Rainfall Infiltration Pattern In The Vadose Zone

Understanding the nature of water movement in the vadose zone and its quantification is essential to solving a variety of problems. Examples of such problems are: prediction of landslides probability induced by rainfall; assessment of the potential for aquifer contamination due to migration of water soluble chemicals present in the vadose zone; control of erosion events from given precipitation; and estimation of water recharge to the underlying aquifer. The key point in understanding this mechanism is the choice of an appropriate method to calculate the volume of rainfall infiltration. Therefore, the objection of this dissertation is to choose one such method, based on which further investigation the mechanism of rainfall infiltration in shallow un-saturated zone is conducted. The main work in the thesis is summarized as follows:1. Based on self-design column test, the progress of rainfall infiltration is revealed and its influencing factors analyzed. Experiments show that: ①For the case where rainfall intensity exceeds the saturated permeability, it can be divided into three stages based on the ponding point and the saturated point. They are the rainfall intensity stage, the infiltration capacity control stage and the saturated infiltration stage.②Rainfall intensity and initial water content are important infiltration factor which influence rainfall infiltration content.2. Based on the experiment results, the Mein and Larson's method that accommodates the boundary condition of uniform rainfall is evaluated. Mein and Larson's method can not be well utilized because enough assumption and the instability parameter(Ψf) which influence the results and is difficult to be defined.3. Adjusted the numerical method that accommodates the boundary condition of rainfall and analyzed sensitivity of unsaturated parameters to the method after understanding the infiltration capacity. The method that judges surface soil layer as turning to saturated stage by numerical calculated infiltration capacity is problematic. In this paper, the measured matric suction of surface soil is taken as the more appropriate criterion.By comparing experiment results with the simulation ones, it is suggested that a more realistic simulation can be made using the method proposed in this thesis to calculate rainfall infiltration content.4. Based on the improved method, numerical simulations are carried out to analyze the sensitivity of infiltration factors and to study the relationship of water movement with the rainfall infiltration. ①With the increase of soil density and the smaller the soil particles, the evident the capillary barrier effect within soil because of the reduction of the capacity of rainfall infiltration.②If the infiltration intensity is less then the saturated permeability coefficient, the surface soil finally reaches to an unsaturated infiltration stage instead of saturated infiltration stage.