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Radiant solar energy interception in the Western Cape
By: John Wooldridge1 and Hein Beukes2
Key words: aspect, insolation, latitude, slope, temperature, terroir, vineyard
Western Cape vineyards are closer to the equator than the vineyards of central France, and receive higher levels of radiant solar energy (insolation). Since vine growth, photosynthate production, wine style and wine quality are potentially affected by insolation, high insolation levels constitute a challenge for South African wine producers. Fortunately, potential insolation levels can be calculated and used as a basis for demarcation, cultivar selection and canopy management. The landscape of the Western Cape is rugged. Because insolation varies across a landscape as a function of aspect and slope angle, the diversity of sites and insolations that are available for possible viticultural utilisation is very wide. This creates many possibilities for vineyard diversification.
1. Introduction
Two of the main terroir-defining criteria (factors) are insolation - a term derived from the phrase 'incoming solar radiation' - and temperature (Laville, 1990). Insolation and temperature are significant in a vineyard context because vines depend on the interception of radiant solar energy to power photosynthate production, and because grapevines only operate efficiently over a limited temperature range, reflecting their origins as temperate climate plants.
Insolation is a direct form of energy. In contrast, temperature is a relative term, indicating the level of heat-driven molecular activity within a substance. Although the energy which flows into a vineyard and converted into heat is ultimately of solar origin, the relationships between insolation and soil or air temperature is always indirect due to the effects of environmental variables such as soil water and stone contents, colour, vegetation cover and air movement.
Consistent with its solar origin, insolation is affected by both extraterrestrial (the motion of the earth relative to the sun, giving rise to seasons and day-night cycles) and terrestrial (latitude and landform) factors. Latitude (angular distance from the equator) is important since the elevation of the sun above the horizon at local noon increases with decreasing latitude. High sun angles lead to shorter path lengths through the atmosphere, to reduced atmospheric attenuation and to high solar radiation intensities. Insolation therefore increases with decreasing latitude. In consequence, Western Cape vineyards, which mostly lie between 32.5° and 34.5° south, receive higher levels of radiant solar energy than those French wine districts which are distributed between 43° and 49° north. This difference may have prompted an observation that ripening would be slower, and wine quality better, if South Africa lay 200 km further south (Platter, 1994). In practice, because the distribution of continents, ocean basins and currents in the northern hemisphere differs greatly from that in the south, no direct comparison can be drawn between vineyards at similar north and south latitudes. Those Western Cape vineyards which benefit from the cooling effect of winds from the Atlantic Ocean with its cold Benguela current are, in fact, likely to be cooler than vineyards in European intra-continental settings at the same latitude. Insolation, however, is unaffected by sea breezes. It therefore remains the case that Western Cape winemakers face the challenge of producing high quality wines under conditions of higher insolation than do their French counterparts.
A common outcome of high insolation, high temperature vineyard conditions are the production of large amounts of photosynthate leading to undesirably high alcohol concentrations in the wine. Paradoxically, another possible outcome is the production of too little photosynthate. This generally occurs under circumstances where high light intensities promote the development of excessively dense canopies, a phenomenon which may be exacerbated by nitrogen fertilisation and irrigation. Since the intensities of photosynthetically active light wavelengths decrease sharply as they pass through the outer leaf layers, high canopy densities invariably lead to sub-optimal intra-canopy light levels and, despite some compensatory responses (Vanden Heuwel et al., 2004), inadequate photosynthate production, low grape sugar levels and low wine alcohol concentrations. Root growth and root : shoot ratios may also be inhibited. An exacerbating factor is that, where temperatures are high, respiration may accelerate to the point where photosynthates are consumed more rapidly than they are produced (Gladstones and Smart, 1994). Amelioration of conditions leading to both over and under production of photosynthate can be achieved through appropriate canopy management practices. Information concerning insolation and its variability across a landscape facilitates decisions regarding the implementation of such practices, and is also relevant to demarcation and cultivar selection.
At any given latitude, season and time of day, insolation varies across the landscape, mainly as a function of slope angle and aspect (slope direction relative to geographic north). On a meso scale, undulating to rugged landscapes are, effectively, patchworks of land areas, each receiving different amounts of solar energy. Such patterns are permanent and, in natural ecosystems, may cause marked variations in the distribution of plant communities (Schulze, 1997). Insolation will also be affected by factors which affect the transparency of the atmosphere, such as cloud, dust and pollutants.
Unlike rainfall, temperature and wind, which must be measured over periods of years before a reasonably reliable pattern can be established, solar radiation influx can be calculated and modelled for individual landscapes using Geographic Information Systems (GIS) software. Such a model was prepared for the Stellenbosch area by Wooldridge & Beukes (2003a, b). The importance of energy budgets in natural ecosystems was recognised several decades ago and a relationship which enables incoming radiation fluxes to be calculated for sloping terrain in Southern Africa was published by Schulze in 1975.
This article presents insolation data which illustrates the effects of latitude, season, aspect and slope on solar radiation intensity, and discusses links between insolation, temperature, vine growth characteristics and wine.
2. Method: derivation of insolation data
Seasonal solar influx values for latitudes at 0.5° intervals from 32.5° to 34.5° south (S), slope angle classes of 0° (horizontal), 5°, 10°, 20° and 30°, and aspect - averaged for each of eight 45° sectors - were calculated for the summer and autumn solstices (longest and shortest day, respectively), and for the spring (vernal) and autumn equinoxes. Calculations were based on the methods of Schulze (1975), and apply to clear sky conditions only. Since near horizontal surfaces do not face in any particular direction, their insolation values are not affected by aspect.
3. Results and discussion
3.1 Latitude
Insolation values for the latitudes occupied by the Western Cape winelands are presented in Table 1. Over this range of latitudes the effect of latitude on insolation is not significant in mid-summer (statistical data not shown). At the equinoxes and winter solstice, however, insolation tends to decrease slightly with increasing latitude. The effect of this decrease is greatest on steep, south-facing slopes in mid-winter when insolation on 30° slopes is 50% lower at 34.5° than at 32.5° S.
3.2 Effects of season
Solar radiation intensities vary smoothly and cyclically over the course of the year, maximum and minimum values occurring on the summer and winter solstices, respectively. At the equinoxes, hours of light and darkness are equal. Since the insolation data are expressed on a MJ / m2 / day basis, an indication of solar intensity can be gained by dividing the daily insolation value by the number of hours over which insolation was received on that specific day.
3.2.1 Summer
In summer, insolation values are higher than at other times of the year. At Western Cape latitudes in mid-summer, horizontal surfaces receive around 2.5 times the insolation which they receive in mid-winter. Insolation values peak on gentle northerly slopes, but otherwise tend to decrease with increasing slope angle. This decrease in insolation with increasing slope angle is small for the north (N), north-west (NW), north-east (NE), west (W) and east (E) sectors, more marked for the south-west (SW) and south-east (SE) sectors, and greatest on the steeper south-facing slopes.
3.2.2 Winter
Apart from the fact that insolation values are lower in winter than in summer, the main difference between summer and winter is that whereas summer insolation values tend to decrease with increasing slope angle at all aspects, winter insolation values show considerable variation due to slope and aspect. Thus, for example, at 33.5° S latitude in mid-winter, the ratios of insolation on 30° slopes / insolation on horizontal surfaces decrease in the sequence: 1.84, 1.60, 0.97, 0.37% and 0.07 for the N, NW and NE, W and E, SW and SE, and S aspects, respectively. Slopes facing N (337.5° to 022.5°) will therefore be appreciably warmer than horizontal surfaces, W (247.5° to 292.5°) and E (067.5° to 112.5°) facing slopes will be little affected by slope angle, SW (202.5° to 247.5°) and SE (112.5° to 157.5°) slopes will be cooler on sloping than on horizontal surfaces and S facing slopes (157.5° to 202.5°) will suffer severe reductions in insolation with increasing slope angle.
3.2.3 Spring and autumn equinoxes
Insolation values at both the spring and autumn equinoxes are the same. For horizontal surfaces average (of latitudes 32.5° to 34.5° S) equinoctial insolation (26.8 MJ / m2) is 28% lower than at the summer solstice, but 83% higher than at the winter solstice.
As was the case at mid-winter, insolation values at the solstices are affected by aspect and slope, tending to increase with slope angle on N facing slopes, and to decrease with increasing slope angle on S facing slopes. Ratios of insolation on 30° slopes to insolation on horizontal surfaces (1.20, 1.14, 0.94, 0.68 and 0.54 for the N, NW and NE, W and E, SW and SE, and S aspects, respectively) are nevertheless less marked than is the case in mid-winter.
3.3 Insolation and landscape
The data presented in Table 1 show that aspect and slope angle have substantial effects on insolation. Both aspect and slope angle are aspects of landscape which, in turn, owes its form and diversity to protracted erosion of the underlying geological structures and materials. Landscape and insolation are therefore linked. In the Western Cape, aspects are generally not random, since they tend to reflect the orientations of the underlying rock structures. These include a north-westerly trend in the granite outcrops and Malmesbury metasediments of the coastal plain, north-south and west-east fold axial trends in the western and southern branches, respectively, of the Cape Fold Mountains, and a north-easterly trend in the hiatus zone where the main fold axes intersect. Dominant aspects therefore vary from area to area.
With the exception of debris slopes in mountainous areas, slope angles tend to reflect the erosion resistance of the underlying materials. Unless hardened by some process, such as heating from underlying granite (metamorphism), as in the case of the Durbanville hills, shales tend to form subdued, undulating landscapes with low slope angles. In contrast, the granites of the coastal plain tend to be elongated in a north-westerly direction, and weather by exfoliation to a rounded outline. Conversely, granites which have been heavily fractured and have undergone deep chemical weathering tend to occur in low lying parts of the landscape, and to be characterised by soils having clay fractions dominated by kaolinite, a clay mineral that appears to be linked with poor vine performance and wine quality in Sauvignon blanc (van Schoor, 2001). Sandstones and quartzites, which are highly resistant to erosion, tend to form stark, often precipitous landscape features with variably steep upper and mid slopes. For a more complete description of the landscape geology, soils and climate of the Western Cape winelands, refer to Saayman (2004).
3.4 Landscape, cloud cover and insolation
Rugged landscapes interact with wind patterns to create patterns of cloud cover. These patterns may be remarkably stable during certain seasons. Since cloud attenuates the intensity of solar radiation before it reaches the earth's surface, such areas are likely to be appreciably cooler, and possibly wetter, than surrounding, cloud-free areas. Such cloudy areas may be beneficial for some cultivars.
3.5 Insolation, temperature and vine physiology
Of the radiant solar energy that falls on a leaf, only parts of the blue and red sectors of the visible spectrum are used for photosynthesis. The rest is reflected or absorbed. Absorbed radiant energy is ultimately dissipated as heat. Depending on the spacing between vine rows, canopy width and row direction, more or less radiant solar energy will fall on the soil surface between the vines. Some of this energy will be reflected, either on to the underside of the canopy, where it may be utilised, or skyward. At high sun angles so much light may be reflected and scattered that the effects of row direction, row width, slope and aspect become small. Solar energy which is absorbed by the soil will contribute to heating, both of the soil profile, and of the overlying air. Since reflection and absorption, as well as the rate at which absorbed heat is subsequently dissipated are affected by soil parameters such as texture, colour, and water content, the relationship between solar radiation intensity and temperature will vary with both vineyard layout and soil characteristics.
Air temperature will also vary due to such factors as turbulence close to the surface, wind over and through the vineyard, and slope. Where little air movement takes place, canopy temperatures may rise to the point where photosynthesis slows appreciably, even though incident solar radiation levels are adequate. Physiological activity in grapevines begins at around 10 °C, peaks in the range 25 °C to 28 °C or even 30 °C, then declines abruptly. This pattern is typical of enzyme-catalysed reactions in which reaction rates increase with temperature until a point is reached where enzymes and proteins begin to denature due to thermal agitation which weakens the hydrogen bonds that hold protein molecules in shape. The longer the period of exposure of the tissue to high temperatures, the lower will be the maximum temperature that can be tolerated.
Even at sub-lethal temperatures, photosynthate production may be curtailed, the cells entering a shocked state in which protein synthesis temporarily ceases. Synthesis of a suite of proteins characteristic of shock situations may then occur. Such 'heat-shock' proteins confer a degree of tolerance to abnormal temperatures, apparently by aggregating to form large multi-unit complexes which act as 'chaperones' to unstable proteins. Heat tolerant forms of enzyme may also be synthesized (Srivastava, 2002). Associated negative effects of high temperatures include incomplete stomata opening, restricted transpiration and gas exchange, desiccation and wilting. Solar radiation is therefore not utilised effectively when temperatures are high. For this reason insolation data for the period between dawn and 11.00 hours, or the local time at which temperatures become excessive, may be more meaningful in physiological terms than that for the whole day. Since wine style and quality are affected by average temperature during ripening and by short-term temperature variability (Gladstones and Smart, 1994), as well as by the direct, photosynthetic effects of insolation itself, radiant solar energy is clearly an important factor in vineyards and wine production.
4. Conclusions
At the latitudes of the Western Cape winelands, levels of radiant solar energy interception are higher than in French wine-producing areas. Patterns of radiant solar energy interception nevertheless vary in accordance with slope and aspect as well as with season. Since the Western Cape is characterised by a complex landscape, sites having a broad range of insolation and other mesoclimatic and soils characteristics are potentially available for viticultural exploitation. Areas which are characterised by specific insolation patterns can be identified and demarcated using GIS techniques. The viticultural potentials of areas thus demarcated must nevertheless be determined by field investigation in accordance with ongoing terroir studies.
For further information contact John Wooldridge at Nietvoorbij on: (021) 809-3330, 083 662 4742, wooldridgej@arc.agric.za.
References
GLADSTONES, J. & SMART, R.E., 1994. Climate and wine quality. In J. Robinson (ed.). The Oxford companion to wine. Oxford University Press, Oxford, pp. 251-254.
LAVILLE, P., 1990. Le terroir, un concept indispensable à la elaboration et à la protection des appellations dé origine comme à la gestation des vignobles: le cas de la France. Bulletin De L'O.I.V. 709-710, 217-241.
PLATTER, J., 1994. South Africa. In J. Robinson (ed.). The Oxford companion to wine. Oxford University Press, Oxford, pp. 896-903.
SAAYMAN, D., 2004. The South African vineyard and wine landscapes: heritage and development. Wynboer (May), 91-95.
SCHULZE, R.E., 1975. Incoming radiation fluxes on sloping terrain: a general model for use in southern Africa. Agrochemophysica, 55-59.
SCHULZE, R.E., 1997. Climate. In R.M. Rowling, D.M. Richardson & S.M. Pierce (eds). Vegetation of southern Africa. Cambridge University Press, Cambridge, pp. 21-42.
SRIVASTAVA, L.M., 2002. Plant growth and development. Academic Press, Amsterdam.
VANDEN HEUVEL, J.E., PROCTOR, J.T.A., FISHER, K.H. & SULLIVAN, J.A., 2004. Shading affects morphology, dry-matter partitioning, and photosynthetic response of greenhouse-grown 'Chardonnay' grapevines. HortScience 39(1), 65-70.
VAN SCHOOR, L.H., 2001. Geology, particle size distribution and clay fraction mineralogy of selected vineyard soils in South Africa and the possible relationship with grapevine performance. M.Sc thesis, University of Stellenbosch, Stellenbosch, South Africa.
WOOLDRIDGE, J. & BEUKES, H., 2003a. Topography and solar energy interception in the Stellenbosch district. A geographic information systems approach. Part 1: Landscape, slope and aspect. WineLand (February), 74-76.
WOOLDRIDGE, J. & BEUKES, H., 2003b. Topography and solar energy interception in the Stellenbosch district. A geographic information systems approach. Part 2: Effect of time of day. WineLand (March), 73-74.
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