Title: Predicting the Gas Diffusion Coefficient in Undisturbed Soil from Soil Water Characteristics
Abstract: The gas diffusion coefficient in soil ( D P ), and its dependency on soil physical characteristics, governs the diffusive transport of oxygen, greenhouse gases, fumigants, and volatile organic pollutants in agricultural, forest, and urban soils. Accurate models for predicting D P as a function of air‐filled porosity (ϵ) in natural, undisturbed soil are needed for realistic gas transport and fate simulations. Using data from 126 undisturbed soil layers, we obtained a high correlation ( r 2 = 0.97) for a simple, nonlinear expression describing D P at −100 cm H 2 O of soil water potential ( D P,100 ) as a function of the corresponding air‐filled porosity (ϵ 100 ), equal to the volume of soil pores with an equivalent pore diameter >30 μm. A new D P (ϵ) model was developed by combining the D P,100 (ϵ 100 ) expression with the Burdine relative hydraulic conductivity model, the latter modified to predict relative gas diffusivity in unsaturated soil. The D P,100 and Burdine terms in the D P (ϵ) model are both related to the soil water characteristic (SWC) curve and, thus, the actual pore‐size distribution within the water content range considered. The D P (ϵ) model requires knowledge of the soil's air‐filled and total porosities and a minimum of two points on the SWC curve, including a measurement at −100 cm H 2 O. When tested against independent gas diffusivity data for 21 differently textured and undisturbed soils, the SWC‐dependent D P (ϵ) model accurately predicted measured data and gave a reduction in root mean square error of prediction between 58 and 83% compared to the classical, soil type‐independent Penman and Millington‐Quirk models. To further test the new D P (ϵ) model, gas diffusivity and SWC measurements on undisturbed soil cores from three 0.4‐m soil horizons (sandy clay loam, sandy loam, and loamy sand) within the 4 to 7 m depth below an industrially polluted soil site were carried out. For these deep subsurface soils the SWC‐dependent model best predicted the measured gas diffusivities.
Publication Year: 2000
Publication Date: 2000-01-01
Language: en
Type: article
Indexed In: ['crossref']
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Cited By Count: 280
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