Water footprint is the volume of fresh water that is used (directly or indirectly) to produce a product, measured over the steps of the production chain at the place where it was actually produced. It is a geographically and temporally explicit indicator of the water consumption and it is expressed as water volume consumed per unit of time or water consumed per tons of product .
For example, on a global average, the production of 1 kg of tomatoes requires 214 liters of water and 1 kg of maize requires 1222 liters. Considering the specific origin; the production of 1 kg of cotton lint requires 6837 liters of water in Turkey, 7307 in the United States and 8705 in Pakistan .
The total water footprint breaks down into three components:
- Green water footprint: it is the volume of water from precipitation that does not run off or recharge groundwater, but that is stored in the soil or temporally stays on the top of soil or vegetation, and it is then lost by evapotranspiration.
- Blue water footprint: it is the volume of surface water or groundwater that is withdrawn and used during the production process; it includes -but it's not limited to- the irrigation provided to the vegetation growth.
- Grey water footprint: it is the volume of freshwater that is required to assimilate the load of pollutants based on natural background concentration and existing water quality standards. Such indicator is considered only occasionally, due to the uncertainties inherent in its definition.
As can be seen in the following picture, total water footprints, as well as each component, are very different from product to product.
An overall indication of the spatial variability of the water footprint of a product is shown in the figure below, which provides the national water footprint of wheat (green+blue+grey) over the period 1996-2005. The global average is 1830 m3/ton (1279 m3/ton green, 343 m3/ton blue, 208 m3/ton grey).
The water footprint of overall wheat production is 1088 Gm3/year (70% green, 19% blue, 11% grey). About the 49% of the global green water footprint is in USA, China, Russia, Australia, and India, while India, China, and Pakistan account for the biggest blue and grey water footprint. 
 Arjen Y. Hoekstra, Ashok K. Chapagain, Maite M. Aldaya, Mesfin M. Mekonnen. ''Water foootprint assessment manual.'' (2011).
 Water Footprint Network (www.waterfootprint.org).
 Mekonnen, M. M., and A. Y. Hoekstra. "A global and high-resolution assessment of the green, blue and grey water footprint of wheat." (2010).