Estimating and Testing the Arya-Paris Scaling Parameter, a, for Egyptian and UNSODA-Database Soils

The Arya-Paris scaling parameter, a (parameter used to scale pore lengths based on spherical particles to natural soil pore lengths) was estimated by logistic growth and linear equations and as a constant for some Egyptian and UNSODA-database soils. Log Ni-log ni curves (where Ni and ni are number of particles, ith fraction in real and ideal soil, respectively) agreed with those reported. Initial log Ni and rate-coefficient () of the logistic equation were greater, while those of final log Ni were slightly smaller than reported. Log Ni and log wi/Ri3 (where wi and Ri are solid mass and particle radius, ith fraction, respectively) correlated strongly (r2 > 0.97). a changed markedly for log ni < 2, and was nearly constant for the rest of log ni range. The scaling parameter decreased as the soil texture became finer. Average a for logistic growth method ranged from 0.977 (silt loam) to 1.464 (sand) for the Egyptian soils and from 1.027 (silt loam) to 1.400 (sand) for pooled soils. For the log-linear fit method, a ranged from 1.010 (silt loam) to 1.349 (sand) for the Egyptian soils and from 0.981 (clay) to 1.311 (sand) for pooled soils; and for the constant a method from 1.112 (silt loam) to 1.464 (sandy loam) for the Egyptian soils and from 1.125 (clay) to 1.463 (sand) for pooled soils. a responded better to sand than to clay content. All three formulation methods showed reasonable capability to predict the soil water characteristic (SWC) from particle-size distribution (PSD) data with minor differences. However, results indicate that estimating a based merely on texture class may be inadequate due to differences in PSD patterns among soils within the same textural class.