derivation of pedotransfer function to estimate parameters of double-exponential equation for soil water retention curve

soil water retention curve is one of the crucial input parameters in any modeling study on water flow and solute transport in soils. direct measurement of soil water retention curve is time-consuming and costly. alternatively, pedotransfer functions (ptfs) may be used to indirectly estimate the soil water retention curve from easily measured soil properties. many equations for the soil water retention curve are presented by researchers. recently, dexter et al. introduced a new double-exponential equation for soil water retention curve that its parameters have distinct physical meaning. the objective of this study was derivation of ptfs to estimate parameters of double-exponential equation from easily obtainable soil properties. this objective was carried out using data of 27 soil samples including particle size distribution (sand, silt and clay), bulk density, and soil water retention curve. ptfs were derived by two softwares spss and datafit using data of 27 soil samples. nine data of 9 soil samples were used to evaluate derived ptfs. the results showed that soil water retention curves estimated using derived ptfs agreed well with measured soil water retention curves. in addition, datafit with the mean absolute error (), modified coefficient efficiency (), and modified index of agreement () 0.020, 0.878, and 0.744, respectively, with respect to spss with , ,  0.025, 0.852, and 0.684, respectively, had better accuracy.