Soils often exhibit a variety of small-scale heterogeneities such as cracks, inter-aggregate macropores and voids which partition flow into separate regions. In this paper an approach is discussed for characterizing the hydrological behavior of heterogeneous soils, in the presence of structural inter-aggregate macropores or even shrinkage cracks. Accordingly, special emphasis is given to the effect of micro-macro-heterogeneity and soil structure on water flow processes at local scale. The discussion is limited to a mechanistic approach, known to be based on physical concepts and laws. Theoretical discussion and experimental evidence of the structural effects on the hydrological behavior of soils are provided. Flexible retention relations in describing the retention data of aggregated soils are evaluated, with the principal aim of assessing their predictive capability for estimating the hydraulic conductivity function and for describing flow in a composite porosity framework. Their predictive capability is independently tested by using a set of water content observations at different depths. The results enable a detailed discussion on model parameterization and related parameter uncertainty to be made.
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