Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
2 Crop Water Availability and Use A s noted in Chapter 1, ethanol production from corn kernels has be- come an established industry and is projected to grow even further in the near future. While much of this increase will occur at the expense of soybeans, other sources of land for increased corn production will likely include cropland used as pasture, reduced fallow, acreage from expiring conservation programs, and shifts from other crops such as cotton (USDA, 2007). In the longer term, the likely expansion of cellulosic biofuel production has the potential to further increase the demand for water re- sources in many parts of the United States. Biofuels expansion beyond cur- rent irrigated agriculture, especially in dry western areas, has the potential to greatly increase pressure on water resources in some areas. 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 overirrigation 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 todayâs water use as a baseline for comparison of future water-availability scenarios. WILL THERE BE ENOUGH WATER TO GROW CROPS FOR THE PROJECTED BIOFUELS DEMAND? In the next 5 to 10 years, increased agricultural production for biofuels will probably not alter the national-aggregate view of water use. However, growing crops for biofuel production is likely to have significant regional and local impacts. Shifting land from an existing crop (or non-crop plant species) to a crop used in biofuel production has the potential to change irrigation water use, and thus the local water availability. Conversion to the 19
20 Water Implications of Biofuels Production in the United States FIGURE 2-1â Regional irrigation water application for various crops for six regions of the United States. Irrigation application is normalized by area, and is in feet. SOURCE: N. Gollehon, U.S. Department of Agriculture (USDA) Economic Research Service (ERS), written 2-1 commun., July 12, 2007. Based on data from USDA Census of Agriculture. different type of biomass will result in increased water use in some cases, in other cases a decrease. As an example, in much of the country, the crop substitution is from soy to corn. The regional effects of this can be seen in Figure 2-1. Corn gener- ally uses less water than soybeans and cotton in the Pacific and Mountain regions. The reverse is true in the Northern and Southern Plains, and the crops use about the same amount of water in the North Central and Eastern regions. Changes in agricultural water use would generally parallel these trends. Another example is in Northern Texas, where annual evapotranspira- tion (ET) rates per year for alfalfa, corn, cotton, and sorghum are estimated to be about 1,600, 760, 640, and 580 mm (63, 30, 25, and 23 inches), respectively. Therefore, regional water loss to ET will likely decrease if
Crop Water Availablity and Use 21 alfalfa acreage is converted to corn, but increase if cotton or sorghum is converted (R. Allen, Univ. of Idaho, written commun., July 12, 2007. Data from Durwood et al. ). Given the regional differences in rainfall and groundwater storage, the feasibility and sustainability of biofuel crop production as a function of water availability may vary significantly by region. Figure 2-2 shows the state-by-state water requirement of irrigated corn in the continental United States. It demonstrates that the amount of rainfall and other hydroclimate conditions in a given area causes significant (10-fold) variations in the water requirement for the same crop. Clearly there will be geographic limits on certain kinds of biofuels feedstock simply based on their water require- FIGURE 2-2â State-by-state water requirements in 2003 of irrigated corn (gallons of ir- rigation water per bushel). SOURCE: N. Gollehon, USDA ERS, written commun., July 12, 2007. Based on data from 2003 Farm and Ranch Irrigation Survey (USDA, 2003). 2-2
22 Water Implications of Biofuels Production in the United States ments. While the preceding discussion focuses on the conversion of one crop type to another, this may simply reflect the earliest phase of biofuels expansion. It seems likely that biofuels will push into a number of other regions, including regions that currently support little agriculture. Biofuels expansion beyond current irrigated agriculture, or even current agriculture in general, especially into dry western areas, has the potential to dramatically affect water use in such areas. The actual impact would be crop specific, and would be especially great where irrigation is introduced to an area that previously did not employ it. There are other local or region-specific factors to address in considering substitution of a crop designed for biofuel production for another crop. The value of the crops relative to their water demand needs to be considered. Water rights can often be bought and sold if the value of the crop is suffi- ciently high. Competing demands for water are another local phenomenon, and the feasibility and sustainability of water diversions for biomass irriga- tion will vary depending on the region. The timing of the water demand of the replacement crop may also be critical. Water is often plentiful in one season but scarce in another. HOW WILL BIOMASS PRODUCTION INTERACT WITH THE OVERALL WATER RESOURCE? While the agricultural water cycle was summarized in Figure 1-1, ag- riculture is only one of many uses of water in a large basin. Water is also used for drinking water and cooling thermoelectric plants, in addition to nonconsumptive uses such as hydropower, fish habitat, and recreation. Conflicts are common: for example, agriculture competes with endangered species in the Klamath River basin of Oregon and California (NRC, 2004), and with urban and other water uses in the Apalachicola-Chattahoochee- Flint (ACF) River Basin in Georgia, Alabama, and Florida. It is important to weigh withdrawal and use of water for crop production against those competing uses, especially since much of the water applied to fields is lost to evapotranspiration. Therefore, the question of how much water is used in biofuel production has societal as well as scientific dimensions. Figure 1-1 makes it clear that crop water may originate from one source, such as rain or groundwater, and be discharged to another, such as surface water. Precipitation, groundwater, and surface water sourcesâand ground- water and surface water dischargesâare not only viewed differently in water law and policy, but also have different consequences for long-term sustain- able use of the resource base. Since groundwater accounts for almost all
Crop Water Availablity and Use 23 of the long-term storage of water on the continents, extracting groundwater for irrigation that is subsequently discharged to streams may decrease the water available for future users of the aquifer. Some of the applied irrigation water from any source runs off immedi- ately into nearby streams, canals, and lakes. This addition to streamflow and water depth in turn may have positive or negative impacts on the ecosystems in and around these waterbodies, in terms of biodiversity, wildlife habitat, and wetlands loss or creation. It may also affect the potential for flooding or water shortages downstream. The fraction of the applied water that seeps into the shallow groundwater system (Figure 1-1) recharges aquifers for other users and in time provides additional base flow to streams. Thus, changes in one part of the agricultural water cycle (e.g., evapotranspiration or run- off) due to conversion of one type of vegetation or management practice to another will have inevitable impactsâfor better or for worseâon the groundwater resource base and streamflow. At a macroscale, the high prices of energy driving the increased produc- tion of biofuels will likely affect water availability and use. For example, Schoengold and Zilberman (2007) show that higher energy prices can lead to increased groundwater pricing, resulting in adoption of modern irrigation technologies and improved pumps. Conveyance costs related to surface waters will also increase with an increase in energy costs. These changes may lead to water conservation that may counter the expansion of water use associated with higher prices for crops. WILL THE WATER REQUIREMENTS OF BIOFUELS CROPS IN THE FUTURE BE DIFFERENT? The introduction of new feedstocksâincluding cellulosic, corn, and other crops optimized for fuel productionâis expected as biofuel production increases. However, there are fundamental knowledge gaps that preclude making reliable assessments of the water impacts of these future crops. While a large body of information exists for water requirements and ET of the nationâs traditional crops grown in their traditional regions, this is not true for non-native species that may be planted in new areas. The same challenge will exist for genetically modified crops that, for example, may be optimized for such things as energy production and water-use efficiency. Water data is also less available for some of the proposed cellulosic feedstockâfor example, native grasses on marginal landsâthan for wide- spread and common crops such as corn, soy, sorghum, and others. Neither the current ET of the marginal lands nor the potential water demand of the
24 Water Implications of Biofuels Production in the United States native grasses is well known. Further, while irrigation of native grass today would be unusual, this could easily change as cellulosic biofuel production gets underway. Thus, there are many uncertainties in estimating quantities such as consumptive water use of the biofuel feedstock of the future. There are additional aspects of crop production for biofuel that may not be fully anticipated using the frameworks existing for food crops. For example, biofuel crops may be irrigated with wastewater that is biologically and chemically unsuitable for use with food crops. In other cases, crops such as safflower may be grown using irrigation water of moderate salin- ity, in effect increasing the available water supply. Overall, there may be opportunities for integrated domestic-agricultural-industrial water, energy, and materials exchange systems that are efficient and beneficial in terms of environmental and ecosystem services. Design and assessment of such systems reinforces the need for assessment tools and understanding that includes the full life-cycle of current and future agroecosystems. HOW MIGHT CLIMATE CHANGE AFFECT THIS PICTURE? Climate change predictions tend to indicate possible wetter and warmer conditions across the major agricultural regions of the continental United States. This is projected to increase aggregate yields of rain-fed agriculture by 5-20 percent, but with important variability among regions. Warming in the western mountains is projected to lead to decreased snowpack, more winter flooding, and reduced summer flows, exacerbating competition for over-allocated water resources (IPCC, 2007). These changes are due to en- richment of the global atmosphere with the greenhouse gases from burning fossil fuels. The net surface water impact of wetter and warmer conditions depends on the land use and seasonally varying factors. Changing climate within the horizon of conversion to biofuels adds an element of uncertainty and warrants extra caution in making assessments. REFERENCES Durwood, M., R. M. Dixon, and O. F. Dent. 1960. Consumptive use of water by major crops in Texas. Texas Board of Water Engineers Bulletin 6019. Intergovernmental Panel on Climate Change (IPCC). 2007. Summary for policymakers. Pp. 7-22 in Climate Change 2007: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. M. L. Parry, O. F. Canziani, J. P. Palutikoff, P. J. van der Linen, and C. E. Hanson, eds. Cambridge, UK: Cambridge University Press. Available online at http://www.ipcc.ch/SPM13apr07.pdf. Accessed July 13, 2007.
Crop Water Availablity and Use 25 National Research Council (NRC). 2004. Endangered and Threatened Fishes in the Klamath River Basin. The National Academies Press, Washington, D.C. Schoengold, K., and D. Zilberman. 2007. The economics of water, irrigation, and development. In Handbook of Agricultural Economics: Agricultural Development: Farmers, Farm Production and Farm Markets, Vol. 3. R. E. Evenson, P. Pingali, and T. P. Schultz. Amsterdam: North-Hol- land. U.S. Department of Agriculture (USDA), National Agricultural Statistics Service. 2003. 2003 Farm and Ranch Irrigation Survey, Vol. 3. Special Studies Part 1, AC-02-SS-1, 176 p. Available on- line at http://www.nass.usda.gov/Census_of_Agriculture/2002/FRIS/fris03.pdf. Accessed July 13, 2007. U.S. Department of Agriculture (USDA), Office of the Chief Economist. 2007. USDA Agricultural Projections to 2016. OCE-2007-1. Available online at http://www.ers.usda.gov/publications/ oce071/. Accessed July 13, 2007.
Courtesy of the Natural Resources Conservation Service Center, U.S. Department of Agriculture