The increasing concern with groundwater pollution and contamination of soils has stimulated the development of numerous mathematical models of pollutant transport in soils. The most important approaches to model transient water and solute transport in the vadose zone are based on the Richards equation. To solve this equation, the knowledge of the soil hydraulic properties, namely, the soil-water characteristic curve (SWCC) and the unsaturated hydraulic conductivity is required. The laboratory measurements show that the value of unsaturated hydraulic conductivity varies considerably from soil to soil with different water content (Khaleel and Relyea, 1995). Indeed, it is found that the unsaturated hydraulic conductivity decreases by one to three orders of magnitude across a small pressure head range even near saturation (0~10cm pressure head), due to the effects of structural macropores (Jarvis and Messing, 1995). Of all hydraulic properties, the unsaturated hydraulic conductivity is most difficult to measure. Therefore the use of indirect methods has become more and more common to estimate the unsaturated hydraulic conductivity from more easily measured soil properties (van Genuchten et al., 1992).