Virtual Water Content
The virtual-water content of a product (a commodity, good or service) is the volume of freshwater used to produce the product, measured at the place where the product was actually produced (production-site definition). It refers to the sum of the water use in the various steps of the production chain. The virtual-water content of a product can also be defined as the volume of water that would have been required to produce the product at the place where the product is consumed (consumption-site definition). We recommend to use the production-site definition and to mention it explicitly when the consumption-site definition is used. The adjective ‘virtual’ refers to the fact that most of the water used to produce a product is not contained in the product. The real-water content of products is generally negligible if compared to the virtual-water content.
The three colors of a product’s virtual water content – The virtual-water content of a product consists of three components, namely a green, blue and gray component.
- The ‘green’ virtual-water content of a product is the volume of rainwater that evaporated during the production process. This is mainly relevant for agricultural products, where it refers to the total rainwater evaporation from the field during the growing period of the crop (including both transpiration by the plants and other forms of evaporation).
- The ‘blue’ virtual-water content of a product is the volume of surface water or groundwater that evaporated as a result of the production of the product. In the case of crop production, the blue water content of a crop is defined as the sum of the evaporation of irrigation water from the field and the evaporation of water from irrigation canals and artificial storage reservoirs. In the cases of industrial production and domestic water supply, the blue water content of the product or service is equal to the part of the water withdrawn from ground or surface water that evaporates and thus does not return to the system where it came from.
- The ‘gray’ virtual-water content of a product is the volume of water that becomes polluted during its production. This can be quantified by calculating the volume of water required to dilute pollutants emitted to the natural water system during its production process to such an extent that the quality of the ambient water remains beyond agreed water quality standards.
It is relevant to know the ratio of green to blue water use, because the impacts on the hydrological cycle are different. Both the green and blue components in the total virtual-water content of a product refer to evaporation. The gray component in the total virtual-water content of a product refers to the volume of polluted water. Evaporated water and polluted water have in common that they are both ‘lost’, i.e. in first instance unavailable for other uses. We say ‘in first instance’ because evaporated water may come back as rainfall above land somewhere else and polluted water may become clean in the longer term, but these are considered here as secondary effects that will never take away the primary effects.
Virtual water flow – The virtual-water flow between two nations or regions is the volume of virtual water that is being transferred from one place to another as a result of product trade.
Virtual water export – The virtual-water export of a country or region is the volume of virtual water associated with the export of goods or services from the country or region. It is the total volume of water required to produce the products for export.
Virtual water import – The virtual-water import of a country or region is the volume of virtual water associated with the import of goods or services into the country or region. It is the total volume of water used (in the export countries) to produce the products. Viewed from the perspective of the importing country, this water can be seen as an additional source of water that comes on top of the domestically available water resources.
Virtual water balance – The virtual-water balance of a country over a certain time period is defined as the net import of virtual water over this period, which is equal to the gross import of virtual water minus the gross export. A positive virtual-water balance implies net inflow of virtual water to the country from other countries. A negative balance means net outflow of virtual water.
Water saving through trade – A nation can preserve its domestic water resources by importing a water-intensive product instead of producing it domestically. International trade can save water globally if a water-intensive commodity is traded from an area where it is produced with high water productivity (resulting in products with low virtual-water content) to an area with lower water productivity.
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