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Topography and solar energy interception in the Stellenbosch district
A geographic information systems approach
Part 2: Effect of time of day
Introduction
It is a matter of common observation that radiant solar energy influx follows a regular daily pattern, and that this pattern slowly changes as the season progresses. However, because solar energy interception is affected by such physical factors as steepness of slope and aspect, as was discussed in Part One of this article, not all locations within a district will receive the same amount of energy per unit area. Neither will all locations in a district receive the bulk of their solar energy input at the same time of day. Since the daily and seasonal patterns of solar radiation to which individual vineyards are exposed are defining factors for those vineyards, and for the terroirs which they represent, it is necessary that those patterns should be known.
Because the effects of aspect and slope on daily patterns of solar energy influx were not easy to quantify and interpret for complex landscapes, the availability of information concerning changing patterns of influx of solar energy into different parts of the landscape was, in the past, extremely sparse. The introduction of geographic information systems (GIS), which enable solar energy influx patterns to be calculated for any given location, time of day or stage of season, and which facilitate the generation of three dimensional landscape models on which the results may be portrayed, has radically changed this situation.
Images of Stellenbosch
In order to illustrate the effects of time of day on solar energy influx for a complex landscape a series of images of a three dimensional landscape model representing the Stellenbosch district were generated using GIS (Figures 1a to 1i). The direction of view in these images is south east from Bottelaryberg across Stellenbosch to the Jonkershoek valley. North lies toward the bottom left corner of each image, as indicated. Colours were ascribed to the images according to the calculated intensity of incident radiant solar energy at the stated time, and graded from blue (low intensity), through green, yellow and orange to red (high intensity). Collectively, the images form a time series representing the pattern of solar energy interception in the Stellenbosch district on 1 January 2002. This arbitrarily-selected date was a few days after the summer solstice (22 or 23 December) which, in the southern hemisphere, is the longest day of the year, and the day on which the sun reaches the highest point of its annual cycle.
Figure 1a shows that, by 07:00, appreciable amounts of solar energy are incident on the north east facing slopes across most of the lowlands, as well as on the north east faces of the Jonkershoekberge, Stellenboschberg and Helderberg. By 09:00 (Figure 1b) shading persists only on the westerly faces of the mountains and of some lowland hills. Solar energy intensity increases progressively as the morning advances.
As the sun climbs higher (Figures 1c and 1d), the length of the path through the atmosphere taken by solar radiation decreases, as does the attendant attenuating effect of the atmosphere on the intensity of the radiant energy. As the sun approaches the highest point of its arc the effects of local topography become progressively more subdued. Shaded areas diminish and, in most lowland localities, disappear. At 13:00 (Figure 1e) solar radiant energy intensity is high and fairly uniform across those areas of valley floor, lower slope and crest where the slope inclinations are low. Solar energy interception by the steeper slopes is nevertheless less than for the lower slopes, due to the more acute angle between the surface and the incoming radiation. Some steep westerly slopes remain shaded. By 14:30 (Figure 1f) a marked reduction in energy intensity is apparent across the north east slopes, whereas north west to west facing slopes begin to receive increasing levels of solar energy. At 15:30 (Figure 1g) high levels of solar energy interception are occurring only on some north west facing slopes. Shadow has begun to fall across north east slopes of the Jonkershoekberge and Stellenboschberg. The lowlands continue to intercept solar energy, although the combined effects of decreasing sun angle and varied topography (inclination and aspect) result in solar energy intensity differing from place to place. By 17:30 (Figure 1h) the angle between the radiant solar energy and the higher, steeper, north west facing slopes of Simonsberg, Groot Drakenstein and Stellenboschberg is high, with the result that energy interception is close to its maximum, despite the fact that the intensity of the radiant energy from the low angle sun is less than at noon due to the longer path traversed by the radiation through the atmosphere. The north eastern and eastern slopes of the Jonkershoekberge and Stellenboschberg are largely shaded. Shading has also spread to the steeper north east facing slopes on the undulating lowlands, although north west facing slopes continue to intercept moderate levels of radiant solar energy. As sunset approaches, shaded areas lengthen and the energy-depleting effects of decreasing sun angle and increasing path length through the atmosphere intensify. This effect is well advanced by 18:30 (Figure 1i) and progresses rapidly thereafter until even the highest west-facing slopes are prevented from receiving radiant solar energy by the earth's curvature.
Discussion
In Figures 1a to 1i the effect of time of day is combined with those of slope and aspect to show the pattern of radiant solar energy influx into the Stellenbosch district over the course of a summer day. From these images it is apparent that some locations receive appreciable amounts of radiant solar energy early in the day whereas energy influx continues into the evening in others.
It is important to appreciate that, because the arc followed by the sun each day is lower in winter than in summer, and because the point on the horizon where the sun rises also changes with season, the pattern of solar energy influx shown in Figure 1 will change progressively during the course of each season. These seasonal changes can also be calculated using GIS.
Since the patterns of radiant solar energy shown in Figure 1 were derived from a mathematical relationship, they represent the maximum values that are potentially available to the landscape at the stated times. Lower radiant energy intensities will occur whenever the clarity of the atmosphere is reduced by haze, smoke or cloud. Where information concerning such obscuration is available it can be incorporated into the GIS database and used to adjust the calculated interception values. Other data, such as that concerning yield, wine quality, soils and climate, may also be added.
Applications of GIS
GIS databases lend themselves admirably to the study of such questions as:
Conclusions
Geographic information systems can be used to calculate and visually present radiant solar energy inflow patterns for any given area, time of day and season. This information is fundamental to the delimitation of terroir units.
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.
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