Porosity Reviewed: Quantitative Multi-Disciplinary Understanding, Recent Advances and Applications in Vadose Zone Hydrology

Abstract

Porosity -one of the most basic mechanical properties of a mediumhas implications in a vast range of disciplines and used for a similar vast range of applications. These include, for instance, the storage and flow of water; the compressible component of earth materials, which can be subjected to consolidation under loading; the variable parameter in the swelling and shrinkage of clays; and possibly a governing parameter in the formation of wetlands and perched water tables. This review notes the relevance of a fourfold quantification of porosity for vadose zone studies, viz. (1) type (matrix or structure), (2) scale (submicro to macro scale), (3) connectivity, and (4) water saturation. This is followed by a review of recent advances in the quantification and description of porosity in porous media (visual and remote sensing methods, porosimetry, geometrical approaches, empirical estimations, densest packing simulations, etc.), the applications to quantification of hydrological parameters, and a brief glimpse into the significance of porosity in a temporary hillslope wetland underlain by Archaean Lanseria gneiss in South Africa. Final comments are made regarding areas where quantification of porosity is problematic.Keywords: porosity; void ratio; vadose zone; effective porosity; fractal; microstructure; porosimetry; ferricrete; temporary hillslope wetland IntroductionAlbeit a straightforward concept (how hard can determining the ratio of voids to solids possibly be?), quantification of porosity is often significantly simplified. The importance of porosity in hydrology as a primary input parameter in almost all subsequent calculations is overlooked and the parameter is estimated without any validation, resulting in significant unquantifiable errors in hydraulic parameters. However, the simple percentage value of porosity is only one such aspect. The importance of type, scale and connectivity of porosity is commonly understood, but rarely evaluated in significant detail.A detailed discussion in Miller and Gray (2002) on the status of groundwater research accentuates a number of aspects requiring more research and where our understanding is often not sufficient. Reference is made particularly to preferential flow paths such as fractures and the resulting complicated accounting of interactions between these fractures and the flow through the primary pore space. They continue to elaborate on the difficulty of accounting for scale ranging from molecular to regional field problems. Fractured systems are singled out due to the difficulty in characterising and modelling fractures with available methods and because of their interaction with the porous matrix.As this is a review paper, basic concepts will be described concisely for the sake of cross-disciplinary agreement. It is impossible to cover the whole range of methods applicable to porosity. The purpose of this paper is primarily to summarise considerations in the quantification of porosity, to review recent advances in the quantification of porosity, to ap...

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