Handbook for Irrigation of Wine Grapes in South Africa

Figure 7.12

TABLE 7.6. Cane mass at pruning, berry mass, yield, overall wine quality and ET of Colombar grapevines subjected to four irrigation frequencies in the medium range in the Lower Orange River valley. Data are means for four seasons (Myburgh, 2007c). Irrigation cycle (days) Cane mass (t/ha) Berry mass (g/berry) Yield (t/ha) Quality (%) ET (mm/year) Sep Oct Nov Dec Jan Feb

7 -90 -80 -70 -60 -50 -40 -30 -20 -10 0 Soil water matric potential (kPa) 50% RAW depletion 75% RAW depletion 3.8 a* 14 3.3 b 21 3.2 b 3.1 b

1.7 a

44.0 a

46.6 a

1020

1.5 b

39.4 b

47.2 a

880

1.5 b

38.3 b

47.1 a

770

28 770 * Values followed by the same letter within a column do not differ significantly (p ≤ 0.05). 1.4 b 36.4 b 52.9 a

Medium frequency drip irrigation caused relatively dry conditions in a fine sandy loam soil in the Breede River valley near Robertson (Fig. 7.13), compared to high frequency irrigation (Fig. 7.11A). As expected, the drier soil induced more water constraints in the Shiraz/110 R grapevines (Table 7.7). This resulted in lower cane mass at pruning, smaller berries and lower yield. The fact that medium frequency irrigation reduced juice pH and increased TTA (Table 7.7) indicates that more exposed leaves dominated the effect on pH and TTA (Fig. 7.10). Similar to low frequency irrigation (Table 7.4), medium frequency irrigation improved overall sensorial wine quality, compared to high frequency irrigation (Table 7.7).

100 120 140 160 180 200

FC

(80) PWP

0 20 40 60 80

Soil water content (mm/60 cm) Sep Oct

Nov

Dec

Jan

Feb Mar

FIGURE 7.13. Seasonal soil water content where Shiraz in a fine sandy loam soil was drip irrigated at ca . 75% PAW depletion in the 2008/09 season in the Breede River valley (Lategan, 2011). Dashed lines indicate field capacity, permanent wilting point and PAW depletion level.

192 CHAPTER 7 – IRRIGATION STRATEGIES