Agriculture Reference
In-Depth Information
briefly described in box 2.1. As an example of the use of these indices, figure 2.3
shows a diagram of the ranges in growing season temperature that are considered
optimum for the production of premium quality wines from some of the best
known varieties. Such a diagram needs to be used with caution, however, because
ongoing climate change may lead to a number of possible outcomes, such as
Box 2.1 Climate Indices for Site Selection
One of the earliest bioclimatic indices for wine grapes, developed in California
by Amerine and Winkler (1944), is based on the summation of heat units
during the growing season, that is, for seven months from October to April
in the southern hemisphere or April to October, inclusive, in the northern
hemisphere. The summation, called heat degree days (HDD), for a site is
calculated as follows:
HDD um of number of days
=
[
×
(
mean daily temperature
−°
10
C)
]
The mean daily temperature is the average of the daily maximum and
minimum temperature, which should be derived from records for at least 10 years.
The 10ºC (50ºF) cutoff was chosen because few grape cultivars grow actively
below this temperature, which is why the term growing degree days, synonymous
with HDD, is used in some countries. Five macroclimatic regions are recognized,
ranging from Very Cool (<1390 HDD, temperature in ºC) to Very Warm/Hot
(>2220 HDD, temperature in ºC).
Another system developed in Australia by Smart and Dry (1980) is based on
the correlation between mean January temperature and HDD for places of similar
continentality. The continentality index is measured by the difference between
the mean January and mean July temperatures (for the southern hemisphere).
More recently, Smart (2001) has promoted the homoclime concept, which is a
way of comparing statistically the mean monthly temperatures and rainfall for a
site with those for established wine regions around the world. For sites without a
record of climate data, the climate can be estimated from national meteorological
databases providing data on a 2
×
2 km grid. After the homoclime of a site has been
established, the most suitable cultivars can be recommended, based on experience
in other parts of the world (see
www.smartvit.com.au)
. Figure 2.3 illustrates
the application of Jones's (2006) growing season temperature, global climate
classification for premium wine production.
Because the HDD concept is relatively crude, Gladstones (1992,
2011) developed a more refined temperature index, expressed as a summation
of “biologically effective” degree days (E
o
days). Based on the strong dependence
of vine phenology (charting a vine's progression from bud burst to maturity),
Gladstones estimated E
o
from the same starting point as HDD but with
adjustments for the diurnal temperature range, day length (dependent on latitude),
and an upper temperature limit of 19
o
C. Full details of this more complex index
are given in chapter 11 of Gladstones (2011).
