Korean J. Soil Sci. Fert. 43 (4), 407-414 (2010) The Simulation of Pore Size Distribution from Unsaturated Hydraulic Conductivity Data Using the Hydraulic Functions Young-Man Yoon*, Jeong-Gyu Kim 1 , and Kook-Sik Shin 2 Biogas Research Center, Hankyong National University, Anseong 456-749, Korea 1 College of Life Science and Biotechnology, Korea University, Seoul 136-713, Korea 2 College of Agriculture and Life Science, Hankyong National University, Anseong 456-749, Korea Until now, the pore size distribution, PSD, of soil profile has been calculated from soil moisture characteristic data by water release method or mercury porosimetry using the capillary rise equation. But the current methods are often difficult to use and time consuming. Thus, in this work, theoretical framework for an easy and fast technique was suggested to estimate the PSD from unsaturated hydraulic conductivity data in an undisturbed field soil profile. In this study, unsaturated hydraulic conductivity data were collected and simulated by the variation of soil parameters in the given boundary conditions (Brooks and Corey soil parameters,   = 1 - 5 L -1 ,  = 1 - 10; van Genuchten soil parameters,   = 0.001 - 1.0 L -1 ,  = 0.1 - 0.9). Then,   (1.0 cm h -1 ) was used as the fixed input parameter for the simulation of each models. The PSDs were estimated from the collected  data by model simulation. In the simulation of Brooks-Corey parameter, the saturated hydraulic conductivity,   , played a role of scaling factor for unsaturated hydraulic conductivity,  . Changes of parameter  explained the shape of PSD curve of soil intimately, and a   affected on the sensitivity of PSD curve. In the case of van Genuchten model,   and   played the role of scaling factor for a vertical axis and a horizontal axis, respectively. Parameter  described the shape of PSD curve and  systematically. This study suggests that the new theoretical technique can be applied to the in situ prediction of PSD in undisturbed field soil. Key words: Soil pore size distribution, Hydraulic conductivity, Matric potential, Soil moisture characteristic curve, Simulation Received : August 2. 2010 Accepted : August 18. 2010 *Corresponding author : Phone: +82316705335 E-mail: yyman@hknu.ac.kr Introduction Hydraulic properties such as soil moisture characteristics curve (SMCC) and pore size distribution (PSD) are very important for the interpretation of soil physical characteristics and for the management of agricultural practices. The relationships between soil water content and water tension or infiltration rate are used to describe water flow in soil commonly. The hydraulic conductivity of a soil is affected by soil structure and pore size distribution. In the comprehension of soil hydraulic properties, the relationships of the hydraulic conductivity and water content,  , and of the matric potential and the water content,  , are very important. Especially  can be used to predict  and pore size distribution in soil is characterized from its soil moisture characteristics curve, indirectly (Childs, 1969). The measurement of  is very difficult and time consuming. Therefore many researches that utilize physical and empirical relation between hydraulic properties and other soil parameters have been studied. As an alternative to measurements, pedo-transfer functions have predicted water retention characteristics and hydraulic properties from the input variables that could be measured more easily such as soil texture, bulk density, or other soil variables (Rawls and Brakensiek, 1985; Ahuja et al., 1989; Vereecken et al., 1989, Saxton et al., 1986). Recent development and application of tension disc infiltrometer (Ankeny et al., 1988; Perroux and White, 1988) has provided a simple, fast, and in situ method measuring infiltration rate. Tension disc infiltrometer is useful instrument that offer estimating methods of soil hydraulic properties at the soil surface. The most widely used method based on tension disc infiltrometer measurement is the Wooding’s (1968) solution of approximate steady-state Article