About 70% of the Earth's surface is covered with water: the oceans hold about 96.5% of all Earth's water, 1% accounts for other saline waters, and 2.5% is freshwater. Over 68% of freshwater is locked up in ice and glaciers, another 30% is stored in the ground, and only 0.1% is directly available for human uses.
Every day we use visible and invisible water. We use visible water for washing, cleaning, cooking, drinking (domestic consumption). We use invisible water every time we use or consume products whose production has required water, thus in particular food. In fact, the 92% of the water we use is hidden in the food we consume every day. The water hidden in food, and in general in all goods, is called "virtual water".
The virtual water content (or water footprint) is the volume of water used to produce a good or a service, measured at the place where the good was actually produced. It is called ‘virtual’ because the water is not physically contained within the product; instead, it is displaced, stored, evapotranspired or polluted, and it is temporarily or definitively not available for other uses at the same location.
The term ‘’virtual water’’ was coined by Tony Allan in 1993, while rising the attention to the fact that local water scarcity in some countries of the Middle East is overcome by importing water-intensive products from other regions of the world. With food commodities trade, in fact, the water required for the production of these commodities is virtually transfered from producing to importing countries, and such solution allows water scarce countries to rely on their political economy to partially face the increasing water deficits.
Exporting and importing countries, exchanging agricultural products such as food items, beverages, bio-fuels, and materials from natural fibers, are linked and intertwined in an overall connected system called virtual water network.