Electrical Resistivity Tomography Monitoring and Modeling of Preferential Flow in Unsaturated Soils

Preferential flow (PF) often occurs in unsaturated soils which allows water, pesticides, and fertilizers to move quickly through the unsaturated soil to the groundwater table with limited degradation and filtration, thereby deteriorating groundwater quality. On the other hand, due to the existence of the PF channel, the soil moisture and nutrient in the farmland are reduced, which not only reduces the utilization efficiency of water and fertilizer, but also degrades the soil function, and even is not suitable for agriculture. In order to reduce groundwater pollution and prevent land degradation, research on the mechanism of PF is of great importance. Therefore, this review presents a comprehensive summary of the main types of PF, influencing factors, model theories, and monitoring techniques to provide accurate simulation and observation options for research on PF. According to the review, soil texture and structure, initial and boundary conditions of infiltration, and instability of water movement constitute the main influencing factors of PF. The review has also revealed that continuity models are relatively easy to carry out but are restricted to characterizing finger flow or flow with high spatial variability. The discrete approaches are capable of modeling macropore and finger flow. However, they only work well for small-scale applications. Fractal approaches have been reported as a powerful tool for describing PF in detail, while in-depth studies on the relationships between fractal parameters and soil characteristics, as well as efficiency, are still greatly needed. Electrical resistivity tomography (ERT) has been increasingly applied to study the detailed characteristics of PF under both field and laboratory conditions. Its advantages include non-invasive PF detection, distinct visibility, and no requirement for expensive and special instruments. This review presents the basic principles that determine how ERT is applied for monitoring preferential flow.