Soil Air Permeability and Saturated Hydraulic Conductivity: Development of Soil Corer Air Permeameter, Post-fire Soil Physical Changes, and 3D Air Flow Model in Anisotropic Soils

Abstract

Air permeability (ka) is a viable alternative to water- and texture-based methods to rapidly map saturated hydraulic conductivity (Ksat). The ability to measure this important hydraulic property without the use of more cumbersome and time-consuming methods may provide a practical approach to generate more complete data to describe hydrologic conditions. This study presents the development of an air permeameter which is suitable for desert soils. The Soil Corer Air Permeameter (SCAP) is compatible with a standard soil corer and employs digital components to measure flowrates under low-pressure gradients to improve accuracy, ease of use, and portability. SCAP allows for the extraction of undisturbed soil samples for laboratory analysis, providing direct comparisons of ka with other soil physical and hydraulic properties. The applicability of a regression equation to estimate Ksat from field-measured ka using SCAP was examined in unburned and burned soils. Ex situ field ka and laboratory Ksat measurements were compared and air to water permeability (ka/kw) ratios were calculated to determine structural changes due to water saturation. The study also characterized changes in permeability due to fire in woodland-chaparral and coniferous soils. For soils that could be extracted with minimal structural changes, results show ka and Ksat measurements for unburned and burned soils were within the 95% confidence intervals of a ka-Ksat regression developed for agricultural soils. However, correlations for in situ ka measurements in some burned soils showed a decrease in accuracy and may be attributed to soil anisotropy. A three-dimensional steady-state finite element air flow model was developed using FEMLAB 3.0A to consider the effects of anisotropy on in situ ka measurements. Results show that anisotropic conditions can introduce an error as high as a factor of 2 especially for air permeameters with high diameter to height (D/H) ratios, however, the error is much smaller than the anisotropy ratio. If anisotropy is important to characterize, it was shown that paired measurements of in situ and ex situ ka can be used to infer the anisotropy ratio.