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Over
the summer, the clusters of grapes grow from
the size of sesame seeds to the size of
green peas. While the fruit has been growing,
we have been continually monitoring the water
usage of the vines. Since 30-odd inches of winter
rains are stored in the soil, the vines initially
have all the water they want. But awareness of
soil- and plant-water content becomes critical
during mid to late summer. That is when the vines
will have exhausted the available water in the
soil.
The loss of available water is not a bad thing
— it’s a natural occurrence that signals to the
vines that it’s time to ripen the fruit. Veraison
is one real sign of ripening. This is the process
of the berries changing from green to purple
(or from light green to yellowish-green for white
wine grapes).
But reducing available soil water too much can
ruin the fruit. So we continuously observe vine
physiology and take regular measurements of soil
water content and vine water demand. The water
content of the soil is monitored by two buried
arrays of sensors. The water status of the vines
is quantified with a pressure chamber, a specialized
field instrument which measures the amount of
suction a vine must exert to extract water from
the soil. We initiate and regulate vineyard irrigations
based on our observations and measurements.
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