Much to the chagrin of soil physicists, convective flow through, and diffusion within field soils do not behave in the uniform and isotropic way that their theoretical descriptions demand. Rather, soil structure in the form of either aggregates, cracks or biopores, serves to create an apparently chaotic flow regime that is rapid and far-reaching because of preferential flow through the connected macropores and wide inter-aggregate spaces.
Yet, thankfully, it would seem, we already know all the physics we need to know about the rapid and far-reaching flow that transports water and solute preferentially through cracked and aggregated soils. For a representative elementary volume (REV) at the Darcy scale of 10 -3 to 10 -1 m 3 , we now have at our disposal the coupled partial differential equations that describe the interactive flows of water and chemical moving simultaneously through a fracture system, and the surrounding more-uniform matrix-even in the presence of dual-domain uptake by plant roots. Numerical solutions of this coupled, dual-domain scheme are available (Gerke and van Genuchten, 1993a), and it would only require tenacity to extend this code of a binary split in the soil's domains into a full description of a soil with a spectrum of macropore classes.
Furthermore, experimentalists have developed new devices and measurement techniques, such as time-domain reflectometry (TDR) and disc permeametry. These tools have not only provided better observations of preferential flow, but they have also allowed apt characterization of the soil's macroporous hydraulic properties at the Darcy scale. So what's the problem?
The problem is simply that neither our detailed theoretical understanding of matrix-fracture flow, nor our hydraulic characterization at the Darcy scale, considers either the spatial topology of, or temporal variation in, the macroporous networks that determine the speed and extent of preferential flows through field soil at larger spatial scales.
This macroporous topology and temporal variation results in an inability to describe, or predict, preferential flow and exchange through connected flow networks, either at the REV of the pedon scale of 10 0 to 10 2 m 3 in the presence of plants, or in sum at the catchment scale of 10 9 to 10 12 m 3 with communities of plants, multifaceted geology and complicated topography. Yet it is the norm that flow at these larger scales is typified by rapid, and far-reaching flow and exchange processes. Damn!