Handbook for Irrigation of Wine Grapes in South Africa

Figure 8.49

Figure 8.49

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Figure 8.49 Figure 8.49

10 Yield (t/ha) 10 Yield (t/ )

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Potential wine quality (%) Potential wine quality (%)

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Merlot - Wellington Shiraz - Robertson Merlot - Wellington Shiraz - Robertson

Merlot - Wellington Shiraz - Robertson Merlot - Wellington Shiraz - Robertson

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-0 6 -0 8 -1 0 -1 2 -1 4 -1 6 -1 8 -2 0 -2 2 Ȍ S (MPa) -0 6 -0 8 -1 0 -1 2 -1 4 -1 6 -1 8 -2 0 -2 2 Ȍ S (MPa)

-0 6 -0 8 -1 0 -1 2 -1 4 -1 6 -1 8 -2 0 -2 2 Ȍ S (MPa) -0 6 -0 8 -1 0 -1 2 -1 4 -1 6 -1 8 -2 0 -2 2 Ȍ S (MPa)

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FIGURE 8.49. Effect of midday Ψ S

at which irrigations were applied on (A) yield and (B)

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potential wine quality of two grapevine cultivars.

10 Yield (t/ha) 10

Yield (t/ha)

0 For example, if maximum yield is the objective, both Merlot and Shiraz should be irrigated when the midday Ψ s is between -0.8 and -1.0 MPa (Fig. 8.50A). Furthermore, it is important that yield will not increase if irrigation is applied when Ψ s is higher than these values (Fig. 8.50A). Therefore, irrigation applied at, e.g. -0.6 MPa, will cause over-irrigation resulting in excessive veget tive growth and poor wine quality (Fig. 8.50B). It is interesting to note that the two cultivars do not really differ with respect to the level of plant water status at which maximum yield is expected. On the other hand, Merlot should be irrigated when the midday Ψ s is between -1.4 and -1.5 MPa to produce optimum wine quality, whereas for Shiraz the best wine quality is expected at ca. -1.9 to -2.0 MPa (Fig. 8.50B). Ȍ S (MPa) Merlot - Wellington Shiraz - Robertson Ȍ S (MPa) Merlot - Wellington Shiraz - Robertson Figure 8.50 Merlot - Wellington Shiraz - Robertson Ȍ S (MPa) Merlot - Wellington Shiraz - Robertson Figure 8.50 5 -0 6 -0 8 -1 0 -1 2 -1 4 -1 6 -1 8 -2 0 -2 2 Ȍ S (MPa) -0 6 -0 8 -1 0 -1 2 -1 4 -1 6 -1 8 -2 0 -2 2 Ȍ S (MPa) Merlot - Wellington Shiraz - Robertson Merlot - Wellington Shiraz - Robertson 30 40 Potential wine quality (%) Potential wine quality (%) -0 6 -0 8 -1 0 -1 2 -1 4 -1 6 -1 8 -2 0 -2 2 Ȍ S (MPa) -0 6 -0 8 -1 0 -1 2 -1 4 -1 6 -1 8 -2 0 - 2 Ȍ S (MPa) Merlot - Wellington Shiraz - Robertson Merlot - Wellington Shiraz - Robertson Figure 8.50 Figure 8.50 0 5 30 40 Ȍ S (MPa)

-2 2 -2 0 -1 8 -1 6 -1 4 -1 2 -1 0 -0 8 -0 6 -2 2 -2 0 -1 8 -1 6 -1 4 -1 2 -1 0 -0 8 -0 6

-2 2 -2 0 -1 8 -1 6 -1 4 -1 2 -1 0 -0 8 -0 6

-2 2 -2 0 -1 8 -1 6 -1 4 -1 2 -1 0 -0 8 -0 6

-2 2 -2 0 -1 8 -1 6 -1 4 -1 2 -1 0 -0 8 -0 6

-2 2 -2 0 -1 8 -1 6 -1 4 -1 2 -1 0 -0 8 -0 6

- 2 -2 0 -1 8 -1 6 -1 4 -1 2 -1 0 -0 8 -0 6

- 2 -2 0 -1 8 -1 6 -1 4 -1 2 -1 0 -0 8 -0 6

Merlot - Wellington Shiraz - Robertson Merlot - Wellington Shiraz - Robertson

Merlot - Wellington Shiraz - Robertson Merlot - Wellington Shiraz - Robertson

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Potential wine quality (%)

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FIGURE 8.50. Relationships to determine at which midday Ψ S

two grapevine cultivars need

to be irrigated to obtain (A) maximum yield or (B) highest potential wine quality.

In practice, it means that soil water status and midday Ψ s need to be measured simultaneously on a regular basis as the soils dries out following rain or irrigation. The soil water content at which the desired Ψ s is reached will be the refill point,

280 CHAPTER 8 – PRACTICAL IRRIGATION SCHEDULING