Abstract
A zone in subsurface where water and air coexist is called vadose zone, and flow in the vadose zone is called unsaturated flow. This chapter reviews steady-state and transient-state unsaturated flow in random porous media of multispace dimensions. Unsaturated flow processes play an important role to determine the pathway of a contaminant plume before it reaches the aquifer—particularly in semiarid and arid regions where the vadose zone is several tens to several thousands of meters thick. In an unsaturated system, flow equations are nonlinear because the unsaturated hydraulic conductivity depends on the pressure head. A steady-state flow is divided into two flow regimes—spatially nonstationary flow and gravity-dominated flow. The transient unsaturated flow in statistically nonhomogeneous porous media is described by the Richards equation. The approach of Kirchhoff transformation is also discussed in the chapter, which attempts to avoid or delay the linearization procedure required to formulate moment equations. Moment partial differential equations for transient, unsaturated flow in nonstationary media are presented in the chapter with several examples.
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