哈佛大学肯尼迪学院-美国的生物燃料和水-能源关系前景（英）-2021.11-58页_12mb

Summary of "Biofuels and the Water-Energy Nexus: Perspectives for the United States"

Core Content

This report explores the relationship between biofuels, particularly corn-based ethanol, and water resources in the United States. It emphasizes the importance of understanding the water-energy nexus and its implications for sustainable biofuel production and policy development.

Main Points

1. Overview of Biofuels and Water Use

• Water Footprint of Biofuels: Both biofuels and thermoelectricity generation have significant water footprints. In the U.S., agriculture and thermoelectric power account for the largest share of water withdrawal.
• Consumptive Water Use: Agriculture is the largest consumer of water in the U.S., accounting for approximately 80% of total water use.
• Biofuel Production and Water Use: Biofuels, especially corn ethanol, are among the most water-intensive energy products. Producing a gallon of corn ethanol requires 11 to 160 gallons of water, depending on the production system and region.
• Biorefinery Water Use: Biorefineries consume about 3 gallons of water per gallon of ethanol produced, mainly for washing, cooling, and fermentation processes.
• Climate Change and Water Stress: As climate change affects water availability, the nexus between biofuel expansion and water sustainability becomes increasingly important.

2. Biofuel Production in the U.S.

• Energy Independence and Security Act (EISA): Enacted in 2007, EISA increased the Renewable Fuel Standard (RFS) from 9 billion gallons in 2008 to 36 billion gallons by 2022.
• Corn Ethanol and Biodiesel: Corn ethanol and soy-based biodiesel are the primary biofuels in the U.S. Corn ethanol production has grown from 6.8 billion gallons in 2007 to 15.8 billion gallons in 2019.
• Advanced Biofuels: Advanced biofuels have not met statutory targets due to technical and cost challenges, especially high capital costs. The EPA has often granted waivers from these targets.
• GHG Reduction: The GHG footprint of ethanol production has decreased significantly, with an average reduction of 46% compared to gasoline. Future improvements may further reduce this footprint through the use of alternative feedstocks.

3. Potential Impacts on Water Resources

• Regional Variability: Water use varies by feedstock type and regional conditions. Corn ethanol grown in Nebraska requires more water per mile driven than corn grown in Iowa.
• Irrigation Requirements: The need for irrigation depends on local factors such as rainfall, soil type, and crop variety. In Nebraska, 70% of corn is grown with irrigation.
• Phenological Stages and Water Use: Corn requires different amounts of water at various growth stages, with the most critical periods being tasseling and silking.
• Water Quality Concerns: Biofuel production can impact water quality through nutrient runoff and soil erosion, though these issues are not the focus of this report.

4. Feedstock Crop Expansion and Land Use Change

• Land Use Dynamics: The expansion of biofuel production can lead to land use changes, which have both direct and indirect impacts on water resources.
• Multiple Cropping: Increasing crop yields and multiple cropping (e.g., double cropping) can help meet biofuel demand without expanding land use.
• Co-Products and Synergies: Biofuel production generates co-products such as DDGS and corn oil, which contribute to the food and feed markets, creating a synergy between food and fuel production.
• Land Use Change (LUC): Land use change can have significant impacts on water availability and ecosystem health, particularly during droughts or low flow periods.

Key Information

• Current Water Availability: Biofuel production in the U.S. has not been limited by water availability.
• Future Expansion: Some simulations suggest that corn-based ethanol production could increase to 19 billion gallons by 2030-31 without additional water or land use.
• Water Stress Considerations: Doubling ethanol production in ten years would require reallocating corn from other uses or expanding land, which could exacerbate water scarcity in some areas.
• Policy Recommendations: Policies should incentivize the use of rain-fed crops in water-stressed areas and account for climate change. They should also support sustainable water management and the development of advanced biofuels to reduce both irrigation and carbon intensity.

Policy Perspectives and Final Considerations

• Current Context: The U.S. biofuel sector is dominated by traditional corn ethanol and soy-based biodiesel.
• Policy Options: To reduce future water impacts, policies should focus on sustainable practices, water-efficient feedstocks, and the integration of biofuel production with food systems.
• Future Trends: As the U.S. transitions to renewable energy, the water-energy nexus will play a critical role in shaping the sustainability of biofuel production.

Conclusion

This report highlights the complex relationship between biofuel production and water resources in the U.S. It underscores the need for integrated policy approaches that consider both the environmental and economic impacts of biofuel expansion. The focus is on sustainable practices, regional water availability, and the role of advanced biofuels in reducing the overall water footprint and carbon intensity of the biofuel industry.