Handbook for Irrigation of Wine Grapes in South Africa

15

15

11 mm

11 mm

10

10

5

5

Cumulative wate

runoff. Therefore, tillage is not recommended for South African vineyard soils except for seedbed preparation to establish winter cover crops (Fig. 2.13A). Another concern is that frequent tillage may destroy biological activity in the soil. For example, birds often prey on insects and earthworms in newly tilled soil (Fig. 2.13B). In many cases, the burrows created by biological activity will enhance water infiltration in the generally poorly structured topsoils in the Western Cape. 0 0 2 4 6 8 10 12 14 16 Time (days) Figure 2.13 0 0 2 4 6 8 10 12 14 16 Cumulative water Time (days) Figure 2.13

A

B

FIGURE 2.13. Examples where (A) a seedbed for a cover crop is being prepared in a vineyard and (B) weeds are being disked in as means of weed control in fallow land.

Trellis system : Although evaporation from the soil was not measured directly, previous studies showed that higher temperature, more wind flow and less shading of the soil surface in the case of bush vines (goblet) probably caused more E s losses, compared to grapevines trained onto a slanting trellis (Table 2.1). Since leaf area of the bush vines were smaller, E s losses must have made a more pronounced contribution to ET than transpiration under the prevailing conditions. Evaporation studies with different soil types showed that E s tended to be higher under a slanting trellis, compared to a vertical trellis (Myburgh, 2015). This agrees with a previous conclusion that higher resistance to air flow by the vertical trellis produces a windbreak effect, whereas a slanting trellis allows air to flow more freely (Van Zyl & Van Huyssteen, 1980b). In fact, the daily wind run measured in the middle of the work row at bunch height was 27.6 km/d under a slanting trellis, as opposed to 15.9 km/day for a vertical trellis. TABLE 2.1. Effect of canopy structure on leaf area, mean shading of the soil surface per day, mean midday air temperature in the canopy, daily wind run at bunch height and seasonal ET of micro-sprinkler irrigated grapevines near Robertson (Van Zyl & Van Huyssteen, 1980a). Trellis system Leaf area (m 2 /grapevine) Shading (%) Mean midday temperature (°C) Wind run (km/day) ET (mm)

Bush vine

10.21

58

26.30

31.4

404.1

Slanting trellis

13.66

87

24.86

27.6

351.3

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