Handbook for Irrigation of Wine Grapes in South Africa

Chapter 8

Another important aspect is that FruitLook ET estimations depend greatly on the vegetative vigour of the vineyard. In other words, vegetative growth response to over- or under-irrigation will reflect in the estimated ET. This means that, if a vineyard is over-irrigated, and the grower irrigates according to FruitLook estimations, the over-irrigation will continue. The opposite will happen if the vineyard is under-irrigated. Therefore, it is recommended that the weekly ET estimations should be validated. In this regard, FruitLook estimations can easily be validated by regular monitoring of grapevine water status, e.g. by means of midday Ψ S . If necessary, irrigation can then be reduced, or increased so that the midday Ψ S falls in the desired water constraint class for a specific end product objective. FruitLook images can also indicate if growth vigour variation occurs within a vineyard. If vigour variation occurs within an irrigation block, i.e. a section of the vineyard that is irrigated via one valve, the grapevine water status must be measured where the lowest vigour occurs. The primary objective of FruitLook is to irrigate crops in such a way that they experience no water constraints. Therefore, an ET deficit in FruitLook terms implies that too little irrigation water is being applied. However, if high wine quality is the objective, irrigation must be reduced as discussed in Chapter 7. Under such conditions, growers must not panic if FruitLook indicates substantial ET deficits. Where irrigation water is limited, and growers are forced to apply low frequency irrigation, it is most likely that FruitLook will also indicate ET deficits. Consequently, there is no need for wine grape growers to be concerned if ET deficits occur. The ideal is to irrigate vineyards at a certain level of plant available depletion to obtain predetermined yield and/or wine quality objectives. Unfortunately, this approach has two major shortcomings. Firstly, determining water holding capacity of soils accurately is time consuming, and therefore not suitable for routine analyses by commercial laboratories. In this regard it must be noted that estimation of plant available water from soil texture is also questionable. Secondly, instrument calibrations might not provide the accuracy required to ensure that the PAW is depleted to the desired level. A solution is to combine soil and plant based measurements. Research has shown that there is a close correlation between grapevine responses, e.g. yield and wine quality, and grapevine water status (Fig. 8.49). Reversing these relationships can be used to determine at which water constraint level grapevines should be irrigated to obtain predetermined yield or quality objectives. 8.5 SETTING REFILL LINES FOR IRRIGATION STRATEGIES BASED ON GRAPEVINE WATER STATUS

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