Soil structure has mostly been investigated in qualitative terms, for example, concerning the characteristic shape and cohesiveness of soil aggregates, clods, or peds. Because the unsaturated hydraulic properties are fundamentally quantitative, to theoretically relate them to soil structure requires the development of concepts and techniques that quantify soil structure.

A related problem is the artificial modification of soil structure, as happens when soil is disturbed for purposes of farming, waste disposal, mining, construction, or other activities.

Our project has demonstrated (Nimmo and Akstin, 1988) the unexpectedly strong influence of soil microstructure on unsaturated K of a sandy soil, even exceeding compaction effects. We also showed how a capillary model can predict this effect on the basis of water retention alone, providing some of the strongest support to date for the soundness of capillary models of unsaturated K.

The data set from our studies at the INEEL contributed to development of a soil structure model (Nimmo, 1997b) which utilizes particle and aggregate size distributions. This model makes physically realistic use of available information, as evidenced by (1) complete retention curves as output, the input requiring no retention data, (2) direct prediction of results, requiring no calibration or optimization of parameters, and (3) excellent fits. This model has potential both as a practical tool, because the required input data are much easier to measure than the retention data, and as a starting point for further quantitative research on soil structure.

An extension of this structure-based water retention model to unsaturated hydraulic conductivity has led to new, practical means of incorporating soil structure (including preferential flow paths) into unsaturated-flow modeling (Nimmo, 1998; Nimmo, 1999). This will potentially afford substantial reduction in cost and effort in making the measurements necessary to mathematically treat unsaturated flow.