Soil is a porous system made up of solid, liquid, and gaseous phases. The liquid phase (soil solution) consists of soil water, which usually contains a variety of dissolved minerals and organic substances. Water in soil may be encountered in three different states: as a liquid, a solid (ice), or a gas (water vapor).
The definitions and discussion in the following sections refer to a macroscopic description of an idealized continuous medium that replaces the actual complex geometry of a pore system. The various state variables (e.g., pressure potential and water content) and soil properties (e.g., bulk density and hydraulic conductivity) are considered to be continuous functions of position and time. They are viewed as macroscopic quantities obtained by volume averages over an appropriate averaging volume referred to as the representative elementary volume (REV) whose characteristic length should be much greater than that of a typical pore diameter but considerably smaller than a characteristic length of the porous system under study . Within this effective continuum, the solid matrix is usually considered as rigid, the liquid phase is Newtonian and homogeneous, air is interconnected at the atmospheric pressure, and the analysis of flow regime is conducted by evaluating the flux density as volume of water discharged per unit time and per unit entire cross-sectional area of soil. Each point of the domain considered is the center of an REV. The macroscopic continuum approach represents a fertile tool for the development of theories applicable to the problem of water movement through porous media.
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