2. Current Research State
2.1. Blue and Green Water Resources Used in the Supply Chain of Food Products Providing a Healthy Diet
2.2. Embedded Information and Environmental Sustainability of a Dietary WF
- The inclusion of blue and/or a green WF component. Studies in the literature report on only the blue WF, the blue and green WF and the total WF (green plus blue) .
- The type and amount of singular food items within a diet, thereby identifying a healthy or unhealthy diet from a nutrition perspective (calories, protein, fats, macro- and micronutrients).
- The origin of these products (local and/or import) and associated WF of production. The latter is the result of production methods (irrigated versus rainfed, conservation agriculture, nutrient application etc.) and resulting yield, and agroclimatological conditions (soil, climate, etc.).
- The inclusion or exclusion of food losses and waste along the supply chain.
- The choice of system boundary (three selected choices are presented in Figure 1).
- Equity: the evaluation of a total dietary WF towards a fair share of globally available blue and green water resources for each global citizen. WF amounts above this share are considered unsustainable. Local environmentally available blue water resources (available blue water resources minus environmental flows as well as sustainably available groundwater resources) [3,8,23,38] are scaled up to river basins  and ultimately a global pool/planetary boundary of blue water [23,40,41], as conceptually displayed in Figure 2. In addition, green water availability is scaled up to a planetary boundary [4,23]. Fair shares are generally defined as per capita equivalents to the global pool of water resources, although other distribution methods exist [42,43]. As already pointed out by Hoekstra , this per capita fair share will decrease due to population growth. In order for a diet to remain within such a fair share, substitution of water-intensive products such as animal products into less water-intensive products such as most vegetal products may be needed. Indeed, healthy diets with less or different animal products have lower total green plus blue WFs [34,45]. When only accounting for the blue or green component separately, this observation is more differentiated . A more comprehensive discussion on this topic is included in Section 3.2.
2.3. Geographical Coverage, Data Used and Modelling Approaches
- Many studies use FAO Food Balance Sheet (FBS) data [14,45], which have the advantage that they are internationally standardized. These data are however food supply data, i.e., food reaching the consumer. They are on an “as purchased” basis, i.e., as the food leaves the retail shop or otherwise enters the household. The quantities are provided on the basis of “primary equivalents”. In order to convert them into actual food intake values, two correction factors are required. The first factor accounts for product equivalent conversions and the second for food waste (by households but also catering) and feed to domestic animals. Often FAO FBS dietary WF assessments therefore include a WF component of consumer food waste [45,75].
- National food supply data from national statistical offices can be used. These often differ in amount and/or food product specifications from FAO FBS data.
- Many dietary WF studies use dietary survey data directly. These are food intake data, often with additional information on socioeconomic factors. They thus provide the possibility to compute dietary WFs according to socioeconomic statistics [34,61]. Such surveys can be very detailed in the type of food item consumed. They can however also be biased due to under-reporting.
2.4. Scarcity-Weighted WF and Human Health Impact (Due to Malnutrition) from Water Stress in LCA
- The value of Falkenmark and Rockstrom [5,7] incorporates green and blue water resources. The approach of Pfister et al.  and Verones et al.  only address blue water scarcity, by assuming a lack of available food due to water stressed irrigation. It is not clear how green water is addressed. Green water is the dominant resource in global food production [14,85]. River basins may be blue water stressed; they can still provide enough rainfed food for a healthy diet.
- The value of Falkenmark and Rockstrom is a rough global average high estimate for producing a balanced diet of 3000 kcal per person per day, with 20% calories from animal products and 80% from vegetal products. Many countries require much less green and blue water resources for a healthy diet [45,52,53,54]. Healthy diets also include pescatarian and vegetarian diets, which are even less water demanding.
- International food trade partly compensates for national food shortages. Water stressed countries can import food. Many river basins cross international borders.
- Trade within countries between river basins can compensate for regional food shortages. As an example, inhabitants in China’s northern water stressed region, coloured yellow to red (high DALY/m3) on the map of Verones et al. , can very well still have access to a healthy diet with food items produced in China’s wet non water stressed southern region, coloured green (low DALY/m3) on the same map.
- This approach ignores socioeconomic differences within a country/river basin. Local water resources may be relevant for the local human health of a small-scale farmer depending on his/her own food, most other inhabitants will purchase their food from the regional, national or international market. Especially middle class or wealthy inhabitants will be able to do this. In many transition and developing countries, proportions of the population now overconsume particular products such as sugar and fats.
- Some river basins are specialized in producing particular food items. The diversity of food items within a river basin can be not enough for a healthy diet.
- Countries or people within river basins can to a large extent depend on marine fish and seafood, for which wild catch does not depend on available water resources.
- Malnutrition has many forms, including overconsumption of specific products.
3.1. Future Clarity in System Boundary and Modelling Assumptions, with Comparison of Results between Different Approaches
3.2. Full Comprehensive Sustainability Assessments
3.3. Dietary Footprint Family Assessments with the WF as One Member
3.4. WF Assessment for Multiple Dietary Regimes with Support to the Development of Local FBDG
3.5. Assessment of the Synergies with and Validity of Lca-Based Indicators
- Future studies on dietary WFs should provide clarity in system boundary and modelling assumptions. Full food supply chain assessments are a topic of future research as well as studies comparing results between different approaches.
- Studies addressing all three sustainability components (equity, efficiency and impact) are currently not abundant in the literature.
- To address trade-offs between different environmental concerns, dietary footprint family assessments with the WF as one member are to be conducted.
- A key research topic is WF assessments for multiple dietary regimes including with the aim to support the development of local dietary guidelines, which account for local agroclimatological, historic and sociocultural conditions.
- This paper confirms previous concerns about the validity of the LCA mid-point indicator scarcity-weighted WF, stressing the need to evaluate the physical meaning of this indicator and whether it conflicts with established indicators on water sustainability. In addition, this paper argues that it is probably impossible to prove an empirical relation between local blue water stress and its effect on local malnutrition (LCA end-point indicator human health impact). Further research should evaluate the validity of these indicators.
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