Handbook for Irrigation of Wine Grapes in South Africa

Irrigation frequency: As mentioned above, E s from a bare soil surface is usually high after a rainfall event or irrigation, after which it reduces quickly. If the irrigation cycles are short ( e.g. 5 days), the E s peaks will occur more frequently and the contribution of soil surface evaporation to ET will be considerably higher, compared to longer periods ( e.g. 21 days), when fewer high E s peaks will occur (Fig. 2.10). If canopies are comparable, E s alone can constitute ca. 80 and ca. 20% of ET for high and low frequency irrigation, respectively.

E s = 18 x 15 mm = 270 mm

PAW

Soil water content

Time

E s = 4 x 15 mm = 60 mm

PAW

Time

Soil water content

FIGURE 2.10. Schematic illustration of E s losses during Phase I under high frequency irrigation (top) and low frequency irrigation (bottom), i.e. if it is assumed that both soils have similar PAW, and that Phase 1 E s is 15 mm.

Mulching and tillage: Organic mulches, e.g. straw or wood chips, will reduce E s losses, compared to bare surfaces (Fig. 2.11). However, the effect of the surface cover is usually effective only for a short period, i.e. the initial Phase I evaporation. Consequently, the water saving effect of mulches will be more pronounced in vineyards where frequent irrigations are required, e.g. in shallow sandy or gravelly soils, compared to deeper, heavier soils which need to be irrigated less frequently. In dryland vineyards, mulches will only reduce E s during Phase I when rainfall occurs. The water saving effect increases with the thickness of the mulch layer (Fig. 2.12). However, thick mulches are only economically viable if the material is readily available from a nearby source. Transport of lightweight material over long distances will not be cost effective. Furthermore, visual observations near Philadelphia and Rawsonville revealed that ca. 8 t/ha wheat straw mulches on the under-vine banks had weathered away within two seasons. In this regard, more durable organic materials such as tree bark or wood chips will probably last longer.

38 CHAPTER 2 – THE DYNAMICS OF WATER IN AND AROUND VINEYARDS