Validation of the Arya and Paris water retention model for Brazilian soils

The Arya and Paris (AP) model predicts soil water retention curves from soil particle-size distribution (PSD) data based on the similarity between these two functions. The AP model estimates pore radius (r(i)) from the radius (R-i) of spherical particles by scaling pore length with a parameter a. This paper evaluates the performance of the AP model with representative Brazilian soil types using three constant a values: alpha = 1.38, 0.938 (literature values), and 0.977, (obtained in the present work); and a a-variable approach, where a is determined as a function of soil water content (0). The study was performed with 104 soil samples collected in three sites. The soil PSD curves were obtained with an automatic soil particle analyzer based on the attenuation of gamma-ray by dispersed soil particles failing in a liquid medium and the soil water retention were measured with tension table and Richard chamber methods. The best mathematical representation of the alpha - f(theta) relationship was obtained with a first-order exponential decay equation [alpha = 0.947 + 0.427exp(- 0/0.129)] that provided values of a in the range from 1.37 (theta = 0 m(3) m(-3)) to 0.96 (0 - 0.6 m(3) m(-3)). The root mean square deviation values of estimated and measured theta were 0.062 m(3) m(-3) for alpha = f(theta), 0.073 m(3) m(-3) for alpha = 0.977, 0.080 m(3) m(-3) for alpha = 0.938, and 0.136 m(3) m(-3) for alpha = 1.38. Therefore, for these set of soils the a-variable approach and the constant ones using 0.977 and 0.938 presented the best estimation for the soil water retention relationships.