The soil vapour flux was introduced as a switch ConvectiveGasFlow which includes the vapour flow as an optional contribution to both the water and energy flow in the soil, see eqs. (1.1) and (2.1). (In equation (2.1) the convective gas flow is written as a diffusion coefficient for vapour in the soil, Dv, times the vapour concentration as a function of depth. Dv corresponds to the factors dvapbfaD0 below.)
Vapour flows between adjacent soil layers will be calculated from gradients in vapour pressure and diffusion coefficient. The diffusion coefficient is adjusted because of deviations from diffusion in free air by use of a parameter dvapb. The vapour flow is given by:
where fa is the fraction of air filled pores (i.e. θs - θ), D0 is the diffusion coefficient in free air, which is given as a function of the soil temperature as:
cv is the vapour concentration, which is given by the vapour pressure. Thus:
where Mwater is the molar mass of water, R is the gas constant, T is the soil temperature and the vapour pressure, ev, is given by:
where es is the vapour pressure at saturation, ψ is the soil water tension and g is the gravitational constant. The later expression is used from the basic assumption that the liquid phase is in equilibrium with the gas phase in the soil.