Water Flow through the Unsaturated Zone

Water present in and moving through the unsaturated zone is subjected to several rules of physics in addition to those influencing the water below the water table. The presence of retained moisture above the water table is due to adsorptive forces between the water molecules and soil particles, in addition to surface tension of the water surface. 

Darcy s equation can be used to describe flow in this region however, the value of permeability varies as a function of saturation. Also, the value of moisture potential is a function of saturation. The total potential for flow (hydraulic gradient in Darcy s equation) can be defined as the difference between the moisture potential (minus) and the elevation potential (plus). When the potential for flow is positive, flow can occur. 

FIGURE 5.5 Liquid-solid interface contact showing advancing and retreating contact angle. 

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