Movement of water under saturated conditions
Poiseuille’s law forms the basis for a number of different equations which have
been developed for determining the hydraulic conductivity of the soil for knowledge of
its pore-size distribution.
Pore size is of outstanding significance, as its fourth power is proportional to the rate of saturated flow. This indicates that saturated flow under otherwise identical conditions decreases as the pore size decreases. Generally the rate of flow in soils of various textures is in the following sequence.
Sand > loam > clay
Moisture movement under unsaturated conditions
As drainage proceeds in a soil and the larger pores are emptied of water the
contribution of the hydraulic head or the gravitational component to total potential
becomes progressively less important and the contribution of the matric potential ψm
becomes more important.
The effect of pressure is generally negligible because of the continuous nature of the air space. The solute potential (osmotic potential) ψs does not affect the potential gradient unless there is unusual concentration of slat at some point in the soil. The negligible effect of solute potential is due to the fact that both solutes and water are moving.
Thus, in moisture moment under unsaturated conditions, the potential ψ (Equation 7.28) is the sum of the matric potential ψm and, to some extent the gravitational potential ψg. In horizontal movement, only ψm applies. Under conditions of downward movement, capillary and gravitational potentials act together. In upward capillary movement ψm and ψg oppose one another. For unsaturated flow may be rewritten as:
∆ (ψm + ψg)
v= – k —————
The direction of I is the path of greatest change in (ψm + ψg)
Sand < loam < clay
It may be noticed that this is the reverse of the order encountered in saturated
flow. However, in the ‘wet range’ the unsaturated conductivity occurs in the same or
similar order as saturated conductivity.
Water vapour movement
Movement of soil water in unsaturated soils involves both liquid and vapour
phases. Although vapour transfer is insignificant in high soil water contents, it increases
as void space increases. At a soil moisture potential of about-15 pars, the continuity of the
liquid films is broken and water moves only in the form of vapour.
Diffusion of water vapour is caused by a vapour pressure gradient as the driving force. The vapour pressure of soil moisture increases with the increase in soil moisture content and temperature, it decreases with the increase in soluble salt content.
Water vapour movement is significant only in the ‘moist range’. In the ‘wet range’ vapour movement is negligible because there are few continuous open pores. In the ‘dry range’ water movement exists, but there is so little water in the soil that the rate of movement is very small.
Water vapour movement goes on within the soil and also between soil and atmosphere, for example, evaporation, condensation and adsorption. The rate of diffusion of water vapour through the soil is proportional to the square of the effective porosity, regardless of pore sizes. The finger the soil pores, the higher is the moisture tension under which maximum water vapour movement occurs.
In a coarse textured soil pores become free of liquid water at relatively low tensions and when the soil dries out there is little moisture left for vapour transfer. But a fine textured soil retains substantial amounts of moisture even at high tensions, thus permitting vapour transfer. It is interesting to note that maximum water vapour movement in soils vapour movement is of greatest importance for the growth and survival of plants.