The functions 0 (h) and K(0), the unsaturated-soil hydraulic properties, are highly nonlinear functions of the relevant independent variables and they characterize a soil from the hydraulic point of view. The unsaturated hydraulic conductivity also can be viewed as a function of pressure head h, because water content and pressure potential are directly related through the water retention characteristic. If one uses this relationship, strictly speaking, unsaturated hydraulic conductivity should be considered as a function of matric potential only but, according to the Richards approximation, the terms matric potential or pressure potential can be used without distinction. The presence of a hysteretic water retention function will cause the K (h) function to be hysteretic as well. However, some experimental results have shown that hysteresis in K(0) is relatively small and negligible in practice.
Typical relationships between unsaturated hydraulic conductivity K and pressure head h under drying conditions are illustrated in Fig. 5.10 for a sandy and a clayey soil. The water retention functions 0 (h) for these two types of soil are depicted in Fig. 5.8. As evident from Fig. 5.10, size distribution and continuity of pores have a strong influence on the hydraulic conductivity behavior of soil. Coarser porous materials, such as sandy soils, have high hydraulic conductivity at saturation Ks and relatively sharp drops with decreasing pressure potentials. This behavior can be explained readily if one considers that coarse soils are made up primarily of large pores that easily transmit large volumes
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of water when filled, but are emptied even by small reductions in pressure potentials from saturation conditions. On the other hand, fine porous materials such as clay soils have lower Ks values and then unsaturated hydraulic conductivity decreases slowly as pressure potentials decrease from h = 0. In fact, the higher proportion of fine pores that characterizes clay soils makes the water transport capacity of this type of porous materials still relatively high even under dry conditions. Intermediate situations can occur for porous materials having soil textures between these two extreme cases. Even if coarse soils are more permeable than fine soils at saturation or close to this condition, the reverse can be true when the porous materials are under unsaturated conditions. Therefore, knowledge of texture, structure, and position of the different layers in a soil profile is of primary importance to accurately assess the evolution of water movement in soil.
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