A Physicoempirical Model to Predict the Soil Moisture Characteristic from Particle‐Size Distribution and Bulk Density Data

Abstract A model to predict the moisture characteristic of a soil from its particle‐size distribution, bulk density, and particle density parameters is presented. The model first translates a particle‐size distribution into a pore‐size distribution. Then, the cumulative pore volumes corresponding to progressively increasing pore radii are divided by the sample bulk volume to give the volumetric water contents, and the pore radii are converted to equivalent soil water pressures using the equation of capillarity. To compute the pore volumes and the pore radii, the particle‐size distribution curve is divided into a number of segments. The solid mass in each segment is assumed to form a matrix with a bulk density equal to that of a natural‐structure sample. For a unit of sample mass, an equivalent pore volume for each segment is computed from V vi = ( W i /ϱ p )e and the corresponding pore radius from: r i = R i [4 en i (1‐α) /6] 1/2 , where V vi is the pore volume, W i is the solid mass, ϱ p is the particle density, e is the void ratio, r i is the mean pore radius, R i is the mean particle radius, n i is the number of particles, and α is an empirical constant ranging in value from 1.35 to 1.40. The formulation for the pore radius is based on spherical particles and cylindrical pores. Model predictions for several soil materials show close agreement with the experimental data.