Field Testing of a Soil Corer Air Permeameter (SCAP) in Desert Soils

Measurements of air permeability are useful both directly, to assess gas phase movement through soils, and indirectly as a proxy for soil hydraulic conductivity. Several designs have been presented for field-based air permeameters; however, none are suitable for use in gravelly desert soils. We designed an air permeameter compatible with a standard soil corer to facilitate insertion into desert soils. The soil corer air permeameter (SCAP) uses digital components to measure flow rates under low-pressure gradients to improve accuracy, ease of use, and portability. The SCAP allows the extraction of undisturbed soil samples for laboratory analysis, providing direct comparisons of air permeability ( k a) with other soil physical and hydraulic properties. The soil sample can be extracted before measuring k a, thus removing the need for a shape factor to account for divergent flow. In an analysis of the SCAP's performance, field testing showed that digital components operate well under field conditions; however, spanner wrench insertion holes must be sealed to provide correct k a measurements. The Jalbert and Dane shape factor for in situ k a is applicable for tilled, unstructured soils. However, significant soil-specific variability exists in the shape factor, so we recommend measuring ex situ k a in the field, where possible. Ex situ field k a and laboratory hydraulic conductivity ( K sat) measurements were compared and air to water permeability ratios were calculated to determine structural changes due to water saturation. For soils that could be extracted with minimal structural change, we found good correlation between k a and K sat and reasonable agreement with previously published results.