Using Dye Tracer for Visualization of Preferential Flow at Macro- and Microscales

The detection of a nonequilibrium water flow and solute transport in structured soil at various scales is essential for better understanding of these phenomena. This study focused on the visualization of preferential flow in a Haplic Luvisol and Haplic Cambisol and their horizons by performing field ponding dye infiltration experiments. In addition, thin soil sections were made and micromorphological images were used to study soil aggregate structure and dye distribution at the microscale. The staining patterns within the vertical and horizontal field-scale sections documented the different nature of preferential flow in different soil types and also within the soil profiles. While preferential flow in the Haplic Luvisol was caused by soil aggregation and biopores, preferential flow in the Haplic Cambisol was caused only by biopores, large soil fractures, and incorporated straw material. Micromorphological images showed that, in the case of the Haplic Luvisol, the dye was primarily distributed either in the interaggregate pores and then in the pores inside the aggregates or in the isolated large pores connected to the dye source and then into the matrix pores. The dye distribution in the soil matrix was uneven as well. Accumulated organic matter, clay coating, and isolated larger capillary pores, which initially did not contain the dye tracer, behaved as less-permeable or impermeable barriers. Uneven distribution was caused by hierarchical pore size distribution of the soil matrix. Results indicated a multimodal character of preferential flow in this soil. In the case of the Haplic Cambisol, the dye pattern studied at the microscale was mostly affected by fractures and the size and shape of mineral grains.