In the spring of 1999 the Rheingau Viticultural Association (“Rheingauer Weinbauverband”) presented to the public the map of the potential must weight for the winegrowing region of the Rheingau on a 1: 25,000 scale which provides the theoretical basis of the "First Growth“ map for the Rheingau on a 1: 5,000 scale which was published one year later. The map of the potential must weight for the grape variety Riesling (scale 1: 25,000) was developed by the branch office of the German Weather Service in the business area agriculture.

The site specific delimitation of the areas (scale 1: 5,000) took place on site in cooperation with the Viticulture Department of Eltville (“Weinbauamt Eltville”) and the Institute of Research Geisenheim branch soil science (“Forschungsanstalt Geisenheim Fachgebeit Bodenkunde”). The cartographic work was conducted by the Department of Geography at the University of Mainz with the help of a geographic information system.

What are the subject-specific basics of this map which is unique in its kind and for winegrowing areas so far?

Rheingau, which, in cooperation with the Institute of Research Geisenheim, the Viticulture Department of Eltville and the German Weather Service started already in 1960 and continued until 1984. The analyses were conducted for the grape variety Riesling in the Rheingau. Thus, it was possible to issue a site guide for the Rheingau as early as 1967 which already contained a first quality map (ZAKOSEK et al. 1967). With financial support by the Ministry of Agriculture, Forestry and Environment Protection of Hesse (“Hessische Ministeriums für Landwirtschaft, Forsten und Naturschutz”) a reissue of the first site map for the Rheingau was issued in 1996 which supplemented and improved the first edition with respect to many content-related topics on water supply and environment protection (LÖHNERTZ 1999).

The insights of the influence of soil and climate on the must weight of the grape variety Riesling have also been developed further since the first edition (HOPPMANN 1988). They could be used for drawing up the must weight map. The analyses over the course of many years form the basis for the development of a must weight model for the Riesling, enabling the calculation of the must weight for each terrain point with the help of a computer if the climatic and soil conditions are known for this point.

The calculations took place in the period from 1961 to 1990. The influence of the climate on the growth and maturity of the grapes changes in the course of individual stages of development. The calculation of site variables is thus assigned to phenological development stages. Quality and harvest of grapes vary rather strongly in the northern cultivation areas from year to year and from site to site whereby the annual variation is significantly higher than the site variations.

That’s why the influence of the annual variations for the time period 1961 to 1990 is analysed first. Climate and vine phenology are tightly linked: solar radiation, temperature, evaporation, precipitation and water budget ( = difference precipitation minus evaporation) determine the vine development and in this way also the maturity degree. The earlier flowering and ripening begin the better are the chances for high must weights. The greatest importance for the must weight is the date of full bloom. If the blooming date is delayed by 7 days the must weight declines by almost 6 degrees Oechsle. During the grapes‘ cell division after blooming the water demand is very high, a positive water budget at that time also increases the must weights significantly. While the grapes are ripening temperature and solar radiation become very important. A rise in temperature by 2 degrees increases the must weights by 2.5 degrees Oechsle.

90% of the Riesling’s quality variations between the years can be explained by the variability of the climatic factors listed in this table and by phenological entry dates

This table lists all variables which influence the must weight as site variables. The listed variables are available for each terrain point. The distance between the terrain points amounts to 20 metres.

Wind
Threat by wind is mapped in 5 stages cartographically. This evaluation does not primarily take into account the exposition to the prevailing wind direction of the Southwest but rather the wind distribution on days of good weather, when a special, stable climate develops in the vineyards favouring growth and maturity. On days with many sunshine hours the wind frequently blows from the east or northeast in the Rheingau. Slopes which are exposed to these directions display a relatively low quality compared to western or northwestern slopes. Vineyards displaying a concave form as for their vineyard shape have little exposure to the wind and are preferred.