3.3 cm2"n dn'1 with n = 1.3, which is of a magnitude similar to values
from the two previously mentioned studies on stony soils. Several
factors may be contributing to the high dispersivity of this soil.
Dispersion increases as the range of small to large pore sizes in a
soil increases, thereby providing a wide range of water velocities
within a single soil sample. Edwards et al. (1984) have shown that the
presence of non-porous gravel increases the total macropore volume at
the expense of the micropore volume. The large reduction in the water
content of the soil under slight tensions (0 to 50 mbar) indicates that
the soil possesses a considerable volume of large pores (Fig. 3-1).
Similarly, the retention of nearly 33% of the total soil water at 15
bars of tension indicates that the soil also contains a large volume of
very small pores.
The values of the dimensionless parameter w, estimated for each
experiment on Column 1, is presented in Table 4-5. It relates the
mass-transfer coefficient, a, to the column length and solution flux
(Eq. [4-16]). The mass-transfer coefficient is a lumped diffusion
parameter that relates the solute diffusion transfer to the molecular
diffusion coefficient, the mobile/immobile-water fraction, the
tortuosity, the radius of soil aggregates, and the solution flux. The
relationship between the mass-transfer coefficient and the solution
flux is shown in Fig. 4-10. The mass-transfer coefficient is not a
constant and has been shown to increase with solution flux using both
theoretical postulations and experimental methods (Rao, 1980a and
1980b; van Genuchten, 1985). The mathematical description of the
diffusion process employed by the MIM model has an underlying