22
considered to occur throughout the soil profile in response to
temperature and water content conditions in the soil. The rate of
phase change of the water (Ev) is considered positive if the water
changes from the liquid to vapor (evaporation), while condensation is
indicated by a negative phase change rate.
The diffusivity of water vapor in the soil air spaces is assumed
to equal that in the atmosphere, however, Schieldge et al. (1982), as
well as others, have made use of a tortuosity factor (a) to account
for the increased path length of the interstitial spaces of the soil
through which the water vapor must traverse. The tortuosity factor
included the effect of the local evaporation and condensation of water
vapor as it passes through the regions of differing temperature and
water content in the soil.
The volumetric water content is typically used to express the
amount of liquid water present in the soil. The mass concentration of
water in the liquid phase is obtained by multiplying the volumetric
water content by the density of water. The time rate of change of the
mass concentration of water is increased by a the rate of diffusion of
water from surrounding soil and decreased by the rate of evaporation.
The mathematical expression of the conservation of mass for the water
is
d 3Ql
(2-18)
where:
Pw
Ql
9
volumetric water content [m3*m~3]
density of water [kg*m-3]
-2 -1
diffusion of liquid water [kg*m *s ]