Comparison of Measured and PTF Predictions of SWCCs for Loess Soils in China

There are significant advantages in using indirect pedo-transfer functions, (PTFs) for the estimation of unsaturated soil properties. The pedotransfer functions can be used for the estimation of the soil-water characteristic curve (SWCC) which in turn is used for the estimation of other unsaturated soil properties. The accuracy of the indirect pedotransfer function method for the estimation of the SWCC depends on the PTF and the equation used to best-fit the particle-size distribution (PSD) data. The objectives of this study are to: (1) evaluate the performance of the Fredlund et al. (Can Geotech J 37: 817-827, 2000) equation for best-fitting the particle-size distribution, (PSD) data, and, (2) compare the predictions made by two of the commonly used PTFs; namely, Arya and Paris (Soil Sci Soc Am J 45: 1023-1030, 1981) and Fredlund et al. (Can Geotech J 39: 1103-1117, 2002), for estimating the SWCC from the PSD. The authors used 258 measured PSDs and SWCC datasets from the Loess Plateau, China, for this study. The dataset consisted of 187 silt-loam soils, 41 loam soils, 11 silt-clayloam soils, 10 sand-loam soils, 6 silt-clay soils, and 3 loam-sand soils. The SWCC and PSD datasets were measured using a Pressure Plate apparatus and the pipette method, respectively. The comparison between the estimated and measured particle-size distribution curves showed that the Fredlund et al. (Can Geotech J 37:817-827, 2000) equation closely prepresented the PSD for all soils in the Loess Plateau, with a lower root mean square error (RMSE) of 0.869%. The comparison between the estimated and measured water contents at the same suction showed that the Fredlund et al. (Can Geotech J 39: 1103-1117, 2002) PTF performed somewhat better than the Arya and Paris (Soil Sci Soc Am J 45: 1023-1030, 1981) function. The Fredlund et al. method had RMSE value of 0.039 cm(3) cm(-3) as opposed to 0.046 cm(3) cm(-3) for the Arya and Paris (Soil Sci Soc Am J 45: 1023-1030, 1981) method. The Fredlund et al. (Can Geotech J 39: 1103-1117, 2002) PTF produced the closest predictions for sand-loam, loam-sand, and loam soils, with a lower RMSE for gravimetric water content ranging from 0.006 to 0.036 cm(3) cm(-3). There were consistent over-estimations observed for silt-loam, silt-clay-loam, and slit-clay soils with RMSE values for gravimetric water content ranging from 0.037 to 0.043 cm(3) cm(-3). The measured and estimated air-entry values were closest when using the Fredlund et al. (Can Geotech J 39: 1103-1117, 2002) PTF. The measured and estimated maximum slopes on the SWCC were closest when using the Arya and Paris (Soil Sci Soc Am J 45: 1023-1030, 1981) PTF.