Soils often exhibit a variety of small-scale heterogeneities such as cracks, inter-aggregate pore network, macropores, decayed root channels, and other types of macropores, which partition flow into separate regions. Due to these heterogeneities, non-uniform (i.e., preferential) flow of water and air and transport of solutes and colloids or dispersed particles occurs thereby creating local-scale non-equilibrium conditions in pressure head and solute concentrations between faster and slower flow pore regions. Similar effects may be caused by temporal heterogeneities of soil physical properties including soil water repellence. Accounting for the various mechanisms, we distinguish several types of preferential flow at the pedon scale, such as macropore flow, fingering or funnelling.
Preferential flow can be observed also at larger scales, for instance, along sediment heterogeneities or layer boundaries, and along the soil surface, in drainage networks or soil pipes.
One common question is how the smaller (i.e., local) scale properties affect the larger (i.e., observation) scale preferential flow process. In addition, the network and properties of flow pathways are dynamically changing as a result of, for instance, local soil dispersion, erosion, and sedimentation of particles and biological or man-made activities.
The objectives of this session are to discuss model approaches, parameterizations, and experimental observations that consider scale effects and to reflect on the relation between structure-dependency and structure-forming effects of flow along preferred pathways.

Solicited speakers to date:
Prof. Nick Jarvis, Swedish University of Agricultural Sciences, Uppsala, Sweden
Dr. John R. Nimmo, U.S. Geological Survey, Menlo Park, CA, USA.
Dr. Jolanta Lewandowska, INPG, Grenoble, France
Prof. Markus Weiler, University of Freiburg, Germany

Publication of contributions:
A Special Issue in Vadose Zone Journal is planned (guest editors: Horst H. Gerke, Peter Germann, and John L. Nieber).
Everybody who wants to submit a manuscript is kindly requested to indicate this intention shortly after abstract submission until January 31, 2009, so that we can contact potential reviewers during February. The deadline for submission of manuscripts is planned for end of April, 2009, so only manuscripts that are mostly finished at the time of the meeting will be included to ensure a timely review and publication.