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Topography and solar energy interception in the Stellenbosch district
A geographic information systems approach
Part 1: Landscape, slope and aspect
Introduction
Vines, like all green plants, are powered by radiant solar energy which drives photosynthesis and provides heat. That solar energy also has more subtle effects was shown by Marais, Hunter & Haasbroek (1999) who found that, in Sauvignon Blanc grapes, the concentrations of certain flavour components were affected by solar radiation and within-canopy temperature. Most Western Cape vineyards lie between 33° and 34°30' south, which is appreciably closer to the equator than is the case in France, where many high quality wines are produced north of the 45° parallel of latitude (Wilson, 1998). Solar energy levels and temperatures in some Western Cape wine producing areas may, in consequence, be too high for quality wine production, particularly from sensitive cultivars such as Sauvignon Blanc. Recognition of this possibility prompted an interest in the factors which affect solar energy interception and led to a need being recognised for a method by which solar energy interception patterns may be determined. The aim of this article is to briefly discuss some of the factors that affect solar energy influx, and to illustrate how geographic information systems can be used to create models which show how topography (land form) affects exposure to radiant solar energy.
Solar energy, aspect and slope
The amount of solar radiation which a unit area of vineyard intercepts at any given time is linked to the direction it faces (aspect), and to the steepness, or angle of slope (inclination). In the Cape winelands the arcuate path traced by the sun each day remains well to the north of the zenith for most of the season, approaching it most closely for a short period around the time of the summer solstice. North facing slopes therefore receive more solar radiation than south facing slopes, particularly during the early and late stages of the growing season. Since the sun rises in the east and sets in the west, east facing slopes receive solar radiant energy earlier in the day than west facing slopes, but may become shaded as the day progresses. Because the strength of incoming solar radiation decreases as the length of the path which it traverses through the atmosphere increases, the energy delivered by sunlight at the earth's surface is less in the early morning, and in the late afternoon and evening when the sun is low, than at mid day when the sun is at its highest point. Paradoxically, due to the fact that the effectiveness of solar radiation increases as the angle between the land surface and the incoming radiation approaches 90°, steep slopes which face directly towards the rising or setting sun receive higher levels of radiation early or late in the day, respectively, than might be expected. The periods of time over which the incoming radiation strikes steeply sloping surfaces at high angles are nevertheless of short duration.
In mountainous terrain, because of the earth's curvature, sunlight falls on the peaks early and late in the day when the sun has either not risen, or has set, over lower-lying areas. Even in areas of fairly low topographic variation, the upper slopes of hills may receive radiant solar energy over appreciably longer periods each day than the adjacent lowlands, particularly in early spring and late autumn when the sun is low. In general, rugged topography contributes to stronger contrasts in solar radiation interception than is the case where relief is subdued.
Landscape images
Slope direction and slope angle can be measured in the field, and may also be deduced from topographic maps. However, such information is not easy to visualise for undulating or mountainous landscapes. This problem is easily overcome by the use of geographic information systems (GIS) in which information, whether a field observation or a calculated value, is linked with co-ordinates which define the location on the earth's surface to which the information relates. Since GIS software keeps track of these co-ordinates, the information can be manipulated in a variety of ways, then reconstructed in visual form without loss of geographic accuracy. The inclusion of height in the database enables three dimensional models of the terrain to be constructed.
For most purposes, images of three dimensional models are easier to interpret than two dimensional images. The satellite overview of the Stellenbosch district shown in Figure 1, for example, gives a visual impression of the Stellenbosch district and its immediate surroundings, but conveys little sense of relative relief in the lowlands. In contrast, topographic variation is clearly apparent in the three dimensional topographic model shown in Figure 2a. Landscape models such as that in Figure 2a, which was generated using GIS, can be rotated and tilted in any desired direction. Scale can also be changed, and details such as roads, rivers and estate boundaries added or removed. Low oblique views (Figure 2a) emphasise differences in height to a greater extent than high oblique views (Figure 2b), but appear to foreshorten the foreground. The choice of angle of view is therefore a compromise.
The images in Figures 2a and 2b are colour coded to represent different landscape positions (crests, upper slopes, lower slopes and valley floors), whereas the colours in Figure 3 indicate steepness, or angle of slope (inclination). Figures 2a, 2b and 3 emphasise the undulating character of the lowlands around Stellenbosch, a feature which is not readily apparent from the satellite image. In Figure 4, the colour coding indicates slope direction relative to north (aspect). This latter image shows that the broad topographic trend in the Stellenbosch wine producing district is north west to south east, which may be a reflection of the orientation of the underlying geological structure of the Coastal Region. A consequence of the north west to south east trend is that few vineyards face either due north or due south.
Collectively, figures 2 to 4 give an excellent visual impression of topographic variability in the Stellenbosch district, and enable locations to be compared in terms of relative altitude and landscape position, slope and aspect, all of which interact to affect exposure to solar radiation and the potential for radiant solar energy interception.
Radiant solar energy and temperature
Although the heating effect of solar radiation increases with the amount of energy intercepted and absorbed and north to north west orientated slopes are warmer than south to south east slopes in the southern hemisphere (Bonnardot, ARC Infruitec-Nietvoorbij, unpublished; Burger & Deist, 1981), the temperature of the vineyard environment may be modified by a variety of factors. These include such characteristics as the colour, texture and moisture content of the soil, and the extent to which the soil surface is covered or shaded. Similarly, cloud or haze may reduce the intensity of the incoming solar radiation, or sea breezes may carry cool air through the vineyard. Temperatures also decrease with altitude, falling by about 0.3°C for each 100 m of height above sea level (Burger & Deist, 1981). The relationship of the site to adjacent topographic features is also important, since the movement of air masses in response to heating and cooling are often controlled by topography . The link between solar radiant energy and temperature is therefore tenuous. For this reason solar radiation and temperature should be regarded by viticulturists as separate, though related, factors.
Conclusions
Landscape images generated using geographic information systems can be used to compare areas in a landscape in terms of their exposure to radiant solar energy, in so far as exposure to, and interception of solar energy is affected by angle of slope and aspect.
The effects of aspect and slope on the calculated pattern of radiant solar energy influx into the Stellenbosch district over the course of a summer day will be illustrated in part two of this article.
More at www.arc.agric.za.
For further information contact John Wooldridge at john@infruit.agric.za or Hein Beukes at vredhb@ plant3. agric.za.
References
BURGER, J. & DEIST, J., 1981. Wingerdbou in Suid-Afrika. Trio-Rand/SA Litho, N'dabeni.
MARAIS, J., HUNTER, J.J. & HAASBROEK, P.D., 1999. Effect of canopy microclimate, season and region on Sauvignon blanc grape composition and wine quality. S. Afr. J. Enol. Vitic. 20, 19-30.
WILSON, J.E., 1998. Terroir. University of California Press, Berkeley.
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