Moving through scales of flow and transport in soil

Flow and transport in soil is governed by the binary geometry of solid and void. This may be described at typical length scales of some millimeters, and even less. In contrast, the problems which are supposed to be solved in soil physics are related to a scale of some meters, the typical distance between soil surface and groundwater. A quantitative understanding of flow and transport, based on measurements of hydraulic properties and transport parameter at a given scale, requires the transfer of information in space, time, and across scales. This is the major challenge in heterogeneous soils and this has motivated many concepts for the organization of heterogeneities, including macroscopic homogeneity, discrete hierarchy and fractal geometry. We propose a conceptual approach termed `the scalewayʹ to predict flow and transport in structured materials, whatever the scale, and whatever the specific type of structural organization. This is based on the explicit consideration of structure that is assumed to be present at the scale of interest, while the microscopic heterogeneities are replaced by averaged, effective descriptions. The three ingredients needed are: a representation of the structure, a process model at the scale of interest, and corresponding effective material properties. We demonstrate the scaleway for one example of solute transport in a soil column and discuss implications for future research.