Preferential flow, an ordinary form of soil water movement, was proposed recently but had not been properly defined at first even though this expression has been adopted by various scientific communities. Preferential flow is a term describing a process whereby most of the water and contaminants move through porous medium following favored pathways from other parts of the medium. Research on preferential flow involves studying the mechanism of soil water movement from homogeneous to heterogeneous fields, and it is inevitable that performing experimental or technological analyses is difficult because this movement is not detected easily. This paper provides a broad overview of preferential flow with respect to definitions, types, basic meanings, basic theory, important characteristics and effectual factors, and systematically outlines the most recent methods to study preferential flow with quantificational experiments such as chemical tracing, mini-tensiometry and electrical resistance tomography, as well as other methods. We also provide a detailed analysis of modern apparatuses used effectively in the above-mentioned technological methods such as time-domain reflectrometry (TDR), tension infiltrometry and ground penetrating radar. Research that takes into account preferential flow can effectively explain the problems of hydrological processes which may not be addressed by Darcy’s law and the Convective-Diffusive Equation, but at the same time, considering preferential flow increases the difficulties and complexity of the research due to the non-equilibrium and regional characteristics and manifold types that are involved. To date, the mechanism of the movement of preferential flow still does not have a proper definition.
There is a lack of systematic standards, which should take into account the spatial and temporal scale. There is also a lack of equipment that can obtain international standard authentication and that has high speed and bypassing characteristics necessary to measure preferential flow. In the end, the paper discusses the direction of the development of future research.