Inferring flow types from dye patterns in macroporous soils

In this study, we exploit dye patterns to identify flow types in macroporous soils as a step forward from using dye patterns simply as qualitative pictures that illustrate preferential flow. Dye tracer experiments at different irrigation rates and initial soil moisture conditions were carried out on three hillslope sites. Several vertical and horizontal soil sections were prepared after each experiment to analyze the patterns of the dye tracer Brilliant Blue FCF. Photographs of the soil sections were processed by image analysis to discriminate between stained and unstained areas and to classify stained areas into three classes of dye concentration. The images of the vertical sections were first analyzed according to conventional approaches (e.g., dye coverage). Then, a new approach was developed using the extent and distribution of the stained objects to classify flow into five types, two of these flow types occur only in the soil matrix, and three of them are related to the degrees of water flow between macropores and soil matrix (interaction). From horizontal sections, the macropore distribution was classified and, in combination with the dye pattern, the interaction quantified based on statistical description of the spatial relationship between macropores and stained areas. Categorized flow type profiles showed a logical sequence in each of our experiments, and we think the concepts will have broad application in soil science and infiltration research to compare the infiltration regimes of soils.