An international consensus on scarcity for the assessment of water footprints

As water has become one of the international priorities from government and NGOs, metrics to quantify its optimal use have become the center of attention. A water footprint is one such metric, which started form the accounting... [ view full abstract ]

As water has become one of the international priorities from government and NGOs, metrics to quantify its optimal use have become the center of attention. A water footprint is one such metric, which started form the accounting of water volumes along the supply chain of a product and evolved into to a more complex assessment of potential environmental impacts associated with water, at each step of the life cycle of a product (or service or organization). These concepts are now framed in a first international consensus on the principles, requirements and guidelines: the international standard ISO 14046:2014. At the core of this assessment, scarcity indexes are used to compare water consumption in different regions of the world facing different water situations. This paper presents the additional consensus work from the UNEP-SETAC Life Cycle Initiative working group on Water Use in LCA (WULCA), which achieved an internationally recommended method to apply the ISO standard and perform a water scarcity footprint.

A scarcity index is based on the comparison between water used and renewable water available in a region, and represents the level of competition present between the different users (ideally human users and ecosystems). Earlier indicators used in LCA were based on withdrawal-to-availability (WTA) ratios. When data became available, indicators emerged which were based on consumption-to-availability (CTA) ratios instead of withdrawals, where consumption refers to the part of the water withdrawn which does not return to the same watershed, e.g. it is evaporated. Later discussions led to the inclusion of environmental water requirements as part of the water demand in order to better represent the total water demand from all users, including ecosystems, and resulted in a ratio based on demand-to-availability (DTA) being proposed. However, these ratios provide information on the fraction of water available in a region that is being withdrawn/consumed/demanded, without explicitly describing how much water this actually represents.

Regions differ largely in terms of absolute water availability (or aridity), from the desert to the jungle, and this information should not be discarded by only looking at the ratio describing the fraction of available water that is being used. This reasoning led to the development of the new consensus-based  indicator in 2016, which is based on the area-specific Available Water Remaining (availability minus demand), AWARE, inverted and normalised with the world average. Ranging between 0.1 and 100, this index assesses the potential to deprive another user (human or ecosystem) of water, based on the relative amount, comparing to the world average, of water remaining per area once the demand has been met. The more water remaining compared to the average, the lower the potential to deprive another user, and vice-versa.

The concept of water footprint is now streamlined and based on consensus, which allow results to be compared and communicated, with the objective to guide better choices for the water resource use, management and protection.