Technical Yearbook 2023

DECEMBER

The effect of mountainous terrains on cold-unit accumulation By Heinrich Schloms

The phenomenon where vineyards bud poorly and/or unevenly in certain areas, with other areas performing well, has puzzled viticulturists for many years. This phenomenon even sometimes occurs within the same block. The objective of this article is to look more closely at possible causes, and whether terrain variation could play a role. Introduction During the last Chardonnay viticultural discussion group, there was a request to look at 15 specific Chardonnay vineyard blocks from the Durbanville, Stellenbosch, Somerset West, Helshoogte and Paarl areas. Some of these blocks budded very poorly last year, while others budded very evenly as desired. The objective was to see if any correlation could be found between the budding pattern of the blocks and their terroir, or their position in the landscape. The process was initiated by collecting the blocks’ coordinates from the respective producers and entering these into the GIS system. All the available climate, terrain, soil and geograpical information that could be found, was connected to these points in an Excel table to establish if there is a specific terroir element that stands out and correlates with the findings. The expectation was that the altitude, distance from the sea and interpolated climate data would show the strongest correlation, but this was not the case. The strongest correlation in fact related more to the position in the landscape. What is the cold-unit requirement of the grapevine? The grapevine (there are differences between cultivars) needs a certain amount of positive cold units (amount of hours between the temperatures of 1.4 and 12.4°C from 1 May to 31 August) in order to uplift dormancy, and this accumulation usually occurs in the early morning hours. Negative cold-unit accumulation takes place during the day when temperatures become too high during this period.

Hourly data from the nearest weather station is used to determine the cold units (Richardson or Utah model), but the question should be asked: how accurate is the model (should we not use the dynamic model which is biologically more correct), how representative is the weather station’s location of your farm, and how do you determine the variation of cold units on the farm due to terrain variation? Is the accumulation period still correct? In essence, cold unit models and interpolation of the data are very complex and not perfect. We also don’t know what the cold-unit requirement is of a specific cultivar or vineyard block. It is known that the low-lying areas become very cold on clear, windless winter evenings and this creates a frost risk in the early spring period, while simultaneously it can be relatively warm on the hill just a few hundred metres away. This is strange, as the opposite temperature variation can be observed during the day. The problem lies therein that cultivar recommendations are often made based on summer day temperatures instead of winter night temperatures. What is cold-air drainage? During the night, the earth’s surface cools down unevenly due to infrared radiation energy on different heights. Some of the radiation energy is absorbed by air molecules close to the surface and prevents the radiation from exiting the atmosphere. More air molecules by volume can be found at sea level than on the high-lying mountain tops (atmospheric pressure) and the ability to absorb the radiation energy, thus causes the mountain tops to cool faster than the lower lying areas. The denser, heavier air that accumulates on the mountain tops drains slowly down the mountain in the early, windless morning hours and displaces the warmer, lighter air in the low-lying valleys, in much the same way water would do. The warmer air is then trapped between the two cold layers and this is called the inversion layer. It can be observed in valleys on cold, windless winter mornings where warm smoke from fires or pollution rises upward to a certain point and then moves into the inversion layer at a 90-degree angle where the temperature is similar. The height of the inversion layer depends on the volume of cold air draining into the valley and it may be possible to determine this through hydrological models. Mountain slopes that are located on the same height as the inversion layer, are called the thermal belt. This thermal belt has an

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TECHNICAL YEARBOOK 2023