The majority of hydraulic processes under a natural condition in a field are carried out under unsaturated flow conditions. The soil moisture retention curve (SMRC) is the most important hydraulic characteristic of an unsaturated soil whose knowledge is of prime importance in soil-water studies such as soil conservation, soil erosion, land evaluation, soil reclamation, and water resources management. SMRC can be determined by two different direct and indirect methods. While there are noticeable developments on direct methods, they are still time- and labor-consuming. As a result, researchers are focusing more on indirect methods. The present research has evaluated some common PTFs for predicting the SMRC for a number of soils in Iran. Fifty soils, the majority of which were loam and clay loam, were taken from Karaj, Amol, and Babol in the north of Iran. Soil water contents corresponding to matric potentials of 0, -5, -33, -100, -500, and -1500 kPa were determined by a pressure plate apparatus. Four common PTFs of Rawls and Brakensiek (RB), Vereeken et al. (VMFD), Wosten (W), and Wosten et al. (WLNL) were used in this study. To evaluate these PTFs, the GMER (Geometric mean error ratio), GSDER (Geometric standard deviation of error ratio), and RMSE (Root mean square error) indices were considered. The results showed that these PTFs functioned better for loam-textured soils. VMFD and WLNL PTFs performed better, while VMFD was better than the others for clay loam soils. In general, better fit was found as the matric potential increased.

Keywords