It is very important to take account of the climate in a given wine region because the different climatic variables take part in the development of the vine, in the genetic expression of the grape and consequently in the character of the wine.

From the four main climatic variables, which define the climate of a region, three cannot be altered by man: temperature, relative humidity, and solar radiation. But rainfall is the only one of these variables that can be modified.

In areas with low rainfall, the winemaker can supply water by means of irrigation and so control the plant and grape quality. But in the areas where rainfall sometimes exceeds the needs of the plant, the producer is unable to eliminate excess of water and that becomes a limiting factor for grape and wine quality.

Rainfall, which can be modified through irrigation, is highly important in the part of the Mediterranean where we are. Here rain mainly falls in some springs and most autumns. However, in the three summer months, it seldom rains. That is why for us it is highly important to know the amount and the moment when water has to be supplied to the vine in every stage of the vegetative cycle. Our studies have been orientated in this direction for years and now we continue in a more detailed way: we want to observe how rainfall in summer is distributed in different prestigious winegrowing regions taking good vintages into account.

We want to observe rain distribution in every stage of the vegetative cycle in Burgundy, Bordeaux, Piemonte and Mas Martinet (Priorat) but focusing on the rain falling from July to mid-October. We are also going to see the total amount of water that our vines get in summer not only from rain but also from irrigation.

We are going to explain in general terms how water acts on the plant (vine) and on the fruit (grape) in every stage of the vegetative cycle:

• April – May (budding – growing)
   In this stage, abundant rainfall is not a negative factor for the plant or the fruit. We want the roots to have enough humidity at the beginning of budding so that the plant can grow more rapidly, the maximum folliar surface can be achieved as soon as possible and the plant can benefit form photosynthesis as long as it is possible.

• June (flowering-fertilization and beginning of berry growth)
   In our experience, in this month, the less water, the better. After fertilization and at the beginning of berry growth, the ground should be close to withering. This way, the berry growing rate decreases and even if it gets more water later on, the grape does not grow that much and the final size will be much smaller during maturation. The ratio of hypodermis surface (skin) to pulp volume will be greater and the contents of quality factors, higher.

• June to mid-August (stop in the vegetative and berry growth)
   This is when the photosynthetic performance is high and this allows the parenchyma cells in reserve to be filled with sugar and have it available at maturation.
  In this stage ground humidity should be between withering and field capacity, so that the vegetative growth can stop and the plant’s shoots start to wither. The vine should be sufficiently hydrated and have enough water to transpire and cool down. In the hot summer days, the plant should have the appropriate conditions to capture the maximum light (photosynthesis) but if the plant suffers from stress because it has not enough water to cool down, the leaves close their stomata and leaf temperature increases from 5ºC to 7ºC in relation to air temperature. Photosynthesis is stopped and the plant’s sun exposure is not used to synthesise sugar, the remaining sugar will be used up through respiration.
  An excess of water in this stage would result in shoot growth, at the expense of the sugar in reserve in the parenchyma cells, reducing their stock. This can negatively affect the maturation period since the plant would not have enough sugar.

• Mid-August to mid-October (veraison and maturation)
   In this stage of the cycle, having water available is essential for the constant maintenance of intracellular osmotic pressure.
  In this period, there is a constant migration of sugar from the parenchyma cells to the grape. If the temperature is high and the plant does not have enough water to transpire and cool down, the intracellular space loses water and the osmotic pressure of the internal medium increases. The plant reacts by trying not to lose any more water: it closes the stomata, eliminates the basal leaves of the shoots (yellow leaves) and gets water from the fruit (grape dehydration). If, on the contrary, the plant has too much water in that stage, the optimum maturation of the grapes would be at risk since their metabolism would focus on the vegetative cycle (hormones: auxins …) instead of on maturation (hormones: abscisic acid)

The analyses that we want to carry out will be based on the aforementioned considerations. We cannot claim that each and every of them is absolutely scientific, they rather have a deductive value: between the already known scientific knowledge and the deductions resulting from observation. We do not intend to speak ex cathedra but we would like this to lead to further reflection.

Josep Lluís Pérez