Virtual water is the volume of water used to make a product, and is the sum of the water use in the various steps of the production chain. Virtual water comprises three components of different colours: the green, blue and grey waters
- Green water is the water transpired by the plant that comes from rain water stored in soil. Soil is a storage reservoir for the green water that falls from the sky, or that which has been added through irrigation from blue-water reservoirs.
- Blue water is the water in our surface and groundwater reservoirs. In irrigated agriculture, blue water is abstracted to maintain transpiration. It is imperative that it is used with a high level of efficiency.
- Grey water is the water that becomes polluted during production, say in agriculture because of the leaching of nutrients and pesticides. Grey-water volume can be quantified by calculating the blue water that would be required to dilute the receiving water body to an acceptable quality standard.