SOIL PREPARATION

There are cases where a perched (temporary) water table is present on impermeable layers of clay, parent material or plinthite and where these layers occur over a large area (Van Woerkom & Streutker, 1971). Accumulation of water occurs above such layers in e.g. Kroonstad, Estcourt, Cartref and Wasbank soils to form a temporary water table after heavy rains or irrigation. The drainage depth of these soils can be improved by breaking up the impermeable layer during soil preparation in a downhill direction (Saayman & Van Huyssteen, 1981a). Under dryland conditions it is important that winter rain saturates the clayey subsoil which can then act as a source of water for vineyards during the warm dry summers of Mediterranean climates. Drainage under such conditions will only serve to unnecessarily extract stored water from the soil. If these soil types occur on low-lying flat land, they may remain wet for too long and then subsurface drainage will be necessary. Saayman & Van Huyssteen (1981a) warned, however, that drainage will only be economically justified if the impermeable layers occur shallower than 80 cm. Waterlogging due to a fluctuating water table can appear even if the impermeable layers are very deep and are covered by more porous soil horizons e.g. in Westleigh and Longlands soils. Capillary rise from the water table can saturate the soil even a further 20-50 cm above the water table depending on the soil texture. On shallow soils that cannot be drained economically, ridging can be considered (see discussion in Chapter 5). 6.4.1 Cut-off Drains Cut-off drains (also called interceptor drains) are used when free water moves laterally in porous layers on impermeable subsoil from a higher elevation to lower lying land (Figure 6.3). This happens typically on shallow soils when water seeps from higher-lying land under irrigation or from leaking dams and canals (Van Woerkom & Streutker, 1971). An advantage of the cut-off drain is the fact that it can be installed outside (up-slope) of the vineyard. The cut- off drain is installed perpendicular to the direction of water flow as a single distributed drain or as a series of parallel drains. According to Reinders et al . (2016) it is usually necessary to estimate the down-slope effect of cut-off drains to determine the number of drains needed to reduce the water table to the desired depth. They suggest that the construction of cut-off drains must often be done progressively, in other words that the effect of a drain first be evaluated before further drains are installed. Cut-off drains will have little effect up-slope of its position.

90 | SOIL DRAINAGE