The guest editors introduce the special section MUSIS, MUlti Scale Interfaces in unsaturated Soil, with contributions that originated from the MUSIS workshop on interfacial phenomena. The presence and complexity of various interfaces limit the predictability of flow and transport processes in the vadose zone.

Prediction of fluid phase dynamics in the vadose zone is hampered by the presence of different types of sharp interfaces, questioning the validity of standard theory that is based on assumption of continuity of air and water phases, uniform distribution of fluids in a control volume, local equilibrium of phase contents and pressures, and slow process velocities. Complexity of fluid front morphology and burst-like redistribution processes may be accentuated across material contrasts or at the interface between soil and atmosphere. This special section presents eleven contributions highlighting the role of interfaces on water and air distributions and outlining methods to improve prediction of interfacial displacement.