Abstract

Abstract Direct measurement of soil water retention curve parameters is a hard, costly, sensitive job. One of the good indirect methods in prediction of soil water retention curve is Arya-paris model which uses the soil Particle Size Distribution (PSD) data. The AP model estimates pore radius ( ri ) from the radius (Ri) of spherical particles by scaling pore with a parameter α. data from soil particle-size distribution, soil water retention curve, density and porosity of 96 different soils, sand, loamy sand, sandy loam and silty loam Were used to help assessing methods of estimating the scale parameter. The results from the inverse method for the sand, loamy sand, sandy loam and silty loam Were 1.32, 1.36, 1.45, 1.23 respectively. Use of  The Linear method for sandy and silty loam soils resulted in amounts of 1.34 and 1.25 respectively and by using the  constant Method ( recommended by Arya et al (2)) for sandy loam and loamy sand, α amounts were1.37 and 1.46 respectively which were the closest to the real calculated amounts. By studying the shifts of α  in different texture and moistures it has been notified that there is no meaningful relationship between α and the amount of sand in the soil, on the other hand increase in the soil moisture increases the amount of α while the correlation coefficient for α and soil moisture was calculated 0.45. Key word: Arya-paris, scaling parameter, soil particle-size distribution, soil water retention curve