A viticultural perspective of Meso-scale atmospheric modelling in the Stellenbosch wine growing area, South Africa

The South African Table grape industry has to expand to new markets with high quality niche products, but limited water availability threatens sustainable production. To overcome this challenge and to obtain high-quality products for the new markets, require constant technological advancement. Currently, limited available scientific information about growth balances and physiology and especially grape quality parameters, hinders technological advancement and thus efficient regulatory management of the morphological, chemical, and pathological status of table grapes, especially in response to abiotic factors.

A « terroir » is a cultivated ecosystem in which the vine interacts with the soil and the climate. Main climatic parameters include temperature, rainfall and reference evapotranspiration

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As the frequency and intensity of drought events increase, understanding the mechanisms of plant resilience to water deficit is crucial. To maintain an appropriate plant yield, a common practice is the application of high amounts of fertilizers with negative environmental impacts. The single and combined effect of water deficit and nutrient availability, namely nitrogen (N) and potassium (K), in Vitis Vinifera L. cv. Cabernet Sauvignon and Grenache was evaluated. Two-year-old grapevine plants grafted on SO4 rootstock were transferred in pots under semi-environmental conditions. During the growing season, plants were either maintained well-watered (100% ETc) or subjected to a controlled water deficit irrigation (33% ETc).

One of the main consequences of global warming is the rise of the mean temperature. Thus, the heat summation by the plants begins sooner in the early spring, and by cumulating growing degree-days, phenological development tends to happen earlier. However, spring frost is still a recurrent phenomenon causing serious damages to buds and therefore, threatening the harvests of the winegrowers. The wind machine is a solution to protect fruit crops against spring frost that is increasingly used. It is composed of a 10-m mast with a blowing fan at its peak. By tapping into the strength of the nocturnal thermal inversion, it sweeps the crop by propelling warm air above to the ground. Thus, stratification is momentarily suppressed. Furthermore, the continuous action of the machine, alone or in synergy, or the addition of a heater allow the bud to be bathed in a warmer environment. Also, the punctual action of the tower’s warm gust reaches the bud directly at each rotation period. All these actions allow the bud to continuously warm up, but with different intensities and over a different period. Although there is evidence of the effectiveness of the wind machines, the thermal transfers involved in those mechanisms raise questions about their true nature. Field measurements based on ultrasonic anemometers and fast responding thermocouples complemented by laboratory measurements on a reduced scale model allow to characterize both the airflow produced by the wind machine and the local temperature in its vicinity. Those experiments were realized in the vineyard of Quincy, in the framework of the SICTAG project. In the future paper, we will detail the aeraulic characterization of the wind machine and the thermal effects resulting from it and we will focus on how the wind machine warms up the local atmosphere and enables to reduce the freezing risk.