Water use is measured in terms of a water footprint.
The water footprint of a product (good or service) is the volume of fresh water used to produce the product, summed over the various steps of the production chain. ‘Water use’ is measured in terms of water volumes consumed (evaporated) and/or polluted.
The ‘water footprint’ includes three components: consumptive use of rainwater (green water), consumptive use of water withdrawn from groundwater or surface water (blue water) and pollution of water (grey water).
As we know, water is used in every process of wine production. AND In sites where irrigation is legally practiced, this is its greatest use of water. 83% of the surface under vine is irrigated in the New World as opposed to 10% in the Old World - Institut national de la recherche agronomique (Montpellier.inra.fr). It is the variables inherent in the practice of irrigation; from country to country; region to region; and micro-climate to micro-climate, plant to plant - that renders determining wine’s water footprint, so difficult.
That said - Incorporating all water sources, the Water Footprint Network reports that it takes an average of 110 litres of water for a 125 ml glass of wine. In drier regions, the average is higher (Australia = 120 litres and California = 131 litres) (WFN, 2014).
It has to be mentioned here that these estimates are challenged by many New World oenologists. They believe that the Dutch researchers at the WFN, failed to consider the higher yields in California and other non-European vineyards, arguing that there is “more wine for the water buck”. And in drought-ridden southern Spain, where limited irrigation is now permitted, researchers argue that the water footprint alone is not a viable enough indicator with which to measure water’s “economic productivity”.
Now … this shared argument overlooks the illogical attempt of justifying increasing irrigation with higher yields and thus, greater economic profitability, when higher yields due to increased irrigation will ultimately lead to lower quality and lower economic profitability in the context of water scarcity. Any profit initially afforded by the greater yields will eventually be consumed.