The scarcity-weighted water footprint provides unreliable water sustainability scoring.

To evaluate the environmental sustainability of blue water use or the blue water footprint (WF) of a product, organisation, geographical entity or a diet, two well-established indicators are generally applied: water efficiency and blue water stress. In recent years, the Life Cycle Assessment (LCA) community has developed, used and promoted the indicator scarcity-weighted WF, which aims to grasp both blue water use and blue water stress in one indicator. This indicator is now recommended in an ISO document on water footprinting and many scholars have used associated scarcity-weighted water use indicators. However, questions on its physical meaning and its ability to correctly evaluate water sustainability have emerged. Here, we analyse for global irrigated wheat production to what extend the scarcity-weighted WF addresses blue water stress and water efficiency. We observe inconsistent results, as a significant proportion of unsustainably produced irrigated wheat has better scarcity-weighted WF scores as compared to sustainably produced irrigated wheat. Using the scarcity-weighted WF or scarcity-weighted water use for policy-making including product labelling, punishes some farmers producing their wheat in a water-sustainable way and promotes some farmers producing wheat unsustainably. Applying the scarcity-weighted WF indicator thereby is contraproductive in reaching the Sustainable Development Goal (SDG) target 6.4 on reducing water stress. In line with the specifications of this SDG target, to evaluate the sustainability of blue water use or the blue WF, the two indicators water stress and water efficiency should be used separately, in a complementary way.