Summary of the Article:

Boundary Conditions for CH4 and O2 Surface Fluxes

- The CH4 and O2 surface fluxes are calculated based on an effective conductance and a gaseous concentration gradient between the atmospheric concentration and the concentration in the first soil layer (for unsaturated soils) or in equilibrium with the water (for saturated soils).
- For the fully explicit and fully implicit cases, the surface fluxes are calculated as w(C1^n - Ca) and w(C1^(n+1) - Ca), respectively, where w is the surface boundary layer conductance.
- If the top layer is not fully saturated, the term (Dm1/Δxm1) is replaced with a series combination: [1/w + Δx1/D1]^-1.
- If the top layer is saturated, the term is replaced with [KH/w + (1/2)Δx1/D1]^-1, where KH is the Henry's law equilibrium constant.
- When snow is present, a resistance is added to account for diffusion through the snow, based on the Millington-Quirk expression and CLM's prediction of the liquid water, ice, and air fractions in each snow layer.
- When the soil is ponded, the diffusivity is assumed to be that of methane in pure water, and the resistance is the ratio of the ponding depth to diffusivity.
- The overall conductance is the series combination of surface, snow, and ponding resistances.
- A zero flux gradient is assumed at the bottom of the soil column.