Estimation of Shape Factor of Soil Hydraulic Conductivity Function of van Genuchten – Mualem Model Using Easily-Available Soil Properties

Proper soil hydraulic functions that accurately predict soil water characteristic curve and hydraulic conductivity are essential for studies of unsaturated flow in soils. In this study, 8 soils from different texture classes of UNSODA soil bank were selected and the optimal validity of shape factor (n) of the RETC software (using retention curve function of van Genuchten-Mualem (VGM) model) was investigated for predicting the value of unsaturated hydraulic conductivity at different moisture contents. Since the predicted results of this model were poor, we attempted to investigate a separate shape factor such as  for the hydraulic conductivity-moisture (K-θ) function of VGM Model. To this end, 24 soil samples from different texture classes of UNSODA were selected and their measured parameters were analyzed by regression analysis to find a suitable pedotransfer function for estimating . The developed function confirmed the relationship of  with both saturated moisture (θs) and organic matter contents with a correlation coefficient of r = 0.745 at significant level of P = 0.0005. Also, to validate the developed pedotransfer function, the hydraulic conductivity values ​​corresponding to the measured moistures for the 8 selected soils in the validation section were calculated based on the two shape factors obtained from the pedotransfer function ( ) and the RETC (n) and the results were compared with the measured values. The statistical indices of root mean square error of model (RMSEM) and Nash-Sutcliff model efficiency coefficient  (NSE) showed that the shape factor of the developed pedotransfer function compared to the RETC, had a better performance in predicting unsaturated hydraulic conductivity values.