Soil Permeability - Page 3
What is soil permeability and how does it affect drainage?
Permeability refers to the ability of a soil to absorb and transmit water.
Soils that allow water to enter and drain from the profile at rates suited to the selected land use are ideal. This means maintaining a soil which allows rapid entry of water (infiltration) and percolation (movement of water downwards through the soil), and is free of layers where permeability decreases.
Deep drainage refers to water that moves below the root zone of plants and recharges groundwater (Figure 1). This has the potential to cause rising water tables (inducing dryland salinity in some areas) and may increase the hydraulic gradient causing saline groundwater to flow into river systems, reducing water quality. Excessive deep drainage can also cause lateral flow and discharge.
Source: Hillel D (2004) Introduction to environmental soil physics. Elsevier, Amsterdam
Figure 1 Hydrological cycle
Water moves into and through soil in response to hydraulic pressure gradients. These gradients may develop as a result of suction forces (water entry into a dry soil) or gravity (water entry in wet soil). This can result in either vertical (downward or upward) or horizontal (lateral) flow. The downward flow of water is primarily driven by gravity but as the soil surface dries out due to evaporation, water can move slowly upwards due to capillary action and diffusion. This kind of movement also occurs to some extent from groundwater and is called capillary rise (when groundwater is under a positive head, water can also be forced upwards as in artesian basins). Lateral flow occurs when water is unable to flow through an impermeable parent material such as rock or dense clay and is forced to flow sideways either on the surface of the impenetrable layer or within a more-permeable band between clay-rich layers.