investigation of relationships between soil texture and scaling parameter to predict soil water content

the arya and paris model is a commonly accepted method to translate a particle-size distribution curve into a corresponding soil moisture characteristic curve using scaling parameter (α). the scaling parameter, in the original model of arya and paris (1981), was assumed constant (α=1.38) for all soil texture classes. in recent years, several studies have shown that α is not constant and its quantity depends on soil particle size distribution. the present investigation was conducted in order to determine the relationships between soil texture and scaling parameter. in this study, α, is considered as a function of scaled number of spherical particles to estimate the pore length in a natural soil. results confirmed that α was not a constant value but changed with increasing particle size, especially for the sand fraction. since the scaling parameter is difficult to determine, a regression equation (r2 = 0.96) was obtained to estimate scaling parameter. the predictive ability of the equation and that of the constant (α = 1.38) were compared on a few soils from amol, babol, and karaj regions. predictions of soil water content showed reasonable to excellent agreement with measured data, while use of a constant value (1.38) led to under-predictions in dry range and over-predictions in wet range.