We used a coupled agro-climatic and life-cycle assessment (LCA) model to estimate the impacts of water resources on bioenergy expansion scenarios in California, a state that suffers from water shortage, a range of agro-ecological conditions and aggressive alternative fuel incentive policies. Unlike greenhouse-gas emissions, which have been the focus of many life-cycle assessments, the impact of water resources depend on the local environment. For example, in California, the amount of water consumed by a given activity varies across the state, as does the effect of this consumption on the resource base.

Our LCA requires that impacts be normalized and reported in terms of a common "functional unit". One important message of this study is that different metrics reveal very different portraits of how water resources impact biofuel production. This is illustrated in figures 1(a) and 1(b), which represent the "embedded water" per unit of biomass ethanol and the per-hectare water needed for producing ethanol, respectively.

Our work reveals that these and other measures of water consumption will be important in different contexts and that there is no one consistently superior functional unit for analysis. Instead, local nuances must be taken into account if policies are to adequately address this important implication when expanding bioenergy. To this end, we are characterizing and assessing many of the metrics that might be important when evaluating how water resources affect biofuel production. This work feeds into larger efforts by governments and non-government actors to manage the broad social and environmental impacts of bioenergy.