#### Water vapour flow

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,
*D*_{v}_{}_{}, times the vapour
concentration as a function of depth. *D*_{v}_{}_{} corresponds to the
factors *d*_{vapb}f_{a}D_{0} 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 *d*_{vapb}_{}_{}. The vapour flow is
given by:

(2.12)

where *f*_{a }is the fraction of air filled
pores (i.e. θ_{s}_{}_{}* *- θ), *D*_{0}_{
}is the diffusion coefficient in free air, which is given as a function of
the soil temperature as:

(2.13)

*c*_{v} is the vapour concentration, which is
given by the vapour pressure. Thus:

(2.14)

where *M*_{water}_{}_{} is the molar mass of
water, *R** *is the gas constant,
*T* is the soil temperature and the
vapour pressure, *e*_{v}, is given by:

(2.15)

where *e*_{s}_{}_{} 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.