Water Flow in Soils and the Vadose Zone

typically, are not fully saturated by water the soil layer and the region reaching to the water table contain water contents below full saturation. These regions usually are referred to as the vadose zone and said to be unsaturated, but they are more correctly considered partially saturated. The degree of saturation is the ratio of the volume of water to the pore volume within the porous medium. Saturation levels usually are a few percent at land surface (or even zero in perpemaUy dry arid zones) and increase slowly with depth until the region of the capillary fringe (water table), where it increases rapidly to 100%. 

Berkowitz et al. Contaminant Geochemistry Transport and Fate in the Subsurface Environment. 

Notwithstanding the natural heterogeneity of the subsurface, we can usefully consider homogeneous (bulk, effective) descriptions for at least some problems, especially for water flow (but less so for contaminant migration see Sect. 10.1). Therefore, two basic approaches to modeling generally are used to describe and quantify flow and transport continuum-based models and pore-network models. We discuss each of these here. 

The traditional, continuum-based approach uses Darcy s law, modified for partially saturated porous media, to quantify the flux of water  

Richards equation (Richards 1931), based on a mass conservation balance, together with Eq. 9.1, can be used to describe the transient flow of water through a partially saturated porous medium. In one dimension (vertically), Richards equation is given as 

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