Relationship between the Hydraulic Conductivity Function and the Particle‐Size Distribution

Abstract

We present a model to compute the hydraulic conductivity, K , as a function of water content, θ, directly from the particle‐size distribution (PSD) of a soil. The model is based on the assumption that soil pores can be represented by equivalent capillary tubes and that the water flow rate is a function of pore size. The pore‐size distribution is derived from the PSD using the Arya‐Paris model. Particle‐size distribution and K (θ) data for 16 soils, representing several textural classes, were used to relate the pore flow rate and the pore radius according to , where q i is the pore flow rate (cm 3 s −1 ) and r i is the pore radius (cm). Log c varied from about −2.43 to about 2.78, and x varied from ≈2.66 to ≈4.71. However, these parameters did not exhibit a systematic trend with textural class. The model was used to independently compute the K (θ) function, from the PSD data for 16 additional soils. The model predicted K (θ) values from near saturation to very low water contents. The agreement between the predicted and experimental K (θ) for individual samples ranged from excellent to poor, with the root mean square residuals (RMSR) of the log‐transformed K (θ) ranging from 0.616 to 1.603 for sand, from 0.592 to 1.719 for loam, and from 0.487 to 1.065 for clay. The average RMSR for all textures was 0.878.