Grapevine Water Management
Vineyard Water Requirements
The water requirement of a mature vineyard over a season varies from 10 to 30 acre-inches (25 to 75 cm) of rainfall or its equivalent in irrigation if grown on medium to heavy textured soils and from 36 to 48 acre-inches (90 to 120 cm) if grown on light, sandy soils. The amount of water required to grow grapes for different wine styles can vary significantly. Generally red grape varieties require less water than white varieties. Grapes for aromatic and light style wines require more water to minimize water stress than grapes producing medium to full bodied wine styles. Grapevine water requirements are influenced by vine density, age of vines, the variety, rootstock-to-scion interaction, cover crops, climate (rainfall and evaporation), and crop load to name a few. Figure 11.1 illustrates the approximate annual percentage of water required by vines at each stage of their growth cycle. In situations where water salinity levels are high, estimates of vine water use should include an appropriate leaching factor for washing salt beyond the effective root zone. If frost is a concern and if sprinkler systems are used that will have to be considered too when determining vineyard water requirements. Depending upon the phenological stage (budburst, flowering, or véraison) water stress has a wide range of effects on grapevine growth, development, and physiology
In order to understand how to manage vines with irrigation water it is important to understand the annual growth cycle of the vine and its water requirements. The main growth stages are:
Budbreak to Flowering
From budbreak to flowering grapevine water requirement during this stage is moderate—approximately 9 percent. This stage is critical for root growth, establishing the vine canopy, and potential yield for the current and the following season.
Flowering to Fruit Set
From flowering through fruit set water consumption for the period is about 6 percent of the season’s total. The most sensitive period to water stress is between flowering and fruit set where severe and prolonged water stress may result in poor flower-cluster development, reduced pistil and pollen viability and subsequent berry set (Hardie et al., 1976).
Fruit Set to Véraison
During the long period from fruit set to véraison water consumption is about 35 percent of the annual water requirements. Insufficient canopy development during this time will limit the photosynthetic capacity of the vine and may restrict fruit development and quality.
Véraison to Harvest
Over the shorter period, from véraison to harvest, water consumption to about 36 percent of the annual water requirements. Irrigation during this period should maintain canopy health and avoid any vine stress.
Harvest to Leaf Fall
Water consumption during the postharvest stage is only about 14 percent of annual water consumption. It is still important to maintain a healthy canopy during this period to ensure that the vine is able to build up sufficient carbohydrate reserves in the wood of the vines for the subsequent season before going into dormancy.
Symptoms of Water Stress
When water demand of the grapevines is higher than supply, drought stress occurs. Often, drought stress is associated with heat stress during the hot summer months. Varieties differ in their response to water restrictions. For example Shiraz growth will slow down and show symptoms of water deficits more readily than some other varieties. Shiraz vines look limp while under a water deficit but have a remarkable capacity to recover when soil moisture conditions improve. Cabernet Sauvignon fruit generally shrivels before any basal leaf loss is observed.
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