In this article we will discuss about the saturated versus unsaturated flow of water into the soil.
Flow of soil water is due to driving force resulting from potential gradient. Darcy law is valid both for saturated and unsaturated flow in that the flow is proportional to potential gradient. When the soil is saturated, all the pores are filled and conducting, so that conductivity is maximal. In unsaturated soil, some of the pores become air-filled and the conductivity decreases.
A sandy soil conducts water more rapidly than a clayey soil. However, the very opposite may be true when the soils are unsaturated. In a soil with large pores, these pores quickly empty and become non-conductive as suction develops, thus steeply decreasing the initial high conductivity.
In a soil with small pores on the other hand, many of the pores remain full and conductive even at appreciable suction, so that the hydraulic conductivity does not decrease as steeply and may actually be greater than that of soil with large pores subjected to the same situation. Unsaturated hydraulic conductivity, also called capillary conductivity is variable and dependent upon the soil moisture content.
As the soil under field condition is unsaturated for most of the times, the flow will be more appreciable and persists longer in clayey than in sandy soil. For these reasons, the occurrence of a sandy layer in a fine textured profile, for from enhancing flow, may actually impede unsaturated water movement until water accumulates above the sand and suction decreases sufficiently for water to enter the large pores of the sand.
Unsaturated hydraulic conductivity is usually measured in the laboratory by applying a constant hydraulic head difference across the sample and measuring the resulting steady flow of water. Soil samples can be de-saturated either by tension plate devices or in pressure chamber.
Irrigation water or rain water enters the soil and moves down. Infiltration determines the relation between water absorption and runoff. Subsurface percolation rate determines internal profile drainage necessary for normal growth and development of crops. Different aspects of water movement in the soil profile and associated flow conditions are given in Table 7.17.