Analysing flow patterns from dye tracer experiments in a forest soil using extreme value statistics

Preferential flow of water in soil is now recognized as a common phenomenon. It results in complex flow patterns that can be visualized by dye tracers and increases the risk of pollutants reaching greater depths. We analysed the behaviour of a risk index for vertical solute propagation based on extreme value theory. This risk index can be calculated from binary images of dye-stained soil profiles and is defined as the form parameter of the generalized Pareto distribution. We did five tracer experiments with Brilliant Blue and iodide under changing initial (variable initial soil moisture) and experimental conditions (different irrigation rates). Our results indicate some persistence of the risk index against small changes of experimental conditions such as the irrigation rate. On the other hand, it seems to be affected by initial soil moisture. Comparisons of Brilliant Blue and iodide patterns show that the form parameter alone is not sufficient to estimate the risk of vertical solute propagation. Therefore we propose to combine the risk index with the scale parameter of the generalized Pareto distribution.