ERT And TDR Combined Inversion Of Layered Soil Water Movement Process

Soil is a three-phase independent body that is composed of solid, liquid and gas. Soil as a kind of porous medium, its skeleton has features with irregular shape, complex arrays, and existing broken solid phased particles, etc. And there are a lot of moving water and air in the pores between the particles, so the study of the soil is more complex. The soil water is the most active part of soil, which plays a very important role in soil formation process. At the same time, soil water occupies an important position in “four water transformation" processes which included atmospheric precipitation, surface water, soil water and groundwater. Soil water is the link between surface water and ground water. At present, the soil is often considered as a homogeneous and isotropic medium in most of the studies of soil water movement, but because of the effects of natural and human factors, they make the field soil often exhibiting layered structure, the layered structure will influence the moving characteristics of soil water and solute migration. Therefore proving the layered soil water and solute transport rules has important theoretical and practical significance for the accurate prediction of pollutant, soil water and nutrient transport in soil.In order to understand the water movement in the layered soil, the study is conducted in Jimo of Qingdao. Water injection infiltration experiment was carried out on the field profile. The experiments monitored the change of soil resistivity with time and space before and after water injection by using DCX- 1 G multifunction and high density electrical instrument, real time imaging system(ERT). Since the resistivity is a basic physical properties of soil and the change of soil water content can cause changes in the soil resistivity, thus combining the soil volumetric water content and laboratory test data related to the measurement of time domain reflectometry(TDR), quantitative research study of 2D vertical section of soil water movement characteristics. Using HYDRUS-1D software to simulate the movement of soil water and comparing with the measured values todetermine that ERT has high resolution monitoring of soil water movement process in spatial and temporal scale. This has an important significance for further optimization study of solutes and contaminants in the soil movement and the saving water irrigation. The research of this paper draws the following conclusions:(1) According to the volume of soil water in the soil resistivity and TDR measurements obtained by ERT monitoring in the process of the infiltration water, established the quantitative relation between the resistivity and water content. Due to the layered distribution of soil in the study area, and the basic physical and chemical properties of the soil layer is different, the three layered soil resistivity and water content can be fitted respectively, and the relationship between water content and resistivity in 0 to 30 cm and 60 to 90 cm layered soil can be obtained, the square of the correlation coefficient are 0.661 and 0.67, respectively. The correlation is good and it can satisfy the calculation accuracy. However, because of the clay content in the soil is higher between 30 to 60 cm, the fitting of the square of the correlation coefficient is 0.509, so the fitting correlation is not very well.(2) By comparing several different algorithms for the inversion of the high density resistivity method, the time-lapse inversion algorithm can better reflect the spatial distribution of soil resistivity than other algorithms. The resistivity of surface soil decreased rapidly after water injection. With the infiltration time, the surface soil moisture content increased in the soil surface layer formed a thin saturated layer, so that the change of the surface resistivity of the soil decreased, and gradually stabilized. Due to the presence of the clay layer, the water is not moving at the interface of the soil to reach the soil surface. But when the soil water content accumulated to a certain value, just continue the downward infiltration. And the rate of movement of water down compared to the surface layer of soil is slower. At the interface of two soil layers, the soil water start to infiltrate to the lower layer when the soil water content of the upper layer reaches 0.183 g/g through the established relationship between soil water content and resistivity. At the same time, it can be seen from the water infiltration process image, the movement of soil moisture is mainly downward movement, and there is still a weak horizontal flow.(3) Based on the determination of water content in soil profile and the change of chloride content with depth, the maximum actual infiltration depth of soil moisture was obtained. By comparing the changes of soil water content, the maximum depth of water infiltration in the point water injection experiment was 60 cm, and the maximum depth of infiltration was 63 cm, which was calculated by ERT. The maximum depth of the linear water injection experiment was 90 cm, and the maximum depth of the infiltration by ERT was 81 cm. So ERT can reflect the maximum depth of water infiltration well.(4) Using HYDRUS-1D software simulate the process of water injection infiltration experiment, the changes of soil water content at every moment were obtained. And they compared with the water content calculated by ERT, it can verify the accuracy of quantitative monitoring of soil water infiltration process by ERT. By comparison, it is found that the change of moisture content with time and depth is consistent with the trend of ERT monitoring, but because the time is not enough the numerical simulation of soil infiltration process, the error is seem larger.