Using particle-size distribution models to estimate soil hydraulic properties

Mathematical models for soil Water retention characteristic [h(theta)] and unsaturated conductivity function [K(theta)] from particle-size distribution (PSD) and bulk density data are indirect and empirical approaches to estimate these hydraulic functions. Often times, mathematical models are fit to sparse PSD data sets to provide the input for h(theta) and K(theta) functions. The objective of this study was to determine whether the choice of a particular mathematical model to represent the continuous PSD significantly affects the predicted soil hydraulic properties. We considered four PSD models with between one and four fitting parameters. For most of the soils, the one-parameter Jaky model for generating PSD input, resulted in the best estimate of soil hydraulic properties. This finding indicates that the linear relationship between the PSD and the void-size distribution (VSD) (or between particle volume and pore volume) defined by the AP model would be not appropriate for most soils. This result suggests that the nonlinear relationship between the PSD and the VSD of the Jaky model would more appropriate. The PSD generated by other models provided better input to the h(theta) and K(theta) functions for clay soils.