Water Transport in the Unsaturated Zone

Beneath the soil profile, the vadose zone may consist of unconsolidated or consolidated materials in this text, only unconsolidated materials are considered. The movement of water in the vadose zone is more difficult to describe than flow in the saturated zone, because water transport occurs only via water-filled pores and the fraction of the pore spaces filled with water (the percent saturation) is highly variable. The water content 6 refers to the fraction of the bulk soil volume that contains water 6 can range between zero 

FIGURE 3-21 Hydraulic conductivity of a certain unsaturated soil as a function of water (moisture) content (0). Higher conductivity occurs when 6 is larger and the pore water is under less suction. Note that the relationship between K and 6 depends on whether the soil most recently was dried or wetted. 

In the unsaturated zone, the pressure of the pore water is less than atmospheric pressure. The symbol i/j is often used to represent the pressure head of pore water, 

Soil suction is a function of capillary forces acting between water and soil grains. Other factors being equal, soil suction is greatest in fine textured media, in which the menisci between soil water and soil gas have the smallest radii and thus produce the greatest pressure difference between water and air. Large pores require larger menisci between air and water these create smaller pressure differences. This phenomenon is easily demonstrated with water in 

FIGURE 3-23 Relationship between water (moisture) content (6) and pore water pressure (if/) in a certain unsaturated soil. Pore water suction is the negative of pore water pressure. As in the case of the K — 6 relationship (Fig. 3-21), this relationship is influenced by whether the soil most recently was wetted or dried. 

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