The influence of hillslope gradient, regolith texture, stone size and stone position on the presence of a vesicular layer and related aspects of hillslope hydrologic processes: a case study from the Australian arid zone

A near-surface layer of vesicles may be found in many dryland soils, often below a stony desert pavement. During and after rain the uppermost part of the regolith develops a surface seal, trapping air which is compressed and which may also subsequently expand as a result of solar heating. This results in the deformation of the soft regolith and the formation of a vesicular layer just below the surface. The work reported here was undertaken to determine the influence of slope gradient, regolith texture, stone size, and depth of stone embedment on the presence of a vesicular layer in arid Australia. This appears to be the first investigation of the role of topography in the formation of soil vesicularity. The four factors listed typically vary systematically with position on a hillslope. The data collected indicate that the development of a vesicular layer is inversely related to hillslope gradient and size of the stone, positively related to the amount of silt and clay in the regolith, and weakly related to the depth to which the stone is embedded. Field evidence indicates that the vesicular layer becomes much more extensive on the lower parts of hillslopes in the study area, and thus so too must the choke on water penetration that such layers create. The spatial distribution of vesicular layers thus constitutes a factor that has traditionally been overlooked in studies of dryland hillslope hydrology.