Field observation and micro-mechanism of roots-induced preferential flow by infiltration experiment and phase-field method

Plant roots potentially contribute to slope instability by causing infiltrating water to be spatially and temporally redistributed in the soil unevenly following rainfall. But the root system's macro-scale impact on water infiltration and the formation and dynamics of pore-scale preferential flow induced by roots are still ambiguous. A field dye infiltration experiment was used to simulate rainwater infiltrating into the soil to quantify the relationship between preferential flow pathways and root characteristics such as root density and root number. A two-dimensional phase-field model was used to describe quantificationally and visually the influence of fluid pressure difference, wall contact angle, and pore throat size on vapor-liquid interface movement in the rootparticle and interparticle pores. Water flows more freely along an extension direction of shallow coarse-root (diameter > 5 mm), facilitating water transfer between adjacent profiles. The dye coverage and preferential flow fraction are found to be significantly positively correlated with the fine-root (diameter < 5 mm) density and the coarse-root quantity. At the pore scale, a low difference of fluid pressure exerts more control from the root system over preferential flow, especially, the vapor-liquid interface in the root-particle pore lowers to the pore outlet about 50 % faster than the inter-particle pore for no pressure difference. The larger wall contact angle of the particle causes a more pronounced lowering rate of the vapor-liquid interface. With smaller pores, the flow rate accelerates. Smaller pores make it harder for water to flow from the interparticle pores than root-particle pores. Our findings can help to more accurately determine the depth of water infiltration in the slope and deeply understand the physical mechanism of the effect of the root system on rainwater redistribution.