"Virtual water" refers to the volume of water needed to produce products and food in agriculture or industry – in other words, water that is not retained within the product itself. Essentially, virtual water is water that is consumed or contaminated during production, processing, or transport. The term "virtual water" describes the volume of water used to manufacture a product – whether in industry or agriculture. 

British geography professor John Anthony Allan (1937–2021) coined the concept of virtual water in the 1990s. Since then, his work has received a great deal of attention from businesspeople and politicians, and he was awarded the prestigious Stockholm Water Prize for it in 2008.

Showering, making coffee, washing our hands, running the washing machine, boiling water for pasta, rinsing dishes, showering again. When you add these all up, every German consumes around 127 liters of water a day. That's a whole lot. We call this "visible water". 

It might make your head spin to consider our "virtual" water consumption, however. If we include in our calculations the amount of water used to manufacture the products that we consume on a daily business, then we find that every German uses 3900 liters per day. This is our daily virtual water balance – for every single one of the 80 million of us. More radical estimates even put the figure at 5300 liters.

Clothing manufacturing is a good example for explaining the concept of virtual water. While it takes around 2700 liters of water to make one t-shirt, 8000 liters of water are needed to produce a pair of jeans. A full 85 % of this is consumed in the cultivation of the cotton and irrigation of the fields. 

The use of fertilizers and pesticides is another water-intensive step in cotton production. There are also further processing steps, such as dyeing and various finishing processes, that account for a proportion of the water needs of such items of clothing. In some cases, consumption can even rise to 15,000 liters per garment.

There are also countless examples of virtual water consumption in the area of food production. For instance, around 15,400 liters of water on average are required to produce a kilo of beef, mainly for feed crops, such as soya and grain. Cocoa production involves particularly extensive use of water, amounting to up to 27,000 liters per kilo. As cocoa cannot generally be artificially irrigated, 98 % of the virtual water in this instance is rainwater.

Consider tomatoes: a single tomato contains around one glass of water, but a full 50 liters of virtual water is needed to produce it. This is not an issue for tomatoes grown outdoors in Germany when there's sufficient rainfall. However, greenhouse tomatoes from Spain are irrigated and therefore need groundwater and water from rivers – which damages the ecosystems. The picture is better for tomatoes from the Netherlands, except that these are grown in heated glasshouses in the winter. 

In order to compare these figures, the concept of the "water footprint" was developed. We take a closer look at that here. 

The water footprint takes the concept of virtual water further. In their calculations, scientists assume that more than two-thirds of the 3900 liters of virtual water consumed by all of us each day have been imported. Accordingly, the aim of the water footprint metric is to establish a water balance between countries in different regions.

The distinctive feature of the concept is that the volume of water consumed, evaporated, or polluted during production in the producing regions is related to the consumption of the produced goods domestically and elsewhere. This quickly shows that a great deal of water is used to manufacture goods in drought-affected developing countries for export to industrialized countries – water that is then lost to the local people and the domestic agriculture sector.

However, it's not just the volume alone that counts when assessing the water footprint. The type of water consumed also plays a role. This is categorized as follows:

Green water 
Green water refers to the volume of rainwater that is stored in the soil. Plants absorb this water during their growth phase. However, the level of precipitation varies greatly depending on the climate zone and this vital resource is finite. Therefore, when consuming goods produced using lots of green water in regions with low rainfall, it's important to remember that this water can no longer be used to serve the needs of local people. 

Blue water
Blue water refers to the volume of water used in industry and domestically for artificial irrigation or the production of goods. This water comes from streams, rivers, lakes, or groundwater. Excessive use of these water resources always has an impact on the natural ecosystem and creates not only environmental but often also social and political problems. 

Gray water
Gray water refers to water that is so heavily polluted during production that it is rendered unusable for further use – whether as drinking water, in agriculture, or in industrial manufacturing. Such gray water has to be diluted with extremely large amounts of fresh groundwater to again meet the quality standards required for usage. Pest protection agents and fertilizers may cause such pollution. Unlike blue and green water, the term "gray water" relates to water quality.

As you can tell, the water footprint is a differentiated indicator of water usage. The green and blue water footprints describe quantitative usage, while the gray water footprint illustrates the impact of usage on water quality. However, the water footprint cannot be used to draw conclusions regarding the number of people with access to clean drinking water, for example. Also, the water footprint only applies to fresh water. Other problem areas, such as ocean pollution, are not considered here at all.

In order to assess direct and indirect water consumption, local availability of water is crucial: a high water footprint in regions rich in water is not as problematic as in water-scarce regions or desert areas where "blue" water often has to be used. If we are to take our international responsibility seriously, we have to define regions with water scarcity in order to shine a spotlight on excessive water consumption in such production locations.

Overexploitation of water resources for export goods that require water-intensive production processes can have negative environmental and social effects. In addition, the production of agricultural and industrial goods may pollute waterways, thereby damaging the health of local people. None of these effects are obvious when looking at the goods that we buy. 

If we are to be in a position to make truly conscious purchasing decisions, then the volume of virtual water required would be a welcome addition to product labels. At the moment, the onus is mainly on the consumer to put the effort in to consider the many pieces of "invisible" information.