The physics of water retention in very dry porous media

Abstract

Very low water contents in porous media are receiving increasing attention as research expands into previously neglected dry environments. Examples include Mars, Antarctica, extreme deserts, and soils exposed to wildfire. Working with very low water contents requires recognition that the generally accepted oven-dryness-based water content scale is a relative scale, analogous to the Celsius scale for temperature, and that oven-dryness is not an absence of water but rather a definable point on a retention curve. The usual definition has ambiguity in that the water content of the oven-dry sample does not depend solely on the oven temperature, but also on unspecified atmospheric conditions outside the oven. An unambiguous definition of zero water content would set it at a specified matric potential, determined by application of the Kelvin equation to pertinent conditions of the method of drying. An absolute, rather than relative, scale is conceivable but is impractical to establish universally. Mathematical representation of dry-range water retention is often done with a two-parameter logarithmic relation interpretable in terms of the energy per unit volume required to remove a particle of water being proportional to the matric potential. Its parameters have strong physical interpretations, related to the specific surface area of the medium’s solid material and the zero point of the relative water content scale. Application of this formula requires attention to the limits of its validity and the means of combining it with formulas for other portions of the water retention curve. These new methods and refined concepts do not constitute a new theory of water retention in porous media, but rather an improved framework for conceptualizing and working with measurements of water retention in the very dry range.