Background The vertical variation and spatial variability of soil moisture have important effects on the process of rainfall, infiltration and runoff.

Methods In order to study the relationship between rainfall and soil moisture response in the Loess Plateau, the vertical variation and spatial variability of soil moisture under 90 mm/h rainfall in Luoyu Valley watershed of Tianshui city was studied using dynamic monitoring and simulated rainfall method.

Results For natural condition, the vertical change in soil moisture could be divided into four levels:the rapid change layer (0-20 cm), the active layer (20-30 cm), the second active layer (30-40 cm), and the relatively stable layer (deeper 40 cm).There was stratification in the vertical distribution of soil moisture, and the slope distribution of soil moisture had significant differences in slope position (P < 0.05). Frequent exchange of soil moisture and air was also concentrated in the 0-40 cm soil layer range. During the rainfall, the activity of soil moisture was obviously enhanced, but the influence was mainly in the range of 0-30 cm soil layer, the influence of soil moisture by rainfall was negatively correlated with the soil depth. The change of soil moisture of 0-30 cm soil layer with time was not uniform, there were 3 periods of the rapid rise period, the stable period and the slight decline period. In deeper soil layer, there was a gradient difference in the vertical variation of soil moisture. Except for the surface soil, the rainfall infiltration only increases the soil moisture of each soil layer, while the proportion of soil moisture did not change, the distribution of soil moisture in the slope was more significant(P < 0.01). With the increase of soil moisture of 0-30 cm soil layer, the runoff rate showed the trend of increase to a stable value. The sediment yield rate increased to a peak value, and then decreased, became stable gradually.

Conclusions  The research of the relationship between rainfall and soil moisture played an important role in explaining the redistribution of soil moisture and the migration of organic matter, and provided scientific basis and theoretical guidance for efficient utilization of water resources and vegetation restoration and ecological reconstruction in hilly-gully region of Loess Plateau.