Developing Pedotransfer Functions to Estimate Soil Water Retention Curve by Regression Method Using Groenevelt and Grant Model

Data on soil hydraulic properties are required to model water movement in soils. Their direct measurements are difficult، time consuming، and expensive، because of high variability and complexity of soil systems. Pedotransfer functions are developed to estimate soil water retention curve (SWRC) using different techniques. In this study، pedotransfer functions were developed to estimate SWRC using the model of Groenevelt and Grant. Sixty nine soil samples were collected from Guilan Province. Soil particles and micro aggregate size distributions، SWRC، and basic soil properties were measured in the laboratory. Fractal and non-fractal models were fitted to the particles and micro aggregate size distributions and their parameters were calculated. Linear regression technique was used to predict the parameters of Groenevelt and Grant’s model. In order to estimate every parameter of the model of Groenevelt and Grant، four pedotransfer functions were developed using each technique and fractal parameters of particles and micro aggregates were used as predictors. Then، estimated parameters were used to simulate SWRC and were compared with the measured SWRC. Fractal parameters of the particle size distribution rendered better predictions of SWRC compared with the other predictors. In spite of the large number of the parameters، using Fredlund’s model did not result in more accurate predictions for SWRC in comparison with using fractal parameters. Probably، these results showed the priority of fractal theory in the particle size distribution and SWRC modeling.