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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

View larger version (27K): [in this window] [in a new window]   FIG. 1. Plots of the (a, d, and g) soil gas diffusivity (Dp/Do) against soil air-content (), and the Buckingham–Currie pore connectivity factor (X) calculated using Eq. [2] (b, e, and h) against and (c, f, and i) against pF = log(–, matric potential in cm H2O) for Japanese soils with different soil texture and structure (100 kPa denotes 100-kPa uniaxial compaction). Fitted X(pF) functions, Eq. [3], for each soil are also shown in (c), (f), and (i). Data from Osozawa (1998).

View larger version (31K): [in this window] [in a new window]   FIG. 2. Behavior of soil water retention and soil gas diffusivity for different soil types: (a and c) fitted soil water retention curves using the van Genuchten (1980) model for Toyoura sand and the bimodal Durner (1994) model for other soils; measured Dp/Do and predictions by Eq. [6] and [7] using the given X(pF) functions are plotted against (a and c) pF (open symbols) (b and d) soil air content, . Data from Osozawa (1998), Resurreccion et al. (2007a), and this study.

View larger version (16K): [in this window] [in a new window]   FIG. 3. (a) Plot of the pore connectivity factor (X) against soil water content, , at pF 3 for Japanese soils. Filled symbols represent Yellow soils (blue), Gray-Lowland soils (red), and Andisols (black). (b) The symmetric X(pF) function, Eq. [3], fitted to the X–pF data of 19 undisturbed Andisol samples (filled circles, data from Resurreccion et al., 2007a,b) is shown. Also shown are the X–pF data from Moldrup et al. (2005) (open circles) and for an undisturbed Tsumagoi Andisol (open triangles, data from Osozawa, 1998). A symmetric X–pF function with A1 = 0.15, A2 = 2, B = 2, and pF* = 3 is also plotted in (b).

View larger version (25K): [in this window] [in a new window]   FIG. 4. (a, d, and g) The two-region Durner (1994) soil–water retention model (solid lines) fitted to the soil water retention data of three undisturbed Japanese Andisols. The contributions of inter-and intra-aggregate pore space regions to soil water retention are shown as dashed lines. The average gas diffusivity (Dp/Do) and Dp/Do models (Eq. [6] and [7]) using the symmetrical X(pF) function (Eq. [3] with A1 = 0.185, A2 = 1.78, B = 2.11, and pF* = 3.13) are plotted against (b, e, and h) pF and (c, f, and i) soil air content, . Also shown are the Dp/Do predictions using X = 2, X = 2 + 0.15 |pF – 3|2, and the commonly used Dp()/Do models of Penman (1940) and Millington and Quirk (1961).

View larger version (26K): [in this window] [in a new window]   FIG. 5. (a) Soil water retention curves at different weight distributions and shape factors, and (b and c) the effect of weight distribution of the inter- and intraaggregate porosity (w1 and w2) and shape factors (n1 and n2) on gas diffusivity, Dp/Do. When not varied, the shape factors are n1 = 1.7 and n2 = 1.3, and weights w1 and w2 are equal to 0.5. A symmetrical X–pF function (Eq. [3] with A1 = 0.185, A2 = 1.78, B = 2.11, and pF* = 3.13) was used in (b) and (c). (d) Effect of parameters of the symmetrical X–pF function on Dp/Do at the given soil water retention parameters. For all plots, saturated water content, s, is 0.75 m3 m–3 and scaling factors 1 and 2 are 0.075 and 0.00005 cm–1, respectively.