Putting It All Together - Understanding Vineyard Soils - page 230

sum of a soil's RAW and DAW . Alternatively, the soil matric suction should not
exceed 100 kPa for a sandy soil or 400 kPa for a clay. In the Californian volume
balance approach, the supply of water should generally not be less than 60% of
full water use. Although too large a deficit may significantly reduce fruit yield in
the current year, the effect is greater in the following year.
With respect to timing, the three main periods of interest are from bud burst
to fruit set, fruit set to veraison, and veraison to harvest. During the first period,
flowering and fruit set are the most sensitive to water stress. An SWD greater than
the soil's RAW causes significant yield reductions, affecting the number of clus-
ters per vine and the number of berries per cluster. Such stress does not normally
occur in regions where the soil is at FC at the end of winter and is topped up by
spring rains, when evaporation rates are still low to moderate. However, a possible
effect of climate change may be that SWDs exceeding a soil's RAW occur more fre-
quently early in the growing season, especially in hot inland regions, so that irriga-
tion will be needed earlier to prevent a yield penalty. In this case, the SWD should
be maintained within the soil's RAW range to restrain early vegetative growth but
avoid a serious yield reduction.
Regulated deficit irrigation that imposes moderate water stress on red variet-
ies from fruit set to veraison generally achieves the best outcome for fruit quality,
although with some reduction in yield. Compared with fully watered vines, berry
size decreases, soluble solids increase, malate concentration is lower, and total
juice acidity is little changed. An increase in the concentration of phenolic (flavor)
and color compounds is often associated with the decrease in berry size.
When RDI is prolonged to harvest, although flavor and color compounds
can increase, the yield reduction is often more pronounced and the concentra-
tion of soluble solids decreased. Research at the University of California, Davis,
has shown that varieties such as Cabernet Sauvignon, Merlot, and Zinfandel
should be moderately stressed only until the berries reach 22º to 23º Brix, whereas
for Syrah (Shiraz), the critical point is 18º to 20º Brix. Methoxypyrazine com-
pounds that cause “green” flavors in wines reach a maximum before veraison and
do not increase with subsequent watering. However, some winegrowers with
well-established vines on deep loam to clay loam soils argue that “turning the taps
off” after veraison is not a problem, provided that water stress does not cause shriv-
eling of the mature berries and premature leaf fall. Implementing this approach
depends on weather conditions during the post-veraison period.
Although PRD can effect similar improvements in fruit quality, the response
is soil and variety dependent. Both RDI and PRD offer the added advantage of
saving water, which is a critical issue for irrigated viticulture in countries as widely
separated as Spain, Australia, the United States (Central Valley, California), and
South Africa. Water savings up to 30% and 50% can be made with RDI and
PRD, respectively. However, with RDI or PRD applied under drip irrigation, soil
salinization and possibly sodicity may occur unless an adequate leaching fraction