Evaluation of water use for bioenergy at different scales

Abstract

This perspective reviews water metrics for accounting total water demand to produce bioenergy at various spatial scales. Volumes of water abstracted, consumed, and altered are estimated to assess water requirements of a bioenergy product, providing useful tools for water resource management and planning at local, regional, and global scale. Blue-water use accounting, integrated over time and space, provides the most direct measurements of the effects of bioenergy production on freshwater allocation among various end-users, and on human and ecosystem health and well-being. Measurement of total water demand for crop evapotranspiration, which includes both blue and green water, communicates vital information of how land and water productivity supports/constrains bioenergy expansion, and helps identify potential areas to increase the productivity of agriculture through improved soil and water conservation, changes in crop choice, and improved crop management. Life-cycle water use accounting provides a useful comparison of water required for production and conversion of feedstock to various forms of energy, and opportunities to improve water use effi ciency throughout the supply chain. In addition, life-cycle water use may be used to account for water use avoided as a result of displacement of products by coproducts of biofuel production; though these applications must be interpreted with caution. Local or regional conditions and the objective of the analysis at hand determine which water accounting metrics are most relevant and the relative importance of water use impact compared to other impacts, such as impacts to soil quality and biodiversity. Perspective: Water use for bioenergy at different scales summarizes selected water use metrics commonly used in the literature. Water use and the associated eff ects on water fl ows and ecosystems are measured by various metrics, depending on the water source, removal from the water cycle via evaporation or transpiration, and qualitative alteration (degradation). For defi nition of green water (GW) and blue water (BW), we adopt the defi nition in Hoff et al. BW withdrawn from surface bodies and aquifers is used both consumptively and non-consumptively. Consumptive use removes water from the current hydrological cycle through evaporation, evapotranspiration and product incorporation. By defi nition, consumption implies that water consumed is not immediately available for use by humans and the ecosystem in the watershed from which water is originally withdrawn. BW used non-consumptively is released back to the environment with or without change in quality, and is available for downstream uses, such as agriculture, industry, and human consumption. Unlike BW, GW use is considered only in a consumptive sense, but modifi cation of GW or soil water storage can infl uence BW availability.Water use requirement is typically expressed as the amount of water use per unit of bioenergy produced (oft en referred to as water intensity). Th e reciprocal of intensity, i.e. the amoun...

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