A Technical Guide for Wine Producers

RECENT ARTICLES   |   WYNBOER HOME Weather stations: Applications for viticulture Bonnardot V1(pictured), Carey VA2 & Strydom J1 1. ARC Institute for Soil, Climate and Water, Stellenbosch. 2. Department of Viticulture and Oenology, Stellenbosch University. Keywords: Weather station, climate, temperature, relative humidity, viticulture Byna enige navorsing in wingerdkunde lei tot 'n gebruik van klimaat data. Verskillende skale van klimaat word bespreek en die monitering van die klimaat word verduidelik. Die opstelling en ligging van die weerstasie is van uiterste belang om die kwaliteit van navorsing te verseker. Twee verskillende opstellings vir die meting van temperatuur en relatiewe humiditeit is ondersoek. Merkwaardige verskille is waargeneem. Sommige implikasies van hierdie verskille word uitgewys. Almost any discussion about grapes or wine will lead to a discussion of the weather, be it of the vintage, the region or the vineyard. It is impossible to separate climate from viticulture, and the significance for viticultural viability and wine style and quality has been well documented - temperature generally being accepted as the parameter having the greatest effect on the functioning of the grapevine and specific reactions that occur during berry maturation (Jackson, 2000), and thus final berry composition. Understanding climate Fig. 1 Climatic scales related to surface and time. In order to discuss climate - or weather stations - and viticulture, it is necessary to understand the different levels of climate monitoring and description. There are generally three scales at which climate is described and these are related to differences in the scales of area (or surface) and time (Fig. 1). The macroclimate describes the climate of a region, extending over hundreds of kilometres (e.g. the South Western Cape) and is studied over a long time-period (usually 30 years or more), using annual, seasonal or monthly data. The macroclimate is influenced by the geographic location (latitude) and proximity to large, climate-moderating bodies of water. The weather may differ from year to year, but the climatic situation over a long period of time is relatively stable in terms of temperature and rainfall patterns. For example, in the Western Cape Province, Cape Town has a Mediterranean-type climate with winter rainfall and moderate temperatures (Fig. 2a), while Robertson has a semi-arid climate (Fig. 2b) with higher temperatures in summer and cooler temperatures in winter than measured in Cape Town. Fig. 2 Monthly temperature and rainfall (1961-1990 period) recorded at (top) Cape Town International Airport and (above) Robertson. Various indices, combining various climatic components (mainly temperature, be it minimum, maximum or mean, but also rainfall, humidity, sunshine duration etc.), may be used to describe the viticultural potential of a macro-region. Some use monthly data or daily data only, while others are a combination of different scales (daily with monthly data). They are usually summed for different periods of time (growth season or whole year), but can also use a single month. They are established for a specific country or region, and then may be adapted to other regions or used for a systematic global classification of the climate. The concept of thermal-time (growing degree-days - GDD) involves heat summation above the physiological minimum of 50°F or 10°C during the growing period of the grapevine (Winkler et al., 1974). The minimum acceptable level of growing season heat accumulation for a vineyard site is 1093 GDD (°C). The closer the macroclimate of a region approaches this threshold value, the more significant the mesoclimatic characteristics of the vineyards become. The mesoclimate describes the climate within smaller areas, extending from less than a kilometer to many hectometres (e.g. vineyards or districts) and over shorter periods of time (using hourly data). It is influenced by the topographic factors of elevation or altitude, slope inclination and aspect, and proximity to bodies of water. Elevation Air temperature generally decreases with an increase in elevation at a mean lapse rate of approximately 0.65°C per 100 m increase in altitude. Aspect The direct effect of slope aspect on temperature is related to the solar radiation angle. Northerly (southerly in the Northern Hemisphere) and westerly aspects are warmer because they receive more direct radiation during the middle and latter part of the day. In a warm region (> 2220°C GDD), southerly (northerly in the Northern Hemisphere) and easterly aspects are desirable for cooler locations. Interception of local winds (e.g. sea breezes) may, however, also have an impact on slope temperature. Proximity to large bodies of water The proximity of a site to large bodies of water leads to moderation of temperature extremes. It reduces heat accumulation in coastal regions during the growing season. In addition to this, the differences in energy balances and properties between the water and the land create temperature differences and corresponding pressure differences, which result in local thermal circulations between the land and the sea (sea breeze during the day and land breeze at night) (Bonnardot et al., 2001). The microclimate is the climate immediately within or surrounding a plant canopy and differences occur within a few metres/centimetres and minutes or seconds. It is influenced by the growth vigour of the grapevine and cultural practices (canopy management, row orientation, row spacing etc.). Why do we monitor climate? Climate monitoring enables us to better understand our natural resources. But what does this mean for a viticulturist? Firstly it helps us to estimate the viticultural potential of a region and to select correct cultivars for a specific situation. It also enables us to estimate timing of growth stages that may be critical for certain viticultural practices (e.g. knowledge of time of budburst for optimum time of pruning, timing of application of fungicides or pesticides, etc.) and assists irrigation scheduling. Knowledge of seasonal and spatial climatic variation will aid harvest planning. Adverse climatic factors can also be determined. These can be used to develop early warning systems for disease incidence (downy or powdery mildew) (Fig. 3) or climatic incidents (late spring frosts) that will lead to increased sustainability of practices and reduced yield loss. Fig. 3 Example of a disease warning report for downy mildew developed by the Agricultural Research Council. Climate is also an integral part of the terroir concept and climatic monitoring is necessary for terroir studies (Carey, 2001; Carey, Archer & Saayman, 2002; Conradie et al., 2002; Hunter & Bonnardot, 2002). These have value in their own right, as better understanding of the terroir/vine/wine system will enable us to produce outstanding and unique wines, improve sustainability of viticultural practices and produce wines to meet consumer preferences. From the economic point of view, consumers have an increased interest in environmental issues and the origin of the product. Both these issues are addressed by terroir studies and they may therefore also be marketing tools. The optimum scale for climate monitoring is at the mesoclimatic level. It is at this scale that one is able to compare the climate across a landscape for disease and frost forecasting and for optimal cultivar choice for specific sites. How is climate monitored? Climate is monitored by means of automatic and mechanical weather stations. Since 1940, the Institute for Soil, Climate and Water (ISCW) of the Agricultural Research Council (ARC) has installed a countrywide network of weather stations aimed at monitoring the climate and satisfying the climatic information requirements of agriculture. This network has grown to the stage where there are 300 mechanical weather stations, 256 automatic weather stations and 800 rainfall stations. In the Western Cape Province, there are 84 automatic weather stations, mostly situated in a vine row (Fig. 4a) or on open ground next to a vineyard (Fig. 4b). Fig. 4 Automatic weather station located (top) in a vine row and (above) on open ground next to a vineyard in the South Western Cape. Landscape strongly affects weather station positioning. The more mountainous the area, the more difficult it is to have a detailed account of the climate and the greater the number of weather stations that are therefore required to characterise the climate of the area. The position of each weather station within the landscape is also of utmost importance. The station surrounds must generally be clear with an angle of no more than 30­ to the horizon. One must ensure that there are no objects (man-made or natural) in any direction likely to affect the wind and radiation records. For example, a weather station situated near a workshed or cellar may be protected from winds from a certain direction and may be in shadow in the early morning or late evening. This will affect the daily weather measurements and give a false indication of the mesoclimate. Another example is a weather station situated in a natural hollow: drainage of cold air may result in night temperatures being recorded as colder than is true for the general locality, as well as affecting the recorded sunlight duration. Fig. 5 Automatic weather station of the ARC-ISCW network in the South Western Cape. Fig. 6 Examples of the standard Stevenson screen and Gill screen for housing of climate sensors. ARC-ISCW only uses weather instruments that are required for agricultural purposes but each weather station still has to conform to certain minimum standards set by the World Meteorological Organization (WMO) (Fig. 5 and Table 1). As there are many factors influencing the performance of automatic weather stations (Estie, 1986), these strict standards ensure that data from all weather stations are comparable. The instruments have to maintain a certain level of accuracy and must be placed in the correct position to measure the weather accurately in the short term and to give an accurate account of the climatic parameters in the long term (Table 1). Table 1 World Meteorological Organization Standards for weather stations (WMO, 1997) used for the ARC-ISCW automatic weather station network. Table 2 Comparison between two temperature and relative humidity sensors in a standard Stevenson screen. (25/11/2002 - 29/12/2002) Table 3 Temperature and relative humidity data: comparison between a Stevenson screen and a Gill screen. (29/11/2002 - 3/12/2002) The importance of standardised conditions can be seen when comparing the data measured by temperature and relative humidity sensors in a WMO standard Stevenson screen and a Gill screen (Fig. 6). The sensors had been previously compared within the same shelter (WMO standard Stevenson screen) over a five-day period at five-minute intervals. The average differences were 0.3°C and 2.1% for temperature and relative humidity respectively (Table 2). These differences were insignificant. This picture changed when data from the same sensors, but now within different screens, were compared. Although the average differences seem low (Table 3), the differences were significant for temperature. The differences were also not constant, the maximum differences of 2.4°C and 13% for temperature and relative humidity, respectively, suggest pronounced variation. The Gill screen resulted in lower temperature and higher relative humidity data than the measurements obtained in the standard Stevenson screen (Fig. 7). Further analysis of the results (frequency analysis) showed that 49.4% of the measurements resulted in temperature differences above 0.5°C and 13% above 1°C, occurring both night and day. With respect to relative humidity, 26.7% of the records showed a difference in relative humidity above 5%, although these differences were mainly at night. Fig. 7 (top) Temperature (°C) and (above) relative humidity (%) recorded under different screens (Stevenson and Gill screens). These differences may have Economic implications: If used in a disease-warning programme, for example, secondary infection of downy mildew may be predicted more often with the set of data having the higher relative humidity values at night time. The additional sprays may not be needed, resulting in unnecessarily high disease control costs. Research implications The measured differences could also result, over a longterm, in significant mesoclimate differences. It is therefore recommended that the WMO standard Stevenson screen should be used for any climate research, especially when international comparisons are made. The above-mentioned tests started in December 2002. Studies will continue to ascertain the degree of difference between the measurements made under the different screens and whether these differences remain throughout the year, across seasons as well as their significance. Address further enquiries to Frans Koch, ARC ISCW, e-mail frans@iscw.agric.za and Dudley Rowswell, ARC ISCW, e-mail Dudley@ infruit1.agric.za. Literature cited BONNARDOT VMF, CAREY VA, PLANCHON O & CAUTENET S, 2001. Sea breeze mechanism and observations of its effects in the Stellenbosch wine producing area. Wynboer 147, 10-14. CAREY VA, 2001. Spatial characterisation of natural terroir units for viticulture in the Bottelaryberg-Simonsberg-Helderberg winegrowing area. M.Sc. Agric Thesis, University of Stellenbosch. 90pp + annexes. CAREY VA, ARCHER E & SAAYMAN D, 2002. Natural terroir units. What are they? How can they help me as a wine farmer? Wynboer 151, 12-14. CONRADIE WJ, CAREY VA, BONNARDOT V, SAAYMAN D & VAN SCHOOR L, 2002. Effect of different environmental factors on the performance of Sauvignon blanc grapevines in the Stellenbosch/ Durbanville districts of South Africa. I. Geology, soil, climate, phenology and grape composition. S. Afr. J. Enol. Vitic. 23(2), 78-91. ESTIE KE, 1986. Factors influencing the performance of automatic weather stations. In: Proc. & recommendations of the symposium on automatic weather stations and data logging systems, Nov. 1986, Weather Bureau, Dept. Environmental affairs, Pretoria, South Africa, 95-115 HUNTER JJ & BONNARDOT V, 2002. Climatic requirements for optimal physiological processes: a factor in viticultural zoning. In: Proc. Int. Sym. Viticultural zoning, June 2002, Avignon, France. JACKSON RS, 2000. Wine Science. Principles, Practice, Perception. Academic Press, San Diego. WINKLER AJ, COOK JA, KLIEWER WM, & LIDER LA, 1974. General Viticulture, 2nd ed. University of California Press, California, 710 pp. WORLD METEOROLOGICAL ORGANIZATION, 1997. Guide to Meteorological Instruments and Methods of Observation. World Meteorological Organization No. 8, 6th edition, Geneva Switzerland.

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