Heterogeneous soil water flow and macropores described with combined tracers of dye and iodine

SummaryThe objectives of this study were to examine applicability of the iodine–starch method to visualize heterogenous soil water flow and to investigate relationships between soil water heterogeneity and macropore structure vs. measurement scale as well as hydraulic boundaries. The food-grade dye pigment Brilliant Blue FCF dye and iodine–starch staining patterns were used to visualize soil macropore and soil water flow patterns, respectively. Totally 10 infiltration experiments were performed in the field, among which three were used to examine the iodine–starch method and seven were for various boundary conditions (i.e., the different initial ponding depths on the soil surface) and measurement scales (i.e., the soil surface areas covered initially by the tracer solutions). The cluster analysis method was used to distinguish preferential flow regions, while information measures were applied to quantify heterogeneity information content and complexity of macropores and flow systems. Results showed that the iodine–starch method was applicable to visualize soil water flow. Heterogeneous soil water flow contained more heterogeneity information than soil macropores. With a low infiltration amount, flow patterns were similar to the macropore patterns and controlled mainly by the macropores. As the infiltration amount increased, flow patterns were influenced by both macropores and boundary conditions. As the measurement scales increased, the preferential regions were more developed.

Research highlights
► Soil water flow contained more heterogeneity information than soil macropores.
► With low infiltration amount, flow patterns were controlled by macropores.
► With high infiltration amount, flow patterns were influenced by macropores and boundary conditions.
► As the measurement scales increased, the preferential regions were more developed.