Quantifying the morphology of flow patterns in landslide-affected and unaffected soils

In mountainous regions, landslides are a common natural phenomenon. They are often triggered by a combination of steep relief and long lasting rainfalls. Therefore, hydrological conditions in the soil are crucial for their initiation. Because direct measurements of water flow are difficult and model predictions uncertain, we used a dye to visualise flow patterns directly. We combined dye tracer experiments with field observations (root density and rock fragment content) and measurements of soil properties (bulk density, soil water retention curve and saturated hydraulic conductivity). We quantified the morphology of flow patterns by image indices and mutual information that measures the overall dependency between variables and compared two shallow landslides of different ages with a landslide-unaffected hillslope. Our results showed that flow patterns on the landslides were more fragmented and contained larger stained objects than on the landslide-unaffected site. We relate these findings to a larger stone content and a stronger macropore–soil matrix interaction. Moreover, we detected a fissure network on the younger landslide that probably affects its hydrological behaviour. Field observations, soil hydraulic properties and dye tracer studies describe different hydrological aspects of the studied sites and complement the information provided by the morphological image indices.