Because of tight dependence of water filled to air filled porosity in soils a close correlation between relative water conductivity (Kr(θ)) and air permeability (Ka), measured at various water contents is expected. The purpose of this investigation was to find out if a reliable relation or function between the two permeabilities can be established. Such a relation greatly facilitates unsaturated water conductivity (K(θ)) prediction. In the current research K(θ) and Ka were measured by pressure plate outflow and variable head methods, respectively, in the range of 0 to -100 kPa matric potential (ψm). A linear regression function as with the correlation coefficient (R) from 0.884 to 0.999 were established for the 22 examined soils. The overall R for 128 data pairs (Kr(θ) and Ka) became 0.821 (being significant at P<0.01) with the slope (b) of -2.54 and intercept (a) of -10.93. For the comparison propose Kr(θ) were also predicted from RETC using experimental SMC data and van Genuchten and Brooks-Corey models. The reliability of the Kr(θ) prediction from Ka based on root mean square error (RMSE), geometric mean error ratio (GMER), and geometric standard deviation of error ratio (GSDER) criteria became considerable greater than those predicted from the two mentioned models.
