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_vapbf_aD₀ 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), D₀ 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.