Identification of methodological challenges remaining in the assessment of a water scarcity footprint: a review

This work presents a systematic review, updating the information on the currently available methods to calculate the water footprint (WF), and addressing the following methodological challenges, as they have not been deeply studied to date: (1) accounting and assessing the environmental impacts related to changes in evapotranspiration (ET); (2) inventory of actual blue freshwater consumption in agriculture; (3) temporal and spatial variation to establish explicit characterisation factors (CFs) and (4) adequate connection between inventory flows and spatio-temporal explicit CFs.

A systematic review relying on the guidelines of Pullin and Stewart (Conserv Biol 20(6):1647–1656, 2006) was conducted. Taking into account five specific formulated research questions in the WF field, WF studies were selected based on two ‘types’ of screening criteria: keyword searches and the WF study filter.

From the 128 papers in peer-reviewed journals on product WF from a life cycle perspective, this literature review shows that major methodological challenges remain partially unsolved, which could degrade the accuracy of product WF assessments. To understand how land use affects ET, and depending on the land cover and size of the land use production system, actual ET can be estimated based on meteorological data on water balance equations embedded in crop and forest growth models, from field measurements at meteorological stations and more recently from remote sensing. For accounting for blue water consumption in agriculture, there are two types of approaches that lead to quite different results: inventory from actual farming records of applied irrigation and inventory from modelled ET associated with irrigation. Depending on the question being addressed, the practitioner can apply either approach. Furthermore, when a single freshwater scarcity CF is determined for large sub-watersheds, especially when the sub-watersheds have non-uniform freshwater availability and demand, uncertainty in the freshwater use-related impacts is introduced. Regarding the connection between inventory flows and spatio-temporal explicit CFs, the difficulty in identifying the exact location of background processes and characterising the local environmental characteristics (e.g. edaphoclimatic conditions, land cover) can hinder the elaboration of an accurate spatially differentiated impact assessment, as more generic CFs can be applied.

This systematic review shows that there are clearly future research needs with respect to the interrelations between freshwater use and potential damages in the areas of protection of resources, human health and ecosystem quality. It is also of paramount importance to understand the effects of land use and land cover change and water irrigation on WF damage.