Estimating Parametrs of Brooks-Corey Soil Water Retention Curve for Drying and Wetting Branches by Pedotransfer Functions

In most studies related to the prediction of soil water retention curve (θ(h)) by pedotransfer functions (PTFs), its drying branch has been worked out and almost no study has been accomplished in developing PTFs for the wetting branch. The latter is required for describing some important natural phenomena such as irrigation and infiltration. Therefore, experimental data presenting the wetting and drying branches were obtained by using respectively Shani field method and pressure plate apparatus in 24 diverse soils. The Brooks-Corey parameters were calculated by fitting the model to the experimental data for the both branches. Each parameter of Brooks-Corey model was regressed against various soil attributes by using of the multivariable curvilinear regression and appropriate PTFs were developed to predict the parameters for both branch. Statistical criteria such as modified index of agreement, relative root mean squares error, geometric mean and geometric standard deviation of error ratio were adopted for accuracy assessment of the PTFs. All the PTFs developed were non-linear multiple regression with R2 ranged from 0.80 to 0.90. According to soil attributes contribution to the developed PTFs, organic matter and bulk density appeared as the two dominant soil properties that played major role in shaping both drying and wetting branches of the θ(h) as well as the hysteresis. Results of sensitivity analysis also confirmed the importance of these two variables.