Characterizing soil preferential flow using iodine–starch staining experiments and the active region model

Preferential flow in the unsaturated soil is an important and common phenomenon, which is difficult to characterize. The major objective of this study was to analyze preferential flow distributions using visualized soil water flow patterns and the active region model (ARM), which has been developed recently. Thirteen infiltration experiments were conducted in the field, including different soils, various initial and boundary conditions controlling the infiltration processes, and measurement sizes. An iodine–starch staining method was applied to visualize flow paths in the soils. Distributions of stained coverage and soil water saturation within the stained region (the active region) were used to determine the key parameter (γ) of the ARM. Results showed that the ARM was able to capture the major features of the observed flow patterns under different infiltration conditions. Low initial soil water contents resulted in more heterogeneous flow distributions, corresponding to larger γ values. The applied amount of infiltrating water affected the flow heterogeneity more complexly. A small amount of infiltrating water resulted in a relatively homogeneous flow pattern. As the infiltration amount increased, the flow patterns became more heterogeneous. With increasing infiltration amount to a certain point, the flow distributions became less heterogeneous and finally relatively homogeneous. Corresponding to the flow pattern changes, γ values of the ARM increased first and then decreased with increasing applied water amount within a relatively narrow range. The relatively small changes of γ values within the measurement sizes seemed to indicate that the soil flow heterogeneity was independent of the measurement scales within a certain range.