Preferential flow, which contributes to the rapid water flow and solute transport in unsaturated soils, is common in the natural unsaturated soils. Preferential flow allows irrigated water and applied agriculture chemicals to move through unsaturated zone to groundwater table quickly with limited degradation and filtration, increasing the losses of applied resources and energy, and making the groundwater under high contamination risks. The non-equilibrium water movement, via preferential flow channel, to deep soil or even groundwater is one of the important factors inducing engineering and geological disasters such as land subsidence and collapse, landslide, debris flow and mountain collapse. However, the capturing of its dynamic process, especially the identification and judgment on the type, position and evolution of preferential flow without destroying soil structure, is still the hot topic and hard nut to crack in both science and technology all over the world. In this research, Electrical Resistivity Tomography (ERT) was applied to monitor the filed infiltration process of NaCl solution in situ. The distribution and change of apparent electrical resistivity of the monitored soil profile was measured at different time during infiltration. And the distribution of Cl- concentration of the monitored soil profile was analyzed in laboratory by soil sampling after infiltration. Based on these measured data, the heterogeneous distribution characteristics of soil structure, and the position, type and evolution of preferential flow in the monitored soil profile were analyzed and identified. Besides, the relationship between apparent electrical resistivity and Cl- concentration of the monitored soil profile was analyzed to evaluate the precision of applying ERT to identify preferential infiltration. The results showed that soil structure and properties affected the movement and distribution of applied NaCl solution, on the contrary, the movement and distribution of applied NaCl solution also affected the soil electrical conductivity and resistivity. Thus, the distribution characteristics of soil structure and properties were able to be detected by comparing the ERT monitored distribution of soil electrical resistivity before and after infiltration. The structure of the soil profile monitored by ERT was not uniform, with the soil within the horizontal direction of 5.0-10.0 m being much denser than that within the horizontal direction of 0-5.0 m. The preferential flow channel that constrained the applied NaCl solution with a greater concentration obviously changed the distributions of soil electrical conductivity and resistivity. And the distributions of soil electrical conductivity and resistivity changed as the preferential flow developed. Thus, the kind, generation position and evolution process of preferential flow were able to be detected by comparing the ERT monitored distribution of soil electrical resistivity at different time during the preferential flow process. During the infiltration process, a heterogeneous fingering flow was developed in the loose soil area within the horizontal direction of 4.0-5.0 m. The fingering channel was completely formed during 9-14 minutes after the application of NaCl solution to the monitored soil surface, and the preferential flow completely degraded to matrix flow no late than 60 minutes after the application of NaCl solution to the monitored soil surface. Power function was capable of establishing relationship equation between apparent electrical resistivity monitored by ERT and the measured Cl- concentration of the monitored soil profile (the coefficient of determination of 0.690). As the preferential flow repeated along the same path once the preferential flow was formed, the ERT monitoring the infiltration process of NaCl solution was of good efficiency in identifying the position and evolution of preferential flow in engineering and geology survey. This results provide valuable information for the prevention and control of losses of applied resources and energy, groundwater contamination and engineering and geological disasters caused by preferential soil water flow. © 2021, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.
