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Comment on “Dual‐energy synchrotron X ray measurements of rapid soil density and water content changes in swelling soils during infiltration” by Patricia Garnier et al.

A theory of water flow and volume change in swelling soils was developed 30 years ago [Philip, 1968; Raats and Klute, 1968a, b; Smiles and Rosenthal, 1968]. This theory has been effectively tested in the chemical engineering context [Smiles, 1986], but an experimental test in unsaturated soils has proved elusive. In part, this is because the theory is based in material coordinates which are not familiar in soil science. There appear also to be difficulties because the approach treats soil volume change as one-dimensional (vertical) at a large enough scale and also because it is experimentally difficult to constrain the material during volume change to mimic field conditions. There has, nevertheless, been renewed interest in the process recently. A specific ase is that of Garnier et al. [1998], who illustrate the use of dual-energy synchrotron X-ray measurements to quickly and accurately determine bulk density and water content profiles during infiltration into swelling soil constrained in a plexiglass cell 4 cm x 4 cm x 3.6 cm. The initial dry bulk density of soil was 1.45 g cm -3. The measurement procedure is powerful, but at least two issues of concern arise in their paper. The first relates to the formulation and use of their flow equations (7a)-(7c) and their equation (8); the second relates to the interpretation of their experimental data, which, we believe, is affected by friction on their cell walls.