Field-scale variation of preferential flow as indicated from dye coverage

Field investigations were performed in semiarid Tunisia to investigate spatial variability of preferential pathways in catchment- and plot-scales. A rainfall simulator with dye was used to simulate effects of a 20-year rainstorm. After infiltration into six one-meter square plots, vertical 2.5-cm thick soil sections were excavated, and dye penetrations were photographed. The results show that different catchment physiographic areas (nose, slope, and hollow) displayed significantly different responses to the infiltrating dye. Nose and hollow areas exhibited larger susceptibility to preferential flow. Here, infiltrated dye was transported to greater depths. Slopes had fewer deep cracks and fissures. The dye distribution within each plot was also investigated. The variability of the dye patterns increased with scale. Cross-correlations and power spectrum analyses indicated that the preferential pathways were randomly distributed. The power-law behavior of the dye patternsʹ spectral density suggested scale invariance and the possibility of random cascade modeling.