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ORIGINAL RESEARCH

Variable Pore Connectivity Factor Model for Gas Diffusivity in Unsaturated, Aggregated Soil

^a Graduate School of Science and Engineering, Saitama Univ., 225 Shimo-okubo, Sakura-ku, Saitama 338-8570, Japan
^b Environmental Engineering Section, Dep. of Biotechnology, Chemistry and Environmental Engineering, Aalborg Univ., Sohngaardsholmsvej 57, DK-9000 Aalborg, Denmark
^c Research Team for Conservation of Agricultural Watershed, National Agricultural Research Center for Western Region, Ikano 2575, Zentsuji, Kagawa 765–0053, Japan
^d Dep. of Land, Air, and Water Resources, Univ. of California, Davis, CA 95616. A.C. Resurreccion, current address: Dep. of Engineering Sciences, Univ. of the Philippines-Diliman, Quezon City, 1101 Philippines
^* Corresponding author (acresurrecci{at}up.edu.ph).

Received 27 March 2007.

The soil gas diffusion coefficient (D_p) and its variations with soil air content () and soil water matric potential () control vadose zone transport and emissions of volatile organic chemicals and greenhouse gases. This study revisits the 1904 Buckingham power-law model where D_p is proportional to ^X, with X characterizing the tortuosity and connectivity of air-filled pore space. One hundred years later, most models linking D_p() to soil water retention and pore size distribution still assume that the pore connectivity factor, X, is a constant for a given soil. We show that X varies strongly with both and matric potential [given as pF = log(–, cm H₂O)] for individual soils ranging from undisturbed sand to aggregated volcanic ash soils (Andisols). For Andisols with bimodal pore size distribution, the X–pF function appears symmetrical. The minimum X value is typically around 2 and was observed close to of –1000 cm H₂O (pF 3) when interaggregate voids are drained. To link D_p with bimodal pore size distribution, we coupled a two-region van Genuchten soil water retention model with the Buckingham D_p() model, assuming X to vary symmetrically around a given pF. The coupled model well described D_p as a function of both and for both repacked and undisturbed Andisols and for other soil types. By merely using average values of the three constants in the proposed symmetrical X–pF expression, predictions of D_p were better than with traditional models.

Abbreviations: SWC, soil water characteristic