Wine Science: Principles and Applications, Third Edition - Vinum Vine

264 5. Site Selection and Climate
Figure 5.23 Diagram of barrier effects on microclimate. Arrows indicate the directions in which the values of different factors
vary relative to unsheltered areas. H refers to barrier height. (From Marshall, 1967, reproduced by permission)
than the normally preferred north–south orientation
may retain heat by limiting the wind fl ow down the
rows (Brandtner, 1974). Thus, in windy climates, slopes
facing other than due south (in northern latitudes) may
be preferable because of the protection provided from
northerly winds.
In contrast, there may be advantages in planting the
rows parallel to the prevailing wind under dry conditions
(Hamilton, 1985). Winds moving directly down
the rows produce less foliage drag and minimize water
loss (Hicks, 1973). It is estimated that this could produce
up to a 10–20% saving in water demand. Wind
moving across rows creates turbulence and greater foliage
movement than moving unimpeded down rows.
Turbulence may increase relative to the density of the
grapevine canopy.
Water
Most of the water absorbed by roots is used as a coolant.
The evaporation of water from the leaves helps
maintain normal physiology by minimizing overheating.
When water loss continues to exceed replacement from
the roots, the stomata close. As the tissues overheat,
most metabolic activity slows. Physiological activity
returns to normal only some time after the water defi -
cit has been overcome. If the defi cit persists, continued
water loss through the cuticle results in cell plasmolysis
and death. Mature vines seldom show marked wilting
due to an extensive root system that can reach water
meters below the soil surface. Nevertheless, physiological
disruption occurs long before wilting. Reduced shoot
growth is one of the most sensitive signs of grapevine
water stress.
Where irrigation is permitted and economically feasible,
the damage caused by naturally occurring water
shortages can be prevented. Inexpensive and reliable
sources of irrigation water are essential for commercial
viticulture in most semiarid and arid climates. Water
sprinkling systems may limit heat stress, but are usually
unnecessary or ineffectual.
Problems with excessive rainfall, fog, or high humidity
are often more serious, or at least more diffi cult
to control. Good soil porosity and drainage diminish
problems associated with heavy rainfall. Basal leaf
removal and divided-canopy training systems, with
their larger surface areas, can further increase water
evaporation. Angling vineyard rows to increase wind