Handbook for Irrigation of Wine Grapes in South Africa

Chapter 7

7.2.6 DEFICIT IRRIGATION The primary objective of a deficit irrigation (DI) strategy is to improve wine quality without compromising on yield by deliberately reducing the irrigation requirement so that the soil dries out gradually (Fig. 7.16). Research has shown that the latter is possible, but that the effects of pre- or post-véraison deficits may be slightly different, depending on the cultivar and viticultural conditions. However, the general trend is that continued DI (Fig. 7.16A), i.e. pre- and post-véraison, will enhance wine quality, but slightly reduce yield (Table 7.9). Given the negative impact of deficit irrigation on shoot growth, more exposed leaves resulted in lower juice pH and higher TTA, compared to high frequency irrigation (Table 7.9). The positive effect on wine quality is primarily caused by reduced vegetative vigour and smaller berries resulting from the longer period of higher water constraints. However, drier irrigation strategies can only be justified if higher grape and/or wine prices will compensate for the possible yield losses.

100 120 140 160 180 200

A

FC

0 20 40 60 80

PWP (90)

Soil water content (mm/60 cm) Sep Oct

Nov

Dec

Jan

Feb Mar

100 120 140 160 180 200

B

FC

0 20 40 60 80

PWP (90)

Soil water content (mm/60 cm) Sep Oct

Nov

Dec

Jan

Feb Mar

FIGURE 7.16. Seasonal soil water content where drip irrigated Shiraz/110 R in a fine sandy loam soil received (A) continued DI throughout the season and (B) where the soil water content was restored to FC at véraison in the 2008/09 season near Robertson (Lategan, 2011). Dashed lines indicate field capacity, permanent wilting point and PAW depletion level.

IRRIGATION OF WINE GRAPES 197