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Improve water use of wine grapes

By Jan van Zyl (Agronomist - Western Cape Business Unit)

The reaction of vineyards to fertilizer inputs is highly dependent on irrigation management in irrigated areas and the two inputs cannot be separated from a management point of view. In general, greater reaction is found were water stress is confined to a minimum during the growing season.

Water is becoming an ever increasingly scarce resource and careful management thereof is a big responsibility of producers in the irrigated areas of the country.

With recent increases in energy costs, irrigation has become a larger part of the input cost of vineyard production. Coupled with increases in fuel, labour and fertilizer costs, it has become more important than ever for vineyard producers to achieve high yields.

It is, however, important for the wine industry that grapes of high quality are produced and therefore imperative that a compromise be found between the production of high quality grapes and yield in the irrigated vineyard production areas.

Researchers, primarily from the ARC Nietvoorbij, have produced valuable research and results in this regard and this article aims to serve as a short summary of this work in recent years.

Two expressions of soil water content are given in these results. The first is readily available water (RAW) which is typically taken as the soil water content between -10kPa and -100kPa. The other is plant available water (PAW) which is typically taken as the soil water content between -10kPa and -1500kPa. In the article percentage of RAW and PAW is given in millimetre water per meter (mm/m) of soil. 

Results from the Olifants River region showed that high water constraints (75% RAW) before véraison reduced yields when compared to the higher irrigation strategy (30% RAW). Sudden high constraints after véraison reduced wine quality in Sauvignon Blanc. Water deficits induced only before or after véraison reduced yield but did not improve wine quality for Pinotage.

In the Breede River region, results showed that higher irrigation frequencies led to higher root densities and had a strong relationship with vegetative growth. In accordance with results from the Olifants River region drier soil conditions from bud break to harvest reduced vegetative growth with Sauvignon Blanc showing higher sensitivity.

Higher irrigation frequency on Colombar in the lower Orange River region tended to increase yields while having very little effect on wine quality when compared to lower irrigation frequencies.

Particularly for white cultivars, irrigation strategies between 40% and 90% PAW had limited influence on wine quality. While it is accepted that increased irrigation will lead to higher yields it is not necessarily related to inferior wine quality when accepted water depletion (50% PAW) is maintained.

In general, it seems that irrigations aimed at maintaining 30% RAW increased yield but reduced quality when compared to 75% RAW strategy and that maintaining 50% PAW is needed to achieve higher yields. It therefore seems that an irrigation strategy aimed at maintaining 40% of PAW should not impact negatively on yield or quality.

To effectively manage any irrigation system at a percentage of PAW or RAW, soil water measuring equipment is vital. There is a large range of soil water measuring equipment available on the market ranging from tensiometers to continuous logging capacitance probes, and producers are encouraged to evaluate and implement any of these systems on their farms in order to manage their water resources as efficiently as possible.