Definition and measures

Blue water footprint

The blue water footprint is the volume of water that has been sourced from surface or groundwater resources (lakes, rivers, wetlands and aquifers) and has either evaporated (for example while irrigating crops), incorporated into a product or taken from one body of water and returned to another, or returned at a different time. Irrigated agriculture, industry and domestic water use can each have a blue water footprint.

Green water footprint

The green water footprint is the amount of water from precipitation that, after having been stored in the root zone of the soil (green water), is either lost by evapotranspiration or incorporated by plants. It is particularly relevant for agricultural, horticultural and forestry products.

Grey water footprint

The grey water footprint is the volume of water that is required to dilute pollutants (industrial discharges, seepage from tailing ponds at mining operations, untreated municipal wastewater, or nonpoint source pollution such as agricultural runoff or urban runoff) to such an extent that the quality of the water meets agreed water quality standards. It is calculated as:

where L is the pollutant load (as mass flux), cmax the maximum allowable concentration and cnat the natural concentration of the pollutant in the receiving water body (both expressed in mass/volume).

Calculation for different actors

The water footprint of a process is expressed as volumetric flow rate of water. That of a product is the whole footprint (sum) of processes in its complete supply chain divided by the number of product units. For consumers, businesses and geographic area, water footprint is indicated as volume of water per time, in particular:

·         That of a consumer is the sum of footprint of all consumed products.

·         That of a community or a nation is the sum for all of its members resp. inhabitants.

·         That of a business is the footprint of all produced goods.

·         That of a geographically delineated area is the footprint of all processes undertaken in this area. The virtual water balance of an area is the net import of virtual water Vi, net, defined as the difference of the gross import Vi of virtual water from its gross export Ve. The water footprint of national consumption WFarea,nat results from this as the sum of the water footprint of national area and its virtual water balance.