Summary:

Aqueous and Gaseous Diffusion

The article discusses the modeling of gaseous and aqueous diffusion in the soil for methane (CH4) and oxygen (O2) in the Community Land Model (CLM). 

Key points:

1. Gaseous diffusion: The temperature dependence of molecular free-air diffusion coefficients (D0) is used, as described by previous studies.

2. Soil gas diffusivity: The relationship between effective diffusivity (De) and soil properties (air-filled porosity, total porosity, and the slope of the water retention curve) is modeled using the Moldrup et al. (2003) and Millington-Quirk (1961) equations.

3. Aqueous diffusion: For the saturated part of the soil column, the effective diffusivity (De) is calculated using the Moldrup et al. (2003) equation.

4. Simplification: The model assumes that gaseous diffusion dominates above the water table, and aqueous diffusion dominates below the water table.

5. Frozen soils: For frozen or partially frozen soils below the water table, diffusion is limited to the remaining liquid, and the diffusion coefficients are scaled by the volume-fraction of liquid.

6. Unsaturated soils: Henry's law equilibrium is assumed at the interface with the water table.

The article provides the specific equations, parameter descriptions, and references for the diffusion modeling approach used in the CLM.