Estimation of soil hydraulic properties and their uncertainty from particle size distribution data

Abstract

A unified approach to the estimation of soil hydraulic properties and their uncertainty from particle size distribution data is presented. Soil hydraulic properties are represented by the parametric models of Van Genuchten and/or Brooks and Corey. Particle size distribution data are used to generate theoretical soil-water retention data using a modified form of the model proposed by Arya and Paris, which was calibrated in this study using a data set of 250 soil samples. Parameters in the Van Genuchten model are fitted to the predicted water content — capillary pressure data by nonlinear regression methods and may be optionally converted to equivalent Brooks-Corey retention parameters using an empirical procedure. Saturated conductivity is estimated from particle size data using a modified Kozeny-Carman equation which was developed from the data set of 250 soil samples. Uncertainty in parameter estimates is evaluated using first-order error analysis methods. Application of the proposed methodology to three soils which were not in the calibration set indicated water content-capillary pressure relations can be predicted with reasonable accuracy and precision. Uncertainty in predicted saturated hydraulic conductivity will be rather large making direct measurement of this variable highly desirable.