|Irrigated land in the US. Click to enlarge. Source: NRC|
Although biofuels production currently entails a marginal additional stress on water supplies at the regional to local scale, the significant acceleration of biofuels production could cause much greater water quantity problems depending on where the crops are grown, according to a new report from the National Research Council of the US National Academies.
Growing biofuel crops in areas requiring additional irrigation water from already depleted aquifers is a major concern. Furthermore, if projected future increases in the use of corn for ethanol production do occur, the increase in harm to water quality could be considerable, the report authors conclude.
The National Academies comprise the National Academy of Sciences, National Academy of Engineering, Institute of Medicine, and National Research Council. The Research Council is the principal operating agency of the National Academy of Sciences and the National Academy of Engineering.
Noting that water is an increasingly precious resource, the National Research Council convened a committee to study how shifts in the nation’s agriculture to include more energy crops, and potentially more crops overall, could affect water management and long-term sustainability of biofuel production. In support of the work, the Water Science and Technology Board (WSTB) of the National Research Council convened a colloquium in July 2007 to address five basic issues:
Crop water availability and use. How much water and land might be required to grow different kinds of biomass in different regions? Where is water availability likely to be a limiting factor?
Water quality. What are the possible, or likely, water quality effects associated with increases in production of different kinds of biomass?
Reducing water impacts through agricultural practices. What promising agricultural practices and technologies might help reduce water use or minimize water pollution associated with biomass production?
Water impacts of biorefineries. What are the water requirements of existing and proposed production plants, and what water quality problems may be associated with them?
Key policy considerations. What policy, regulatory, and legal changes might help address some of these water use and water quality issues?
The report is based on the findings presented at the colloquium.
Crop water availability and use. The committee found that agricultural shifts to growing corn and expanding biofuel crops into regions with little agriculture, especially dry areas, could change current irrigation practices and greatly increase pressure on water resources in many parts of the United States.
In the next 5 to 10 years, increased agricultural production for biofuels will probably not alter the national-aggregate view of water use. However, there are likely to be significant regional and local impacts where water resources are already stressed.
The water resource is already stressed in many agricultural areas. For example, large portions of the Ogallala (or High Plains) aquifer, which extends from west Texas up into South Dakota and Wyoming, show water table declines of over 100 feet. Colorado River reservoirs are at their lowest levels in about 40 years. And over-irrigation in areas such as the San Joaquin Valley of California has led to salinization of the soils. This should be kept in mind when utilizing today’s water use as a baseline for comparison of future water-availability scenarios.
|Regional irrigation water application for various crops for six regions of the United States. Irrigation application is normalized by area, and is in feet. Click to enlarge. Source: NRC|
The amount of rainfall and other hydroclimate conditions from region to region causes significant variations in the water requirement for the same crop, the report says. For example, in the Northern and Southern Plains, corn generally uses more water than soybeans and cotton, while the reverse is true in the Pacific and mountain regions of the country. Water demands for drinking, industry, and such uses as hydropower, fish habitat, and recreation could compete with, and in some cases, constrain the use of water for biofuel crops in some regions. Consequently, growing biofuel crops requiring additional irrigation in areas with limited water supplies is a major concern.
Even though a large body of information exists for the nation’s agricultural water requirements, fundamental knowledge gaps prevent making reliable assessments about the water impacts of future large scale production of feedstocks other than corn, such as switchgrass and native grasses. In addition, other aspects of crop production for biofuel may not be fully anticipated using the frameworks that exist for food crops. For example, biofuel crops could be irrigated with wastewater that is biologically and chemically unsuitable for use with food crops, or genetically modified crops that are more water efficient could be developed.
Water Quality Impacts. The quality of groundwater, rivers, and coastal and offshore waters could be impacted by increased fertilizer and pesticide use for biofuels, the report says. High levels of nitrogen in stream flows are a major cause of hypoxic regions, commonly known as “dead zones”. (Earlier post.)
The report notes that there are a number of agricultural practices and technologies that could be employed to reduce nutrient pollution, such as injecting fertilizer below the soil surface, using controlled-release fertilizers that have water-insoluble coatings, and optimizing the amount of fertilizer applied to the land.
One metric that can be used to compare water quality impacts of various crops are the inputs of fertilizers and pesticides per unit of the net energy gain captured in a biofuel. Of the potential feedstocks, the greatest application rates of both fertilizer and pesticides per hectare are for corn. Per unit of energy gained, biodiesel requires just 2 percent of the N and 8 percent of the P needed for corn ethanol. Pesticide use differs similarly. Low-input, high-diversity prairie biomass and other native species would also compare favorably relative to corn using this metric.
The switch from other crops or non-crop plants to corn would likely lead to much higher application rates of highly soluble nitrogen, which could migrate to drinking water wells, rivers, and streams, the committee said. When not removed from water before consumption, high levels of nitrate and nitrite—products of nitrogen fertilizers—could have significant health impacts.
Agricultural practices. The reports points to many agricultural practices and technologies that, if employed, can increase yield while reducing the impact of crops on water resources.
Many of these technologies have already been developed and applied to various crops, especially corn, and they could be applied to cellulosic feedstocks. Technologies include a variety of water-conserving irrigation techniques, soil erosion prevention techniques, fertilizer efficiency techniques, and precision agriculture tools that take into account site-specific soil pH (acidity, alkalinity), soil moisture, soil depth, and other measures.
Biorefineries. For biorefineries, the water consumed for the ethanol production process—although modest compared with the water used growing biofuel crops—could substantially affect local water supplies, the committee concluded. A biorefinery that produces 100 million gallons of ethanol a year would use the equivalent of the water supply for a town of about 5,000 people. Biorefineries could generate intense challenges for local water supplies, depending on where the facilities are located. However, use of water in biorefineries is declining as ethanol producers increasingly incorporate water recycling and develop new methods of converting feedstocks to fuels that increase energy yields while reducing water use, the committee noted.
Policy considerations. As total biofuels production expands to meet national goals, the long-term sustainability of the groundwater and surface water resources used for biofuel feedstocks and production facilities will be key issues to consider, according to the report.
From a water quality perspective, it is vitally important to pursue policies that prevent an increase in total loadings of nutrients, pesticides, and sediments to waterways. It may even be possible to design policies in such a way to reduce loadings across the agricultural sector, for example, those that support the production of feedstocks with lower inputs of nutrients.
The authors suggest that creative alternatives to a simple subsidy per gallon produced could help protect water quality. For example, performance subsidies could be designed to be paid when specific objectives such as energy conversion efficiency and reducing the environmental impacts of feedstock production—especially water quality—are met.
Biofuels production is developing within the context of shifting options and goals related to US energy production. There are several factors to be considered with regard to biofuels production that are outside the scope of this report but warrant consideration. Those factors include: energy return on energy invested including consideration of production of pesticides and fertilizer, running farm machinery and irrigating, harvesting and transporting the crop; the overall “carbon footprint” of biofuels from when the seed is planted to when the fuel is produced; and the “food vs. fuel” concern with the possibility that increased economic incentives could prompt farmers worldwide to grow crops for biofuel production instead of food production.
The study was sponsored by the McKnight Foundation, Energy Foundation, National Science Foundation, US Environmental Protection Agency, and National Research Council Day Fund.