Handbook for Irrigation of Wine Grapes in South Africa

Chapter 7

7.2.10 IRRIGATION STRATEGIES DURING CRITICAL DROUGHT PERIODS During dry winters, storage dams for irrigation water might not fill to their full capacity. In the long run, low winter rainfall could also cause the level of groundwater, e.g. in boreholes, to fall. The drought conditions will become worse if spring rainfall is also lower than usual. Although rainfall could occur in summer, it will most probably be insufficient to meet the water requirements of vineyards. Where vineyards have deep, well-developed root systems, winter water stored in the root zone will normally be adequate to to meet the grapevines’ needs until the end of December. Lower than usual rainfall during winter can cause low soil water content, particularly in the layers below the root zone, when grapevine growth begins in September. In summer, water stored in these deep soil layers can move into the root zone via capillary rise to serve as a buffer against grapevine water constraints during drought periods. In certain areas, water quality can decline drastically during periods of drought, particularly with respect to the level of salinity or sodicity. If the water quality is outside the norms presented in Chapter 5, it should not be used for irrigation to avoid permanent damage to the soil and the grapevines. In such cases, a possible solution would be to “mothball” the vineyards, i.e. to remove all bunches and only leave three or four shoots to keep the rest of the grapevine physiologically active. When the water resources are limited, irrigation should be aimed at reducing grapevine water constraints during critical phenological phases. Sufficient water at budbreak will allow even budding and vegetative growth. An irrigation before budbreak will be crucial if the preceding winter was exceptionally dry. If the water resources are limited, e.g. only enough for one irrigation, it should be applied as close as possible to véraison (Fig. 7.26). The irrigation must be applied to wet the root zone as deep as is practically possible. Cultivars that ripen first should be irrigated first, followed by cultivars that ripen later. The objective of this approach is to reduce the time between the irrigation and picking of the late cultivars. If the winter rainfall is below normal, or in regions where winter rainfall is usually low, the single irrigation must be applied before flowering to reduce water constraints during berry development. This may reduce the drought impact on yield, i.e. if heat waves do not occur later in the season.

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