UC Davis ITS researchers take a detailed look at water consumption and withdrawal requirements for ethanol
|Water consumption intensity of ethanol from corn grain and crop residue and the avoided/displaced water use credits assigned to coproducts: DGS and electricity. Credit ACS, Mishra and Yeh. Click to enlarge.|
While a number of studies have tired to assess the water consumption required for ethanol production, the results differ by orders of magnitude, with estimates ranging from 1.1 to 335 L/vehicle kilometer traveled (VKT) for Iowa and from 59 to 214 L/VKT for Nebraska.
The major difference between these studies stems from the debate existing in the water life cycle analysis (LCA) literature regarding whether, and how, to include consumption of green water (GW), which comes from precipitation before and during the crop season and is stored as soil moisture, note UC Davis Institute of Transportation Studies (ITS) Gouri Shankar Mishra and Sonia Yeh in a new paper published in the ACS journal Environmental Science & Technology.
In their paper, Mishra and Yeh analyze the lifecycle water requirement consumption and withdrawal requirements of ethanol produced from corn and from crop residue.
To address the controversy regarding GW use, this study explicitly states the sources of water inputs (GW versus BW [blue water, i.e., surface water and groundwater] and surface water versus groundwater). Our water accounting system also considers different types of uses (consumptive, nonconsumptive, and withdrawal) and accounts for application losses, conveyance losses, water use of direct energy inputs throughout the life cycle, and coproduct credits.—Mishra and Yeh
|Comparison with fossil fuels|
|Water is required for crude oil recovery by water flooding, enhanced oil recovery via steam injection, and steam extraction of bitumen from oil sands and during refining of crude oil to produce gasoline.|
|BW consumption intensities of gasoline from conventional crude oil and Canadian oil sands range from 0.41 to 0.78 and from 0.29 to 0.62 L/VKT, respectively. A recent GAO report suggests the water intensity of gasoline from large oil shale deposits in the western United States could range from 0.29 to 1.01 L/VKT.|
|Assessing the differences in water impacts of biofuels and fossil fuels is more complicated than simply comparing the total water intensities, Mishra and Yeh note. The BW consumption of biofuels from rain-fed crops and residue is lower than that of gasoline, but orders of magnitude higher for those from irrigated crops.|
|Though the water intensity of fossil fuels is on average low compared with that of biofuels, oil sand production and shale oil development could result in substantial streamwater withdrawals and significant alteration of water flows during critical low river flow periods, groundwater depletion and contamination, and wastewater discharges.|
Mishra and Yeh’s estimates of corn grain ethanol’s BW and GW consumption are lower than those of previous studies, due to the accounting of coproduct credits for water use, which they estimated to be 5% and 45% of the total BW used to produce ethanol from rain-fed and irrigated corn, respectively, and around 50% of GW in both cases.
They found that corn ethanol consumes 50-146 L/vehicle kilometer traveled (VKT) of BW and 1-60 L/VKT of GW for irrigated corn and 0.6 L/VKT of BW and 70-137 L/VKT of GW for rain-fed corn after coproduct credits. Extending the system boundary to consider application and conveyance losses and the water requirements of embodied energy increases the total BW withdrawal from 23% to 38% and BW + GW consumption from 5% to 16%.
They estimated that, in 2009, 15-19% of irrigation water was used to produce the corn required for ethanol in Kansas and Nebraska without coproduct credits and 8-10% after credits. Harvesting and converting the cob to ethanol reduces both the BW and GW intensities by 13%.
The BW and GW requirements of ethanol from corn grown in different regions provide useful information for local water resource management; for example, water use by ethanol can be compared with a region’s total water budget to identify potential water availability constraints and risks. However, such volumetric estimates do not consider differences in ecosystem or socioeconomic trade-offs as a result of differences in local hydrological conditions—specifically water “scarcity”.
Our method necessarily employs spatial and temporal aggregation. It sums across types of water consumption (BW and GW consumption and avoided water credits) in locations where the relative importance of water-related aspects may differ; thus, some results may carry no clear indication of potential social and/or environmental harm or trade-offs. Similarly, temporal aggregation of water use estimates ignores the interseasonal variability of water use and water scarcity and can therefore yield erroneous conclusions concerning seasonal water use competition.
Recent literature on freshwater LCA has developed regionally differentiated characterization factors that measure water scarcity at a watershed level and also account for temporal variability in water availability. Volumetric estimates of GW and BW may be converted using characterization factors to provide “stress-weighted” or “ecosystem-equivalent” water footprint estimates that can be compared across regions. Work is ongoing to use the explicit water inventory results to undertake impact analysis and accurately assess the effects of biofuel production on water resources.—Mishra and Yeh
Gouri Shankar Mishra and Sonia Yeh (2011) Life Cycle Water Consumption and Withdrawal Requirements of Ethanol from Corn Grain and Residues. Environmental Science & Technology Article ASAP doi: 10.1021/es104145m