The function of irrigation in viticulture is considered both controversial and essential to wine production. In the physiology of the grapevine, the amount of available water affects photosynthesis, shoot growth and grape development. While climate and humidity play important roles, a typical grape vine needs 635-890mm of water a year, mainly during the spring and summer months of the growing season to avoid excessive stress. Not receiving necessary water, the vine will have its growth and grape quality affected in several ways.
In many Old World wine regions, natural rainfall is the only source for water allowed to ensure the vineyard retains its 'terroir' characteristics. The use of irrigation is viewed by some as manipulative with the potential for poor wine quality due to high yields that can be increased with irrigation. Historically it has been banned by the European Union's wine laws, though in recent year's countries such as Spain has been relaxing their regulations and France's wine authority have also been reviewing the issue.
In dry regions that have little rainfall, irrigation is considered essential to any viticultural success. Many New World wine regions such as Australia and New Zealand regularly apply irrigation. Advances and research in these wine regions (as well as some Old World wine regions), have shown that wine quality can improve where irrigation is carefully managed.
The principle behind controlled irrigation is to ensure the vine receives sufficient water during the budding and flowering period, then scaled back during the ripening period where the grape vine directs more of its limited resources into developing the grape bunches instead of foliage. If the vine experiences too much water stress, photosynthesis and other processes can be impacted with the vine potentially shutting down. The availability of irrigation means that in drought conditions, sufficient water can be provided so that the balance between water stress and grape development is kept at favourable levels.