Indices of Water Footprint on the Basis of the Concept of Acceptable Delay in Water Use: Their Application to Evaluation of Agricultural Production

Abstract

Objective. An extended water footprint (EWF) and an occupied water footprint (OWF) are proposed in this study as indices of water resource consumption. EWF represents the burden of water resource consumption in flow base, and has a unit of area in which the water supply equals to the subject consumption. Similarly, OWF is derived in stock base and has a unit of area multiplied by occupied time period. They are based on the concept of acceptable delay of water use, instead of fixed period such as a month and a year which is the basis of conventional water footprint. EWF and OWF enable us to analyze the sustainability of water resource consumption in higher detail, in particular in temporal variation. Similar to an ecological footprint, the regional and temporal differences in water scarcity are comparable to the total carrying capacity. In addition to being exact water footprint indices, EWFs and OWFs can be treated as impact categories in midpoint analysis of life cycle assessment (LCA), similar to land use and land occupation with the same dimensions. To evaluate the possibility of using the EWF and the OWF in the assessment of water use in agricultural production, sugarcane production for biofuel on Tokunoshima Island was examined as a case study.
Results and Discussion. EWF exhibited a higher value in blue water (BW) in comparison to green water (GW). In particular, higher values were shown during the summer, when evapotranspiration was active. The EWF exceeded the actual cultivation area in some periods, and it was eight times higher at its maximum, which indicates temporal unsustainability. However, the total annual OWF was less than the actual land occupation, showing that the process can be sustainable with adequate water resource management. The corresponding value from conventional water footprint calculation showed significant variation by day, which indicates the difficulty of analyzing the daily variation in water stress using conventional methods. An analysis of all the processes of bioethanol production showed that the impact of sugarcane cultivation dominated them in terms of water resource consumption.
Conclusions. The current work showed the potential of EWFs and OWFs as new indicators for analyses of water resource consumption. They are expected to be utilized as impact categories in LCA, especially in fine-grained analysis, although some challenges remain with regard to both database preparation and analysis methodologies.