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Gas Transport Parameters in the Vadose Zone

Gas Diffusivity in Field and Lysimeter Soil Profiles

^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 Dep. of Agroecology, Danish Inst. of Agricultural Sciences, Research Centre Foulum, P.O. Box 50, DK-8830 Tjele, Denmark
^d Dep. of Land, Air, and Water Resources, Univ. of California, Davis, CA 95616

View larger version (29K): [in a new window]   Fig. 1. Locations of soil sampling: three lysimeter soils came from Rønhave, Foulum, and Jyndevad; two agricultural field soils were sampled at Gjorslev and Mammen. Note that soils from Rønhave and Jyndevad were transported to lysimeters at Research Centre Foulum.

View larger version (23K): [in a new window]   Fig. 2. Pore volume distribution for (a) lysimeter soils (Rønhave, Foulum, and Jyndevad) and field soils at (b) Gjorslev and (c) Mammen. Average data for each soil layer are given; d is equivalent pore diameter calculated from d = 3000(–)–1 where is soil-water matric potential (cm H2O) (Schjønning, 1985b).

View larger version (31K): [in a new window]   Fig. 3. Soil-gas diffusivities Dp/D0 as a function of air-filled porosity : (a) all matric-potential () measurements for lysimeter soils (Rønhave, Foulum, and Jyndevad); (b) all measurements for field soils (Gjorslev and Mammen); (c) measurements from C horizon samples with different soil textures at Mammen; and (d) measurements from sandy clay loam layers from Foulum, Gjorslev, and Mammen. Note different axes and log scale.

View larger version (23K): [in a new window]   Fig. 4. Soil-gas diffusivities Dp,100/D0 as a function of air-filled porosity 100 for (a) lysimeter soils (Rønhave, Foulum, and Jyndevad), (b) field soils (Rønhave, Foulum, and Jyndevad) (Schjønning, 1985b), and (c) field soils (Gjorslev and Mammen). For (c), matric potential () = –100 cm H2O data are plotted in both normal (left axis) and log scale (right axis). 100 = air-filled porosity at  = –100 cm H2O; Dp,100/D0 = soil-gas diffusivity at  = –100 cm H2O.

View larger version (35K): [in a new window]   Fig. 5. Scatterplot comparison of predicted and measured soil-gas diffusivities (Dp/D0) for lysimeter soils (Rønhave, Foulum, and Jyndevad) and field soils (Gjorslev and Mammen). All measured data are given. Model predictions are: (a, b) the Millington and Quirk (1961) model (MQ) and (c, d) the three-porosity model (TPM). Calculated RMSE values using Dp/D0 data and log-transformed Dp/D0 data (values in brackets), Eq. [1], are given. Note different axes and log scale.

View larger version (14K): [in a new window]   Fig. 6. Relation between the Campbell (1974) pore-size distribution parameter b and the tortuosity–connectivity parameter X for 96 soils. The X = 2 + 3/b curve is also drawn.

View larger version (16K): [in a new window]   Fig. 7. Comparison of the three-porosity model (TPM) and the Millington and Quirk (1961) model (MQ) with measured data for field soils at Gjorslev at the (a) 0.43-m, (b) 2.15-m, and (c) 4.75-m depth. The average of soil-gas diffusivities (Dp/D0) at each matric-potential () stage are shown in the plots. The standard deviations of both air-filled porosity and Dp/D0 are shown in error bars. Average values of the Campbell pore-size distribution parameter b are also given.

View larger version (19K): [in a new window]   Fig. 8. Depth distribution of soil-gas diffusivities Dp,100/D0 for Gjorslev and Mammen (field soils) at matric potential = –100 cm H2O. Average and standard deviation of Dp,100/D0 at each depth are shown. Predicted Dp/D0 by the three-porosity model (TPM) are given by dotted and solid lines.