Handbook for Irrigation of Wine Grapes in South Africa

Since grapevine yield generally declines with the volume of irrigation water, irrigation strategies where the irrigation water is reduced are unlikely to improve WUE. However, comparing different pre- and post-véraison irrigation frequencies, PAW depletion levels and DI showed that the WUE of drip irrigated Shiraz near Robertson in the Breede River Valley region only increased where less grapes were produced with less water (Fig. 7.46). In reality, the WUE can only be higher if (i) more grapes are produced with the same amount of irrigation water without compromising on wine quality, or if (ii) the same grape yield and quality are maintained with less irrigation. One way to achieve these objectives is to adapt the trellis and/or training systems in order to increase the bearing capacity per grapevine. For example, the cordon arm length of grapevines trained onto a two- tier vertical trellis will be almost double, compared to single cordon grapevines on a six-wire vertical trellis (Fig. 7.47). The two levels of cordon arms will increase the bearing capacity substantially (Fig. 7.48).

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10 15 20 25

WUE (kg/m 3 )

WUE (kg/m 3 ) 0 5

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0 5

Yield (t/ha)

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Yield (t/ha)

FIGURE 7.46. Relationship between irrigation WUE and yield of Shiraz that were subjected to different irrigation strategies in a field trial in the Breede River valley (after Lategan, 2011).

Figure 7.47

A

B

Figure 7.47

65 c m

70 c m

65 c m

FIGURE 7.47. Examples of grapevines trained onto (A) a six-wire vertical trellis and (B) a two-tier vertical trellis. Dashed lines indicate trunks and cordon arms of the two-tier trellis.

70 c m

230 CHAPTER 7 – IRRIGATION STRATEGIES