What is virtual water, and how much water is in food?

Virtual water is the water used to produce a good, embedded in it invisibly. The concept, developed by Tony Allan, reframes trade in food as trade in water — and it explains how water-scarce countries feed themselves.

Why it matters. Direct household use — drinking, washing, flushing — is a small fraction of an individual's total water footprint. The overwhelming majority is embedded in food, and secondarily in clothing and other goods.

The three water types, which the concept separates usefully:

Green water — rainfall stored in soil and used by plants. Not extracted from rivers or aquifers, so its use is generally far less consequential.

Blue water — surface and groundwater, extracted for irrigation. This is the number that matters, because it competes with other uses and can deplete aquifers.

Grey water — the volume needed to dilute pollution produced to acceptable standards.

Typical figures, which vary enormously by region and method:

Beef is the highest by a wide margin — commonly cited around 15,000 litres per kilogram, though the great majority is green water from grazing and feed rainfall.

Nuts, particularly almonds, are high in blue water and are grown in water-stressed regions such as California, which makes them consequential out of proportion to the headline figure.

Rice is high, and frequently irrigated.

Vegetables and cereals are generally far lower.

The crucial nuance most coverage misses: where the water came from matters more than how much. A kilogram of something grown with rainfall in a wet country is very different from the same figure irrigated from a depleting aquifer in a dry one. Comparing total litres across products without distinguishing green from blue, and without location, is close to meaningless.

Water-scarce countries import virtual water by importing food rather than growing it — which is rational, and creates dependency.

Reducing food waste is among the most effective ways to reduce water footprint.