Handbook for Irrigation of Wine Grapes in South Africa

1.3.3 HELIOTHERMAL INDEX The HI can be calculated to describe the thermal character of the climate with respect to potential for viticulture within a specific wine growing area (Bruwer, 2010 and references therein). In South Africa, the HI is based on the summation of the mean and maximum monthly temperatures from October to March (Table 1.6). The HI includes a length of day coefficient to compensate for the greater photosynthetic active radiation occurring during longer days at latitudes higher than 40º. For latitudes lower than 40º, a value of one is used. An example where thermal character of the Lower Olifants River region was analysed by Bruwer (2010) is presented in Figure 1.5. This classification clearly shows how the climate becomes warmer as the distance to the Atlantic Ocean increases. TABLE 1.6. The HI used to describe the thermal character of the climate (Tonietto & Cabonneau, 2004 and references therein). Viticultural climatic class Class code Class interval (ºC) Very cool HL -3 <1 500 Cool HL -2 > 1 500 < 1 800 Temperate HL -1 > 1 800 < 2 100 Temperate warm HL +1 > 2 100 < 2 400 Warm HL +2 > 2 400 < 3 000 Very warm HL +3 > 3 000

Figure 1.5

2 200 2 400 2 600 2 800 3 000 3 200 3 400 3 600 3 800

Klawer

Class HI +3

Vredendal

Lutzville

HI (ºC)

Class HI +2

Ebenaeser

R 2 = 0.9984

Class HI +1

0

10

20

30

40

Distance (km)

FIGURE 1.5. Effect of distance to the Atlantic Ocean on the thermal character in the Lower Olifants River region according to the HI (Bruwer, 2010). Figure 1.6

17

Klawer

22 CHAPTER 1 – CLIMATE

16