Handbook for Irrigation of Wine Grapes in South Africa

Chapter 1

1.3 CLIMATIC INDICES FOR VITICULTURE Climate classifications such as the Köppen-Geiger system only provide a broad regional to global generalization of climates. Unfortunately, they do not provide specific details on atmospheric variables that are important for viticulture and wine production. Atmospheric variables that affect grapevine growth, yield and wine quality are solar radiation, air temperatures, day-night temperature fluctuations, heat accumulation, wind speed, precipitation and humidity. Consequently, the following classification systems have been developed specifically for viticulture potential. 1.3.1 GROWING DEGREE DAYS This index describes the potential for wine quality based on heat summation (Amerine & Winkler, 1944). The criteria were adapted for the Western Cape wine producing regions by Le Roux (1974). The growing degree days are calculated as the summation of the daily mean air temperature above 10ºC through the seven months growing season, i.e. from September to March (Table 1.4). The GDD can be used to identify different areas for potential wine quality within a wine region (Fig. 1.3). For instance, wine quality potential in the Lower Olifants River improves as the distance to the Atlantic Ocean decreases (Bruwer, 2010).

TABLE 1.4. Wine quality potential classification according to the GDD as proposed by Le Roux (1974).

GDD

Class

Wine quality potential

< 1 389

I

Quality red and white table wine

1 389 - 1 666

II

Good quality red and white table wine

1 667 - 1 943

III

Red and white wine and port

1 944 - 2 220

IV

Dessert wine, sherry and standard wine

> 2 220

V

Dessert wine and brandy

IRRIGATION OF WINE GRAPES 19