From a climatic point of view much can still happen this season to change the dry situation, but many producers are already making plans to try and cope with less water.
The dry conditions which have prevailed since August 2014 have caused very low levels in large catchment dams, as well as farm dams, at the end of September. Soil profiles are also drier than usual in areas in the coastal region that are generally at field capacity at the time of bud burst.
Water management and irrigation scheduling are obviously specific to the area and region and dependent on the wine objectives and production levels to be achieved. The following principles may be important, however, in the development of a strategy where water resources are very limited:
It has already been proven that an increase in water supply will cause an increase in production in a given situation. The relationship is not absolute, however, and with excessive supply production will level out and may even decrease. In the case of intensive irrigation, a small decrease in water supply will not cause large losses in production.
Under local conditions it was found that wine quality and water consumption efficiency (kg grapes per cubic metre water produced) improved when irrigation took place at increased soil moisture deficiency levels, in other words drier soil.
Moisture stress during flowering and set are extremely detrimental to production, but at other stages in the course of the season the grapevine is able to handle a fair amount of moderate moisture stress. Moderate stress during the period after set until véraison is especially beneficial to curb excessive growth.
A large canopy surface also results in high water consumption. Unnecessary shoots may be removed to reduce demand points. Likewise timely tipping and topping actions may be used to prevent excessively large canopies.
Effective, deep soil preparation and a well-established and distributed root system enlarges the buffer capacity of the grapevine and are invaluable in dry years. The root depth and distribution should be known, because excessive irrigation past the root zone will result in losses.
The value of a good cover crop should not be underestimated; not only is run-off restricted, but so too evaporation. With regard to evaporation, losses are also bigger if small irrigations are given at short intervals. Evaporation is also higher if drops fall directly onto warm plastic strips.
It goes without saying that the grapevine’s competitors should be eliminated and in dry years it makes sense to kill weeds and cover crops at an earlier stage.
The size of the crop will begin to play a role after véraison. If the grapevines bear too heavily, delayed ripening will ensue or grapes may even be unable to reach the desired degree of ripeness. Crop adjustments should therefore be done before véraison.
This soil profile in the Wellington area shows soil moisture to be below field capacity at the time of bud burst. Winter rainfall was therefore insufficient to moisten the soil as usual to full capacity.
In this vineyard weeds compete with scarce nutritional and water resources. An unnecessarily large canopy surface results in high water consumption.
The most important principle is nevertheless to measure the soil and plant water status. Only with sufficient, reliable information can the choice of irrigation strategy be implemented successfully.
For more information contact Hanno van Schalkwyk at firstname.lastname@example.org.
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