The occurrence of preferential flows is commonly associated with high contrasts in hydraulic properties and/or geometric features of the porous medium, and hence with a partitioning of water flow into several sub-domains (macropores, fractures). In the vadose zone preferential flows are an important driver for rapid recharge dynamics, specifically in fractured (karstified) porous media, where the unsaturated zone may reach a thickness of several hundred meters. Under such conditions preferential pathways, such as fractures, fracture networks, faults and macropores may strongly affect travel time distributions, system vulnerability, connectivity of surface- subsurface ecosystems, and require adapted strategies in the context of groundwater management.

As preferential flows occur on various scales, ranging from pore scales to catchment scales, unified scale-continuous concepts for their onset and magnitude are extremely difficult to define. The accessibility of thick vadose zones for in-situ field studies is often limited or impossible to realize and studies often rely on limited data, commonly obtained at the system boundaries, e.g., precipitation/recharge, water table fluctuations or spring discharge. This session welcomes studies with a focus on elaborate analytical/numerical methods, field studies and laboratory experiments that target the complex processes within preferential pathways and their interaction with the surrounding porous matrix under saturated and variably-saturated conditions.