Handbook for Irrigation of Wine Grapes in South Africa

1.3.5 COMPARISON BETWEEN THE KÖPPEN-GEIGER

CLASSIFICATION AND VITICULTURE-BASED INDICES According to the Köppen-Geiger classification, Stellenbosch, Robertson and Lutzville have three distinct climates as discussed in Section 1.2. This implies that grapevine growth, yield and wine quality are likely to be affected differently in the three localities. However, in the case of GDD, MFT, HI and CI ′ , some of the indices are the same in more than one region, although the climate differs (Table 1.8). This means that grapevines could respond differently in regions that have the same viticultural index. Furthermore, the viticultural indices only focus on the wine quality and do not give an indication of grapevine water requirements. In contrast, the Köppen-Geiger classification provides an indication of the intensity of irrigation needed. At Stellenbosch with its “dry summers”, low frequency irrigation, e.g. once a month, would be adequate for sustainable viticulture. In fact, vineyards with deep, well-developed root systems in sandy loam or clay loam soils might be able to produce economically viable yields and wine quality. On the other hand, vineyards in similar soils under “arid” conditions in the Breede River Valley will require irrigation at a medium frequency, i.e. every two to three weeks. Under the almost “desert” conditions in the Lower Olifants River region, grapevines will need high frequency irrigation, e.g. more than once a week, particularly in the sandy soils away from the river. TABLE 1.8. Comparison of the climate classification according to Köppen-Geiger and the climatic indices for viticulture at three localities in the Western Cape. Climate classification Stellenbosch (Coastal region) Robertson (Breede River Valley) Lutzville (Olifants River Valley) Köppen-Geiger Temperate, dry, warm summer (Csb) Arid, steppe, cold (BSk) Arid, desert, hot (BWh) Growing degree days III (1914°C) IV (2029°C) IV (2217°C) Mean February temperature Moderate (21.8°C) Moderate (22.4°C) Warm (23.0°C) Heliothermal index Temperate warm (HI +1 ) Warm (HI +2 ) Warm (HI +2 ) Cool night index Temperate nights (CI ′ -1 ) Temperate nights (CI ′ -1 ) Cool nights (CI ′ +1 )

1.4 INTERACTION BETWEEN CLIMATE, WINE QUALITY AND IRRIGATION

The climate plays a prominent role in the wine style or quality produced in a specific terroir. Comparing results obtained by means of irrigation studies carried out with Merlot near Wellington (Myburgh, 2011a & b), Ashton (Lategan & Howell, 2010b) and Lutzville (Myburgh, 2011i) showed that the sensorial overall wine quality seems to be better where the HI is higher and the CI ′ is lower (Fig. 1.7). However, within a

24 CHAPTER 1 – CLIMATE