WINETECH Technical Yearbook 2020

WITHIN ALL THIS RESEARCH, WHAT SHOULD A VITICULTURIST BE LOOKING AT, AT FIELD LEVEL FROM SEASON TO SEASON? Using the traditional indices, it was clear that over the four growing seasons there was a general trend of warming. The Geoviticulture MCC System provides more detail on the site and seasonal behaviour as more elements, other than only daily maximum and mean temperatures are considered, a weighting of cool night index and dryness index are integrated into the index. This improved the quantification of the site and seasonal variability of the study sites (table 2). The extremity of sites identified in the classification with the dryness index variable added, showed the cooler site (Elgin) to have the lowest dryness index compared to Vredendal with the highest. The extreme sites (Elgin and Vredendal) did not highlight seasonal variation, however, the intermediate sites in the coastal region had more pronounced seasonal variability driven either by temperature or dryness. The additional variables affect biological systems, emphasising the importance of considering more variables when quantifying the impact of climate shifts (at regional, local and plant levels) even though the index is still limited, as it is a seasonal summary. Additional climatic analyses at higher temporal resolution (hourly) improves the

understanding of daily kinetics of change at the vineyard level to explain grapevine responses spatially, and over vintages. Hourly threshold classes provides more detail for each site as a seasonal summation in hours observed at specific temperature classes (level 1-9). Temperature differences driven by site and season variability are shown in more detail with frequency analysis (figure 2). This frequency data can improve in-season grapevine management, as more hours at high temperatures in the critical growing periods with lower relative humidity at a site would allow more precise management to also optimally accommodate water constraints. The analysis showed more information of hours at warmer temperature classes, with the cooler sites showing more hours at lower temperature classes, as expected. Overall, the Somerset West site had more than double the observed hours at 10-15°C compared to the other sites. This could be due to more windy conditions. Within the study, Vredendal which is the warmest site, reached the highest maximum values compared to the other sites in the study, but the duration at higher temperatures was shorter than that of the other sites . Understanding the diurnal cycle of a day in terms of temperature, wind and relative humidity is important to match the correct grapevine cultivar and to optimise the physiological functioning of the grapevine.

FIGURE 1. Western Cape map with districts highlighted and main study areas A and B. Zoomed in sections A and B show the main study areas with the selected sites (encircled areas).

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