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, theta, directly from the particle-size distribution (PSD) of a soil. The model is based on the assumption that soil pores can be rep resented by equivalent capillary tubes and that the water flowrate is a fun ction of pore size. The pore-size distribution is derived from the PSD usin g the Arya-Paris model. Particle-size distribution and k(theta) data for 16 soils, representing several textural classes, were used to relate the pore flow rate and the pore radius according to qi = crt, where qi is the pore now rate (cm(3) s(-1)) and ri is the pore radius (cm). Log c varied from ab out -2.43 to about 2.78, and x varied from approximate to 2.66 to approxima te to 4.71. Howe ver, these parameters did not exhibit a systematic trend w ith textural class. The model was used to independently compute the K(theta ) function, from the PSD data for 16 additional soils. The model predicted K(theta) values from near saturation to very low water contents. The agreem ent between the predicted and experimental theta(theta) for individual samp les ranged from excellent to poor,,vith the root mean square residuals (RMS R) of the log-transformed K(theta)ranging from 0.616 to 1.603 for sand, fro m 0.592 to 1.719 for loam, and front 0.487 to 1.065 for clay. The average R MSR for all textures was 0.878.