T
he
growing degree day or heat unit theory assumes that plant
growth commences at a particular base temperature. It has
been determined, for grapes, that the growth begins at the
mean temperate of 10 degree C. The accumulation of growing
degree days throughout the growing season above the 10 degree
C. base temperature permits the assessment of grape growing
potential in a region.
The method of calculation
of growing degree days is very simple. The growing season,
during which degree days accumulated must be specified which
in the case of the Napa it is assumed to be the period from
April first until the end of August. Winkler(1974) in his
work with French and german wine grapes in California found
that the accumulation of 1390 degree days was the optimum
for wine grape production. In German Rhine area the 944 was
found as the lowest accumulates degree day acceptable for
commercial wine grapes.
Four growing degree
day suitability classes are shown below
Class
|
Degree Day
|
Ranking
|
1
|
1390 and above
|
most suitable
|
2
|
1165 to 1389 |
good suitability
|
3
|
945 to 1164 |
fair uitability |
4
|
less than 945
|
questionable suitability
|
The use of grape degree day concept provides information
as to the varieties of grapes that can be grown and the quality
of the wine. Vineyards in class 4 on average will experience
a shorter growing season and the wines tend to be acidic.
Grapes grown in class 1 will have the longest growing season.
wines produced in these regions will have a higher alcohol
content and will be somewhat sweeter and more mellow than
wines produced from less suitable areas. The grapes in class
2 and 3 will have higher acid concentration than if they had
been grown in a class 1 region. In many cases the grapes grown
in class 2 have higher intensity of aromas
The degree day concept is not without drawbacks because there
is no accounting for a lessening in plants response at temperatures
more than 27C. Also there is no accounting for different between
maximum and min temperature for of each day.
Growing degree days in Napa.
Archive temperature recordings of 4 climate stations were used
to determine the growing degree day climate in Napa. These
stations each were at different elevations and since the
growing degree day varies with the elevation I produced a
regression model based on average degree days (for years from 1995
to 2002) for each station and the corresponding elevation
of each station. The resulting function of degree days with
respect to elevation was as following.
Degree Day = -2.58108 * Elevation(ft) + 4061.09
Then using the growing degree day function I determined the
elevations for boundaries on the growing degree day classes.
| DD |
1390 |
1165 |
945 |
| Elevation |
473.388 |
1621.969 |
1054.132 |
ArcGIS Spatial Analysis extinction -> Raster Calculator and
the table above was then used to make a grid layer for each
of the classes 4. finally I used the raster calculator again
to overlay the 4 gird to make the final map.
click here to see the map
Soil Suitability, Solar Radiation Suitability,
Final Result
| 
