Characterization of macropore-matrix mass transfer parameters in two-domain preferential flow models

The macropore-matrix mass transfer of water and solutes is an important aspect of non-equilibrium-type of preferential flow in structured soils. For a representative soil volume, effective mass transfer parameters depend on heterogeneous local properties of the soil macropore structure, its geometry and shape, and on properties at macropore walls that can differ from those of the matrix with respect to texture, organic matter, bulk density, and porosity. These affect the soil pore system locally with respect to hydraulic, mechanic, bio-geo-chemical, and other processes. Clayey aggregate skins, for example, may be more due to plastic deformation but can restrict water exchange; solutes may become adsorbed along macropore surfaces and released under changing condition. Still relatively little is known about formation of such local biological hotspots in soil, on how to determine the local mass transfer parameters, and how to upscale to the scale of the soil volume, and on the interrelations between all the individual local properties and the combined effect on relevant bulk soil transport processes. The present contribution reviews recent experimental and modeling work including field and lab percolation experiments using the movement of bromide, Brilliant Blue, iodide, and Na-Fluorescein to identify the flow paths and parameter optimization approaches for determining such parameters. It seems that preferential transport of reactive solutes depends even more strongly on the geometry and properties at flow paths surface than the water flow itself or the movement of conservative solutes. The identification and determination of effective mass transfer parameters in two-domain models remains a challenge when considering that local changes in the soil structure are highly dynamic during the vegetation, the seasons, and due to soil management.