Understanding water footprint: a glossary

Water is such a familiar element that we might think we understand it well. Didn’t we already study the water cycle in elementary school? Yet, its complexity quickly becomes apparent when we examine our impact on this precious resource. To fully grasp it, we need to define some key concepts.

The water cycle

Like carbon, the Earth's water cycle is a closed loop. It never disappears but constantly transforms. It exists in four distinct forms, shifting between them:

• Liquid water: found in lakes, rivers, and oceans
• Water vapor: present in the air and clouds
• Ice: found on mountain peaks and in polar ice caps
• Hydrogen (H) and oxygen (O) molecules, which can combine and separate to form… water

Although water never vanishes, its distribution across the planet and its quality (presence or absence of pollutants) significantly impact the inhabitability of Earth for both humans and other living beings.

Studying our water footprint helps us understand how our activities disrupt the natural water cycle and threaten the inhabitability of many regions worldwide.

Water footprint

There are several methodologies to measure one's water footprint. We chose to base our approach on the AWARE method, developed by Boulay et al. in 2018. Currently, this is the most widely accepted method and is recommended by the European Commission for water footprint calculations.

This method has the advantage of quantifying and taking into account the water stress of regions where water withdrawals occur.

However, it does not incorporate pollutant emissions.

Definitions

Consumed water vs. withdrawn water

Withdrawn water refers to freshwater abstraction from the surface or underground sources. The portion of withdrawn water that is not returned to its ecosystem, meaning it does not re-enter the same source after use, is considered consumed water. This includes water that has evaporated or been absorbed into the soil, plants, or manufactured products.

Withdrawn, consumed, and returned water: the case of a washing machine

From the water footprint perspective

When calculating a water footprint, we focus on consumed water because it represents our impact on the natural water cycle.

Blue, green and gray water

These color classifications are particularly important in assessing the water footprint of agricultural activities.

• Blue water refers to withdrawn water, as defined above.
• Green water is rainwater absorbed by the soil and plants or lost through evaporation.
• Gray water represents the amount of water required to dilute pollutants introduced into the ecosystem.

From the water footprint perspective

In our water footprint calculations, we focus on blue water. Gray water is not included in the AWARE methodology we use for footprint assessment.

Virtual water vs. domestic water

Domestic water is the water that comes out of the tap. It accounts for your direct water consumption, which is reflected in your water bill. In France, the average domestic water consumption is 150 liters per person per day, used as follows:

Virtual or indirect water is the water included in everything we consume. It includes the water used to grow the vegetables we eat, produce the soy for animal food or manufacture the cotton in our t-shirts.

From the water footprint perspective

The water footprint calculation accounts for your virtual water consumption.

Other key concepts

Water stress

Water stress occurs when water demand exceeds the available supply.

Evapotranspiration

This refers to the combined process of direct water evaporation and transpiration from plants, essentially, all water that enters the atmosphere as vapor.