Using Geometric Mean Particle Diameter to Derive Point and Continuous Pedotrasfer Functions

The possibility of using geometric mean dg and geometric standard deviation σg of particle diameters instead of soil particle size distribution to derive some Pedotransfer Functions (PTFs) was investigated. For this purpose, 34 soil samples were collected randomly having different particle size distributions. The variables were separated into two groups: (i) particle-size distribution and bulk density, (ii) bulk density, geometric mean and geometric standard deviation of particle diameters. Two point PTF types and two parametric PTF forms were developed to predict six points at the retention curve and the parameters of van Genuchten and Mualem-van Genuchten models, using the stepwise regression method. The derived PTFs were compared with ROSETTA database. The results indicated that the second specified variables (ii) could describe 94 and 69 percent of α and θs variations. The results were also indicated that these variables could better predict n and l parameters and the water contents at –30,-100,-300,-500 and –1500 kPa. It was concluded that dg and σg can better predict the water contents at drier parts of the retention curve than the soil bulk density. This can be attributed to the fact that at near saturation the water content is mainly affected by total soil porosity, while at lower water contents, the soil moisture is more influenced by geometric pore size distribution. The validation tests indicated better accuracy of the derived parametric PTFs using the second specified variables (ii) than the other parametric PTFs.The reliability test indicated that the derived PTFs are more accurate than what can be obtain from the ROSETTA database to predict the soil hydraulic properties. Key words: Pedotransfer function, retention curve, soil hydraulic function