The effect of irrigation on yield is not a linear relationship (Figure 3). Theoretically, there is an irrigation threshold where yield reaches maximum productivity. Any irrigation above this level results in wasted water to deep drainage and evaporation that can also lead to excessive vegetative growth, nutrient leaching, pest and disease problems and poorer fruit quality (Figure 4).
Currently the Australian wine industry has widely adopted various irrigation management strategies such as regulated deficit irrigation (RDI), partial rootzone drying (PRD) and sustained deficit irrigation (SDI) to improve and sustain water use effciciency (WUE-tonnes of fruit per megalitre of water) (Kreidemann & Goodwin, 2003). A common feature of these irrigation techniques is the reduction in available soil water, but how the water is applied is fundamentally different.
In order for growers to make a decision whether to focus on irrigating select patches (high value varieties) or applying reduced water across all patches there needs to be information on the immediate impact of water deficit on yield and quality not only in the current season but also preceding years. There are some general concepts that can be considered when trying to budget and prioritise water when water restrictions are imposed. This section describes a number of scenarios ranging from no irrigation at all to those aiming to achieve maximum yield.
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Scenario 1:
No irrigation and likely abandoning of the vineyard. Vines may have to be removed because of the potential of pest and disease carryover. After one season without irrigation it may be possible to revive the vineyard in the subsequent season. Survival will depend on rootstock and variety. Ramsey grafted vines are more likely to survive than vines on their own roots.
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Scenario 2:
Irrigation for survival. This scenario suggests keeping the vines alive to protect the future production capacity under very limited water supplies. To maximise the potential for a crop in the next season, it is important to maintain bud fruitfulness and retain as much of the vine’s carbohydrate reserves as possible. This could be achieved by winter pruning to reduce the number of buds retained so as a small canopy is maintained.
Furthermore, the removal of bunches shortly after berry set will reduce the tranpsirational demand required to ripen a crop. Irrigation should only be applied once shoot growth ceases, which tends to be near flowering. Where possible, limit shoots to approximately 30-40cm in length (6-8 healthy leaves per shoot) either by withholding irrigations or summer hedging. Large amounts of leaf loss should be avoided during veraison to harvest, as it will lead to reduced carbohydrate reserves and poor fruitfulness of the basal buds. Careful observation of the vines for signs of early water stress in conjunction with soil moisture monitoring will help in avoiding the vines shutting down.
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Scenario 3:
Irrigate to minimise the loss in yield. Most appropriate is the application of either a RDI or SDI irrigation strategy. Severe stress prior to flowering and set should be avoided because of its negative impact on fruiting potential in the current and subsequent season. Post-set RDI has less impact on fruiting potential but may reduce berry size and yield. The least sensitive period to apply RDI is after fruit set around mid to late November to veraison in mid to late January.
Applying a sustained deficit below the estimate plant water requirement has been shown as a successful strategy for conserving water. However it should probably not be less than 70% of crop water requirement and some reduction in yield will be inevitable.
Author
Mark Krstic
Grape and Wine Research and Development Corporation
