Handbook for Irrigation of Wine Grapes in South Africa

Chapter 8

i.e. when irrigation needs to be applied. With this approach, the soil water measurement is actually calibrated against grapevine water status. Once the refill lines are set, growers can continue only with soil based measurements. Calibration of soil water measurements against grapevine water status will also allow growers to use electronic instruments with questionable calibrations. In drip irrigated vineyards where the positioning of sensors with respect to the wetted soil volume is problematic, midday stem water potential will also be useful for setting refill lines. As a practical demonstration, the refill line for a drip irrigated commercial Cabernet Sauvignon vineyard was set by means of midday Ψ s . Since the estate produced the grapes for high quality wine, it was decided to set the refill line at the soil water content where midday Ψ s reached -1.4 MPa, as discussed above. Soil water content was measured by means of a neutron probe calibrated for the specific soil. Measurements commenced before budbreak when the soil profile was close to field capacity, i.e. following the winter rainfall (Fig. 8.51). The “Full” line was set at this soil water content. Midday Ψ s measurements began in middle October when there were enough mature leaves. These measurements were repeated regularly until the Ψ s was close to -1.4 MPa, i.e. around véraison. This served as a guide to set the “Refill” line for the irrigations. At that stage, the grower applied a 24-hour irrigation. Due to a misunderstanding, the grower applied a second, 12-hour irrigation before the soil water content reached the refill line. Following this irrigation, the soil was allowed to dry out until the refill line was reached. Since it was so close to harvest, the third irrigation was also only applied for 12 hours (Fig. 8.51).

Figure 8.51

250

Véraison

Budbreak Full line

Harvest

200

24 h

12 h

12 h

-0.66 MPa

-0.60 MPa

150

-0.88 MPa

100

-0.93 MPa

Refill line

SWC (mm/80 cm)

-1.09 MPa

-1.15 MPa

50

-1.37 MPa

-1.41 MPa

0

Aug Sep

Oct

Nov

Dec

Jan

Feb

Mar

FIGURE 8.51. An example where the irrigation refill line for a commercial Cabernet Sauvignon vineyard near Stellenbosch was set by means of midday Ψ S in the 2016/17 season (unpublished data).

IRRIGATION OF WINE GRAPES 281