Calculating Water Footprint of Food

The water footprint of your food is important. But if you really want to understand the impact that your diet has on the water cycle, you need to dig a little deeper. Discover the differences between green, blue, and grey water footprints, and why those differences matter.

The impact that any food has on the world’s freshwater resources depends not only on the total volume consumed but also on the form of that water usage, which falls into one of 3 categories:

1. Green Water Footprint. Any rainwater utilised – either directly or from that stored in the soil. Although still calculated into the overall footprint of that food, it is significantly less detrimental to the world’s water resources than the following two categories.
2. Blue Water Footprint. Any further water consumption - extracted from surface water (e.g. rivers, lakes) or groundwater (e.g. boreholes).
3. Grey Water Footprint. Rather than referring to water physically consumed to grow or process a product, the grey water footprint refers to water that gets polluted at any step of the production chain. In terms of food, a large proportion of this typically comes from the field and farm run off polluting nearby fresh waterways, as well as in any water used to dilute pollutants produced or used in manufacturing.^1,2

Agriculture currently accounts for about 70% of global freshwater withdrawals.

Differences Between Water Footprints: Green, Blue, & Grey

Even if two foods have a seemingly identical water footprint in terms of volume, their impact on water availability and the environment might differ drastically, depending on how that water usage is spread across these three categories.

Blue vs Green Water Footprint: Environmental Damage

Blue and green water consumption both reduce freshwater availability. But blue water consumption also increases the chance of problems such as waterlogging, salinization, soil degradation and water depletion, the repercussions of which can be much wider reaching and longer lasting than reduced water availability alone.^3,4 Thus, any food with a predominantly green as opposed to a blue water footprint is likely a more sustainable choice.

Grey Water Footprint: Not Consumed, but No Longer Useable

In contrast to the former two categories, a ‘grey’ water footprint does not directly impact water levels but rather reduces the quality of that water. This not only has the potential to leave the water unsafe to drink (thus having an impact on water availability) but can also cause widespread damage to the environment with which it comes into contact. The extent of this further damage will depend on several factors, including what specific contaminants are at play, the concentration of those pollutants, what other uses that water might have had, and where the contaminated water ultimately ends up.^5,6

Find out how mercury pollution in the ocean affects fish stocks and people 

So, which is the “best” water footprint?

While it is pretty clear that in any instance, ‘green’ water usage is preferable over 'blue' or 'grey', determining which of the latter two categories is a more sustainable or ethical choice varies on a case-by-case basis. A range of variables, such as geographical location, season, and the context of the ever-changing climate, all have significant impacts on the water availability in a given location at any given time.

For this reason, we often see trade-offs taking place depending on which type of water use is considered more detrimental in that exact location at that specific time. For example, to maximise irrigated crop yields, nitrogen is commonly administered as a fertilizer. Nitrogen is essential for producing chlorophyll in plants and greatly enhances photosynthetic capacity to help boost crop yields. Whilst this 'intensification’ increases land productivity and efficiency of water use – thus lowering the blue water footprint - it comes at the cost of drastically increasing the grey water footprint.^7,8

Find out more about how chemical fertilisers are feeding the world

Beyond the Numbers

It is for this exact reason that we must look beyond just the numbers. By reducing the blue water component, the food’s overall footprint in terms of litres consumed will drop. That is not to say, however, that in every situation, a lower blue but higher grey water footprint would be the most sustainable and least damaging choice.

Considering the wider impact of your food choices is, however, easier said than done. Theoretically, we can calculate the impact that any action could or did have on our environment. When standing in the supermarket aisle faced with hundreds of choices of what to buy for dinner, we cannot conceivably know how one piece of food fares against the next.

There are, however, a few generalised rules which can help you eat better for the environment. You can find them in the article linked below.

6 Tips to Reduce the Water Footprint of Your Food