- Analysis of flow patterns and flow mechanisms in soils (2009)
- Matrix flow and preferential flow can occur concurrently in the same soil. Both flow regimes produce typical flow patterns that can be visualised in dye tracer experiments. To extract quantitative information from dye tracer studies a vast variability of approaches exists. One of them is to describe dye patterns by the so called dye coverage function, i.e. the percentage of stained area per soil depth. Based on extreme value statistics the dye coverage function can be reinterpreted as a probability function to find the tracer in a certain depth. Therefore, the two-parametric probability distribution 1 – H, H being the generalised Pareto distribution, can be fitted to the dye coverage function. The form parameter of this distribution serves as a risk index for vertical solute propagation. We did tracer experiments with Brilliant Blue FCF at three study sites: in a Norway spruce forest in southeast Germany, in a tropical mountain rainforest in southern Ecuador and on an agricultural field in southern France. We tested the ability of the risk index to summarise main information obtained in dye tracer studies and characterise flow patterns in different soils. Our results suggest that the risk index is to some degree invariant to changing experimental conditions (such as irrigation rate). The initial soil moisture, however, seems to have a large influence on the risk index. It is difficult to adjust the parameters of the generalised Pareto distribution when the dye coverage function fluctuates or does not decrease monotonically. This might be due to tortuosity of paths, varying flow mechanism or changing soil physical properties. Thus, we restricted the analysis to the lowest part of the profile. Since the theory of the risk index is based on extreme values of vertical solute propagation it is the lowest part of the profile that is the most interesting. We propose to combine the two parameters of the generalized Pareto distribution and to use the complete distribution 1 - H to estimate the risk of vertical solute propagation in soils. Despite a certain resistance to changes of experimental conditions, the risk index is not an intrinsic soil parameter. Since the flow regime in the same soil can be dominated either by preferential flow or by uniform matrix flow, the risk of vertical solute propagation will change. The adjusted parameters of the generalised Pareto distribution will capture the dominant flow regime as reflected by tracer flow patterns. Bearing in mind the boundary conditions of the tracer experiment like irrigation rate, the tracer employed, soil initial moisture or type of vegetation (permanent or seasonal, deep rooted or shallow rooted) it is possible to compare different study sites or to consider the same site at different boundary conditions and to access the risk of vertical solute propagation. Pattern analysis based on the risk index for vertical solute propagation revealed the occurrence of preferential flow at the German study site. To gain insight in flow mechanisms and possible impacts on soil chemistry we analysed soil texture, fine root density, soil bulk density, exchangeable cations, pH and total C and N contents in preferential flow paths and soil matrix. Results from linear mixed-effects models suggested that at this study site roots constituted main preferential flow paths and induced macropore flow, especially in the topsoil. In the subsoil root density decreased and inhomogeneous infiltration from preferential flow paths into the soil matrix caused non-uniform flow. There were no textural differences between the flow domains, but smaller bulk densities in preferential flow paths. This is probably due to a higher soil organic matter content in preferential flow paths. We found smaller pH values, more Ca, more Mg, more C and more N in preferential flow paths. Compared to the adjacent soil matrix, more Al and more Fe (but small absolute amounts) were found in the subsoil where macropore flow along root channels decreases and heterogeneous matrix flow dominates. These distinct chemical properties can be explained by root activity and translocation of solutes and DOC (dissolved organic carbon) via preferential flow paths. During transport along preferential flow paths contact time between DOC and soil is reduced so that DOC is transported to greater depth where it potentially forms organo-mineral associations. If this holds true, preferential flow is a mechanism that promotes C sequestration in subsoil and does not only influence its immediate environment around paths, but also underlying subsoil horizons.