Modeling diffusion and reaction in soils: iii. predicting gas diffusivity from the campbell soil-water retention model
1996
Moldrup, P. | Kruse, C. W. | Rolston, D. E. | Yamaguchi, T.
Improved prediction of gas diffusivity in soils is essential to the development of better gas transport and fate models. Empirical equations analogous to three well known capillary tube models for unsaturated hydraulic conductivity, based on the Campbell soil-water retention function, were used to predict gas diffusivity as a function of soil-air content. Using the measured gas diffusivity at the highest soil-air content as a matching point value, good agreement between predicted and measured gas diffusion coefficients as a function of soil-air content was found for 16 undisturbed soils of varying texture. An improved prediction was obtained using a Burdine-Campbell type of equation but with a reduced tortuosity factor. All four suggested retention-dependent diffusivity equations gave overall better predictions (smaller root mean square error) compared with using a simple power function model. If neither the soil-water characteristic curve nor a single measurement of gas diffusivity at a high soil-air content are available, it is suggested that the Campbell model parameter, b, be estimated from soil texture and the matching-point value of diffusivity be estimated at maximum air-filled porosity from the Millington-Quirk model. Using this concept, the new b-dependent diffusivity equations gave an overall better prediction of gas diffusion coefficients for an additional 27 undisturbed soils compared with the traditional Millington-Quirk model.
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