SOIL PREPARATION

SOIL DRAINAGE

6

6.1 Subsurface water In order to understand drainage, a grasp of the relationship between the different components of subsurface water is necessary (Figure 6.1). i) Groundwater: Consists of a water body in the saturated zone and sometimes it is confined between strata with low permeability. If groundwater is under pressure, it gives rise to artesian wells or boreholes. ii) Water table: This is the upper surface of the saturated free groundwater. The water pressure at this imaginary level that separates the free water from the capillary fringe will be equal to atmospheric pressure. In some soils a perched or hanging water table can be present. A perched water table is the upper surface of a water body above a restricting layer and not in contact with the permanent body of groundwater below. The perched water table may fall gradually if the restricting layer is permeable enough for water to seep through to the permanent water table. iii) Zone of water and salt accession: The water table and capillary fringe move upwards in this zone due to irrigation and rainfall, but fall due to natural and artificial drainage. iv) Soil water: This is the water in the root zone. Under normal growing conditions soil water content varies between field capacity and permanent wilting point. v) Capillary fringe: This is the soil layer affected by capillary rise from the water table. Close to the water table this layer is almost saturated and at its top the water content will be below field capacity.

86 | SOIL DRAINAGE PHOTO: J.E. HOFFMAN, STELLENBOSCH UNIVERSITY.