Three‐Porosity Model for Predicting the Gas Diffusion Coefficient in Undisturbed Soil

Abstract

The soil gas diffusion coefficient (D_P) and its dependency on air‐filled porosity (ε) govern most gas diffusion‐reaction processes in soil. Accurate D_P(ε) prediction models for undisturbed soils are needed in vadose zone transport and fate models. The objective of this paper was to develop a D_P(ε) model with lower input parameter requirement and similar prediction accuracy as recent soil‐type dependent models. Combining three gas diffusivity models: (i) a general power‐law D_P(ε) model, (ii) the classical Buckingham (1904) model for D_P at air saturation, and (iii) a recent macroporosity dependent model for D_P at −100 cm H₂O of soil–water matric potential (ψ), yielded a single equation to predict D_P as a function of the actual ε, the total porosity (Φ), and the macroporosity (ε₁₀₀; defined as the air‐filled porosity at ψ = −100 cm H₂O). The new model, termed the three‐porosity model (TPM), requires only one point (at −100 cm H₂O) on the soil–water characteristic curve (SWC), compared with recent D_P(ε) models that require knowledge of the entire SWC. The D_P(ε) was measured at different ψ on undisturbed soil samples from dark‐red Latosols (Brazil) and Yellow soils (Japan), representing different tillage intensities. The TPM and five other D_P(ε) models were tested against the new data (17 soils) and data from the literature for additional 43 undisturbed soils. The new TPM performed equally well (root mean square error [RMSE] in relative gas diffusivity <0.027) as recent SWC‐dependent D_P(ε) models and better than typically used soil type independent models.