Global review: the water footprint of diets
FCRN member Francesca Harris has co-authored this paper, which systematically reviews the water footprint of different types of diets around the world. The paper distinguishes between the use of blue water (ground and surface) and green water (rain).
The paper assessed the water use of four major dietary patterns identified in the 41 studies reviewed:
- Average: current diets in the region covered by a study
- Healthy: diets based on national or international dietary guidelines
- Reduced animal source foods (ASF): diets with lower ASF consumption than the average diets
- No animal source foods: vegan diets.
The figure below shows green and blue water footprints of average diets in different areas of the world. Green dietary water footprint is highest in Europe (2999 L/day per person), which the authors partly attribute to high consumption of animal source foods such as meat, which are major contributors to green water use.
Blue water footprint is highest in Asia (382 L/day per person). Some parts of the Middle East also have high blue water footprints due to low rainfall and therefore a high need for irrigation. Other factors contributing to blue water footprints are consumption of crops such as cereals, nuts, sugar and fruit.
Image: Adapted from Figure 3, Harris et al. National dietary green, and blue dietary water footprints. Values are the mean for the respective country including national and subnational estimates.
On average, “healthy” diets recommended in national guidelines use 6% less green water than “average” diets actually consumed, but use the same amount of blue water, as shown below. The lack of difference in blue water footprint is because “healthy” diets tend to include crops with relatively high blue water use, such as fruits, oils and nuts. Reducing or removing animal source foods from diets reduces both green and blue water use (even after accounting for substitution with other foods).
Image: Adapted from Figure 4, Harris et al. Percentage differences in dietary water footprint relative to “average” diets for (upper graph) green water footprint and (lower graph) blue water footprint. In all graphs the 0 line represents the “average” dietary pattern.
Agricultural water requirements differ between foods. Population-level dietary preferences are therefore a major determinant of agricultural water use. The “water footprint” (WF) represents the volume of water consumed in the production of food items, separated by water source; blue WF represents ground and surface water use, and green WF represents rain water use. We systematically searched for published studies using the WF to assess the water use of diets. We used the available evidence to quantify the WF of diets in different countries, and grouped diets in patterns according to study definition. “Average” patterns equated to those currently consumed, whereas “healthy” patterns included those recommended in national dietary guidelines. We searched 7 online databases and identified 41 eligible studies that reported the dietary green WF, blue WF, or total WF (green plus blue) (1964 estimates for 176 countries). The available evidence suggests that, on average, European (170 estimates) and Oceanian (18 estimates) dietary patterns have the highest green WFs (median per capita: 2999 L/d and 2924 L/d, respectively), whereas Asian dietary patterns (98 estimates) have the highest blue WFs (median: 382 L/d per capita). Foods of animal origin are major contributors to the green WFs of diets, whereas cereals, fruits, nuts, and oils are major contributors to the blue WF of diets. “Healthy” dietary patterns (425 estimates) had green WFs that were 5.9% (95% CI: −7.7, −4.0) lower than those of “average” dietary patterns, but they did not differ in their blue WFs. Our review suggests that changes toward healthier diets could reduce total water use of agriculture, but would not affect blue water use. Rapid dietary change and increasing water security concerns underscore the need for a better understanding of the amount and type of water used in food production to make informed policy decisions.
Harris, F., Moss, C., Joy, E.J., Quinn, R., Scheelbeek, P.F., Dangour, A. and Green, R., 2019. The Water Footprint of Diets: A Global Systematic Review and Meta-analysis. Advances in Nutrition: An International Review Journal, nmz091.
While some of the food system challenges facing humanity are local, in an interconnected world, adopting a global perspective is essential. Many environmental issues, such as climate change, need supranational commitments and action to be addressed effectively. Due to ever increasing global trade flows, prices of commodities are connected through space; a drought in Romania may thus increase the price of wheat in Zimbabwe.