This book focuses on statistical approaches to air/water flow in heterogeneous media, particularly for applications in soil hydrology where the unsaturated flow model of Darcy-Richards can be used. Beyond the assumptions of unsaturated flow, we present also a more general description of immiscible two-phase flow, with a wetting phase (water) and a non-wetting phase (air or oil), each with its own viscosity and density. Thus, we describe quasi-analytically 2-phase flow dynamics for axial flow in tubes/joints, possibly with axially variable diameter/aperture.