Abstract: Preferential flow, which contributes to the rapid water flow and solute transport, is common rather thanexceptional in natural unsaturated soils.Preferential flow allows irrigated water and applied agriculture chemicals to movethrough unsaturated zone to groundwater table quickly with limited degradation and filtration, increasing the losses ofapplied resources and energy, and making the groundwater under high contamination risks.There are different kinds ofpreferential flow, Macropore flow and finger flow are two of the most importance and receive tremendous of studies.Pipeflow, channel flow, funnel flow, bypass flow, oscillatory flow, short circuiting flow, heterogeneity-driven flow, subsurfacestorm flow, partial displacement flow, depression-focused recharge flow, and gravity-driven unstable flow also receive a lotof research interests because of the environmental problems and risks they induced.There are a number of factors toinduce preferential flow.Soil structure and texture, the initial and boundary conditions, incorporating with the instability ofinfiltration front, affect the generation and development of preferential flow patterns.Because of the variety of preferentialflow generating and impacting factors, and the diversity of preferential flow patterns, incorporated with the high-speed andnon-equilibrium characteristics of preferential flow, the description and simulation of preferential flow is always the hottopic and big question in vadose zone hydrology.A variety of modeling approaches have been developed to describepreferential flow in soil.These are mainly continuum, discrete, and fractal approaches.The continuum approaches arerelatively simple and straightforward to implement, but they are incapable of characterizing preferential flow paths causedby fingering and the spatial variability of soil properties.On the contrary, the discrete approaches have been successfullyused to represent field observations of preferential flow.However, the discrete approaches are limited to small-scaleapplications and the physical mechanisms underlying these approaches are still not totally clarified.To properlycharacterize heterogeneous water flow processes in the soil and benefit from the combined advantages of the continuum anddiscrete approaches, models based on fractal theory are developed recently.While some previous field studies support thefractal approaches, in-depth studies have not yet been conducted on physical mechanisms underlying these approaches,determination of the fractal parameters, relation between fractal parameters and soil characteristics, and efficiency ofapplying fractal approaches for representing practical preferential flow processes.To study the characteristics ofpreferential flow and to evaluate the efficiency of numerical models for representing preferential flow, it is essential tovisualize preferential flow from flow background.For this purpose, a variety of experimental approaches, such as micro-tension measurement, acoustic sounding, breakthrough-curves, non-invasion tomography, ground penetrating radar, andelectrical resistance tomography, have been developed.However, these approaches either require very expensive andpreferential machines or consume too much of time and labor.With the advantages of low price, distinct visibility, highwater solubility, and requiring no special detecting machines, tracing(e.g.dye tracing and iodine-starch staining tracing)experiments are increasingly applied to study the detail characteristics of preferential flow in both field and laboratory.Within all the dyes, the food-grade dye pigment Brilliant Blue FCF, which is with some other advantages as limited toxicityand inactive, is most commonly used.However, as the dye is adsorbed by soils with high clay and organic carbon contents,iodine-starch staining tracing experiment is determined as a much more effective technique to visualize preferential flowpathways, as the anionic properties of iodide ion with high mobility and low adsorption even in heavy clay soils.Althoughpreferential flow has received increasing studies these years, it is still far behind fully studied.Unifying the discriminationstandard, increasing modeling efficiency and developing special equipments for preferential flow study are the mainresearch directions in this field.