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Several existing equations for solving the non-linear soil-hydraulic properties are introduced and validated to field and
laboratory measured data. Models for non-linear hydraulic properties of unsaturated porous media arise from statistical
and mathematical fit through the measured data and they can be expressed in forms of unsaturated permeability versus
either pressure head or volumetric moisture content. This paper presents the difference models: Gardner, Knuze et al.,
Haverkamp et al., van Genuchten and Saxton et al. for calculation of hydraulic properties coefficients, typically
unsaturated permeability. The accurate and computational efficiency of these five existing models are evaluated for a
series of study cases simulating hydraulic properties of unsaturated porous media. The results indicate that all existing
models can be applied to homogenous and heterogenous unsaturated porous media, dry and wet cycles and laboratory
and field measuring data. Besides, the statistical fit model is inefficient compared to mathematical fit models. Among
the mathematical fit models, van Genuchten model is the most promising model. Gardner model can be competitive
with van Genuchten model and Haverkamp et al. model is less efficient than others. The mathematical fit models
appear to be attractive alternatives to estimate the unsaturated permeability, although there are concerns regarding the
stability behaviour of the occupied air in pores, which need to be resolved. The air movement in unsaturated porous
media affected the unsaturated permeability, which gives the difference results between wet and dry cycle. Both of
unsaturated permeability and volumetric water content of dry cycle were higher than ones of wet cycle. This suggests
that the velocity of air-releasing during a wet process was higher than the velocity of air-entering during a dry process.
The infiltration is the most important land applications. So, the wet cycle hydraulic properties test might be concerned.
Moreover, most of infiltration fields locate on the mixed grain media. So too, the pore-size distribution could affect the
unsaturated permeability of porous media. It was observed that the finer material, the lower unsaturated permeability.
