are developed from systems of law, tradition, and cus - tom that vary in important ways across countries, re- gions, and time periods. Consequently, how water rights are defined varies as well, depending on a num- ber of economic, social, political, and cultural factors. American definitions of water rights contain elements of English common law and French civil law. Background To govern rights regarding surface waters, two funda- mental systems of water law have emerged over time: riparian law and appropriative law. Water rights are defined quite differently under the two systems. The key issues concerning the definition of water rights are how they are acquired, how they are maintained over time (or conversely, lost), and to what precisely the owner has title. Riparian Law Where the legal system is based either on civil law or on traditional English common law, surface water rights are based largely on riparian principles. Such regions include France, England, and the eastern United States. Under riparian regimes, water rights are based on ownership of adjacent land, which enti- tles the claimant to the use of the water rather than ac- tual ownership of a specific quantity of water. In most cases, that use must not unreasonably infringe upon the rights of other claimants to the same source. Fur- thermore, the water may not be diverted for use on nonriparian lands. All riparians along a waterway en- joy coequal status, in the sense that all share in the benefits, and all must absorb the hardships inflicted by shortages when they occur. Riparian rights are maintained over time simply by maintaining ownership of the adjacent land. It is im- portant that riparian rights are typically not forfeited if the water is not actually being used. Riparian rights may, however, be condemned for public uses if a pub- lic entity exercises its right of eminent domain. In ad- dition, a riparian right may be forfeited if the riparian does not contest an upstream diversion that inflicts damages. In such a case, the upstream diverter may gain a so-called prescriptive right. Contesting such a diversion doesnotguaranteethat itwillbe disallowed: The court may permit it anyway if it decides that the competing use is more valuable. Voluntary transfers of rights between riparians are effected simply by sell - ing theriparianlands. Riparian rightscanin principle be transferred to nonriparian uses, but this typically requires that other riparians along the waterway not be adversely affected. On most waterways, this pre- sents an insurmountable obstacle to such transfers in practice. Appropriative Law In the western United States, where water is generally scarce, a very different system predominates; it is based on the doctrine of prior appropriation. Under appropriative law, rights to surface waters are ac- quired for specific quantities, which may be diverted for use on nonriparian land. Furthermore, appropri- ative rights are issued on a first-come, first-served ba- sis. That is, claimants earlier in time (senior appropri- ators) acquire rights superior to those of subsequent claimants (junior appropriators). The practical im- port of this “first-in-time, first-in-right” principle is that junior appropriators bear the brunt of water shortages, as their allocations are the first to be re- duced. In contrast to riparian rights, appropriative rights are maintained through actual use, typically with the requirement that such use not unreasonably damage other claimants. This“use it or lose it”provision some- times induces appropriators to use water unnecessar- ily for fear of losing their right. As with riparians, appropriative rights may be condemned for public uses or forfeited if the appropriator fails to protest a damaging diversion by a competing claimant. Finally, in principle, appropriative rights may be transferred voluntarily. In practice, however, many states have im- posed restrictions on transfers that adversely affect other appropriators, particularly interbasin transfers or ones that cross state lines. It should be mentioned that the courts have on occasion struck down restric- tions on the exportation of water imposed by individ- ual states because such restrictions were deemed to vi- olate the“commerce clause”of the U.S.Constitution. Groundwater Rights The legal rulesgoverninggroundwater rights parallel those governing surface water. In regions where ap- propriative law governs surface water rights, it typically governs groundwater rights as well; again, use must be reasonable given the competing needs of other claim- ants. In such regions, aheavierburden is placed on ju- nior claimants, who are often liable for any damages inflicted on claimants with senior rights. In states where riparian law governs surface water rights, no such temporal priority governs groundwater use, and 1316 • Water rights Global Resources simple ownership of overlying lands typically confers the right to pumpgroundwater. However, suchpump- ing is typically regulated, and pumpers may be liable for damages inflicted on other claimants drawing from the same aquifer. As with appropriative regimes, groundwater use must be reasonable given compet- ing claims for groundwater from the same source. Some states have gone further and mandated specifi- cally that groundwater use must be correlative, mean- ing that during periods of scarcity, all groundwater us- ers overlyinganaquifer must reducetheir use of water proportionately. Indian In the and Federal Reserved Water Rights United States, Indian and federal reserved rights constitute another type of water right that does not fit into any category discussedso far. Indianwaterrights are aspe- cial kind ofright accruingonly to Indian tribeson fed- eral reservations, mainly for purposes of irrigation. Like appropriative rights, they possess a priority date, namely, when the reservation was established. Since many reservations were established long ago, Indian rights are often quite senior in the appropriative rights hierarchy. Unlikeappropriative rights, however,the In- dian right is not specifically quantified but rather de- pends upona vague notion ofthe “practicably irrigable acreage” contained in the reservation. Furthermore, Indians need not actually use the right in order to maintain it. The basic reason is that Indian rights are considered to have been reserved by the federal gov- ernment for use by Indians when they established In- dian reservations. Similar reasoning has been applied to federal reservations of water for other uses, espe- cially environmental uses such as the establishment of wilderness areas, national parks, or wildlife refuges. Interstate Water Rights In the United States, many conflicts over water rights have occurred among users in different states, be- cause many rivers and groundwater aquifers cross state lines. The water rights of competing users in dif- ferent statesare governedby thedoctrine of equitable apportionment, which was first enunciated by the Global Resources Water rights • 1317 The California aqueduct transports water from the Sierra Nevada to Southern California, a procedure that upsets many residents of North - ern and Central California. The issue of water rights in the West has been and continues to be contentious. (Getty Images) U.S. Supreme Court in the landmark case Kansas v. Colorado (1907). Equitable apportionment superseded earlier notions that states enjoyed absolute sover- eignty overall water resourcesoriginating within their borders. Under equitable apportionment, states must share equitably in the benefits derived from interstate watercourses, regardless of where the water originates and irrespective of the particular water law regimes which have been adopted by the individual states. This notion serves as a legal basis for both interstate compacts and international conventions that have been written to allocate and resolve conflicts over water resources. International Water Rights Superimposed upon systems of water law that hold within countries is an amorphous system of interna- tional law that more or less governsinternationalcon- flicts over water. By broad international agreement, most countries have consented to abide by certain water allocation principles based on a notion of lim- ited territorial sovereignty. Specifically, countries have agreed in principle to cooperate to divide interna- tional water resources equitably and to abide by the rule that water use within a country should not cause substantial damage to other countries. These general principles are supplemented with bilateral or multi- lateral agreements that apply to the particular river system in question. These agreements can be as for- mal as treaties, which are subject to ratification by the respective parties, or as informal as joint statements of principle. In cases where countries cannot reach agreement, disputes may be submitted to the Interna- tional Court of Justice or to tribunals specially consti- tuted to resolve a particular dispute. The rulings of the court, while nominally binding on the parties to the dispute, have no force of precedent for future cases, ensuring that disputes are resolved on a case-by- case basis. Mark Kanazawa Further Reading Caponera, Dante A. Principles of Water Law and Admin- istration: National and International. 2d ed. Revised and updated by Marcella Nanni. New York: Taylor & Francis, 2007. Dunbar, Robert G. Forging New Rights in Western Waters. Lincoln: University of Nebraska Press, 1983. Getches, David H. Water Law in a Nutshell. 4th ed. St. Paul, Minn.: Thomson West, 2009. Getzler, Joshua. A History of Water Rights at Common Law. New York: Oxford University Press, 2004. Hodgson, Stephen. Modern Water Rights: Theory and Practice. Rome: Foodand Agriculture Organization of the United Nations, 2006. Johnson, John W. United States Water Law: An Introduc- tion. Boca Raton, Fla.: CRC Press, 2009. McCaffrey, Stephen C. The Law of International Water- courses. 2d ed. New York: Oxford University Press, 2007. McCool, Daniel. Command of the Waters: Iron Triangles, Federal Water Development, and Indian Water. Berke- ley: University of California Press, 1987. Reprint. Tuscon: University of Arizona Press, 1994. Sax, Joseph L., et al. Legal Control of Water Resources: Cases and Materials. 4th ed. St. Paul, Minn.: Thom- son West, 2006. Sprouole-Jones, Mark, Carolyn Johns, andB. Timothy Heinmiller, eds. Canadian Water Politics: Conflicts and Institutions. Montreal: McGill-Queen’s Univer- sity Press, 2008. Web Site U.S. Geological Survey Water Science for Schools http://ga.water.usgs.gov/edu/ See also: Drought; Irrigation; Streams and rivers; Takings law and eminent domain; United States; Water. Water supply systems Category: Obtaining and using resources A watersupply system is the collection of interconnected structures and machinery that permit the delivery of water of adequate quantity and acceptable quality to a home, public or private building, or industry. As a community grows in size, the importance of its water supply system becomes more apparent because itsignifi- cantly affects future economic growth, the quality of life, and the general cost of living. Background A water supply system is composed of several vital components. These components, in their order within the system, include sources of water for the system, 1318 • Water supply systems Global Resources such as streams and rivers (called surface water) and aquifers (groundwater); reservoirs or primary storage structures; pumps and conveyance channels (aque- ducts, pipelines, and open channels) to water treat- ment or conditioning facilities; treatment or condi- tioning facilities; pumps and conveyance channels to intermediate storage reservoirs from treatment facili- ties; and pumps and pressurized water distribution facilities to the end user. The actual components in a given water supply sys- tem depend on whether treatment is necessary and whether pumps are needed to boost pressure and the supply flow rate in the system. For example, when water is withdrawn from groundwater sources, it may not need to be treated, so the water supplysystem may not include treatment facilities or intermediate stor- age facilities. Considerations for Planning a Water Supply System Generally, water supply systems can be categorized as publicly owned or privately owned. Privately owned water supply systems refer to those that are developed by private companies through the sale of stocks and bonds. Publicly owned systems are funded through taxes paid by the citizens of a community (usually a city or county) and are under the jurisdiction of the government of the community. Publicly owned systems significantly outnumber private ones in the United States and serve a major part of the popula- tion. Because only one water supply system can practi- cally be developed and operated to serve a given com- munity, it constitutes a monopoly. It is thus subject to governmental regulation. Regulation can be by water pollution control boards, public utility commissions, or the county or city government. Some external in- fluence isexertedby safetyandinsurance groups such as the National Board of Fire Underwriters, the Insur- ance Services Office, the American Insurance Associ- ation, and the U.S. Environmental Protection Agency (EPA). Other professional groups, such as the Ameri- can Society of Civil Engineers (ASCE), the American Water Works Association (AWWA), and the Ameri- can Water Resources Association (AWRA), influence the design and construction of water supply systems through published design and public safety guide- lines and codes. In the planning and eventual construction of a water supply system, a number of issues must be con - sidered. First, the population of the community at the projected end of the service life of the system is esti - mated. The future population is used to project the future demand forwater in the community. The prod- uct of the future population and the projected aver- age water use per person provides a good estimate of future demand. In addition, since water supplies are designed to meet demand 95 percent of the time, a generally acceptable risk of 5 percent failure (not meeting demand, or drought) every year is often used. If this risk level is too high, a larger water supply reservoir must be provided. Second, sources of water of adequate quantity and acceptable quality must be identified. In particular, the chemical, physical, and biological properties of the available water should be established. Third, a storage reservoir must be de- signed, including the conveyance system from the res- ervoir to the community. Fourth, water treatment fa- cilities that match the water quality characteristics of the raw water supply and the water quality require- ments of the community must be designed. Fifth, a plan ofthedistribution network anditslayout must be developed. In this regard, the following components of the distribution system must be considered: water distribution reservoirs, pumps, size of water mains, lo- cation of fire hydrants (considering required pres- sures as dictated by appropriate guidelines and ordi- nances), and the size and optimal location of elevated storage tanks. Water Uses Adequate water supply is imperative in a well-func- tioning modern community. Water is used for numer- ous domestic, public, and industrial activities. Irriga- tion is critical to agriculture and therefore is a major consumptive use of water. According to the United Nations Educational, Scientific and Cultural Organi- zation World Water Assessment Programme the total world consumptive use of water for irrigation is ap- proximately 2,500 cubic kilometers, or 70 percent of the total world consumption. The consumptive use for domestic and municipal purposes is about 285 cubic kilometers. A comparable number for indus- trial uses is about 785 cubic kilometers. Domestic and municipal uses of water include drinking, food preparation, washing, cleaning, sanitation, watering lawns and gardens, service industry requirements (for example, restaurants, pools, medical services, and laundry facilities), and public buildings and facilities (churches, parks, civic buildings, schools, and col - leges). Global Resources Water supply systems • 1319 Water use is affected by many factors. It varies from community to community and depends on climate, environmental awareness and concerns, population characteristics, level and type of industrialization, stan- dard of living, and the availability of water. Water use also varies seasonally, daily, weekly, and hourly. About 80 percent of industrial water usage is for cooling pur- poses and hence is often returned without much deg- radation in chemical or biological quality; it also does not need to be of very high quality. Water conservation programs have become popu- lar because significant water savings can be achieved from well-planned and -administered conservation efforts, primarily involving incentives and the basic education of a community in conservation strategies. Some programs target large apartment complexes and provide water-saving devices at subsidized prices. It has been reported that when drought conditions have caused a community to reduce water use, volun- tary waterconservation has reducedwater use by 10to 40 percentwithout muchhardship to thecommunity. Water Quality Water quality is measured by the quantification of sev- eral parameters that are used as indicators. These pa- rameters include, but are not limited to, the amount of dissolved oxygen, the biochemical oxygen demand (which measures the demand for oxygen, indicating pollutional potential), the concentration of solids (suspended and dissolved solids), nitrogen content (not dissolved nitrogen, but ammonia nitrogen and organic nitrogen),and bacteriological measurements that attempt to establish the potential presence or ab- sence of pathogenic bacteria and viruses. Sizing of Water Reservoirs Because of fluctuations in the quantity of surface water suppliesfrom astream, providinga storage facil- ity so thatanticipated demand for watercan be met re- liably is necessary. A reservoir that is sufficiently large to provide dependable water supplies is required. To determine the size of the reservoir needed to achieve a certain level of reliability, a method called the “mass curve analysis” is often used. This method involves summing the total historical flows in the stream at the site of the proposed reservoir and then plotting these cumulative flows against time (usually months or years). On the same graph, the proposed cumula - tive demand is plotted. The maximum vertical depar - ture of one curve from the other is the size of the reservoir that will guarantee withdrawals at a rate equal to the proposed demand used in the analysis. Water Distribution Networks The network consists of the machinery and conduits that permit the delivery of water from the source or sources of water to the user. Water distribution sys- tems may deliver water by gravity alone, by pumps alone, or by pumps and intermediate storage reser- voirs. Gravity distribution systems are possible only when the water supply source is located at a substan- tially higher elevation than the community. Pumping with storage is the most common type of water distri- bution network. The layout of a network can be a branched or tree- like system, a grid or looped system, or a combination branched and looped network. A grid or looped sys- tem is the preferred configuration because water can be supplied to a given point from at least two direc- tions. A treelike system often has many dead ends and is not very satisfactory because of stagnation of water at its extremities. Also, repairs in a segment may result in the disruption of service to a large portion of the network. Networks consisting of intermediate or distribu- tion reservoirs (elevated tanks) provide storage to meet fluctuations in use, provide storage reserves for firefighting, and provide a means for stabilizing pres- sures in the distribution system. Typically, these stor- age tanksshould be locatedat the approximatecenter of the system and should be located at higher eleva- tions. Physically, thenetwork is madeup of pipes(consist- ing of arterial or primary lines and secondary lines), pumps, valves, hydrants, and tanks. In some cases, pressure-regulating valves are used to segment the network into several pressure zones. Recommended water supply network pressures assure that the system can be used for firefighting and that moderately tall buildings can be serviced without booster pumps. Ad- equate water pressures also help to safeguard the water from infiltration and possible pollution. The American Water Works Association recommends pres- sures of about 400 to 500 kilopascals. Emmanuel U. Nzewi Further Reading American Water Works Association. Water Transmis - sion and Distribution. 3d ed. Denver, Colo.: Author, 2003. 1320 • Water supply systems Global Resources Bhave, P.R., and R.Gupta. Analysis of Water Distribution Networks. Oxford, England: Alpha Science Interna- tional, 2006. Linsley, Ray K., et al. Water Resources Engineering. 4th ed. New York: McGraw-Hill, 1992. McGhee, Terence J. Water Supply and Sewerage. 6th ed. New York: McGraw-Hill, 1991. May, Larry W., ed. Water Distribution Systems Handbook. New York: McGraw-Hill, 2000. Purcell, Patrick J. Design of Water Resources Systems. Lon- don: Thomas Telford, 2003. Swamee, Prabhata K., and Ashok K. Sharma. Design of Water Supply Pipe Networks. Hoboken, N.J.: Wiley- Interscience, 2008. Viessman, Warren, et al. Water Supply and Pollution Control. 8th ed. Upper Saddle River, N.J.: Pearson/ Prentice Hall, 2009. Weiner, Ruth F., and Robin A. Matthews. Environmen- tal Engineering. 4th ed. Boston: Butterworth-Heine- mann, 2003. See also: Dams; Groundwater; Hydrology and the hydrologic cycle; Irrigation; Los Angeles Aqueduct; Streams and rivers; Water; Water rights. Watt, James Category: People Born: January 19, 1736; Greenock, Renfrewshire, Scotland Died: August 25, 1819; Heathfield Hall, near Birmingham, Warwick, England Watt’s improvements to the steam engine created an im- portantsourceof power forthe Industrial Revolution. Biographical Background As a boy, James Watt worked in his father’s shop, where he learned woodworking, metalworking, smithing, instrument making, and model making. By the age of eighteen, he had opened his own instru- ment-making shop in Glasgow, and he received an appointment as instrument maker to the University of Glasgow in 1757. Impact on Resource Use In 1764, Watt was asked to repair a model of a Newcomen steam engine in use as a classroom dem - onstrator. Upon examining the engine, he saw that the engine wasted a great amount of energy because the cylinder had to be completely heated and then completely cooled on each cycle. Watt pondered this problem for many months, and he spoke with several noted heat theorists before realizing the problem could be solved by drawing the steam from the cylin- der into a separate vessel in which it would be con- densed. The separate condenser, as it was known, would be kept permanently cool, while the cylinder was kept permanently hot, thus eliminating the need to reheat the cylinder on each stroke. Watt patented this invention in 1769 and then continued to invent ways to improve the steam engine’s efficiency. He also thought of ways to convert the steam engine’s recipro- cating motion to the smooth rotary motion necessary to powerthe machinery ofthe comingindustrial age. Brian J. Nichelson See also: Industrial Revolution and industrialization; Steam engine; Steel; Textiles and fabrics; Under- ground mining. Weather and resources Categories: Ecological resources; environment, conservation, and resource management Weather systems are a pervasive aspect of the natural environment. Impacts of weather on resources range from soil and beach erosion to the health of forests and livestock. Access to resources can be limited by weather, and the wind and precipitation that weather systems bring are important resources themselves. Background Weather is the state of the atmosphere at a given place and time as described by parameters such as air tem- perature, pressure, moisture content, wind direction and speed, visibility, sky conditions, and the occur- rence of phenomena such as rain, snow, and thunder- storms. In contrast, climate is a more long-term de- scription of the average weather and its expected variability for a particular location or region. Changing weather conditions are caused by moving weather systems that form in, and cansubsequently al - ter, atmospheric circulations of many spatial and tem - poral scales. These circulations serve a purpose in the Global Resources Weather and resources • 1321 Earth-atmosphere system. Like their oceanic counter - part, atmospheric circulations systematically trans- port fluids (air in the case of the atmosphere, and water in the case of the oceans) of varying energy con- tent in an attempt to balance the uneven distribution of solar energy. In so doing, weather systems can have notable impacts on natural resources and on the hu- man ability to obtain resources. In addition, weather itself canbe considereda resourcein several regards. Impact of Weather on Resources Soil erosion, an ongoing natural process, is consid- ered to be a problem when it happens too quickly. Weather systems exacerbate the erosion of topsoil when strong surface winds scour bare land and when heavy rains or rapid snowmelt produce runoff. Ero- sion by winds, a global concern, turned the American Plains region into the Dust Bowl in the 1930’s. In the late twentieth century, satellite-borne sensors tuned to detect atmospheric aerosols as a method of tracing airflow showed a notable plume of dust moving west- ward off Africa. Moving water is an equally efficient cause of soil erosion. Heavy rainfall can quickly produce gullies and ravines, even on gently sloping surfaces. Sedi- ment loading of the runoff can lead to siltation of res- ervoirs and increases the need for filtration at indus- trial water intakes. Contaminants other than silt can also be introduced to waterways.Duringthe extensive 1993 Midwest flooding along the Mississippi River, it was expected that the floodwaters would dilute any pesticides washed in from agricultural lands. How- ever, the concentration of pesticides proved to be higher than typically observed. Coastal erosion can be greatly accelerated by land- falling hurricanes and extratropical cyclones that bring both strong winds and heavy precipitation. The northeastern coast of the United States is occasion- ally battered by low-pressure systems known as “nor’- easters.” These storms are named for the damaging northeasterly winds found on the north side of the storm. The wind speeds in this sector are especially strong because they experience reduced friction in traveling over the Atlantic Ocean and can strike land 1322 • Weather and resources Global Resources Dust storm approaching Stratford, Texas, town and fields, 1935. (NOAA) with near hurricane force. One of the most destruc - tive nor’easters in U.S. history struck on March 7, 1962, producing waves more than 10 meters high and causing more than $300 million in property damage along 1,000 kilometers of the Atlantic coast. Weather systems also have an impact on forest re- sources. Forest fires are frequently initiated by light- ning from storms. Lightning is a particular problem in arid regions, where the layer of air between the cloud base and the surface, known as the subcloud layer, may be so dry that any rain falling from the storm evaporates before reaching the ground. Thus, many thunderstorms cannot extinguish fires they have started. Another problem formed in and transported by weather systems is acid precipitation, which has been associated with killing or reducing forests in both Europe and North America. Excessive heat and high relative humidity can place dangerous stress on livestock. This is particularly true for animalsthat are confinedor in transport.Hogsare especially susceptible, because they lose the majority of their body heat through respiration. The heat in- dex (also know as the apparent temperature), though developed as a measure of human discomfort, works equally well as an indicator of danger to confined live- stock. Weather systems can also have less obvious yet sig- nificant impacts on natural resources. The tropo- pause is the interface between the troposphere and stratosphere and is typically a smooth transition zone between these layers. However, the tropopause has been observed to fold back on itself and extrude downward during the formation of fronts in the mid- dle and upper troposphere. This phenomenon, known as tropopause folding, is accompanied by a flow of stratospheric air into the troposphere and a compensating flow of tropospheric air into the strato- sphere. While there is little or no mixing between these layers under typical conditions, tropopause folds can produce significant mixing. Folds are believed to be associated with the rapid development of intense surface low-pressure systems as well as with the down- ward transport of ozone and radioactive debris (from past airborne nuclear tests) into the troposphere and the upward transport of ozone-destroying chlorofluo- rocarbons into the stratosphere. Weather and Access to Resources Adverse weather conditions can increase the costs as - sociated with resource extraction. One location where weather is a primary consideration is in the oil fields of the North Sea. This stretch of ocean is one of the world’s roughest bodies of water and is subject to se- vere storms. Waves have reached 30 meters in height. Constructionand operation of oil rigs can be both ex- pensive and hazardous. Drilling is normally feasible for only about 175 days per year. Rough weather re- quires that equipment be exceptionally robust, and installations that transport oil and gas must be well protected. In spite of these difficulties, development in this region continues—partly because procedures for resource extraction that succeed under the ex- treme conditions of the North Sea generally prove more than adequate for developments in other parts of the world. Weather as a Resource Weather systems themselves can be viewed as a re- source or as providing resources. This aspect of weather can perhaps be best illustrated by consider- ing water and energy. Much of the world’s population is directly depen- dent upon rainfall and snowmelt for its drinking water as well as for irrigation. Weather systems on all scales contribute to the natural purification of ocean water through evaporation, and they facilitate its transport away from the source region and its even- tual deposition in the form of rain and snow. In the United States, the majority of the water that falls as rain and snow east of the Rocky Mountains originates from evaporationoff the Gulfof Mexico. Weather pat- terns that disrupt the poleward transport of moist air can result in extended droughts, as was observed in the centralUnited States duringthe summerof 1988. In semiarid agricultural regions where irrigation may not be a viable option, cloud seeding has been used in attempts to produce precipitation in clouds that might otherwise dissipate. Often the limiting fac- tor in the production of precipitation in a cloud out- side the tropics is an adequate amount of ice in the cloud. Clouds with a mixture of ice and liquid water can produce precipitation, since air that is unsatu- rated with respect to liquid water is oversaturated with respect to ice. As a result, water molecules will evapo- rate from the liquid cloud droplets and deposit them- selves on the ice crystals. In clouds lacking ice for this transfer, the cloud droplets simply evaporate, and the cloud dissipates. Cloud seeding in clouds with subfreezing tempera - tures is accomplished in one of two ways. In the first Global Resources Weather and resources • 1323 method, the cloud is cooled by adding dry ice (solidi - fied carbon dioxide) in an attempt to convert super- cooled water (liquid water below the freezing point) to ice. In the second, a substance such as silver iodide that promotes the formation of ice crystals by provid- ing freezing nuclei is introduced. The effectivenessof cloud-seeding experiments is very difficult to measure, but results suggest that an increase in precipitation of about 10percent canbe expected ifclouds areseeded. The kinetic energy of the wind can be extracted by modern wind machines and converted to electricity. The greatest number of operating wind machines in the United States were once in California; however, Texas has become the leader in wind-power capacity. Energy derived from wind greatly increased in the first decade of the twenty-first century, and countries such as Denmark, Spain, and Germany garnered sig- nificant amounts of their total energy production through wind power. Alan C. Czarnetzki Further Reading Ahrens, C. Donald. Meteorology Today: An Introduction to Weather, Climate, and the Environment. 9th ed. Belmont, Calif.: Brooks/Cole, Cengage Learning, 2009. Cotton, William R., and Roger A. Pielke. Human Im- pacts on Weather and Climate. 2d ed. New York: Cam- bridge University Press, 2007. Cowie, Jonathan. Climate Change: Biological and Hu- man Aspects. Cambridge, England: Cambridge Uni- versity Press, 2007. Frederick, Kenneth D., and Norman J. Rosenberg, eds. Assessing the Impacts ofClimate Change on Natural Resource Systems. Boston: Kluwer Academic, 1994. Gautier,Catherine. Oil, Water,and Climate: AnIntroduc- tion. New York: Cambridge University Press, 2008. Leary, Neil, et al., eds. Climate Change and Vulnerability. Sterling, Va.: Earthscan, 2008. Morhardt, J. Emil, ed. Global Climate Change and Natu- ral Resources: Summaries of the 2007-2008Scientific Lit- erature. Claremont, Calif.: Roberts Environmental Center Press, Claremont McKenna College, 2008. _______. Global Climate Change and Natural Resources: Summaries of the 2008-2009 Scientific Literature. Claremont, Calif.: Roberts Environmental Center Press, Claremont McKenna College, 2009. Schneider, Stephen H., and Terry Root, eds. Wildlife Responses to Climate Change: North American Case Studies. Washington, D.C.: Island Press, 2002. Solomon, Allen M., and Herman H. Shugart, ed. Vege - tation Dynamics and Global Change. New York: Chap- man & Hall, 1993. See also: Acid precipitation; Atmosphere; Biosphere; Climate and resources; Drought; Dust Bowl; Floods and flood control; Hydrology and the hydrologic cy- cle; Water; Weathering; Wind energy. Weathering Category: Geological processes and formations Weathering comprises the processes that decompose and disintegrate earth materials at or near the Earth’s surface.Weathering occurs becauserock or soilis physi- cally or chemically unstable in the surface environ- ment and is therefore subject to attack. Weathering continues until minerals achieve equilibrium with prevailing environmental conditions. If those condi- tions change, however, a new episode of weathering may begin. Background The term “weathering” derives from the fact that it is the elements of weather—for example, heat and rain- water—that are the agents of weathering. Definitions of weathering generally include only those processes that precede erosion and transport of materials from the site of weathering. Weathering is the initial step in the production of sediments, soils, and sedimentary rocks from preexisting rock material. The majority of rock materials weathered and eroded from the Earth’s surface in the past now reside below the sur- face in sedimentary rocks. Types of Weathering All types of weathering phenomena fall into two broad categories: physical weathering and chemical weath- ering. Physical and chemical weathering produce dif- ferent results. Physical weathering simply generates smaller fragments of the original materials. Chemical weathering often significantly changesa rock’s miner- alogy, by either dissolving minerals or altering them. Physical, or mechanical,weathering breaks up earth materials in three ways: increased pressure, release of pressure, and abrasion. If water freezes within the pores in a rock, expansion of the water may cause the 1324 • Weathering Global Resources rock to fracture, a process known as frost wedging. Other processes that cause physical weathering by increasing pressure include thermal expansion, root growth, and salt crystal formation. Removal of overly- ing materials by erosion, or unloading, releases pres- sure on underlying rocks, causing them to expand and fracture.Abrasion occurs whenrocks grind against or strike one another—for example, at the base of a gravel-filled stream channel. Physical weathering does not alter the composition of earth materials; it merely causes them to disaggregate. This also makes more surface area available to chemical weathering. Chemical weathering either chemically alters or dissolves components of earth materials. For exam- ple, feldspar, a common mineral, produces the clay mineral kaolinite if altered by hydrolysis, a decompo- sition reaction involving water; liquid water dissolves soluble minerals such as halite (salt), a process known as “solution.” As with physical weathering, a variety of chemical weathering processes exist. The most com- mon include hydrolysis, solution, oxidation (addition of oxygen to a mineral), and hydration (addition of water to a mineral). Products of Weathering Weathering produces two general types of materials: chemical or mineral solutions, and mechanical sedi- ments. Chemical weathering produces mineral solu- tions, which consist of solids or gases dissolved in water. Fresh water, which some people mistakenly call “pure” water, actually consists of water and an assort- ment of chemical weathering products. If a glass of tap water is allowed to evaporate, a thin layer of chem- ical sediments will accumulate in the bottom of the glass—a process known as precipitation, or chemical sedimentation. Evaporation of a glass of seawater pro- duces a thicker layer of chemical sediment because of its higher dissolved mineral content (salinity). Dis- solved minerals may precipitate out to form a mineral deposit, may chemically react with (weather) other mineral solids, or may remain in solution for long pe- riods of time. A combination of chemical and physical weather- ing produces most mechanical sediments. Mechani- cal, or clastic, sediments range in size from clay par- ticles (smaller than 1 256 millimeter) to boulders (smaller than 256 millimeters) and include the fol- lowing intermediate grain sizes: silt ( 1 256 to 1 16 milli - meter), sand ( 1 16 to 2 millimeters), pebbles (2 to 64 millimeters), and cobbles (64 to 256 millimeters). Sediments may remain in place at the site of weather - ing for long periods of time or be transported else- where bygravity, wind, water,or ice. If mechanicalsed- iments accumulate at the surface, they form soil and undergo further weathering. If deeply buried, chemi- cal solutions buried along with the sediments pre- cipitate minerals into the pores between the grains, cementing the sediments together. Thus, when com- bined in the subsurface, chemical and physical weath- ering products form sedimentary rocks. Rates of Weathering Numerous factors control the rate at which rocks weather. Two of the most important factors are cli- mate and mineralogy. Of all climatic factors, precipi- tation and temperature affect the rate of weathering most. Water is by far the most important agent of chemical weathering; without it, chemicalweathering effects are minimal. Humid regions therefore experi- ence higher rates of chemical weathering. Warm tem- peratures also generally produce more rapid rates of chemical weathering. Overall, the most rapid rates of chemical weathering occur in humid, tropical re- gions, whereas the slowest rates occur in cold areas with low rainfall, such as the polar regions. However, any arid region will have relatively low rates of chemi- cal weathering. Because of the absence of chemical weathering, physical weathering dominates arid landscapes. Bro- ken, angular rock fragments often cover the ground surface.However, physicalweathering rates reach their maximum where climatic conditions favor frost forma- tion and therefore frost wedging. As a result, physical weathering is mostsignificant in subpolar regions and at high altitudes in middle to low latitude regions. Minimal rates of physical weathering occur in humid, tropical regions where rapid rates of chemical weath- ering discourage physical weathering activities. The mineral composition of a rock is also integral to weathering rates. Rocks that contain an abun- dance ofchemically unstable mineralsquickly decom- pose and disintegrate when exposed to the atmo- sphere. Because of its dissolved oxygen content and slight acidity, rainwater readily attacks most mineral grains. For example, upon exposure at the Earth’s surface, iron- and manganese-rich (ferromagnesian) minerals such as olivine (FeMgSiO 4 ) and pyroxene (FeMgSi 2 O 6 ) quickly oxidize, and rainwater easily dis - solves calcite (CaCO 3 ). However, quartz (SiO 2 ) effec - tively resists chemical weathering in most settings and Global Resources Weathering • 1325 . Court of Justice or to tribunals specially consti- tuted to resolve a particular dispute. The rulings of the court, while nominally binding on the parties to the dispute, have no force of precedent. eventual construction of a water supply system, a number of issues must be con - sidered. First, the population of the community at the projected end of the service life of the system is esti - mated and resources Categories: Ecological resources; environment, conservation, and resource management Weather systems are a pervasive aspect of the natural environment. Impacts of weather on resources