MODELING DIFFUSION AND REACTION IN SOILS: VIII. GAS DIFFUSION PREDICTED FROM SINGLE-POTENTIAL DIFFUSIVITY OR PERMEABILTY MEASUREMENTS

Variations of gas diffusivity (ratio of gas diffusion coefficients in soil and free air, D s /D o ) with air-filled porosity (∈) influence the transport, degradation, and volatilization of reactive gasses in soil systems. We show that the prediction accuracy of the Penman-Millington-Quirk (PMQ) diffusivity model (introduced in Part VII of this series) is often improved significantly by including as a reference point a measured value of the gas diffusion coefficient (D fc ) at a single soil-water potential, ψ, between -100 and -500 cm H 2 O. As a result, the root mean square error of prediction was reduced by 45% (based on individual D s /D o measurements) and by ≥ 65% (based on mean values of 6 to 9 closely-spaced D s /D o measurements) for undisturbed soil samples from six differently textured surface soils. Gas permeability is measured more easily and more rapidly than gas diffusivity, and we suggest that a measured value of gas permeability (k fc ) at a single soil-water potential, combined with a tortuous tube permeability model and the PMQ diffusivity model, can also be used to improve D s /D o predictions. For practical use, a relation between the equivalent tube radius (r fc ) at ψ = -100 cm H 2 O and clay content, taken to represent the soil structure-forming ability, is proposed for surface soils. Gas diffusive transport simulations using the DARC numerical model (Part I of this series) verified that the inclusion of a single (D fc or k fc ) measurement in the D s /D o (∈) predictions can improve simulation accuracy significantly. D fc - and k fc -based diffusivity models require limited measurement effort and seem promising for site-specific simulations of gas diffusion and reaction.