The Water Footprint of Fuel Ethanol

[摘要] Biofuels (Le., biomass-derived fuels) play a key role in discussions in theUnited States about energy security, agriculture, taxes and the environment.Although their potential to reduce our dependence on foreign oil and to mitigateclimate change is still being debated, biofuels constitute a renewable domesticresource, offer advantages to air quality improvement, and provide alternativerevenue for agricultural producers. In 2007, Congress enacted the EnergyIndependence and Security Act (EISA), which mandates the production of 36 billiongallons per year (BGY) of biofuels by 2022, including 15 BGY of corn-derivedethanol. This large increase in demand for biofuels requires immediateconsideration and mitigation of unintended environmental impacts.Specifically, there is concern that the potentially high water demand forbiofuel production could result in added pressure to already scarce water resourcesacross the country and become, in many cases, the main limiting factor to biofuelproduction. The extent of the impact created by different crops and acrossagricultural regions is unknown but could potentially be large. In addition, climatechange could ameliorate or worsen the water footprint of biofuels through severalmechanisms. First, it could either reduce or increase rainfall and water availability.Secondly, water use by crops will change as a result of the combination of severalfactors influenced by climate change (notably temperature, precipitation and C02concentration in the air), which interact in complex ways. Finally, climate changeswill be markedly regional and biofuels production is also highly concentrated in oneparticular region of the United States, potentially magnifying the effects on waterresources of large scale production.To answer these important questions, we calculated the water requirementsfor biofuel production from mUltiple cash crops (i.e., corn, soybean, switchgrass,sorghum, potatoes, and sugarbeet), taking into consideration the region they arecurrently grown. This is done through a life cycle analysis (LCA) methodology andbased on existing US Department of Agriculture (USDA) and industry statistics. Wealso estimated the effects of climate change in the water demand of corn, the mostprominent biofuel crop, using a large-scale distributed agricultural model andprojections of climate change from coupled General Circulation Models (GCMs).Climate projections from five different models were used to include a wide range offuture climate scenarios. This approach is necessary given the large differences inprojected precipitation that exist between different climate models. The magnitudeof projected increase in water requirements varied across the five simulations butthe trend was consistently upwards in all of them.Overall, this thesis will enhance decision making by contributing with a toolthat can provide spatially distributed projections of water requirements for biofuelcrop agriculture. The location of future biofuel crop acreage is unknown at this time,which precludes accurate discernment of where and to what extent water shortagesare likely to occur. Nevertheless, model simulations underscore the importance toconsider irrigation requirements and water resources availability prior to selectingbiofuel crops and where to grow them to avoiding straining regional water resources andjeopardizing future biofuel production.Specifically, our analyses show that the consumptive water demandsassociated with biofuel crop agriculture ranges from 500 to 4,000 liters of water forliter of fuel ethanol produced under current climatic conditions. Simulations withcorn showed that by mid century corn crops in traditionally irrigated areas of theHigh Plains might require significantly more water (up to 40% in some areas) andthat biofuels production now taking place in traditionally rainfed areas of theMidwest might require irrigated water supplies, potentially placing a major strainon water resources in that region, if not analyzed and managed properly. Thisanalysis suggests that u.S. biofuels policy will have to be adjusted in the comingyears to avoid exacerbating the substantial pressures that climate change areexpected to have on certain regions of the United States and on national foodproduction. In particular, the Ogallala Aquifer, the main source of irrigation water inthe High Plains, is already experiencing significant water table drops and could besignificantly threatened by a continuation of current biofuels policy in the context ofprojected climate change outcomes.

[发布机构] Rice University

[效力级别] engineering