Abstract: It is an indisputable fact that rainfall can induce landslides in loess area; however, groundwater levels are always very deep in loess area. At present, it is not clear that how does rainfall move in loess and whether it affects groundwater directly when infiltrating ground below. In order to simulate the natural rainfall condition, manual drip experiment is carried out. By using soil moisture meters which are inserted into the wall at a 10 m-deep exploratory well, the changes of moisture content with time of soil layers at different depths in the process of artificial rainfall infiltration could be easily observed; the range affected by rainfall could be confirmed later. The result shows that moisture content of soil layers within 0.5 m changed obviously when the precipitation is 3.82 mm/d (light rain), while that of below have almost no changes; the moisture content within 1 m increases when the precipitation is 10.31 mm/d (moderate rain); the moisture content of the layers within 1 m increases significantly, and that of between 1 m to 1.6 m have a slight rise when the precipitation reached 25.21 mm/d (heavy rain), the changes of moisture content lag and the growth decreases gradually with depth. It is shown that if there are no obvious water paths in arid loess region, the infiltration depth of short-term rainfall is limited and it is difficult to reach the groundwater level, but according to the results of paleosol in deeper, the moisture content of paleosol has a considerable increase even though that of upper loess is relatively weak, which indicates that unsaturated seepage or vapor migration do exist in loess. It is also found that the circulation of soil moisture is primarily occurred in evaporation zone; that within shallow 0.7 m in Longdong Loess Plateau, if there is no recharge from follow-up rainfall, the rainfall infiltrated within evaporation zone will be excreted upward by evaporation; however, it will continue to move downward without the influence of evaporation when infiltrating beyond the evaporation zone; the water will gather on the surface as encountering the impervious layer, which is also reflected preferably by finite element simulation.

Key words: soil moisture, volumetric water content, infiltration, rainfall, loess, soil-water characteristic curve, permeability coefficient