The effect of parameters of particle and aggregate size distribution on the point estimation of soil water retention curve

Due to high variability and complexity of the soils, direct measurement of hydraulic parameters is difficult, time-consuming and expensive. Therefore, these properties are estimated using indirect methods. In this study, 74 soil samples were taken from Guilan province, and particle (PSD) and micro-aggregate size distribution (MASD), bulk density and water content at matric suctions of 2, 5, 33, 100 and 1500 kPa were measured. Fractal parameters of soil PSD and MASD and particle’s size distribution parameters of Fredlund’s model were calculated and used to estimate water content at different matric suctions. The results showed that entering fractal parameters of PSD did not improve the estimation of water content. By adding fractal parameters of MASD to the model, estimation of water contents at different matric suctions was improved significantly (P<0.05) and the root mean square error (RMSE) value decreased by 6.5% for the validation step. This may be the result of the influence of soil structure on water retention. But using of PSD parameters of Fredlund’s model was more effective in estimation of water content and decreased the RMSE of validation step up to 27.0%. The best estimation of water content was observed at the matric suction of 5 kPa, in which the coefficient of determination of training step increased by 0.36. It may be due to the large number of parameters of Fredlund’s model and better simulation of PSD. In general, using parameters of PSD and MASD improved the estimation of soil water retention curve.