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diffusivity, or coefficient of diffusion, is a measure of the number of
collisions and molecular interaction between molecules of water vapor
and the other constituent components of the air (ASHRAE, 1979) and is a
function of temperature, total air pressure and partial pressure of
water vapor. Eckert and Drake (1972) presented the following equation
to calculate the diffusivity of water vapor in air.
Dva 2.302 ( (T X 10-5 (2-33)
va p
where
Dva = diffusivity of water vapor in air [m2.s-1]
p = atmospheric pressure [Pa]
Po = reference atmospheric pressure [Pa]
= 0.98 X 105 Pa
T = air temperature [OK]
T = reference air temperature [OK]
= 256 OK
However, for water vapor in the soil air space, the path is more
convoluted, thus increasing the probability of collisions with other
particles and lowering the kinetic energy of the water vapor molecules.
To account for the increased path length within the soil, a tortuosity
factor (a) has been introduced to reduce the effective coefficient of
diffusion in other porous materials. De Vries (1958) used a value for
the tortuosity of 0.667. Using this information the coefficient of
water vapor in the soil (Dv) becomes
Dv = Dva
= 1.535 (P T 1.81 X 10-5 (2-34)
~p ) To)