Pest Control Since plants are besieged by a panoply of biological agents that utilize plant tissues as a food source, plant protection from pests is a major concern in the horti- culture industry. Microbial organisms, nematodes, in- sects, and weeds are the major plant pests. Weeds are defined as unwanted plants and are considered to be pests because theycompete with crop plantsfor water, sunlight, and nutrients. If left unchecked, weeds will drastically reduce crop yields because they tend to produce a large amount of seed and grow rapidly. Weed control is generally accomplished either by re- moving the weed physically or by use of a variety of herbicides that have been developed to chemically control weeds. Herbicides are selected on the basis of their ability to control weeds and, at the same time, cause little or no damage to the desired plant. Plant protection from microbes, nematodes, and insects generally involves either preventing or restrict- ing pest invasion of the plant, developing plant variet- ies that will resist or at least tolerate the invasion, or a combination of both methods. The application of chemicals, utilization of biological agents, isolation of an infected crop by quarantine, and cultural practices that routinely remove infected plants or plant tissues are examples of the different types of control meth- ods. A large number of different bactericides, fungi- cides, nematocides, and insecticides have been devel- oped in recent years, and the use of these pesticides has been particularly useful in plant protection. Since many of these chemicals are harmful to other ani- mals, including humans, the use of pesticides, and insecticides in particular, requires extreme caution. There is an increasing interest in the use of biological control methods because many of the chemical pesti- cides pose a threat to the environment. The develop- ment and use of pest-resistant crop varieties and the introduction of natural enemies that will not only re- duce the pest population but also live harmoniously in the existing environment are two of the more promising biological measures employed. Harvest A crop must be harvested once it has grown to matu- rity. Harvesting is oneof the most expensiveaspects of crop production because it is usually extremely labor intensive. For almost all crops, there is a narrow win- dow between the time the plants are ready to harvest and the time when the plants are too ripe to be of eco - nomic value. Hence, the process requires consider - able planning to ensure that the appropriate equip - ment and an adequate labor supply are available when the crop is ready to be harvested. Predicting the harvest date is of paramount importance in the plan- ning process. The length of the harvest window, the length of the growing season that is necessary for a given plant to mature under normal environmental conditions at a given geographic location, and the in- fluence of unexpected weather changes on the grow- ing season all have to be considered in the planning process. Since nature is unpredictable, even the best planning schedules sometimes have to be readjusted in midseason. Some crops are picked from the plant by hand and then mechanically conveyed from the field, while other crops are harvested entirely by hand. New me- chanical harvesting equipment is continually being developed by agricultural engineers, and crops that lend themselves to mechanical harvesting are grow- ing in importance as the manual labor force contin- ues to shrink. After harvest, most crops are generally stored for varying lengths of time, from a few days to several months. Since postharvest storage can affect both the quality and appearance of the product, con- siderable care is given as to how the crop is stored. Sometimes storage improves the quality and appear- ance, while in other cases, it causes them to deterio- rate. The ideal storageconditions are those that main- tain the product as close to harvest condition as possible. Future of the Resource In order for horticulture to remain a viable resource in the future, advances in horticulture technology have to continue to keep pace with the needs of an ever-increasing population. However, horticulturists also have to be mindful of the fragile nature of the en- vironment. New technologiesmust be developedwith the environment in mind, and much of this new tech- nology will center on advances in genetic engineer- ing. New crop varieties that will both provide higher yields and reduce the dependency on chemical pesti- cides by exhibiting greater resistance to a variety of pests will have to be developed. The future develop- ment of higher-yielding crops that can be harvested mechanically and the production of new types of equipment to facilitate the harvesting process will also be important improvements in the horticulture industry. D. R. Gossett 578 • Horticulture Global Resources Further Reading Acquaah, George. Horticulture: Principles and Practices. 4th ed. Upper Saddle River, N.J.: Pearson Prentice Hall, 2009. Adams, C. R., K. M. Bamford, andM.P.Early.Principles of Horticulture. 5th ed. Boston: Butterworth-Heine- mann, 2008. Bailey, L. H. The Standard Cyclopedia of Horticulture.2d ed. 3 vols. New York: Macmillan, 1963. Hartmann, Hudson T., et al. Hartmann and Kester’s Plant Propagation: Principles and Practices. 7th ed. Upper Saddle River, N.J.: Prentice Hall, 2002. Janick, Jules. Horticultural Science. 4th ed. New York: W. H. Freeman, 1986. Reiley, H. Edward, and Carroll L. Shry, Jr. Introductory Horticulture. 7th ed. Clifton Park, N.Y.: Thomson Delmar Learning, 2007. Rice, Laura Williams, and Robert P. Rice, Jr. Practical Horticulture. 6th ed. Upper Saddle River, N.J.: Pearson Prentice Hall, 2006. Ward, Janet D., and Larry T. Ward. Principles of Food Science. Tinley Park, Ill.: Goodheart-Willcox, 2002. Web Sites Agriculture and Agri-Food Canada Horticulture http://www4.agr.gc.ca/AAFC-AAC/display- afficher.do?id=1204824463519&lang=eng U.S. Department of Agriculture Horticulture http://www.csrees.usda.gov/horticulture.cfm See also: Agricultural products; Agriculture indus- try; Biotechnology; Hydroponics; Monoculture agri- culture; Plant domestication and breeding. Hot springs. See Geysers and hot springs Hydroenergy Category: Energy resources The first recorded uses of hydroenergy, or water power, occurred during the first century B.C.E. Water eventu - ally drove mills for grinding grain, powered machine tools in factories, and, finally, in the twentieth cen- tury, became animportant source of energy for generat- ing electricity. Background Although devices for moving water have existed since prehistoric times, apparently no one realized that water could be used to power mills or other equip- ment until approximately two thousand years ago. Farmers throughout the ancient Middle East used primitive waterwheels, known as noria, to transfer water from one level to another, as from a flowing river to an irrigation canal. Similar devices, which consist of jars or buckets attached to a wheel that is turned by the pressure of water flowing against it, can still be seen in use in Egypt and Iraq. Sometime around 100 b.c.e. an unknown inventor harnessed the power of the moving water to a mill for grinding grain. The Roman Empire Through the Nineteenth Century Following the invention of the waterwheel, its use for moving millstones spread throughout the Roman Empire. The water-powered mill made possible a dra- matic increase in the production of flour. Sixteen to twenty man-hours were required to grind sixty kilo- grams of grain. Even a primitive waterwheel, one with the equivalent of perhaps three horsepower in motive power, could produce two and one half times that amount in only one hour. Waterwheels and milling techniques remained rel- atively unchanged untiltheMiddle Ages. Between the years 800 c.e. and 1200 c.e., innovations in waterwheel technology exploded across Europe. Millwrights re- fined waterwheels for greater efficiency and adapted wheels for use in a wide variety of applications. In addition to milling grain, waterwheels drove fulling hammers for processing wool in manufacturing felt and softened hides at tanneries. Townsgrew up around milling complexes in European cities. Millers con- structed dams to regulate the flow of water, while land- owners became wealthy through the lease fees col- lected for choice mill sites on rivers and streams. A narrow stream might be dammed to provide water for one wheel, while wider rivers, such as the Seine in France, were spanned by a series of waterwheels and mills all constructed side by side. Artisans devised var - ied types of waterwheels and gearing to use with dif - Global Resources Hydroenergy • 579 ferent levels of available water, such as undershot, overshot, and breast wheels, and they built ingenious systems of stone dams and timber crib weirs to exploit every conceivable source of moving water, from tidal flows to the smallest freshwater streams. Waterwheels were also built in the Middle East, In- dia, and China, but these never reached the level of complexity common in Europe even before the Re- naissance. In the 1600’s, European colonists brought waterwheel technologies with them to the New World, and, not surprising, patterns of settlement fol- lowed streams and rivers inland from the ocean. Al- though the eighteenth century invention ofthesteam engine and its contribution to the Industrial Revolu- tion changed patterns of industrial development in Europe and elsewhere, thesteam engine did notelim- inate the importance of water power to manufactur- ing. While steam engines quickly found applications in the mining industry, it took many years for steam power to displace water power elsewhere. Steam en- gines eventually allowed industry to develop factory sites located away from sources of moving water, but did not reduce the importance of water power to many factories already in place. In fact, the rapid ex- pansion of the textile industry in the United States re- lied far more on water power than it did on steam, even though steam engines were commonplace by the 1820’s. Textile factories, such as those located in Lowell, Massachusetts, used waterpower by developing elabo- rate systems of drive belts that extended through fac- tories that were several stories high and hundreds of meters long. Dams on the river above the town di- verted water into multiple canals, allowing factory construction well back from the original banks of the river. The development of the water-powered Lowell 580 • Hydroenergy Global Resources Hydroenergy has been used for centuries. The power station above, in Vienna, Austria, is a modern example of how hydroenergy is produced. (©Richard Kittenberger/Dreamstime.com) sites began in the early years of the nineteenth cen - tury and continued for almost one hundred years. It was not until the twentieth century, following the invention of the electric motor and the widespread distribution of electrical power, that factories began to abandon water power as a motive source. Even then, only the presence of other factors, such as the buildup of silt in mill ponds and the movement of in- dustry from the New England states to the South, may have pushed factory owners to implement changes in sources of motive power. Twentieth Century Developments At the beginning of the twentieth century, industry moved away from direct exploitation of hydroenergy through the use of waterwheels and began instead to use electricity generated from hydroelectric power plants. Hydroelectric power plants generate electric- ity by converting the motive power of the water into electrical current. The water enters the plant through a power tunnel or penstock that directs the water into a casing. The casing, which looks like a gigantic snail, narrows as it spirals in and directs the water toward the blades of a turbine that turns the shaft an electric generator. Early hydroelectric plants utilized designs that converted the force of the water striking the waterwheel directly into electrical energy, but engi- neers and scientists quickly developed more efficient turbines to take advantage of available water resources. The amount of energy potential in a water power site depends on two factors. First is the effective head, or the height difference between thelevelofthe water standing behind the dam (before the water enters the power tunnel) and where it will exit at the tailrace on the downstream side of the turbine. Second is the vol- ume of water. A large volume ofwatercancompensate for a low effective head, just asanextremelyhighhead can compensate for alowvolume of water. High-head, low-volume hydroelectric plants generally rely on im- pulse wheels. Water enters the casing around the wheel under tremendous pressure and strikes the wheel buckets with incredible force. As the wheel spins in response to the force of the water striking it, it turns the shaft of a generator to convert kinetic energy to electricity. Impulse wheels have a fairly low efficiency rating, but they are often the only practical turbines for use in situations where water is in short supply. These impulse wheels, also known as Pelton wheels, are vertical water wheels that to the observer share an obvious ancestry with the old-fashioned wa - terwheels seen in bucolic illustrations of gristmills and ponds. Impulse wheels were once widely used throughout the western United States, where effec- tive heads of several hundred meters are common. Most large modern hydroelectric plants use a dif- ferent type of turbine, a reaction turbine, that exploits the pressure differential between the water entering the turbine casing and the tailrace below. Engineers such as James B. Francis turned the vertical water- wheel on its side. In the process, Francis designed a turbine that creates a partial vacuum in the space between the turbine and the tailrace. The Francis tur- bine and other reaction turbines work, in effect, by sucking the water through the turbine casing, causing the water to flow faster and to increase the overall effi- ciency of the system. Reaction turbines can be used in settings that have extremely low heads if a suffi- cient volume ofwater exists tocreate an effective pres- sure differential. Reaction turbines are especially well suited for applications in run-of-the-river power plants in which the dam diverting the water into the turbine may be only a couple of meters high. The Early Promise of Hydroenergy Noted conservationists of the early twentieth century, such as Gifford Pinchot, unabashedly pushed for the widespread exploitation of hydroelectric sites. Pin- chot and others in the conservation movement en- couraged the U.S. government to take a more active role in thedevelopment of hydroelectricity. Thealter- native to hydroelectricity was electricity generated by steam turbines,and steam required a fuelsource such as coal or oil. Even before World War I first created shortages of fossil fuels, conservationists advocated greater use of renewable resources, such ashydroelec- tricity. Because hydroelectricity does not permanently remove water from a watershed—it merely diverts the flow to pass it through a powerhouse and then returns the water to the system—conservationists argued that hydroelectric sites should be exploited in order to conserve nonrenewable energy sources, such as coal. Conservationists devoted almost twenty years to lob- bying for awater power bill, finally succeedingin 1920 with the passage of the Federal Water Power Act, which created the Federal Power Commission. Not surprisingly, the following decades witnessed an explosion ofhydropower development. The sizeof early hydro development had been limited by the available technology, but engineers quickly solved problems that had restricted turbine and generator Global Resources Hydroenergy • 581 size. Construction journals and the popular press alike regularly reported on new dams and power plants that would be the largest in theworld,witheach gigantic project quickly supplanted by a newer,bigger project. In the United States,thisfascinationwith ever bigger hydroelectric projects became a physical real- ity with the construction of Hoover Dam on the Colo- rado River and the Bonneville Power Project along the Columbia. The arrival of the Great Depression in 1929 didnot slow the construction boom. If anything, it may have accelerated it. In a time when millions of Americans were unemployed, massive construction projects such as Bonneville in the Pacific Northwestor the Tennessee Valley Authority dams in theSouthpro- vided meaningful work. Reassessing Hydro By the 1950’s, the enthusiasm for large hydroelectric projects had abated. Conservationists who had once advocated hydroelectricity because it was clean and renewable began to realize that it nonetheless posed significant environmental problems. Construction of a high dam such as Ross Dam on Washington’s Skagit River or Glen Canyon on the Colorado inevitably required that hundreds of square kilometers of land be permanently covered with water. Deserts, forests, farmland, and entire towns were all lost forever as res- ervoirs filled. Nor were hydroelectric plants neutral in affecting aquatic life. The percentage of dissolved oxygen pres- ent in water changes as it passes through turbines, as does the water temperature. Water downstream from a hydroelectric plant may flow faster than before, vary widely in volume depending on power demands, and be warmer than it would be naturally. Some species of fish may disappear or be displaced by other species that find the changed conditions more favorablethan the original native fish do. Upstream from the dam, the water on the surface of the reservoir will be both calmer and warmer than prior to construction, while the water at the bottom will be colder. Again, these changed conditions affect which fish will thrive and which fish will gradually disappear. Construction of a hydroelectric plant can change a stretch of a river from a trout stream into a bass lake. The dam and power plant themselves present a physical barrier to spawning fish, abarrier that techni- cal solutions such as fish ladders only partially solve. Fish may make it past the dam going upstream via a fish ladder, for example, but then be killed by pres - sure changes as they inadvertently pass through the turbines as they swim downstream. In addition,twentieth century dam builders had to relearn what the mill owners of the Middle Ages and the early Industrial Revolution knew: Dams stop sedi- ment as well as water. Mill owners in past centuries had learned to drain mill ponds periodically to re- move accumulated silt, but such a procedure is im- practical for a mammoth hydroelectric power plant. The effective life of dams has also begun to be exam- ined: If a 90-meter dam wasdesignedandbuiltin 1920 to last for fifty years, what happens when it is time to replace it? About six hundred dams havebeendecom- missioned in the United States. The Promise of Hydroenergy Despite the problems inherent in hydroelectricity, many environmentalists and advocates forsustainable development believe that the creation of small-scale hydroelectric power plants could significantly reduce reliance on nonrenewable fossil fuels. A typical small- scale hydroelectric plant mighthavea turbine rated at only 3,000 horsepower, as opposed to the 60,000 horsepower capacity of a large plant. On the other hand, where a large hydroelectric development, such as Glen Canyon, may cost millions of dollars, take many years to complete, and have a devastating envi- ronmental impact, small-scale hydro can be easily and cheaply implemented. Diversion dams for small-scale hydro need not even block the entire flow of a stream. That is, if a stream or river has a steady flow of water, a diversion dam to steer water into the power tunnel or penstock can be constructed that extends only part- way across the streambed, allowing the water and aquatic life to continue their normal passage almost free from restriction. Such small dams can utilize in- digenous materials, such as timber or rocks available on the site, making construction in underdeveloped regions easy and affordable. In the United States, development of small-scale hydroelectric power plants has been explored by in- dependent power producers. Changes in federal en- ergy regulations require public utilities to purchase electricity produced by independent power produc- ers, which can be companies that generate excess electricity as part of their normal manufacturing pro- cess as well as firms that have chosen to develop alter- native energy sources rather than using fossil fuels. Small hydroelectric plants once existed in many small towns throughout the nation but were abandoned as 582 • Hydroenergy Global Resources economies of scale pushed public utilities to invest in larger plants or steam turbines. Exploiting these sites suited for small-scale run-of-the-river hydroelectric power is both possible and desirable. Hydroenergy harnessed by a 200-meter-high dam can be an envi- ronmental disaster, but hydroenergy behind a 2-meter dam has few negative side effects. Nancy Farm Männikkö Further Reading Alternative Energy Institute, and Kimberly K. Smith. “Hydropower.” In Powering Our Future: An Energy Sourcebook for Sustainable Living. New York: iUni- verse, 2005. Boyle, Godfrey, ed. Renewable Energy. 2d ed. New York: Oxford University Press in association with the Open University, 2004. Craddock, David. Renewable Energy Made Easy: Free En- ergy from Solar, Wind, Hydropower, and Other Alterna- tive Energy Sources. Ocala, Fla.: Atlantic, 2008. Gimpel, Jean. The Medieval Machine: The Industrial Rev- olution of the Middle Ages. 2d ed. London: Pimlico, 1993. Gordon, Robert B., and Patrick M. Malone. The Tex- ture of Industry: An Archaeological View of the Industri- alization of North America.New York:Oxford Univer- sity Press, 1994. Raphals, Philip. Restructured Rivers: Hydropower in the Era of CompetitiveMarkets, a Report. Montreal: Helios Centre, 2001. Reynolds, Terry S. Stronger than a Hundred Men: A His- tory of the Vertical Water Wheel. Baltimore: Johns Hopkins University Press, 1983. Twidell, John, and Tony Weir. “Hydro-Power.” In Re- newable Energy Resources. 2d ed. New York: Taylor & Francis, 2006. U.S. Bureau of Reclamation. Hydropower 2002: Recla- mation’s Energy Initiative. Denver, Colo.: U.S. Dept. of the Interior, Bureau of Reclamation, 1991. Web Site U.S. Geological Survey Water Science for Schools: Hydroelectric Power Water Use http://ga.water.usgs.gov/edu/wuhy.html See also: Dams; Electrical power; Energy storage; Federal Energy Regulatory Commission; Streams and rivers; Tidal energy; Water rights. Hydrogen Category: Mineral and other nonliving resources Where Found Hydrogen is the most abundant substance in the uni- verse and is the principal constituent of stars such as the Sun. Because of its low molecular weight, gaseous hydrogen is not retained in the Earth’s atmosphere, and it must be produced by the decomposition of its chemical compounds. The principal source of hydro- gen is water, from which the hydrogen must be ex- tracted by chemical reaction or electrolysis. Primary Uses Hydrogen is useful both as a chemical reactant and as a source of energy. Hydrogen is used in the commer- cially important Haber-Bosch process for the produc- tion of ammonia. It is added to oils and fats to raise their melting points. It is also used as a fuel in certain engines and in fuel cells. The production ofenergy by the controlled fusion of hydrogen nuclei has been ex- plored as an alternative to fossil and nuclear (fission) energy sources. Technical Definition Hydrogen (chemical symbol H), atomic number 1, is the simplest chemical element, existing under normal conditions as a diatomic gas or in chemical combina- tion with other elements. It has three isotopes. The lightest isotope, atomicmass1.00797, is sometimes re- ferred to as protium to distinguish it from the much rarer deuterium, or heavy hydrogen, with atomic mass 2.014. The third isotope, tritium, with atomic mass 3.016 and a half-life of 12.26 years, is produced in trace amounts by cosmic rays bombarding the atmosphere. Hydrogen has amelting point of−259.14° Celsius and a boiling point of −252.87° Celsius. Description, Distribution, and Forms Nearly all the hydrogen that exists on Earth is found in chemical combination with other elements. Since the vast majority of chemical compounds involve hydro- gen, there is little point in trying to identify a separate chemistry of hydrogen. As the supply of hydrogen available is inexhaustible for all practical purposes, the main reason for including it in a discussion of nat - ural resources is the effect of hydrogen-based technol - ogies on the use of more limited resources. Global Resources Hydrogen • 583 History Credit for the discovery of hydrogen is generally awarded to the English scientist Henry Cavendish, who collected the flammable gas released when iron and other metals reacted with acid and reported its properties in 1766. Later, English surgeon Anthony Carlisle and English chemistWilliam Nicholson made use of the newly developed voltaic pile to produce hy- drogen through the electrolysis of water. Because of its inherently low density, hydrogen was used to pro- vide buoyancy for balloons and other lighter-than-air craft, a practice that ended with the destruction by fire of the zeppelin Hindenburg in 1937. Helium re- placed hydrogen for buoyancy applications. Much research in the later third of the twentieth century was directed toward achieving hydrogen fu- sion under controlled conditionson Earth. The princi- pal engineering challenge has been the containment of the extremely hot plasma necessary for sustained nuclear fusion, but atleastpartial success has beenob- tained with the tokamak,a device that uses strong mag- netic fields to confine the plasma. Considerable excite- ment was generated within the scientific community in 1989 when two electrochemists at the University of Utah announced that they had achieved deuterium fusion by electrochemical means in a table-top appa- ratus. Numerous attempts were made to repeat their experiment, with disappointing results. Within a few years most scientists had come to consider the evi- dence for “cold fusion” to be inconclusive at best. Obtaining Hydrogen Hydrogengas may be produced by the action of an acid on a reactive metal, by the electrolysis of water, or by the reaction of water withcarbon or hydrocarbons athigh temperature. Because of its small size, hydrogen can enter the lattice structure of many metallic elements. This creates a problem in steels, particularly in oil- drilling equipment, in which hydrogen embrittlement can cause mechanical failure. On the other hand, a number of transition metals, notably palladium, can absorb large quantities—up to one hydrogen atom per metal atom—of hydrogen and release it under controlled conditions, thus offering the potential for safe and compact storage of this high-energy fuel. Uses of Hydrogen Hydrogen is a very dense energy source in the sense that the combustion of a few grams of hydrogen in air releases a great deal of heat energy. The usefulness of hydrogen as a fuel is somewhat limited by its low boil - ing point and the fact that it readily forms an explo- sive mixture with oxygen from the air. Hydrogen tends to be used as a fuel only in situations in which weight is anoverriding concern. Thusit is used to pro- vide electrical power in spacecraft. There is some in- terest in using hydrogen as a fuel for motor vehicles, because the only combustion product is the environ- mentally acceptable water. Use of hydrogen in the load leveling of power-generating systems has also been proposed. In this case it would be produced by electrolysis when demand for electrical energy is low and used to power fuel cells during peak demand pe- riods. Hydrogen can be produced from solar energy either by using photovoltaic cells to electrolyze water or directly by a photogalvanic process in which light energy absorbed byasemiconducting material is used to split the hydrogen-oxygen bond in water. Steam reacts with coal to form synthesis gas, a mixture of hydrogen, carbon monoxide, carbon dioxide, and methane that can be burned as a fuel or exposed to a catalyst to form further hydrocarbons. Donald R. Franceschetti Further Reading Eubanks, Lucy Pryde, et al. Chemistry in Context: Ap- plying Chemistry to Society. 6th ed. New York: Mc- Graw-Hill Higher Education, 2009. Greenwood, N. N., and A. Earnshaw. “Hydrogen.” In Chemistry of the Elements. 2d ed. Boston: Butter- worth-Heinemann, 1997. Gupta, Ram B., ed. Hydrogen Fuel: Production, Trans- port, and Storage. Boca Raton, Fla.: CRC Press, 2009. Henderson, William. “The Chemistry of Hydrogen.” In Main Group Chemistry. Cambridge, England: Royal Society of Chemistry, 2000. Holland, Geoffrey B., and James J. Provenzano. The Hydrogen Age: Empowering a Clean-Energy Future. Salt Lake City, Utah: Gibbs Smith, 2007. Hordeski, Michael Frank. Alternative Fuels: The Future of Hydrogen. 2d ed. Boca Raton, Fla.: CRC Press, 2008. _______. Hydrogen and Fuel Cells: Advances in Transpor- tation and Power. Boca Raton, Fla.: CRC Press, 2009. Rifkin, Jeremy. The Hydrogen Economy: The Creation of the Worldwide Energy Web and the Redistribution of Power on Earth. New York: J. P. Tarcher/Putnam, 2002. Rigden, John S. Hydrogen: The Essential Element. Cam - bridge, Mass.: Harvard University Press, 2002. 584 • Hydrogen Global Resources Romm, Joseph J. The Hype About Hydrogen: Fact and Fic - tion in the Race to Save the Climate. Washington, D.C.: Island Press, 2004. Web Sites Universal Industrial Gases, Inc. Hydrogen (H 2 ) Properties, Uses, Applications: Hydrogen Gas and Liquid Hydrogen http://www.uigi.com/hydrogen.html U.S. Department of Energy Hydrogen http://www.energy.gov/energysources/ hydrogen.htm U.S. Department of Energy, Alternative Fuels and Advanced Vehicles Data Center Hydrogen http://www.afdc.energy.gov/afdc/fuels/ hydrogen.html See also: Coal gasification and liquefaction; Fuel cells; Haber-Bosch process; Nuclear energy; Solar en- ergy. Hydrology and the hydrologic cycle Categories: Geological processes and formations; scientific disciplines Hydrology is the study of the Earth’s water. It involves a number of scientific disciplines related to its acquisi- tion, planning, and management. The hydrologic cycle is the cycle that water passes through as it is trans- formed from seawater to atmospheric moisture to pre- cipitation on land surfaces and its eventually to water vapor or the sea. Background Unlike any other planet in our solar system, the Earth has a vast abundance of water. More than 70 percent of the Earth’s surface is covered by water. Therefore, the life that has evolved on the Earth is extremely de- pendent on water for continued survival. The Ameri- can Geologic Institute’s Dictionary of Geological Terms defines hydrology as “the science that relates to the water of the Earth.” It can also be described as the study of the Earth’s water in all its forms and areas of occurrence. This study includes an array of scientific disciplines, such as civil engineering, geology, ocean - ography, chemistry, geography, and ecology, to name only a few. Importance of Water as a Resource On a casual appraisal, that water would be considered an important natural resource seems unlikely given its abundance on the Earth. However, as Benjamin Franklin observed, “When the well’s dry, we know the worth of water.” Despite the vast volumes of water on our planet, fresh water is in fact one of our most im- portant natural resources. Without it, much terres- trial life, including humans, could not exist. Water fit for human consumption is an absolute necessity, and much of the Earth’s water is too salty to be consum- able by humans. Although desalinization is used in some areas, it is often not economically feasible on a large scale. Al- though not readily consumable by humans, the water in the oceans is of unquestionable importance as a re- source. It supports the biodiversity of the oceans, and all creatures of the Earth are either directly or indi- rectly dependent on it for survival. Water of accept- able quality is necessary for irrigation and livestock operations. Huge quantities ofwater are necessary for certain industrial processes and as a coolant for vari- ous industrial processes. Forms of Water Although estimates vary, more than 97 percent of the Earth’s water exists in the form of the seawater found in the oceans. Of the remaining percentage, much is tied up in icecaps, glaciers, saline lakes, and soil mois- ture. Freshwater lakes, rivers, and streams account for a surprisingly small percentage of the total of the Earth’s water, about 0.01 percent. Fresh groundwater accounts for roughly 0.76 per- cent of theoveralltotal. It can beseen by this compari- son that fresh groundwater sources far outweigh sur- face water sources. In reality, only a small portion of the Earth’s water is readily available in the form of fresh water. Although the amountsoffresh groundwa- ter and surface water are comparatively small, much of the study of hydrology involves these two forms be- cause of their crucial importance. The search for new sources of groundwater is primarily accomplished by exploratory drilling coupled with a knowledge of hydrologic and geologic processes. Artificial lakes and reservoirs increase the supply of water by length - ening the residence time of surface water. Global Resources Hydrology and the hydrologic cycle • 585 Importance of Water Since World War II, agricultural, residential, and in- dustrial demands on water supplies have increased dramatically. In areas such as California and Idaho, where groundwater is used extensively for irrigation, some sources of fresh water appear to be dwindling rapidly. Although its full extent is not known, human pollution of water resources is also a major concern. The U.S. Environmental Protection Agency has indi- cated that roughly 40 percent of assessed rivers and lakes and more than 30 percent of assessed estuaries were not suitable forfishing, swimming, orother uses. Civil engineers, geologists, chemists, and others work in concert with cities and other governmental agen- cies to expand water supplies, to provide better plan - ning for future water use, and to protect remaining sources of water. The Hydrologic Cycle Although there is no true beginning or end to the hydrologic cycle, descriptions often begin with the oceans. Solar radiation provides the energy for the cy- cle. It not only transforms some of the Earth’s liquid waters to watervapor but also leadsto a planetary heat imbalance. In general the Northern Hemisphere has a net heat loss to space, and equatorial areas have a net heat gain. To counteract this imbalance, heat is transferred in the form of ocean currents and atmo- spheric currents. As water evaporates from the oceans, it leaves be- hind many of its impurities, including salts. As water vapor collects in clouds it is carried along by atmo- spheric currents. When conditions are right, atmo - spheric water vapor precipitates as rain, snow, sleet, and so on. Some of this precipitation falls back on the 586 • Hydrology and the hydrologic cycle Global Resources Cloud formation Rain clouds Precipitation w h i l e f a l l i n g Evaporation t r a n s pi r a t i o n f r o m v eg e t a t i o n f r o m t s r e a m s f ro m s o i l f r o m o c e a n Infiltration t r a n s p i r a t i o n Soil Percolation Zone of saturation Rock Groundwater Deep percolation Ocean S u r f a c e r u n o f f Water table The Hydrologic Cycle Source: U.S. Department of Agriculture, Yearbook of Agriculture (Washington, D.C.: Government Printing Office, 1955). oceans to begin the cycle again, butsome falls on land surfaces. Of the precipitation that falls on land surfaces, much becomes locked up in ice caps and glaciers, but some falls in the form of rain (orsnowthatmeltswhen temperatures rise). The majority of the precipitation that falls on land surfaces runs off in the form of sur- face flow, referred to as overland flow. This flow is ob- served in the complex surface drainage systems of streams, creeks, rivers, and lakes. The residence time of surface water can be as short as a few days or weeks. Surface water is a major area of study. Evaporation from surface water adds to atmospheric moisture, as does water vapor that transpires from the leaves of trees and other plants. Although the majority of precipitation takes the form of overland flow, in areas where surface soil or rock is porous and permeable, water can move down- ward into the ground by the process of infiltration. This water of infiltration becomes groundwater. Groundwater flows through void spaces in soil or rock; therefore its flow is restricted by the porosity and per- meability of the material it enters. The residence time of groundwater can be on the order ofmonths, centu- ries, or even thousandsofyears.In essence the water is stored for a time. The soil zone or rock stratum in which the water is stored is called an aquifer. Aquifers are further categorized as major or minor and as con- fined or unconfined. An unconfined aquifer, also called a water table aquifer, is said to have a water table. A confined aquifer has a potentiometric surface, or level to which water will rise, rather than a water table. Since precipitation and infiltration have seasonal variability, the height of the water table in an uncon- fined aquifer alsohas seasonal variability. There is com- plex interaction between groundwater and surface water, based on gravity and the height of the watercol- umn, expressed as hydrostatic head. An axiom is that water moves from highheadto low head. Anotherway to view this is by picturing a lake. Water tries to move from high elevation to low elevation; the ultimate level is sea level. The same is true for groundwater. In the absence of geologic complexity, the water ta- ble in an unconfined aquifer tends roughly to follow the topographic surface. This creates areas of higher hydrostatic head and areas of lower hydrostatic head, providing a gravitation impetus for groundwater flow, expressed numerically as the gradient. As groundwa - ter flows from higher elevations to lower elevations, it encounters incised streambeds that may have a base level lower than the level of the water table. In this in - stance, groundwater will discharge to the streambed, creating the base flow of the stream. In this situation, the stream is considered a gaining stream. If the in- cised bed of the stream has a higher elevationthanthe groundwater, the stream can lose surface water to groundwater by the process of infiltration; in this in- stance the stream would be considered a losing stream. Because of the seasonal variation in the water table, streams can change seasonally from gaining to losing and vice versa. Because of geologic processes, many beds of rock, or strata, are not flat. As the strata composing an aqui- fer dip away, groundwater can become confined un- der a less permeable layer such as a shale. In this type of aquifer the recharge area of the strata exposed at the surface is at a higher elevation than down-dippor- tions of the strata under the confining bed. The water table at higher elevations exerts hydrostatic pressure on the confined portion of the aquifer at lower eleva- tions. A well penetrating the confined portion of an aquifer is said to be artesian because the hydrostatic pressure causes the water column in the well to rise above the confining layer, and, in many cases, water from confined aquifers will flow to the surface. Raymond U. Roberts Further Reading Brutsaert, Wilfried. Hydrology: An Introduction. New York: Cambridge University Press, 2005. Davie, Tim. Fundamentals of Hydrology. London: Rout- ledge, 2003. Dingman, S. Lawrence. Physical Hydrology. 2d ed. Up- per Saddle River, N.J.: Prentice Hall, 2002. Fetter, C. W. Applied Hydrogeology. 4th ed. Upper Sad- dle River, N.J.: Prentice Hall, 2001. Freeze, R. Allan, and John A. Cherry. Groundwater.En- glewood Cliffs, N.J.: Prentice-Hall, 1979. Manning, John C. Applied Principles of Hydrology. Illus- trated by Natalie J. Weiskal. 3d ed. Upper Saddle River, N.J.: Prentice Hall, 1997. Ward, Andrew D., and Stanley W. Trimble. Environ- mental Hydrology. 2d ed. Boca Raton, Fla.: Lewis, 2004. Web Sites U.S. Geological Survey Water Science for Schools: The Water Cycle http://ga.water.usgs.gov/edu/watercycle.html Global Resources Hydrology and the hydrologic cycle • 587 . discussion of nat - ural resources is the effect of hydrogen-based technol - ogies on the use of more limited resources. Global Resources Hydrogen • 583 History Credit for the discovery of hydrogen. series of waterwheels and mills all constructed side by side. Artisans devised var - ied types of waterwheels and gearing to use with dif - Global Resources Hydroenergy • 579 ferent levels of available. compact storage of this high-energy fuel. Uses of Hydrogen Hydrogen is a very dense energy source in the sense that the combustion of a few grams of hydrogen in air releases a great deal of heat energy.