Estimation of Fredlund and Xing Soil Moisture Equation Using One-point Measurement

Quantitative knowledge of soil hydraulic properties such as the soil moisture characteristics curve (SMC) and the hydraulic conductivity curve is crucial for flow and transport modeling in hydrologic and agricultural engineering. However, many laboratory and field methods are currently available for direct measurement of the soil hydraulic properties, but, most or all of direct methods are too time consuming and costly. Chin et al. have developed a one-parameter method for estimation of Fredlund and Xing SMC model using regression analysis. This study was carried out for evaluation of this method for estimation of SMC and available water (AW). Forty five soils from UNSODA database and forty two soils from Iranian soils with a wide range of hydraulic properties were selected. Smallest root mean square error (RMSE) was obtained when the basic point of SMC was set at the matric suction of 33 kPa. Large coefficient of determination (R2=0.96) and small RMSE (0.026) between estimated and measured soil moisture confirmed that the studied approach was a suitable method for estimation of SMC in quite divers soils. Small RMSE and positive and significant correlation between measured and estimated available water showed that the approach properly estimates the AW as well. Comparison of the results demonstrated that one-parameter method is more accurate than the several previously developed pedotransfer functions for the Iranian soils.