Evaluation of PTFs Developed From Large Databases for Iranian Soils To Predict SMRC

The majority of hydraulic processes under a natural condition in a field arecarried out under unsaturated flow conditions. the soil moisture retention curve (smrc) is themost important hydraulic characteristic of an unsaturated soil whose knowledge is of primeimportance in soil-water studies such as soil conservation, soil erosion, land evaluation, soilreclamation, and water resources management. smrc can be determined by two different directand indirect methods. while there are noticeable developments on direct methods, they are stilltime- and labor-consuming. as a result, researchers are focusing more on indirect methods. thepresent research has evaluated some common ptfs for predicting the smrc for a number ofsoils 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 of0, -5, -33, -100, -500, and -1500 kpa were determined by a pressure plate apparatus. fourcommon ptfs of rawls and brakensiek (rb), vereeken et al. (vmfd), wosten (w), andwosten et al. (wlnl) were used in this study. to evaluate these ptfs, the gmer (geometricmean error ratio), gsder (geometric standard deviation of error ratio), and rmse (root meansquare error) indices were considered. the results showed that these ptfs functioned better forloam-textured soils. vmfd and wlnl ptfs performed better, while vmfd was better thanthe others for clay loam soils. in general, better fit was found as the matric potential increased.