Discrimination of flow regions on the basis of stained infiltration patterns in soil profiles

Dye tracer experiments have often been used to study the prevailing flow regime, such as in investigating the role and extent of preferential flow of water. Flow patterns in two-dimensional profiles provide qualitative information on the infiltration regime but are difficult to analyze and compare quantitatively. The scope of this study was to develop a quantitative method to analyze the spatial distribution of the stained areas in vertical profiles, to identify differing transport mechanisms on the basis of the pattern information, and to analyze how the discriminated patterns correspond with soil properties and structure. Dye tracer infiltration experiments were performed on 25 plots at eight sites. The spatial distribution of the stained areas in vertical profiles was analyzed and compared using digital image processing. We first split the flow patterns into similarly stained horizontal layers based on the width distribution of stained areas. All of these layers identified in the flow patterns of all 25 plots were then partitioned into groups of layers with similar patterns by hierarchical clustering. The sequence of layers found in the pattern was finally interpreted with respect to transport mechanisms and qualitatively compared with the sequence of morphological layers observed in the soil profiles. The obtained classification reliably distinguished between zones of homogeneous infiltration and zones of preferential flow, but also between zones of narrow stained structures and zones of lateral spreading (e.g., sand or gravel lenses). Dye coverage and mean width of stained structures were the most indicative factors for the different clusters. We often found an agreement between the sequences of layers found in the flow patterns and the soil horizons. However, in all profiles we observed layers in the flow patterns that did not correspond to textural and structural layers observed in the filed. It seems that knowing the pedological horizons is important but not sufficient to understand the observed flow patterns. Since the flow pattern in a given layer always depends on the overlying soil layers, similarly textured soil layers do not necessarily exhibit equal patterns. However, soil layers with a given textural sequence (e.g., fine-course-fine) are reflected by typical flow patterns