used to determine the characteristics and dominant patterns of spatiotemporal variations in the measured surface soil moisture. The results reveal that the predominant pattern of temporal variation is related to large-scale rainfall and that this was spatially uniform over the entire study area. In terms of both temporal and spatial patterns, the spatial heterogeneity of rainfall activity was a minor factor, accounting for less than 33% of the total observed variance. The most dominant pattern in the spatial distribution of surface soil moisture showed a correlation with the spatial distribution of the mean or minimum water content, and the minimum water content was largely equivalent to the, wilting point, which is a soil-hydraulic index. Thus, we conclude that the most dominant pattern of surface soil moisture distribution over the study region is regulated by soil-hydraulic properties. Rainfall events acted to disturb or reset this pattern, but the pattern was re-established during drying of the soil.