Estimation of soil moisture characteristic curve based on simple one parameter model

Measurement of soil moisture characteristic curve in laboratory and field is costly, time consuming and difficult. Since, this curve is necessary for studying water movement in unsaturated soil, therefore using the estimated methods for this curve is common. The model of Grykson et al. (1987) is a single one parameter model based on the logarithmic shape. This model has two coefficients with negative correlation and the other unknown parameter. In this study, soil bulk density and geometric standard deviation of the particle-size diameter were used for determining the unknown parameter of model. For this purpose, 160 soils from UNSODA soil data bases and 32 soils from Fars province were used, and four conditions have been considered: a) Calibration the results for different soil textural classes and validation the results for remained soils in each soil textural class. b) Calibration the results for UNSODA soils and validation the results for soils of Fars province. c) Calibration the results for soils of Fars province and validation the results for UNSODA soils. d) Calibration the results for about 80 percent of total soils and validation the results for about remained 20 percent of soils. The obtained results in condition one showed the good accuracy for estimating soil moisture characteristic curve in different soil textural classes. Also, the comparison of conditions two, three and four indicated that the calibration results based on UNSODA soil data bases were better for estimating soil moisture characteristic curve.