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

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 q i = cr i x , 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.