In-situ observation on rainfall infiltration in loess

By many artificial rainfall tests,the depth of rainfall infiltration in loess area was 4 meters atmost. So,it is considered that the rainfall is hard to reach groundwater through the normal leaking way,but through vertical fissures,sinkholes or some other water paths to recharge groundwater. However,accord-ing to our investigation,these water paths only occur in unloading areas which always in the edge of loesstableland,but rare in the center. In order to make clear surface water recharge groundwater in loess area,a monitoring site was set up in Zhengning county,Gansu Province. By using the soil moisture meterswhich inserted into the wall of a 10-meter-deep exploratory well,the changes of volumetric moisture con-tent of soil layers with different depth were observed continuously for one year under the condition of natu-ral rainfall,the daily precipitation during the monitoring period was also recorded by rain gauge. The re-sults show that the moisture content of the shallow layers within 2 meters changed in annual cycle,andthe trend is consistent with the change of evaporation. When the precipitation is less than 18 mm/d,the cir-culation of soil moisture occurs mainly in surface layers,and has little effect on the moisture content be-low 20 centimeters. But when the precipitation is more than 18 mm/d,the moisture content may have asudden increase. The greater the rainfall,the higher the growth of moisture content and the deeper the af-fected range will be. With the increasing depth,the growth decreases and the change of moisture contentlags behind the depth. The infiltration zone of the loess is about 2 meters. In the unsaturated soil below 2meters,water moves mainly in the form of unsaturated seepage or vapor form,the motion of vapor is verysmall but cannot be ignored,the water would gather on the surface as encountering paleosol layer with lowpermeability,weak zone will be formed after long-term accumulation,inducing loess landslides eventually.