Handbook for Irrigation of Wine Grapes in South Africa

Chapter 2

TABLE 2.3. Effect of PAW and leaf exposure on stomatal resistance of Colombar/99 R grapevines measured during berry ripening on 4 February 1982 near Robertson in the Breede River Valley (adapted from Van Zyl, 1987). Leaf exposure Level of PAW depletion (%) Stomatal resistance (cm/s)

10:00

12:00

14:00

Sunlit

10%

2.0 a*

2.5 a

3.0 a

75%

2.5 ab

9.0 ab

16.5 b

Shaded

10%

3.5 b

5.0 ab

18.5 b

75% 35.0 c * Values followed by the same letter within a column do not differ significantly (p ≤ 0.05). TABLE 2.4. Effect of weekly irrigation and PAW per soil preparation depth (mm/cm) on transpiration of young Pinot noir/99 R grapevines in a gravelly soil measured around noon prior to harvest in the 1987/88 season near Stellenbosch (Myburgh et al ., 1996). Treatment Transpiration (mmole/m 2 /s) 17.6 mm/40 cm 35.7 mm/80 cm 54.2 mm/120 cm Non-irrigated 2.75 b* 4.93 b 5.53 a Irrigated weekly 6.36 a 7.17 a 6.78 a * Values followed by the same letter within a column do not differ significantly (p ≤ 0.05). As discussed above, the total water use of vineyards consists of transpiration and evaporation from the soil surface. In the case of full surface wetting, e.g. under micro-sprinklers, overhead sprinklers or flood irrigation, ET is basically the sum of soil evaporation over the full surface plus grapevine transpiration. However, sometimes confusion exists around ET when the total soil surface is being wetted, compared to fractional wetting. This section is aimed at explaining what happens to ET if only a fraction of the soil is wetted. Typical examples of fractional wetting are drip and furrow irrigation (Fig. 2.22). The substantial difference in wetted area between micro-sprinkler and drip irrigation is illustrated in Figure 2.23. 7.5 c 13.5 b 2.4.3 EFFECT OF WETTED SOIL VOLUME ON EVAPOTRANSPIRATION

IRRIGATION OF WINE GRAPES 47