5.2 How do food systems affect water use?
5.2.1 Agricultural water usage
Agriculture uses high volumes of fresh water
Food production requires significant amounts of fresh water.
Some foods are more water intensive than others, e.g. livestock products (livestock have extensive direct and indirect water demands – e.g. drinking/washing and irrigation of feed crops, respectively), many horticultural products, rice and processed foods
The amount of water the production, distribution and consumption of a product uses can be expressed as its total water usage.
However, the ‘type’ of water used and the geographical context of its use are very important
Blue water usage expresses the amount of water diverted or drawn from stored water sources – e.g. ground sources, rivers or lakes. Excessive abstraction can deplete these stores. Agriculture is responsible for 70% of these water withdrawals (primarily for irrigation).
However, these metrics do not account for water scarcity – whether water is abundant or scarce within a region – nor, in a related term, whether the region is experiencing water stress (a concept that encompasses not just the abundance of water, but its quality and accessibility for human use).
There is huge variation in current scarcity of water and the impacts of water use will therefore vary widely.
In a world of potentially increasing water stress, how we use ‘blue water’ is significant. Agricultural water needs in coming years will increasingly face competition from other sectors.
The water footprinting approach is increasingly used: it provides a way of quantifying and understanding the water use of a product, and its potential impacts on the environment.
5.2.2 Types of water usage, water scarcity and water stress
Water usage terminology
In terms of food system water use, we may consider two categories of water: green and blue
- GREEN water refers to water from rainfall or other forms of precipitation that would be falling on the land anyway.
- BLUE water refers to water taken from ground or surface water stocks (i.e. it is water that is extracted or abstracted).
It is also important to note the difference between water stress and water scarcity: For a useful explanation of the difference, see http://pacinst.org/water-definitions/.
In short: water scarcity refers to a shortage in the absolute volume of available water, while water stress is a broader and more inclusive concept encompassing considerations of human need for the water in that area. For example, two areas may have similar levels of water scarcity (i.e. a similar lack of water), but if one area has a much higher local human population depending on its water supply, that area could be said to be more water stressed than the other.
The stress-weighted blue water use of a product refers to the blue water use of a product adjusted to take account of the water stress level of the region from where its embedded water derives.
Note that most literature on this topic will refer to water ‘footprints’ or ‘virtual water use’. However, as these terms are used slightly differently by different stakeholders, leading to some ambiguity about what is and is not being included in the phrase, we have opted here to refer to water ‘use’ or ‘usage’ as a more general term encompassing both direct water use and – where context indicates – indirect water use.
Rainfall, water extraction and water stocks are interrelated
The sustainability of water use depends not only on the absolute volumes of water required to produce a product, but also on the relationship between green water, blue water, and the reliability, maintenance and abundance of blue water sources.
Regions with high rainfall do not need to rely on blue water to irrigate crops, but regions with lower rainfall will need to extract water to use for irrigation. When extraction rates are greater than replenishment rates, then water stress will increase. Water stress potentially undermines future food production and potentially causes other problems such as reduced availability for water, sanitation and other non food uses. Over exploitation and drying of rivers and aquifers also has negative environmental consequences including damage to aquatic and terrestrial ecosystems, eutrophication, organic matter pollution and saline intrusion.
When weighted for impact on water resources (i.e. for how water-stressed the production region is), a product that uses a large volume of green water in its production may have a lower water usage than a product that uses a smaller volume of blue water in a water-stressed region (see below).
5.2.3 Factors determining the impact of water scarcity and water stress
The relationship between blue water usage, water scarcity and water stress is important
It is important to understand how much blue water is used in relation to how scarce the water is in the region of production. The relationship between the two is key. There is huge variation in the scarcity of water use and therefore in the impacts of water use. Stress-weighted water usage can show more clearly whether products are being produced in ways that increase the risk of water scarcity.
Graph produced from data in Ridoutt and Pfister (2010)
This study used water-stress indicators to produce stress-weighted water usage of 2 different products and compared them with their total, green and blue water use. The products compared are here used to illustrate the importance of water stress, rather than to imply that these specific products have any particular role to play in sustainability.
While the blue water usage of the peanut snack was very similar to that of the tomato pasta sauce, the stress-weighted water usage for the tomato pasta sauce was much higher. This was because generally the tomatoes used in production were grown in hot, dry, irrigated and water stressed environments while the ingredients for the chocolate peanut snack were produced in regions that were less water stressed.
Thus the significance of a given product’s water usage will depend upon a). Whether abstracted water or ‘green’ water was used; b). whether it was grown in a water scarce area, c). whether it was using blue water at levels that depleted overall water stores faster than they were being replenished, and d). whether there could arguably be an alternative, more societally valuable use for that water.
As such, the simplified metric of ‘blue water usage’ does not capture the full impact of water scarcity relative to the product since in principle if blue water is extracted at a sustainable rate (no faster than it is being replenished) and there is no competition with competing activities, its use is not a problem. However, if it is being extracted at an unsustainable rate, problems clearly arise. Unlike the carbon footprint (GHG emissions – see Chapter 3) where greenhouse gas emissions are of global importance, water scarcity is therefore a more locally specific concern. As such, generalisations about water usage can be misleading if local stress indicators are not included.
Note that the impact of the activity on the water discharged also needs to be considered: regions with abundant water that is nevertheless contaminated will still be water stressed. Potential agricultural impacts include pesticide, heavy metal or bacterial contamination and eutrophication.
Exporting water scarcity: food consumption in one region/country affects water use and water scarcity elsewhere
One study that examined the blue (irrigation) water impact of current UK food consumption found that, two-thirds (~67%) of the UK’s blue water requirement for food production is met from overseas.
But when considering the water scarcity footprint – that is, considering how food consumption potentially affects water stress – a higher 78% of the scarcity burden is borne overseas, while the UK only carries 22% of the blue water scarcity impact of its own consumption.
Graph produced from data in Hess et al. (2014)
This illustrates several important points:
- That consumption within a country affects water use abroad
- That consumption within a country may therefore be responsible for water scarcity elsewhere
- And that, crucially, there is often a disparity between the absolute amount of water that is ‘imported’ (in the form of embedded water use in food) and the impact on water scarcity that this causes. In other words, as shown above it is not just the amount of water used but also the water stress in an area which determines the impact of that water use.
The same product can use very different amounts of irrigation water, depending on region and context
The water extraction requirements of agricultural produce can vary enormously between regions. Here, a study showed that the blue water usage of livestock varied greatly between regions. Regions with lower rainfall need to rely more on water extraction and irrigation to produce livestock.
Source: Ran (2010)
5.2.4 The relationship between dietary patterns and water stress
Dietary patterns influence water usage
Adapted from Hess et al. (2014)
See Chapter 9 for more on healthy diets.
A study on how UK eating habits affect water requirements in the UK and in the countries it imports from (Spain, Egypt, India, South Africa, Pakistan, Belgium, US, Israel, Morocco) found that changes in food consumption towards healthier dietary patterns potentially has a greater effect on blue water scarcity overseas than in the UK. This is because under a healthy diet scenario fruit and vegetable intakes would increase – and much of the fresh produce consumed in the UK is imported from water stressed regions. Globally, shifts in dietary patterns towards increased consumption of highly water-intensive foods (e.g. increased livestock and processed foods in developing countries experiencing the nutrition transition – see Chapter 7) are expected to increase blue water use and subsequently to increase pressure on water-stressed regions.
Demand for water is increasing globally – and agriculture competes with other sectors
Source: OECD (2012)
With rising industrialisation and population growth, water demand across all sectors is predicted to increase globally by 55% between 2000 and 2050. The main demand pressures will come from domestic, manufacturing and electricity sectors in the emerging BRICS economies (Brazil, India, China and South Africa).
As such, there will be more competition for water, allowing for no significant increase in water use for agricultural irrigation. In the context of predicted rises in demand for food, water will need to be used more efficiently in agricultural production if water scarcity is to be avoided.
Greater efficiencies can be achieved through both production and consumption side shifts. Production- and consumption-side changes (in relation to food and GHG emissions) are discussed in Chapter 4.