Encyclopedia of Global Resources part 69 doc

duction reached 81.6 million metric tons, a 3.6 per - cent rise, while livestock, poultry, and dairy output in- creased by 7 percent. Livestock production increased to 10.2 million metric tons during 2007. While agri- cultural programs aimed at modernization have raised the production levels, problems remain; these in- clude poor weather conditions, outdated equipment and farming techniques, and shortages of viable seed and water. Moreover, the combined burdens of gov- ernment subsidies and price controls in the food sector remain burdensome to Iran’s economy. Wheat, Iran’s most important crop, is grown mainly in the western and northwestern regions of the coun- try. From 1999 to 2004, wheat imports in the Middle East began to contract, especially in Iran. In 2007, Iran was self-sufficient in wheat production and be- came a net exporter of wheat for the first time. How- ever, this gain was short-lived after poor weather con- ditions in the second half of 2008 damaged Iran’s wheat crop, resulting in the need to import a mini- mum of 1.8 million metric tons of wheat, increasing Iran’s budget deficit. Acording to the U.N. Food and Agriculture Organization (FAO), Iran hadto lower its wheat production forecasts from (13.6 to 11) metric tons and significantly reduce its wheat exports. In the past, Kazakhstan was able to meet Iran’s demand for wheat, but it too had problems with its wheat crop in 2008, and Iran had to rely on wheat exporters such as the European Union, Canada, Australia, and the United States. Despite U.S. trade sanctions, in early 2009, Iran spent $96 million on imports from the United States—including wheat, soybeans, and medi- cal supplies.Previously, rice, the major crop cultivated in the Caspian Sea region, did not meet domestic needs and resulted in substantial imports. In 2008, Iran imported 19 percent of its foodstuffs and other consumer goods. Cynthia F. Racer Further Reading Axworthy, Michael.A History of Iran: Empire of the Mind. New York: Basic Books, 2008. Hyne, Norman J. Nontechnical Guide to Petroleum Geol- ogy, Exploration, Drilling, and Production.2ded. Tulsa, Okla.: PennWell Books, 2001. Louër,L. Transnational Shia Politics: Religious and Politi- cal Networks in the Gulf.New York: ColumbiaUniver- sity Press, 2008. Sagar, Abbuj D. “Wealth, Responsibility, and Equity: Exploring an Allocation Framework for Global GHG Emissions.” Climatic Change 45, nos. 3/4 (June, 2000): 511-527. See also: Agricultural products; Agriculture indus- try; Copper; Iron; Nuclear energy; Oiland natural gas reservoirs; Organization of Arab Petroleum Ex- porting Countries; Steel. Iron Category: Mineral and other nonliving resources Where Found Iron is one of the most abundant metals in the world, constituting 35 percent of the entire planet and 5 per- cent of the Earth’s crust. It combines with other ele- ments in hundreds of minerals,the most important of which are hematite and magnetite. Australia, Brazil, China, India, and Russia have been the top five pro- ducers of iron ore. Primary Uses Iron and its principal alloy, steel, are widely used in tools, machines, and structures. Historically, discover- ies and inventions involving the many uses of iron have been cruciallyimportant.Iron is also essential to biological metabolism. Technical Definition Iron is a chemical element (symbol Fe, from the Latin ferrum) and a metal of the transition Group VIII on the periodic table. Its atomic number is 26 and its atomic weight 55.487. Iron’s melting point is 1,535° Celsius, its boiling point 3,000° Celsius,and its density 7.86 grams per cubic centimeter. Description, Distribution, and Forms Iron is the cheapest and most widely used metal in the world. Itis used inthree mainproducts: wroughtiron, steel, and cast iron. Although each is approximately 95 percent iron and is produced with the same fuel, each has vastly different properties, arising from dif- ferent production methods. Wrought iron, contain- ing negligible amounts of carbon, has a melting point so high that it was not achieved by humans until the nineteenth century. When hot, wrought iron can be forged and welded, and even when it is cold it is duc - tile—capable of being shaped and hammered. Steel 628 • Iron Global Resources contains 0.25 to 1.25 percent carbon, with a lower melting point than wrought iron. It can be forged when hot and is extremely hard when quenched (cooled quickly by plunging into water or another cooling medium). Cast iron, with approximately 2 to 4.5 percent carbon, is easily melted and poured into molds. When cool it is soft and easily machined, but it is brittle and does not withstand tension forces well. The principal iron ores are hematite, magnetite, li- monite, pyrite, siderite, and taconite. Hematite and magnetite are the richest and most common ores. They are known as iron oxides because they are com- pounds of iron and oxygen. Hematite (Fe 2 O 3 ) can contain as much as 70 per- cent iron but usually contains closer to 25 percent. Significant deposits are found near Lake Superior, and in Alabama, Australia, Belgium, and Sweden. It may appear in colors ranging from black to dark red and may occur as shiny crystals, grains of rock, or loose particles. Magnetite (Fe 3 O 4 ) is a black magnetic material of- ten called black sand. Limonite (2Fe 2 O 3 3H 2 O), or brown hematite, is a hydrated variety of hematite; it is also called bog-iron ore. It can contain as much as 60 percent iron ore and is yellowish to brown in color. It is foundin Australia, France, Germany,the former So- viet Union, Spain, and the United States. Pyrite (FeS 2 ), also called fool’s gold because of its shiny yellowish surface, is about half sulfur. Siderite (FeCO 3 ) is a gray-brown carbonate ore that was once found inlarge deposits inGreat Britain and Germany. Taconite is a hard rock that contains specks or bands of either hematite or magnetite. History Iron was probably discovered accidentally in the late Bronze Agewhen it was found in the ashes of fires that had been built on top of red iron ore. Artifacts of iron weapons and tools have been found in Egypt (includ- ing the Great Pyramid of Giza) dating to 2900 b.c.e. Iron has probably beenmade ona regularbasis since at least 1000 b.c.e. The Chinese had independently de- veloped their own furnacesand techniques for produc- ing cast iron by the sixth century b.c.e. The Romans acquired ironworking technology from the Greeks and spread it throughout northern Europe. Because iron ore was readily available throughout the Near East and Europe, iron was less expensive than copper and bronze, the “metals of aristocracy.” As a result, it was used to make many everyday tools and utensils, earning its later nickname, “the democratic metal.” Through the Middle Ages, the common method of producing iron was the bloomery method. A bloom- ery may have been as simple as a circular hollow in the ground, several meters deep and several me- ters across. The iron ore was heated in a bed of burning charcoal within this hollow, often with the use of bellows to increase the fire’s tempera- ture. Asthe heatreached about800° Celsius (nor- mally the highest temperature attainable in early bloomeries), the oxygen in the ore separated from the iron and combined with carbon to form slag. The iron changed to a pasty mass called the “bloom.” The operator removed the bloom when he judged it was ready and alternately hammered and reheated it to remove the slag and to consoli- date the iron. The final product was wrought iron, produced at temperatures below iron’s melting point, a process referred to as the “direct” method. Sometimes the iron would accidentally melt in the bloomery; this was undesirable, because pro- longed exposure allowed the iron to absorb car- bon from the charcoal, creating cast iron. Be- cause of its lack of ductility and low resistance to abrasion, cast iron was unsuitable for working into tools and weapons and was therefore consid - ered worthless. Global Resources Iron • 629 Iron and Steel: World Production, 2008 Metric Tons Nation Pig Iron Raw Steel Brazil 37,000,000 36,000,000 China 478,000,000 513,000,000 France 12,000,000 19,000,000 Germany 30,000,000 48,000,000 Italy 11,000,000 32,000,000 Japan 88,000,000 123,000,000 Russia 52,000,000 74,000,000 South Korea 31,000,000 55,000,000 Ukraine 34,000,000 40,000,000 United Kingdom 11,000,000 14,000,000 United States 36,000,000 94,000,000 Other countries 138,000,000 312,000,000 Source: Data from the U.S. Geological Survey, Mineral Commodity Summaries, 2009. U.S. Government Printing Office, 2009. The major limitation of the bloomery was its low volume of output per unit of labor. Even when bloom- ery technology had fully matured, a large bloom might weigh only 90 kilograms, and the annual out- put of that bloomery would probably have been less than 20 metric tons of wrought iron. In an effort to in- crease output, the blast furnace was developed (by building up the walls of the bloomery, according to some sources). This new technology was so successful that by the middle of the sixteenth century the blast furnace had replaced the bloomery as the prevalent method of iron production. Early blast furnaces stood about 4.5 meters high, later reaching 10 meters or more. (The use of coke— made by heating coal in an airtight container to drive out gases and tar—as a fuel, beginning in the early 1700’s, allowed taller furnaces, since it did not crush as easily as charcoal and could be stacked higher.) The interior cavity widened as it descended from the top opening for about two-thirds of the furnace’s height. At that point the cavity began to narrow, cul- minating in a chamber at the very bottom of the fur- nace, called the crucible. The structure of the furnace created a chimney ef- fect, drafting air through it to accelerate combustion; waterwheel-powered bellows usually supplemented the draft. The ore, charcoal, and limestone (a flux) were dumped into the blast furnace from above. As the ore meltedand thelevel ofraw materialsdropped, more would be added on top of them. In this way, it was possible to keep a furnace in continuous opera- tion for months at a time. As the ore slowly worked its way toward the crucible, it was exposed for a pro- longed period to heat, which melted it (at about 1,400° Celsius), and carbon, which it absorbed. The molten iron collected in the crucible, and the slag, floating ontop ofthe iron, waspulled off through side openings. Theend product wasa largevolume ofmol- ten iron with a high carbon content—cast iron. The molten iron could be tapped directly from the crucible. Some of it would be poured into oblong molds pressed intodamp sand.These moldswere usu- ally laid out with several smaller molds attached at right angles to the largest mold, reminding the iron- workers of a sow and suckling pigs—hence the term “pig iron.” The pig iron would later be converted to wrought iron at a forge. The molten iron might also have been cast directly into molds for stove and fire - place parts, pots and pans, cannons, cannon balls, and many other products. In the nineteenth century, cast iron was also used for machine parts, railroad tracks, and structural elements. By that time cast iron had found many uses, and the demand for iron prod- ucts increased dramatically. A blast furnace couldproduce, typically,180 metric tons of iron per year—a tenfold increase over the bloomeries. In producing a larger output for less la- bor, however, a trade-off was necessary: the addition of another step in the process. To create wrought iron—the most desirable iron product until the late nineteenth century—from the cast iron coming from the blast furnace, the carbon had to be removed. This was done in a refinery hearth in which the bloom was heated indirectly without coming in contact with the fuel. In this way, the carbon already present burned off, and no additional carbon was absorbed from the fuel. Despite this added step, the blast furnace pro- duced a much larger volume of iron, and for less labor, than previous methods had. As a result, the de- velopment of the blast furnace was the key to making iron products much more common beginning in the fifteenth century. Even with the blast furnace, the production of good wrought iron was limited by the use of coke. Coke introduced more impurities to the cast iron than charcoal had, making it more difficult to pro- duce high-quality wrought iron. In 1784, an English- man, Henry Cort, devised a new process to address this problem. Known as the “puddling process,” it be- gan by heating the pig iron in a coke-fired rever- beratory furnace (one in which the heat was reflected off the roof of the furnace in order to keep the iron from coming in contactwith thecoke). Workers stirred the molten metal to expose more of it to the air, thus burning offcarbon. As the carbon content decreased, the melting temperature increased, and the metal gradually stiffened, separatingit fromthe moreliquid slag. When the process was complete, workers gath- ered the low-carbon iron in a “puddle ball” and shaped it in a rolling mill. Thanks to Cort’s puddling process, wrought iron became an important factor in the In- dustrial Revolution. Its dominance of the iron market lasted until the 1860’s, when steel production began on a large scale via the Bessemer process. Obtaining Iron Ore An ore’s quality for commercial purposes depends on several factors. While a pure ore may contain as much as 70 percent iron, ores are seldom found in their pure state. It is more realistic to expect a 50 to 60 per - 630 • Iron Global Resources cent iron content. At less than 30 percent, an ore is probably uneconomical. Other factors in determin- ing an ore’s quality include the amount of constitu- ents such as silicon and phosphorus in the ore, the geographical location of the ore, and the ease with which it can be extracted and processed. In prehistoric times iron ore was probably gath- ered from meteorites, high-grade outcroppings, and other sources that required little or no work to ex- tract. As the demand increased and those sources were exhausted, mining techniques had to be devel - oped to extract iron ore from the Earth. Global Resources Iron • 631 Data from the U.S. Geological Survey, . U.S. Government Printing Office, 2009.Source: Mineral Commodity Summaries, 2009 12,000,000 12,000,000 110,000,000 42,000,000 27,000,000 80,000,000 54,000,000 20,000,000 50,000,000 Metric Tons 900,000,000750,000,000600,000,000450,000,000300,000,000150,000,000 Venezuela Sweden South Africa Russia Mexico Mauritania Ukraine United States Other countries Note: Estimates for China are for crude ore, other estimates are for usable ore. 330,000,000 390,000,000 35,000,000 770,000,000 200,000,000 32,000,000 26,000,000 India China Canada Brazil Australia Iran Kazakhstan Iron Ore: World Mine Production, 2008 Most iron ore is obtained either by the open-pit mining process or by hard-rock shaft mining. Open- pit mining is employed when the ore is lying near the surface. Large machinery removes the overlying soil and rocks (called overburden) to expose the ore. It is then broken up with explosives and loaded onto a transportation system (usually large earth-moving trucks) by huge power shovels. As the process contin- ues, the equipment digs deep into the Earth, creating a large pit often several square kilometers in area and 150 meters or more deep. Most of the world’s iron ore is mined in this way. Ore that lies deep below the surface is removed via the moretraditional hard-rock shaftmining. A shaft is sunk near the deposit from which tunnels and addi- tional shaftsbranch out into the deposit. Shaft mining is muchmore expensive and dangerous thanopen-pit mining and is normally used only for very high-grade ore that cannot be reached in any other way. All ores must be processed before being sent to the blast furnace; theore’squality and iron content deter- mine the degree and type of processing needed. At a minimum, ore must be crushed, screened, and washed prior to reducing in a blast furnace. In the screening process, ore is separated into lumps that are large enough to be put into the blast furnace (7 to 25 millimeters across) and smaller par- ticles called fines. Fines are not suitable for use in a blast furnace because the particles will pack together and hinder the efficient flow of hot gases. To cor- rect this, a process called sintering is used to make larger particles out of the fines. Sintering begins by moistening the fines to make particles stick together. Coke is then added to the mixture. After passing un- der burners, the coke ignites, heating the fines until they fuse into larger particles suitable for use in the blast furnace. As the best ore deposits become exhausted (or be- come uneconomical to mine because of their inacces- sibility), methods of upgrading low-quality ore become necessary. Collectively, these processes are known as beneficiation. The first step in beneficiation is to con- centrate the ore by one of several techniques. The general objective is to concentrate the iron and re- move the silica. Most techniques rely on the differ- ence between the density of iron and that of the surrounding rock to separate the two materials. Ore might be leached and dried, pulverized and floated in a mixture of oil,agglomerated into largerparticles, or separated magnetically. Concentrating the oreby these techniques reduces both the shipping costs and the amount of waste at the blast furnace plant. After beneficiation, the concentrated ore is a very fine powder that would not work properly in a blast furnace. Since the concentrate is too small even for sintering, the pelletizing process is used. In pelletiz- ing, the concentrate is moistened and tumbled in a drumor on an inclined disk, and the resulting balls of ore are fired to a temperature of about 1,300° Celsius to dry and harden them. These pellets are usually about 10 to 15 millimeters across and are then ready for the blast furnace. Although the exact chemical processes have been fully understood beginning only during the twentieth century, the goalof iron making has always been to re- lease oxygen from its chemical bond with iron. The blast furnace is the most efficient and common way to do this. Modern blast furnaces work on the sameprin- ciples as those developed in the fifteenth century, but they are larger and have benefitedfrom centuriesof re- finement to the design, materials, and process. A mod- ern blast furnace may be asmuch as 30 meters tall and 10 meters in diameter. Because of improvements in materials, ablast furnace may stay in continuousoper- ation for two years, requiring maintenance only when its brick lining wears out. Some of the most important advances involve the use of mathematical modeling and supercomputers to provide more accurate and timely control over the process. The output of a mod- ern furnace mayexceed 10millionkilograms perday. A modern blastfurnacehas fivereadily identifiable sections; from the top down they are: throat, stack, barrel, bosh, and hearth (or crucible). The ore, coke, and limestone (collectively called the charge) enter the furnace through the throat. The distribution and timing ofthe charge is carefully monitored atall times to ensure proper operation. The throat opens onto the stack, which resembles a cone with the top cut off. The stack widens as it descends because the tempera- ture of the charge increases as it works its way down the furnace, causing the charge to expand. The next section, the barrel, is a short, straight section that con- nects thestack to the bosh, ashorter, upside-downver- sion of the stack. The bosh narrows as it descends be- cause the iron is beginning to liquefy and compact by the time the charge reaches the bosh. At the bottom of the bosh are nozzles called tuyeres through which blast air is blown into the furnace. The air coming through the tuyeres has been preheated to about 1,000° Celsius or higher, and oxygen is sometimes 632 • Iron Global Resources added to it. This hot air causes the coke in the charge to burn. The oxygen in the air combines with carbon from the coke to create carbon monoxide gas, which in turn removes the oxygen from the ore. The burn- ing coke also produces temperatures up to 3,000° Cel- sius to melt the iron. The liquid metal collects in the bottom section, called the hearth or crucible. Just as in earlier furnaces, the slag floats on the molten iron, and workers periodically pull it off through openings in the side of the furnace. Several direct reduction processes (in which the temperature never exceeds iron’s melting point) were developed in the twentieth century but are used only in special circumstances. The basic process relies on hot gases to reduce the iron ore in a way roughly anal- ogous to the process of the earlier bloomeries. Since the iron is never completely melted, slag never forms, and the final product contains impurities that must be removed during the steelmaking process. Direct reduction furnaces can be built more quickly and cheaply than blast furnaces, and they produce less pollution. The disadvantages are that they re- quire a supply of cheap natural gas and the iron ore must be processed to a very high grade. Uses of Iron Ore The vast majority of iron produced in blast fur- naces is converted to steel. The remainder is cast as pig iron and later converted to either cast iron or wrought iron. At a foundry, the pig iron is melted to a liquid state in a cupola (a small ver- sion of a blast furnace) and then cast in molds (some of them are still made with damp sand) to make machine parts, pipes, engine blocks, and thousands of other items. Wrought iron is now made in limited quantities. Its production begins by meltingpig iron and removing impurities.The molten iron is then poured overa silicate slag and formed into blooms which can then be shaped into products. Iron is used in a vast range of special-purpose alloys developed for commercial applications. The major classifications of these alloys are discussed below only in broad outline; within each group- ing there remains anenormousvariety becauseof the wide range of special needs. Magnetic alloys are either retentive (hard) or nonretentive (soft) of magnetism. The hard al - loys remain magnetized after the application of a magnetic field, thus creating a permanent mag - net. One family of hard alloys contains cobalt and mo - lybdenum (less than 20 percent of each), while an- other contains aluminum, nickel, cobalt, copper, and titanium. Once magnetized they are used in such ap- plications as speaker magnets, electrical meters, and switchboard instruments because of the constancy of their magnetic field and their resistance to demagne- tization. The soft alloys also fall into two families: those with nickel and those with aluminum. The nickel alloys are used in communications and electric power equipment, while those containing aluminum are used to carry alternating current. High-temperature alloys, used in high-temperature environments such as turbine blades in gas turbines and superchargers, are generally referred to as either iron-based, cobalt-based, or nickel-based. They are formulated to retain their chemical identity, physical identity, and the strength required to perform their intended function, allat extreme, hightemperatures. The most common electrical-resistance alloys are best known as heating elements in toasters, radiant Global Resources Iron • 633 Service centers &distributors 24% Construction 20% Transportation 13.5% Other & undistributed 40% Containers 2.5% Source: Historical Statistics for Mineral and Material Commodities in the United States U.S. Geological Survey, 2005, iron and steel statistics, in T.D.KellyandG.R.Matos,comps., ,U.S. Geological Survey Data Series 140. Available online at http://pubs.usgs.gov/ds/2005/140/. U.S. End Uses of Iron and Steel heaters, water heaters, and so on. They usually con - tain nickel (as much as 60 percent), chromium (ap- proximately 20 percent), and sometimes aluminum (approximately 5 percent). Alloys without the nickel have higher resistivity and lower density and are used in potentiometers, rheostats, and similarapplications. Corrosion-resistant alloys are designed to resist corrosion fromliquids andgases otherthan air oroxy- gen and usually contain varying amounts of nickel and chromium along with combinations of molybde- num, copper, cobalt, tungsten, and silicon. No one al- loy is capable of resisting the effects of all corrosive agents, so each is tailored to its intended purpose. The powdered iron technique employs iron that has been finely ground and mixedwith metals or non- metals to form the desired alloys. After a binder is added, themixture ispressed tothe desiredshape in a mold. This process has the advantages of precise con- trol over the makeup of the alloy and the ability to form iron pieces to precise dimensions with little or no working required afterward. Iron is important to almost every organism and is used in a variety of ways. It is involved in oxygen trans- port, electron transfer, oxidation reactions, and re- duction reactions. Iron is a constituent of human blood. Some iron compounds have medical uses, such as stimulating the appetite, treating anemia, co- agulating blood, and stimulating healing. Brian J. Nichelson Further Reading Dennis, W. H. Foundations of Iron and Steel Metallurgy. New York: Elsevier, 1967. Gordon, Robert B. American Iron, 1607-1900. Balti- more: Johns Hopkins University Press, 1996. Greenwood, N. N., and A. Earnshaw. “Iron, Ruthe- nium, and Osmium.” In Chemistry of the Elements.2d ed. Boston: Butterworth-Heinemann, 1997. Harris, J. R. The British Iron Industry, 1700-1850. Basingstoke, England: MacmillanEducation, 1988. Hillstrom, Kevin, and Laurie Collier Hillstrom, eds. Iron and Steel.Vol.1inThe Industrial Revolution in America. Santa Barbara, Calif.: ABC-CLIO, 2005. Krebs, Robert E. The History and Use of Our Earth’s Chemical Elements: A Reference Guide. Illustrations by Rae Déjur. 2d ed. Westport, Conn.: Greenwood Press, 2006. Lewis, W. David. Sloss Furnaces and the Rise of the Bir - mingham District: An Industrial Epic. Tuscaloosa: Uni - versity of Alabama Press, 1994. Moniz, B.J. Metallurgy. 4th ed.Homewood, Ill.: Ameri - can Technical Publishers, 2007. Web Sites Natural Resources Canada Canadian Minerals Yearbook, Mineral and Metal Commodity Reviews http://www.nrcan-rncan.gc.ca/mms-smm/busi- indu/cmy-amc/com-eng.htm U.S. Geological Survey Iron and Steel: Statistics and Information http://minerals.usgs.gov/minerals/pubs/ commodity/iron_&_steel U.S. Geological Survey Iron and Steel Scrap: Statistics and Information http://minerals.usgs.gov/minerals/pubs/ commodity/iron_&_steel_scrap U.S. Geological Survey Iron and Steel Slag: Statistics and Information http://minerals.usgs.gov/minerals/pubs/ commodity/iron_&_steel_slag/index.html#mcs U.S. Geological Survey Iron Ore: Statistics and Information http://minerals.usgs.gov/minerals/pubs/ commodity/iron_ore See also: Alloys; Australia; Belgium; Bessemer pro- cess; Brazil; China; Coal; France; Germany; India; In- dustrial Revolution and industrialization; Metals and metallurgy; Mineral resource use, early history of; Open-pit mining; Russia;Steel; Steel industry; United States. Irrigation Category: Obtaining and using resources Because agriculture is basic to human existence, irri- gation has been practiced since prehistoric times. Es- sentially, irrigationis theapplication of waterto soil to overcome soil moisture deficiency so that crops can have adequate water supply for optimal food produc - tion. Irrigation is essential to sustained large-scale food production. 634 • Irrigation Global Resources Background Irrigation systems were important to many ancient civilizations. They were the basis of life in the ancient civilizations of Egypt, India, China, and Mesopotamia (modern day Iraq). Some irrigation works in the Nile Valley that dateback to around 3000 b.c.e. still playan important role in Egyptian agriculture. In the United States, the first irrigation systems were developed by American Indians, and traces of ancient water distri- bution systems, made up of canals, were still visible at the beginning of the twenty-first century. Scope and Land Requirements In 1977, the Food and Agriculture Organization (FAO) of the United Nations estimated that the total global land area under irrigation was 223 million hectares. By 2000, about 270 million hectares were ir- rigated worldwide. In the United States, more than 20 million hectares are irrigated for crop production. Some form of irrigation is practiced in every country in the world. Although irrigation results in increased food production, it is extremely water intensive. For example, to grow 1 metric ton of grain (adequate for 50 percent of an average person’s supply for five years and six months) requires as much as 1,700 cubic kilometers of water per person per year. In the United States, 40 percent of total freshwater withdrawals is for irrigation. The value of irrigationis that it greatly in- creases agricultural productivity. For example, in 1979, the FAO reported that although irrigated agriculture represented only about 13 percent of global arable land (agricultural land that, when properly prepared for agriculture, will produce enough crops to be economically efficient), the value ofcrop production from ir- rigated land was 34 percent of the global total production. For irrigation to be economically viable, theland inconsideration must be able to produce enough crops to justify the investment in irrigation works. The land must be arable and irrigable; that is, sufficient water for irrigationmustexist. Soilsuitable for irrigation farming has the following attributes. The soil must have a rea- sonably high water-holding capacity and be readily penetrable by water; the rate of infiltration (percola- tion) should be low enough to avoid excessive loss of water through deeppercolation beyondthe rootzone of the crops. The soil must also be deep enough to allow root development and permit drainage of the soil, and it must be free of harmful (toxic) salts and chemicals—especially those that tend to bond to soil and reach dangerouslyhigh concentrations. Finally, it must have an adequate supply of plant nutrients. Land slopes should permit irrigation without ex- cessive runoff accompanied by high erosion rates. The land should be located in an area where irriga- tion is feasible without excessive pumping or convey- ance costs. Generally, the land should permit the planting of more than one type of crop so that the investment in irrigation works can be utilized year- round, and ideally shouldallow theflexibility ofplant- ing more economically viable crop types should eco- nomic conditions dictate such changes. Types of Irrigation Systems Generally, irrigation systems can be classified as non - pressurized systems (also known as gravity or surface Global Resources Irrigation • 635 An example of a field irrigation machine. (©Edward Homan/Dreamstime.com) systems) and pressurized systems. Historically, non - pressurized systems, in which water was flooded onto the soil surface via open channels, were the first to be constructed. In fact, nonpressurized systems preceded pressurized ones by thousands of years. Nonpres- surized systems include canals, open channels, and pipes that are not flowing full. Pressurized systems in- clude all types of sprinkler systems and low-pressure nozzle systems. There are five basic methods of implementing ir- rigation systems: flooding, furrow irrigation, subir- rigation, trickle irrigation, and sprinkling. Several subcategories exist within these five basic categories. Flooding systems include wild flooding, controlled flooding, check flooding, and basin flooding applica- tions. In all cases the irrigated area is flooded with water. The degree to which flooding is controlled or administered differentiates the types of flooding. For example, in wildflooding thereis not muchcontrol or preparation of the irrigated land. In contrast, check flooding is accomplished by admitting water into rela- tively level plots surrounded by levees. In check flood- ing the check (area surrounded by levees) is filled with waterat a fairly rapid rate and thewater isallowed to infiltrate into the soil. Furrow irrigationis used for row crops—hence the name (a furrow is a narrow ditch between rows of plants). In this method evaporation losses are mini- mized and only about 20 to 50 percent of the area is wetted during irrigation, in contrast to flooding irri- gation. In sprinkler application water is sprinkled on the irrigated land. The sprinkling is possible because the water is delivered under pressure. Sprinkler sys- tems provide a means for irrigation in areas where the topography does not permit irrigation by surface methods. Subirrigation methods are useful in areas where there is permeable soil in the root zone and a high water table. In this method irrigation water is applied below the ground surfaceto keep the water table high enough so that water from the capillary fringe is avail- able to crops. Subirrigation has the advantages of minimizing evaporation loss and requiring minimal field preparation. In trickle (or drip) irrigation a plas- tic pipe with perforations is laid along the ground at the base of a row of crops. The water issuing from the perforations is designed to trickle. Excellent control is achieved, and evaporation and deep percolation are minimized. Emmanuel U. Nzewi Further Reading Albiac, Jose, and Ariel Dinar, eds. The Management of Water Quality and Irrigation Technologies. Sterling, Va.: Earthscan, 2009. Cuenca, Richard H. Irrigation System Design: An Engi- neering Approach. Englewood Cliffs, N.J.: Prentice Hall, 1989. Heng, L. K., P. Moutonnet, and M. Smith. Review of World Water Resources by Country. Rome: Food and Agriculture Organization of the United Nations, 2003. Linsley, Ray K., et al. Water Resources Engineering. 4th ed. New York: McGraw-Hill, 1992. Morgan, Robert M. Water and the Land: A History of American Irrigation. Fairfax, Va.: The Irrigation As- sociation, 1993. Postel, Sandra. Pillar of Sand: Can the Irrigation Miracle Last? New York: W. W. Norton, 1999. Shortle, James S., and Ronald C. Griffin, eds. Irrigated Agriculture and the Environment. Northampton, Mass.: Edward Elgar, 2001. Zimmerman, JosefD. Irrigation.NewYork: Wiley,1966. Web Site U.S. Geological Survey Irrigation Water Use http://ga.water.usgs.gov/edu/wuir.html See also: Dams; Hydrology and the hydrologic cycle; Streams and rivers; Water; Water rights; Water supply systems. Isotopes, radioactive Category: Mineral and other nonliving resources Where Found All the known elements have at least one radioactive isotope, eithernatural or artificiallyproduced. There- fore, the radionuclides are found in the Earth’s crust, in its surface waters, and in the atmosphere. Primary Uses Radioisotopes are used in many areas of science and industry as tracers or as radiation sources. They pro- vide fuel for the nuclear generation of electricity and have found both diagnostic and therapeutic uses in medicine. 636 • Isotopes, radioactive Global Resources Technical Definition Radioactive isotopes are unstable nuclides that decay ultimately to stable nuclides by emission of alpha, beta, gamma, or proton radiation, by K capture, or by nuclear fission. Description, Distribution, and Forms Alpha, beta, and gamma radiation are the three types of naturally occurring radioactivity; they result in the transmutation ofone chemical nucleus to another.Al- pha decay isthe ejectionfrom the nucleusof aparticle equivalent in size to a helium nucleus. The daughter nucleus has an atomic number (Z) two less than that of the parent and a mass number (A) four less than the parent. The equation below represents the emis- sion of an alpha particle from a polonium nucleus to produce an isotope of lead (a gamma ray is also emit- ted in rare cases). 210 Po → 206 Pb + 4 He + γ 84 82 2 Beta decay results from the change within the nu- cleus of a neutron into a proton. Z increases by one, while A is unchanged. The equation below illustrates beta emission by phosphorus to become sulfur. 32 P →β – + 32 S 15 16 In gammadecay, electromagnetic radiationis emit- ted as a nucleus drops to lower states from excited states. It is the nuclear equivalent of atomic line spec- tra that show wavelengths of visiblelight emitted by at- oms whenelectrons dropfrom higher to lower energy levels. Nuclear fission is an extremely important pro- cess by which isotopes of the heavy elements such as uranium 235 capture a neutron and then split into fragments. 235 U + 1 n → 140 Ba + 94 Kr + 2 neutrons 92 56 36 The neutrons produced are captured by other nuclei, which in turn fission, producing a chain reaction. This is the process that resulted in the first atomic bomb and is now used in nuclear plants to produce electric power. History The story of radioactivity begins with Wilhelm Conrad Röntgen’s work with cathode-ray tubes. Roentgen al- lowed cathode rays to impinge on various metal sur - faces and observed that highlypenetrating radiations, which hecalled Xrays, were produced.He notedsimi - larities between the X rays and sunlight in that both could expose a photographic plate and could cause certain metals and salts to fluoresce. This fluorescence was of interest to Antoine-Henri Becquerel, who discovered by accident that crystals of uranium salt left on a photographic plate in a drawer produced an intense silhouette of the crystals. Al- though his understanding of the phenomenon was limited at the time, what Becquerel had observed was the effect of uranium radioactivity. Marie and Pierre Curie pursued the study of this phenomenon with other minerals. They worked to isolate and characterize the substances responsible and were able to isolate and purify samples of polo- nium andradium. Other scientists worked atthe same time to characterize the radiations emitted. In 1903, Ernest Rutherford and Frederick Soddy proposed that the radiations were associated with the chemical changes that radiation produced, and they character- ized three types of radiation: alpha (α), beta (β), and gamma (γ) rays. Obtaining Radioisotopes The use of nuclear fission to produce energy is based on a principle formulated by Albert Einstein, E = mc 2 . E is energy, m refers to mass, and c is a constant equal to 3.0×10 8 m/c. Thecomplete conversion of one gram of matter per second would produce energy at the rate of nine trillion watts. The main particles contained in the nucleus of an atom are protons and neutrons. The mass of a given nucleus is less than the sum of the masses of the con- stituent protons and neutrons. This mass defect has been converted, according to the equation above, to energy (binding energy) in the process of forming the nucleus. The separation of the nucleus into its constituent particles would require replacement of this energy. The binding energy per nucleon is a mea- sure of the stability of a particular nucleus. Those nu- clei having mass numbers between 60 and 80 have the highest binding energy per nucleon and are there- fore the most stable. A large nucleus such as uranium can split into fragments with sizes in the 60 to 80 mass range. When this happens, the excess binding energy is released. Uses of Radioisotopes Radioisotopes are used in a number of ways in the fields of chemistry and biology. Radioimmunoassay (RIA) is a type of isotopic dilution study in which la - Global Resources Isotopes, radioactive • 637 . conversion of one gram of matter per second would produce energy at the rate of nine trillion watts. The main particles contained in the nucleus of an atom are protons and neutrons. The mass of a given nucleus. nucleus. The separation of the nucleus into its constituent particles would require replacement of this energy. The binding energy per nucleon is a mea- sure of the stability of a particular nucleus released. Uses of Radioisotopes Radioisotopes are used in a number of ways in the fields of chemistry and biology. Radioimmunoassay (RIA) is a type of isotopic dilution study in which la - Global Resources