Irrigation

The problem of salinization

Irrigation systems

Surface irrigation

Sub-irrigation

Overhead irrigation

Resources

Irrigation is the process of bringing a water supply to a dry region especially with regards to the growing of crops. The practice of diverting water from natural resources to crops has been practiced for at least 7,000 years. The earliest methods, as practiced in places like the areas surrounding the Nile river basin, included digging channels to allow water from the river during flood periods to reach cultivated fields along the river’s banks. Ancient farmers also built dikes to help retain the water on the flooded land. Other early irrigation techniques included the construction of diversion dams and the use of machinery to lift the water and irrigate land that was higher than the flood plains. Evidence of early irrigation systems has been found in North America, South America, the Middle East, and in China and India.

Surface irrigation system techniques include surface flooding, furrow flooding, and dead-level surface flooding. In surface flooding the whole land area to be irrigated is flooded with water. This technique is good, for instance, for growing rice. Furrow flooding involves planting trees or crops between shallow trench-like channels and flooding the area. In arid regions, dead-level surface irrigation, where fields are leveled to a zero slope, is practiced.

Closed-conduit irrigation includes sprinkler systems, bubbler irrigation, and drip or trickle irrigation. Gardeners, as well as farmers, commonly use these techniques. Sprinkler systems pump water through pipes or hoses to the sprinkler, which can be fixed or mobile. Bubbler and drip systems periodically supply water to the roots of one or more plants. These systems are constructed of tubing or pipes. Drip systems deliver water slowly and are the most conservative users of water resources. They are particularly favored in arid regions, such as the southwest area of the United States, Australia, and the Middle East.

Around the beginning of the nineteenth century, there were about 20 million acres (8 million ha) of the world’s land under the use of irrigation. By the start of the twentieth century, the figure had risen to 99 million acres (41 million ha). In 2005, now in the twenty-first century, there is about 675 million acres (273 million ha) worldwide, about 18% of agriculturally productive land that is routinely irrigated. More than 60% of the irrigated land is contained within a few countries—China, India, Pakistan, the United States, and parts of the countries of the former Union of Soviet Socialist Republics (USSR). Since becoming independent in 1947, India has developed over 700 irrigation projects, more than doubling the amount of land they irrigate, which exceeded 100 million acres (41 million ha) by the late 1980s.

In China, where irrigation has been used since the third century BC, irrigated land doubled and tripled in some areas after the completion of dam projects undertaken since World War II (1939–1945). The primary irrigation crop in China is rice, but they also irrigate their wheat and cotton fields. One dam, the Tujiang on the Min River, was built around 300 BC and is still in use. Tujiang Dam is the source of water for 50,000 acres (202,000 ha) of land.

While the purpose of irrigation is to produce a better crop yield, the need for irrigation varies depending upon seasonal and climatic conditions. Some regions need crop irrigation all year, every year; some only part of the year and only in some years; and others need to irrigate only during seasons of water shortage from rainfall. In Iraq and India, for instance, irrigation is absolutely necessary in order to grow crops, since rain cannot be depended upon in those regions. In other areas, irrigation may be used only as a backup in case there is not sufficient rainfall during a crop’s growing season. This is termed supplemental irrigation.

The problem of salinization

Salinization is a major problem associated with irrigation, because deposits of salts build up in the soil and can reach levels that are harmful to crops. In addition, the salts can make ground water, which may be in use for drinking, saltier and unsuitable for drinking. It is mostly in arid and semiarid regions where the problem of high salt content deposited from irrigation threatens crops.

Drip irrigation is a technique that can be used in areas where the ground water level is high and in danger of suffering from a high salt content. Where salinization is a problem to plants, enough water can be added to the irrigation process to leach salts away from plant roots. When the danger of salinization is to the water table, it is necessary to add drainage to the irrigation system away from the water table.

Crops have different salt tolerances and their selection in relation to the salinity of the soil is an advisable practice. Among the common crops that have a high salt tolerance are red beets, spinach, kale, asparagus, sugar beets, barley, cotton, date palms, and some grasses used for animal feed, such as wild rye and wheat grass. Crops that have a low tolerance for salinity include radishes, celery, green beans, fruits such as pears, apples, oranges, grapefruit, plums, apricots, peaches, strawberries, lemons, and avocados, and a number of clovers that are used for animal grazing.

Areas in the world where farming is threatened by high salinity include the Indus Basin in Pakistan where they also face the problem of a rising water table. The Imperial Valley in California, formerly productive agricultural lands in South America, China, India, Iraq, and many other regions throughout the world are all facing the threat of losing fertile land because of salinization. After the building of the Aswan Dam in Egypt, the Nile River and the surrounding fields that had been irrigated successfully for over 5,000 years became threatened by high salinity in the water.

The main technique used to reclaim land that has developed a high salt content from irrigation is a leaching process. This is based on a careful analysis of the soil and the amount of water that must be applied to reach a level of acceptable salt content. One problem of leaching is that other nutrients needed by the crops, besides the undesirable salts, may also be leached from the soil. Consequently, nutrients often need to be replaced after an area is reclaimed from high salinization.

Irrigation systems

The planning of irrigation systems is highly specialized and requires the help of agricultural engineers who understand not only the design and construction of irrigation systems, but also farm management and mechanization, soil science, crop husbandry, and the economics of farming. The engineer’s education in these related fields is important so that he or she is able to design an irrigation system that is appropriate to the type of farming in the area that is to be irrigated.

Before an irrigation system can be built, a number of important studies must be made. Among them would be a survey of land and water resources, a study of the current uses of the area, a proposal for an irrigation system, cost estimates of the project, and a projection of its economic benefits. A large regional or national project might also include the economic and material resources for the project that are available by the particular entity, the cost of construction and administration of the project, the financing and marketing of the project to individual farmers, and the training of personnel to carry out the project.

Among the specific surveys that must be made before an irrigation system is constructed are soil, water, and topographic surveys. Sometimes critical decisions must be made about the destruction of monuments from antiquity. For instance, when the Aswan Dam was built, some important statues from ancient Egyptian culture were lost because they were covered by water. Another important consideration for building a new irrigation project is whether it will change the current farm practices, and if so, how to educate farmers to new methods. Foremost at issue is the consideration of how an irrigation system will impact the farmers and farming in the area.

Surface irrigation

In surface irrigation systems, the area to be covered with water is sloped away from the supply channel so that the water will flow over the entire area with the water moving both across the surface to be irrigated and filtering down to the root bases of the plants in the field. Among the variations of surface irrigation are the techniques of furrow irrigation, border strips, basins, and wild flooding.

Furrow irrigation has the advantage of allowing the crops to be tended shortly after watering periods. The system is useful for crops that are grown in rows that can be separated by furrows (shallow ditches) along the rows. The furrows are usually dug along the line of the slope, but sometimes they run perpendicular to it. The problem with cross cutting the furrows on a slope is that it may collapse during irrigation periods from the force of the water.

The preferred method of supplying water to the furrows is to siphon water from a main source and carry it through plastic or aluminum pipes set in a main ditch at the head of the field. Another ditch at the end of the furrows collects excess water and runs it along to lower lying fields. The best incline for furrow irrigation is one from 0 to 5% slope. Crops are usually planted on the rise between the furrows, but sometimes trees are planted at the bottom of the furrow. Since there is less water surface in this method, evaporation of water is less than in surface flooding.

In pastures where there are crops that grow closely together border strips may be used. In this system, a main ditch is constructed along the highest end of the slope and banks, called checks, which can be built as much as 70 ft (21 m) apart. Water is then siphoned from the main ditch onto the strips where the crops are grown. Sometimes the banks are replaced with border supply ditches, which allow more control over the release of water. This system is often used in research studies. In hilly areas contour ditches are built that follow the contour of a hill. They are carefully graded to control the flow of water.

For landscapes, gardens, and the watering of individual trees, the use of basins may be a suitable method of irrigation. The area to be irrigated is surrounded by banks (checks) and then watered from a main source along a high point in the basin. A drain is also placed along the major depression of a basin to allow water to run off. This system is easy to build since it requires very little movement of earth. It is usually built around the natural contours of the area. Where the land is extremely steep, an adaptation of basin irrigation called terracing can be used. Here basins are created in a step fashion along the slope of the hill. At the end of each basin step, a check is built. Basin watering is not generally recommended for flat ground.

While wild flooding is still practiced, it is not recommended by agricultural engineers because the water distribution is uneven and can lead to high saline contents in the soil and to waterlogging. The crop yields are consequently unpredictable.

Sub-irrigation

In areas where the topsoil is of high quality and porous and there is an underlay of clay soil that absorbs water slowly, conditions exist for natural sub-irrigation, provided that the water table is high. Ditches dug along the fields can be used to monitor the water level and to also replenish the water supply when it is low. Where there is little or no rainfall and the salts in the water build up on the surface of the soil, leaching is carried out. To overcome excess rain in areas where sub-irrigation systems are in use, water can be removed by pumping or using natural gravity features available in the terrain, that is, slopes and depressions in the ground.

When sub-irrigation is desired but the conditions are not available naturally, pipes with evenly distributed punctures can be buried underground. A difficulty involved with these systems is that they can be damaged when the soil is being cultivated. These systems also work by the use of natural sloping features in the terrain or by pumping water through the pipes.

Drip irrigation, which is not actually sub-irrigation, but uses some of the same principles as in sub-irrigation,

KEY TERMS

Checks —Banks used to contain water in surface irrigation systems.

Closed-conduit irrigation —Systems that use pipes to distribute water.

Emitter —A device that releases water, such as those used in drip irrigation or sprinklers used in overhead irrigation.

Furrows —Ditches running along rows of crops where water is siphoned for irrigation.

Overhead irrigation —The distribution of water above ground, as through the use of sprinklers.

Salinization —The accumulation of salt compounds in water.

Sub-irrigation —The distribution of water from below ground to plants from natural or by artificial means.

Supplemental irrigation —Periodic distribution of water to agricultural crops.

Surface irrigation —When fields are flooded with water or distributed through shallow ditches, basins, or channels.

Terracing —The creation of step-like basins on hilly ground in order to irrigate crops grown there.

delivers water slowly to the root areas of plants. Here, too, pipes are used as the channels for transporting the water and emitters are placed to water plants directly. While it is economical to use because there is little waste of water and evaporation is at a minimum, initial costs of installing drip irrigation systems are higher than other methods. There is also a tendency for emitters to become clogged by the salts in the water. Salts, however, do not build up around the roots of plants in drip irrigation systems.

Overhead irrigation

These systems use a pumping unit, conveyor pipes, and some form of sprinkler mechanism. Of all the irrigation systems, they most resemble natural rainfall. Some systems are fixed and use pipes laid on the ground with risers that have a sprinkling nozzle at the top that rotates 360 degrees. The size of the water droplets, the speed of rotation, and the evaporation rate are considerations in selecting sprinkler systems, since these all have an effect on the soil. An added use of sprinkler systems is that they can in some situations be used for frost protection.

Besides fixed systems, mobile sprinkling systems are in use in the United States and Great Britain. Portable systems use a pump at the water source to pump the water into a main line that is laid throughout the field. The sprinkler units are moved from field to field for irrigation of crops. Other mobile sprinkling systems use a device called a rain gun, which has a nozzle with a large diameter.

Resources

BOOKS

Albinson, B, and C.J. Perry. Fundamentals of Smallholder Irrigation: The Structured System Concept. Columbo, Sri Lanka: International Water Management Institute, 2002.

Deficit Irrigation Practices. Rome, UK: Food and Agriculture Organization, 2002.

Doolittle, William E. Canal Irrigation in Prehistoric Mexico. Austin: University of Texas Press, 1989.

Peterson, Gary A., Paul W. Unger, and William A. Payne, eds. Dryland Agriculture. Madison, WI: American Society of Agronomy, 2006.

Shortle, J. S., and Ronald C. Griffin, eds. Irrigated Agriculture and the Environment. Northampton, MA: Edward Elgar, 2001.

Simmers, Ian, ed. Understanding Water in a Dry Environment: Hydrological Processes in Acid and Semiarid Zones. Lisse, UK: A.A. Balkema, 2003.

Svendsen, Mark. Irrigation and River Basin Management: Options for Governance and Institutions. Wallingford, UK, and Cambridge, MA: CABI Publishing, 2005.

Vita Richman

Irrigation

IRRIGATION TECHNIQUES

SUCCESSES AND FAILURES

BIBLIOGRAPHY

Irrigation refers to techniques for augmenting the moisture content of soil to grow crops. These techniques have played an important role in intensifying agriculture, increasing production, and improving the productivity of land and labor. Early civilizations in China, Egypt, Mesopotamia, Peru, and India relied on them to support large and complex populations. One of the earliest and most successful large-scale hydraulic works was the Dujiangyan irrigation project built in the third century BCE in southwest China’s Sichuan Province. The project was designed to simultaneously solve the problem of the incessant flooding of the Minjiang River, a tributary of the Yangtze River, in the summer, and provide water during the winter when it was beset with drought. Working without a dam, the river was divided by a long bank in the middle, with the inner river serving as a channel for delivering water for irrigation and the outer river used as a floodway. This enabled the delivery of water during the dry season and the return of excess water during the winter to the mainstream of the Minjiang River. A weir made of bamboo cages filled with stones balanced the channel’s inflow. The project fed a grid of irrigation canals watering 160,000 hectares of arable land in the Chengdu Plain.

Many ancient large-scale irrigation systems were accompanied by elaborate, complex, social organization. Descriptions of these systems in the nineteenth century intrigued the social theorists Karl Marx (1818–1883) and Max Weber (1864–1920). Marx’s idea of the Asiatic mode of production, based largely on irrigation-based societies in China, included state control and collection of rents, a despotic political system, and societal organization obtained through religion, rather than economics and exchange. Village life, rather than cities, circumscribed the social spheres of people. Weber, in contrast, drew attention to the peculiar “hydraulic-bureaucratic official-state” in China and India. Although flawed due to limits in the data then available, these constructs were quite influential in the subsequent development of social theory.

Karl Wittfogel, inspired by the writings of Weber, revisited the materials and proposed in Oriental Despotism (1957) that large-scale waterworks required centralized direction, bureaucratic organization, and disciplined armies. He distinguished such “hydraulic societies” from smaller-scale “hydroagricultural societies” relying on less provident water sources in regions where geographical features hydraulically compartmentalized the countryside. Wittfogel’s hydraulic society thesis generated various criticisms, of which the most telling were examples of locally controlled and managed irrigation in societies where irrigation authority was formally centralized. More recently, Donald Worster in Rivers of Empire (1985) reinvigorated the discussion of large-scale irrigation, arguing that Wittfogel’s hydraulic societies take a somewhat different form in the modern world. In his analysis of river-based irrigation in the western United States he finds parallels between the archaic centralized regimes of ancient hydraulic societies and the centralized state agencies of the Bureau of Reclamation and the Army Corps of Engineers.

IRRIGATION TECHNIQUES

The motivation for irrigation stems from the difficulties of farming in arid and semiarid areas of the world beset with insufficient or unreliable precipitation. Where precipitation is insufficient, irrigation may be the only way farmers can supply moisture for growing crops. In areas with sufficient, but otherwise unreliable, precipitation, it can provide insurance against crop failure. And in lands with adequate moisture, irrigation can be used to grow water-intensive subsistence crops such as rice, or highvalue market crops such as sugar beets or beans.

Irrigators commonly draw on gravity to move surface water from a source through canals and furrows to fields and to store water in reservoirs and cisterns. Techniques to move surface water run the gamut from simple and unsophisticated counterbalanced poles mounted with buckets to complex, labor-intensive feats of engineering such as large-scale dams and canal systems. Surface-water irrigation often works in concert with the control and manipulation of floodwater. In one frequent form, as soon as the flood in a perennial river reaches a sufficient level, inundation canals start to flow and water is led over fields. In another, recession irrigation, a rising perennial river overflows its banks and inundates the plains alongside the river. Crops are grown on the rising or receding flow or on the residual moisture.

In spate irrigation, found throughout semiarid environments of the Middle East, North Africa, East Africa, West Asia, and parts of Latin America, seasonal water is used as a source. Seasonal floods are contained in mountain catchments or diverted from riverbeds and spread over large areas. Seasonal floodwater may last only a few hours or a few days, and sophisticated local knowledge is required to organize and manage the accumulation and distribution of the floodwater. Spate irrigation is associated with low returns to labor, great variability in productivity between good and bad years, and a high degree of social organization. Its uncertainty restricts its appeal to only the very poorest.

In areas with high water tables or surface depressions, human or animal energy has been used to raise groundwater for irrigation. Gravity usually has only limited potential in this regard. One exception is the complex “horizontal wells” that tapped subterranean aquifers through filtration galleries in the ancient Middle East (qanats ), and the Andes (puquios ). In the nineteenth century pumps driven initially by steam, and later by electricity and gasoline or diesel, were adopted to irrigate with groundwater from shallow aquifers and deep groundwater basins. The availability of small, portable, inexpensive, submersible pumps in the late twentieth century expanded groundwater irrigation dramatically. In groundwater-rich spate-irrigated areas, hydraulic infrastructure has been neglected and land use intensified through perennial cropping.

The adoption of industrial irrigation altered significantly the scale, impact, and productivity of agriculture, and its reliance on inanimate mechanical converters of energy, especially fossil fuels. Irrigation played a key role in the Green Revolution of the 1960s, helping to ensure the high yields of “miracle seeds.” It remains instrumental in realizing the potential of genetic engineering and precision agriculture, the application of space-age technologies to tailor soil and crop management to local conditions. Two techniques now in widespread use include drip irrigation, relying on emitters to release carefully calibrated amounts of water, and sprinkler irrigation, with relatively permanent or portable sprinkler systems.

Despite new and more efficient irrigation technologies, today’s demands for water produced by the Green Revolution, population growth, urbanization, and industrialization outstrip supply. Scarcity of water provokes conflicts in many areas of the world; they are endemic in the Middle East over access to the water of the Euphrates, Jordan, and Nile Rivers. At the heart of the Arab-Israeli conflict is the allocation to Israel, Jordan, the Palestinian Territories, and Syria of the water of the Jordan River, its tributaries, coastal rivers, and two aquifer systems.

SUCCESSES AND FAILURES

Although many regions in China, Egypt, Mesopotamia, and India have enjoyed continuous and sustainable irrigation for centuries, it has not always been an unmitigated success. Archaeological and historical research has uncovered significant evidence of cases of agrarian collapse due to environmental degradation and mismanagement. Poorly managed surface-water irrigation can lead to the excessive buildup of salts, exacerbated by shallow soils, water with relatively high salt content, aridity, and high water tables. For example, the Aral Sea, located in inland Central Asia, has been shrinking since the 1960s as the U.S.S.R. diverted the rivers that feed it for irrigation.

Often, inadequate drainage is at the root of the irrigation failures. Drainage does not always have to be a problem, however. Some societies have devised techniques for draining heavily waterlogged soils to levels sufficient to support intensive agriculture. Raised-bed agriculture in Lake Titicaca of southern Peru and Bolivia achieved levels of productivity comparable to irrigated land.

In the industrial era, irrigation has increased the risks of ground and surface-water pollution from the intensive use of pesticides and nitrate fertilizers. Groundwater extraction poses an additional problem; it is much more difficult to regulate than surface-water irrigation and in many regions of the world, overpumping has contributed to the drying up of aquifers and groundwater basins.

SEE ALSO Development, Rural

BIBLIOGRAPHY

Glick, Thomas. 1970. Irrigation and Society in Medieval Valencia. Cambridge, MA: Harvard University Press.

Mabry, Jonathan B., ed. 1996. Canals and Communities. Tucson: University of Arizona Press.

Watson, Andrew M. 1983. Agricultural Innovations in the Early Islamic World. Cambridge, U.K.: Cambridge University Press.

Wittfogel, Karl. 1957. Oriental Despotism. New Haven, CT: Yale University Press.

Worster, Donald. 1985. Rivers of Empire: Water, Aridity, and the Growth of the American West. New York: Pantheon Books.

David W. Guillet

Irrigation

The practice of diverting water from natural resources to crops has been practiced for at least 7,000 years. The earliest methods, as practiced in places like the areas surrounding the Nile river basin , included digging channels to allow water from the river during flood periods to reach cultivated fields along the river's banks. Ancient farmers also built dikes to help retain the water on the flooded land. Other early irrigation techniques included the construction of diversion dams and the use of machinery to lift the water and irrigate land that was higher than the flood plains. Evidence of early irrigation systems has been found in North and South America , the Middle East, and in China and India.

Surface irrigation system techniques include surface flooding , furrow flooding, and dead-level surface flooding. In surface flooding the whole land area to be irrigated is flooded with water. This technique is good, for instance, for growing rice . Furrow flooding involves planting trees or crops between shallow trench-like channels and flooding the area. In arid regions, dead-level surface irrigation, where fields are leveled to a zero slope, is practiced.

Closed-conduit irrigation includes sprinkler systems, bubbler irrigation, and drip or trickle irrigation. Gardeners, as well as farmers, commonly use these techniques. Sprinkler systems pump water through pipes or hoses to the sprinkler, which can be fixed or mobile. Bubbler and drip systems periodically supply water to the roots of one or more plants. These systems are constructed of tubing or pipes. Drip systems deliver water slowly and are the most conservative users of water resources. They are particularly favored in arid regions, such as the southwest area of the United States, Australia , and the Middle East.

There are more than 600 million acres (243 million ha) worldwide, about 17% of agriculturally productive land, that are routinely irrigated. More than 60% of the irrigated land is contained within a few countries—China, India, Pakistan, the United States, and parts of the former USSR. Since becoming independent in 1947, India has developed over 700 irrigation projects, more than doubling the amount of land they irrigate, which exceeded 100 million acres (41 million ha) by the late 1980s.

In China, where irrigation has been used since the third century b.c., irrigated land doubled and tripled in some areas after the completion of dam projects undertaken since World War II. The primary irrigation crop in China is rice, but they also irrigate their wheat and cotton fields. One dam, the Tujiang on the Min River, was built around 300 b.c. and is still in use. Tujiang Dam is the source of water for 500,000 acres (202,000 ha) of land.

While the purpose of irrigation is to produce a better crop yield, the need for irrigation varies depending upon seasonal and climatic conditions. Some regions need crop irrigation all year, every year; some only part of the year and only in some years; and others need to irrigate only during seasons of water shortage from rainfall. In Iraq and India, for instance, irrigation is absolutely necessary in order to grow crops, since rain cannot be depended upon in those regions. In other areas, irrigation may be used only as a backup in case there is not sufficient rainfall during a crop's growing season. This is termed supplemental irrigation.

The problem of salinization

Salinization is a major problem associated with irrigation, because deposits of salts build up in the soil and can reach levels that are harmful to crops. In addition, the salts can make ground water, which may be in use for drinking, saltier and unsuitable for drinking. It is mostly in arid and semiarid regions where the problem of high salt content deposited from irrigation threatens crops.

Drip irrigation is a technique that can be used in areas where the ground water level is high and in danger of suffering from a high salt content. Where salinization is a problem to plants, enough water can be added to the irrigation process to leach salts away from plant roots. When the danger of salinization is to the water table, it is necessary to add drainage to the irrigation system away from the water table.

Crops have different salt tolerances and their selection in relation to the salinity of the soil is an advisable practice. Among the common crops that have a high salt tolerance are red beets, spinach , kale, asparagus, sugar beets, barley , cotton, date palms , and some grasses used for animal feed, such as wild rye and wheat grass. Crops that have a low tolerance for salinity include radishes, celery, green beans, fruits such as pears, apples, oranges, grapefruit, plums, apricots, peaches, strawberries, lemons, and avocados, and a number of clovers that are used for animal grazing.

Areas in the world where farming is threatened by high salinity include the Indus Basin in Pakistan where they also face the problem of a rising water table. The Imperial Valley in California, formerly productive agricultural lands in South America, China, India, Iraq, and many other regions throughout the world are all facing the threat of losing fertile land because of salinization. After the building of the Aswan Dam in Egypt, the Nile River and the surrounding fields that had been irrigated successfully for over 5,000 years became threatened by high salinity in the water.

The main technique used to reclaim land that has developed a high salt content from irrigation is a leaching process. This is based on a careful analysis of the soil and the amount of water that must be applied to reach a level of acceptable salt content. One problem of leaching is that other nutrients needed by the crops, besides the undesirable salts, may also be leached from the soil. Consequently, nutrients often need to be replaced after an area is reclaimed from high salinization.

Irrigation systems

The planning of irrigation systems is highly specialized and requires the help of agricultural engineers who understand not only the design and construction of irrigation systems, but also farm management and mechanization, soil science, crop husbandry, and the economics of farming. The engineer's education in these related fields is important so that he or she is able to design an irrigation system that is appropriate to the type of farming in the area that is to be irrigated.

Before an irrigation system can be built, a number of important studies must be made. Among them would be a survey of land and water resources, a study of the current uses of the area, a proposal for an irrigation system, cost estimates of the project, and a projection of its economic benefits. A large regional or national project might also include the economic and material resources for the project that are available by the particular entity, the cost of construction and administration of the project, the financing and marketing of the project to individual farmers, and the training of personnel to carry out the project.

Among the specific surveys that must be made before an irrigation system is constructed are soil, water, and topographic surveys. Sometimes critical decisions have to be made about the destruction of monuments from antiquity. For instance, when the Aswan Dam was built, some important statues from ancient Egyptian culture were lost because they were covered by water. Another important consideration for building a new irrigation project is whether it will change the current farm practices, and if so, how to educate farmers to new methods. Foremost at issue is the consideration of how an irrigation system will impact the farmers and farming in the area.

Surface irrigation

In surface irrigation systems, the area to be covered with water is sloped away from the supply channel so that the water will flow over the entire area with the water moving both across the surface to be irrigated and filtering down to the root bases of the plants in the field. Among the variations of surface irrigation are the techniques of furrow irrigation, border strips, basins, and wild flooding.

Furrow irrigation has the advantage of allowing the crops to be tended shortly after watering periods. The system is useful for crops that are grown in rows that can be separated by furrows (shallow ditches) along the rows. The furrows are usually dug along the line of the slope, but sometimes they run perpendicular to it. The problem with cross cutting the furrows on a slope is that it may collapse during irrigation periods from the force of the water.

The preferred method of supplying water to the furrows is to siphon water from a main source and carry it through plastic or aluminum pipes set in a main ditch at the head of the field. Another ditch at the end of the furrows collects excess water and runs it along to lower lying fields. The best incline for furrow irrigation is one from 0-5% slope. Crops are usually planted on the rise between the furrows, but sometimes trees are planted at the bottom of the furrow. Since there is less water surface in this method, evaporation of water is less than in surface flooding.

In pastures where there are crops that grow closely together border strips may be used. In this system a main ditch is constructed along the highest end of the slope and banks, called checks, which can be built as much as 70 ft (21 m) apart. Water is then siphoned from the main ditch onto the strips where the crops are grown. Sometimes the banks are replaced with border supply ditches, which allow more control over the release of water. This system is often used in research studies. In hilly areas contour ditches are built that follow the contour of a hill. They are carefully graded to control the flow of water.

For landscapes, gardens, and the watering of individual trees, the use of basins may be a suitable method of irrigation. The area to be irrigated is surrounded by banks (checks) and then watered from a main source along a high point in the basin. A drain is also placed along the major depression of a basin to allow water to run off. This system is easy to build since it requires very little movement of Earth. It is usually built around the natural contours of the area. Where the land is extremely steep, an adaptation of basin irrigation called terracing can be used. Here basins are created in a step fashion along the slope of the hill. At the end of each basin step a check is built. Basin watering is not generally recommended for flat ground.

While wild flooding is still practiced, it is not recommended by agricultural engineers because the water distribution is uneven and can lead to high saline contents in the soil and to waterlogging. The crop yields are consequently unpredictable.

Sub-irrigation

In areas where the topsoil is of high quality and porous and there is an underlay of clay soil that absorbs water slowly, conditions exist for natural sub-irrigation, provided that the water table is high. Ditches dug along the fields can be used to monitor the water level and to also replenish the water supply when it is low. Where there is little or no rainfall and the salts in the water build up on the surface of the soil, leaching is carried out. To overcome excess rain in areas where sub-irrigation systems are in use, water can be removed by pumping or using natural gravity features available in the terrain, that is, slopes and depressions in the ground.

When sub-irrigation is desired but the conditions are not available naturally, pipes with evenly distributed punctures can be buried underground. A difficulty involved with these systems is that they can be damaged when the soil is being cultivated. These systems also work by the use of natural sloping features in the terrain or by pumping water through the pipes.

Drip irrigation, which is not actually sub-irrigation, but uses some of the same principles as in sub-irrigation, delivers water slowly to the root areas of plants. Here, too, pipes are used as the channels for transporting the water and emitters are placed to water plants directly. While it is economical to use because there is little waste of water and evaporation is at a minimum, initial costs of installing drip irrigation systems are higher than other methods. There is also a tendency for emitters to become clogged by the salts in the water. Salts, however, do not build up around the roots of plants in drip irrigation systems.

Overhead irrigation

These systems use a pumping unit, conveyor pipes, and some form of sprinkler mechanism. Of all the irrigation systems, they most resemble natural rainfall. Some systems are fixed and use pipes laid on the ground with risers that have a sprinkling nozzle at the top that rotates 360 degrees. The size of the water droplets, the speed of rotation , and the evaporation rate are considerations in selecting sprinkler systems, since these all have an effect on the soil. An added use of sprinkler systems is that they can in some situations be used for frost protection.

Besides fixed systems, mobile sprinkling systems are in use in the United States and Great Britain. Portable systems use a pump at the water source to pump the water into a main line that is laid throughout the field. The sprinkler units are moved from field to field for irrigation of crops. Other mobile sprinkling systems use a device called a rain gun, which has a nozzle with a large diameter.

Resources

books

Crouch, Dora P. Water Management in Ancient Greek Cities. New York: Oxford University Press, 1993.

Doolittle, William E. Canal Irrigation in Prehistoric Mexico. Austin: University of Texas Press, 1989.

Guillet, David. Covering Ground: Communal Water Management and the State in the Peruvian Highlands. Ann Arbor: University of Michigan Press, 1992.

Kluger, James R. Turning on Water with a Shovel: The Career of Elwood Mead. Albuquerque: University of New Mexico Press, 1992.

Shortle, J. S., and Ronald C. Griffin, eds. Irrigated Agriculture and the Environment. Northampton, MA: Edward Elgar, 2001.

Wallace, Henry A. Henry A. Wallace's Irrigation Frontier: On the Trail of the Corn Belt Farmer. Norman: University of Oklahoma, 1991.

other

American Society of Agricultural Engineers. National Irrigation Symposium: Proceedings of the Fouth Decennial Symposium. November 14-16, 2000.

Vita Richman

KEY TERMS

Checks

—Banks used to contain water in surface irrigation systems.

Closed-conduit irrigation

—Systems that use pipes to distribute water.

Emitter

—A device that releases water, such as those used in drip irrigation or sprinklers used in overhead irrigation.

Furrows

—Ditches running along rows of crops where water is siphoned for irrigation.

Overhead irrigation

—The distribution of water above ground, as through the use of sprinklers.

Salinization

—The accumulation of salt compounds in water.

Sub-irrigation

—The distribution of water from below ground to plants from natural or by artificial means.

Supplemental irrigation

—Periodic distribution of water to agricultural crops.

Surface irrigation

—When fields are flooded with water or distributed through shallow ditches, basins, or channels.

Terracing

—The creation of steplike basins on hilly ground in order to irrigate crops grown there.

IRRIGATION

IRRIGATION, the delivery of water to grow crops, has been a factor in North American society and agriculture since long before the existence of the United States. Mostly practiced in the arid western regions of the country, its expansion in the twentieth century dramatically altered the national landscape and food production.

Possibly as early as a.d. 300, the Hohokam erected the first large-scale irrigation systems in the area that later became the southwestern United States. Although their rawhide and basket tools were simple and their dams small by modern standards, these indigenous societies maintained thousands of acres under irrigation for centuries. The Papago and Pueblo nations later practiced similar techniques, though they generally irrigated only smaller fields near arroyo mouths and seasonal stream-beds. The coordinated efforts to construct and maintain this sophisticated infrastructure required these early irrigators

to develop political institutions and tribal affiliations larger than those of their hunter-gatherer neighbors.

Spanish and Mexican settlers in New Mexico created similar irrigation systems to support their own agriculture. Many of their villages and fields were actually built around an acequia madre or "mother ditch," and they boasted well-articulated lines of command and labor expectations to maintain the ditch. This social and physical system still existed in some of these villages at the end of the twentieth century.

The Mormon settlers arriving in the Salt Lake area in the 1840s drew on these precedents in the erection of their own irrigation networks. Using the cooperative religious institutions that characterized their society, by 1850 they grew such diverse crops as potatoes, wheat, hay, and oats on more than sixteen thousand irrigated acres.

The westward expansion of the United States in the nineteenth century brought the regions where irrigation was needed to practice extensive agriculture under American control. At first, however, migrating Americans were slow to recognize the challenge that aridity posed to their traditional agricultural practices. At less than twenty inches a year, the average rainfall west of the one-hundredth meridian—roughly the line that runs north and south through the middle of the states of Texas, Oklahoma, Kansas, Nebraska, and the Dakotas—is just below the amount needed to grow wheat, and ten inches less than that needed by corn.

The enormous challenge of the arid West was initially difficult to recognize. During the 1880s, when thousands of farmers settled on the Great Plains, rainfall was significantly above average, in some cases twice as heavy as the long-term pattern. Farmers and policymakers were thus lulled into a false sense of security. One theory even held that the plowing of so much virgin territory had in fact fundamentally changed the natural patterns of rainfall, increasing precipitation to facilitate the conquest of the continent.

Private efforts to irrigate the arid regions of the West met with very limited success. Irrigation was generally outside the reach of individual farmers for the simple reason that it required the control of large stretches of rivers and streams and the erection of sizable dams for storage. In the 1870s and 1880s, private land companies entered the irrigation business, constructing dams, building extensive canal systems, and then selling nearby lands to farmers who would remain dependent on the companies for their water. High capital costs, however, constrained these efforts. Only the most opportune sites were irrigated, the total acres under irrigation soon stagnated, and by 1900 nearly nine out of ten of these irrigation companies were in financial jeopardy.

The Federal Role

The failure of private efforts created an opening for those who thought that the federal government should build massive irrigation works. John Wesley Powell, a pioneering scientist and ethnographer who headed the United States Geological Survey, had made the most radical proposals in this regard. Surveying the lands of the arid West in the 1870s, Powell came to the conclusion that the country's model for the settlement of newly acquired territory was deeply flawed. Extinguishing the public domain by giving settlers 160-acre tracts (under the provisions of the Homestead Act) might work where enough rain fell to grow crops, but the development of the West hinged on water rather than land. Since very little of the West could be farmed in the traditional way, the government, Powell believed, should divide the region by watershed. Much like the Mormons—whose communal irrigation made quite an impression on Powell—settlers should govern themselves by watershed, forming a cooperative to raise the capital for the necessary irrigation network. Irrigated farms, more productive and labor intensive, would be smaller than farms back east, probably about 80 acres. Unirrigated lands, which he thought would always comprise the vast majority of the West, would be reserved for ranching in large tracts of 2,500 or more acres.

Powell's vision was at once too radical and too modest to gain the political support it needed to be implemented. Western boosters were enraged by his assumption that little of the region's land was fit for agriculture, and even fewer were willing to accept the drastic revision in territorial laws for which his watershed proposal called. After the failure of private irrigation in the West, the form that federal intervention took was much more modest. The 1902 Newlands Reclamation Act created the Bureau of Reclamation, a federal agency charged with building dams, reservoirs, and irrigation canals for the benefit of private farmers. The West, in other words, was meant to resemble the East, with a little more help from the federal government.

The Newlands Act married conservation's technical expertise with its emphasis on antimonopoly. Farmers were to repay the construction costs through annual charges for their water. Individuals could buy water for a maximum of only 160 acres. The Newlands Act thus extended the provisions of the Homestead Act, seeking to create egalitarian farming communities with dispersed land ownership.

The Bureau of Reclamation was remarkably successful in its goal of irrigating the West. Whereas in 1906, fewer than thirty thousand acres west of the one-hundredth meridian were under irrigation, by 1992 that number had skyrocketed to more than 45 million. The Bureau of Reclamation, the Army Corps of Engineers, and other federal agencies erected more than one thousand dams in the West. These massive structures not only provided water for crops, but also generated much of the electricity that lit the region's cities and towns. For decades, the politics of irrigation proved irresistible. The construction of dams,

aqueducts, and canals created numerous jobs, and the lands that they opened up for agriculture benefited real estate speculators and the local tax rolls alike. While the New Deal saw a significant expansion in the scope of these projects, the fact that their ultimate goal was to support private agriculture kept them attractive to more antistate politicians.

In the 1930s, affordable pumps and low-cost electricity opened up a new dimension in irrigation: ground-water pumping. By 1970, such pumping watered more than 40 percent of the nation's irrigated acreage, most of it on the Great Plains. This irrigation differed from federal projects in that it drew upon generally unrenewable aquifers and was easily affordable by individual farmers.

Social and Environmental Issues

Such intensive irrigation, however, generated its own social and environmental effects. From its inception, the Bureau of Reclamation operated much differently than its founders had envisioned. Very few irrigation projects were actually paid for by their beneficiaries, and so these public works quickly became subsidies. The proliferation of modest homesteads that had been so important to justify giving the federal government primary responsibility for irrigation never came to be. Land speculators bought much of the land where they anticipated dams might be built, and the bureau showed little interest in enforcing its 160-acre limit on what came to be some of the most powerful political interests in the West. In most areas served by the bureau's projects, it was actually impossible to purchase small tracts of land. The high productivity and costs of irrigated lands meant that such agriculture tended to be more market-oriented, more mechanized, and to employ more migrant labor than elsewhere. In practice, then, irrigation helped to solidify the dominance of large-scale agribusiness in the West.

Environmental problems have increasingly limited the effectiveness of irrigation and reduced its public support. The damming of most of the West's major rivers has decimated their salmon runs. Natural river flows have been dramatically altered. The Colorado River, for example, once mighty enough to carve the Grand Canyon, was so heavily drawn on for irrigation that it did not reach the Pacific from 1964 to 1983. Proposals to build further dams on the Colorado sparked an environmental backlash as early as the 1950s. The buildup of silt behind reservoir walls quickly became a problem; by 2000, most reservoirs built before 1945 had lost from 7 to 15 percent of their capacity. Salinization, the accretion of salt in perpetually water-logged soil, puts thousands of acres out of production each year. Groundwater pumping on the Great Plains seemed headed for extinction, with the aquifer predicted to dry up within a few decades.

No large federal irrigation projects were approved from the late 1970s to the turn of the century. In part this was because so many of the most feasible dam sites had been taken, but the loss of support for federal irrigation also reflected the growing political power of more environmental-minded urbanites. Nevertheless, irrigation continues to be a decisive force in American agriculture and the landscape of the West.

BIBLIOGRAPHY

Hundley, Norris, Jr. The Great Thirst: Californians and Water—A History. Rev. ed. Berkeley: University of California Press, 2001.

Pisani, Donald J. To Reclaim a Divided West: Water, Law, and Public Policy, 1848–1902. Albuquerque: University of New Mexico Press, 1992.

Reisner, Marc. Cadillac Desert: The American West and Its Disappearing Water. Rev. ed. New York: Penguin, 1993.

Walton, John. Western Times and Water Wars: State, Culture, and Rebellion in California. Berkeley: University of California Press, 1992.

Worster, Donald. Rivers of Empire: Water, Aridity, and the Growth of the American West. New York: Pantheon, 1985.

Benjamin H.Johnson

See alsoAgriculture ; Agriculture, American Indian ; Reclamation ; Water Supply and Conservation .

Irrigation

Irrigation, the artificial watering of crops. Much of Latin America receives insufficient precipitation for farming. To expand and improve arable land, pre-Hispanic horticulturalists had developed water management systems by the first millennium bce in both Mesoamerica and the Andes. The simplest method, pot irrigation, is familiar to all gardeners and leaves few, if any, archaeological traces. Water is carried by hand and applied to individual plants. Very effective use is made of limited amounts of water, but labor demands are high, so early farmers began to devise other methods of redirecting water's natural flow.

In central Mexico, rubble or earthen storage dams may have been built across natural channels to collect ephemeral surface runoff at the site of Teopantecuanitlán, in northern Guerrero State between 1200 and 1000 b.c., and in the Tehuacán Valley, between 750 and 600 b.c. However, the earliest undisputed remains of a Mexican irrigation system are at Santa Clara Coatitlán, now within metropolitan Mexico City. Here unlined canals branched off a channelized gully and took water to fields from about 900 b.c. Near the present city of Puebla, canal networks carried water from ephemeral stream channels by the middle of the first millennium b.c. In Tlaxcala State, there is evidence that terraced fields were watered by a dam, reservoir, and canal system around the same time.

A canal fed from a dammed reservoir is found just below the archaeological site of Monte Albán, Oaxaca. Part of this canal is cut into bedrock, demonstrating that its builders had the theoretical knowledge to plan water flow without trial and error. The Monte Albán canal carried water from about 550 to 250 b.c. By a.d. 200, mortared masonry storage dams and aqueducts were functioning near Monte Albán.

By 1519, when Hernán Cortés arrived in the Valley of Mexico, hydraulic works had reached monumental proportions. Tenochtitlán, the Mexica (Aztec) capital, occupied an island in Lake Tetz-coco. The Mexica redirected rivers and constructed aqueducts, dams, dikes, and open canals. Mortared aqueducts carried sweet water from springs at Chapultepec (later a park in Mexico City) and Coyoacán across the lake to Tenochtitlán, where it was used for drinking, bathing, and watering gardens.

The Peruvian and north Chilean coasts receive little or no precipitation, so agriculture is dependent upon irrigation. Dozens of small rivers and streams, many seasonally dry, flow down the western slopes of the Andes, through the coastal deserts, and into the Pacific. Pre-Hispanic farmers drew water directly off these rivers; directed water in long-distance canals from higher, moister elevations; diverted spring flow; and dug wells and trenches down to the water table. The early prehistory of Andean irrigation is unclear, but simple canals were probably built as early as the second millennium b.c. to extend floodwater farming of coastal valleys.

On Peru's north and central coasts, attempts were made in the mid-first millennium a.d. to build open canal irrigation networks supplied by two or more rivers. However, some of the long-distance linkages, including a 71-mile canal joining the Chi-cama and Moche drainages, may never have functioned. In the Chilca Valley, and elsewhere on Peru's central coast, ancient farmers dug a few meters to the water table, creating moist sunken fields called mahames or hoyas. This practice extended agriculture into areas without surface water.

The Andean Central Highlands receive unreliable precipitation during a wet season from November to April. To extend the growing time, or to produce two crops per year, agriculturalists made extensive use of open canals combined with terraces. These techniques were well established by the mid-first millennium a.d., the apogee of the Wari state. By 1500 the Incas had brought basic irrigation to near perfection. Water was so important to the lords of Cuzco that they conceptualized their whole social structure in terms of its natural and artificial flow.

In parts of Latin America, waterlogged or flooded fields are a major problem for farmers. Both in lowland Mesoamerica and the Lake Titicaca basin, indigenous cultivators built extensive systems of ridges and furrows for planting and water containment. Some Maya cities and the early Andean state of Tiwanaku depended heavily upon such raised fields. However, construction of some Andean raised fields predated the rise of the Tiwanaku state by more than one thousand years. Remnants of chinampas, or "floating gardens," can still be observed in the Valley of Mexico. These are artificial planting platforms constructed and maintained in shallow lakes.

The aridity of Spain and Portugal stimulated the development of Iberian irrigation and made the conquistadores appreciative of the sophisticated indigenous hydraulic systems they encountered in the New World. Native waterworks and concomitant social structure were often retained. However, the requirements of sugarcane, water mills, and horses meant that Iberians needed more water than the pre-Conquest Indians. The most remote branches of canals were often abandoned to provide greater volume and flow elsewhere. This has preserved remnants of ancient systems, especially in the valleys of Peru's north coast. The Spanish introduced many Old World hydraulic devices, including the arched aqueducts that were a hallmark of ancient Roman technology. Examples can still be seen in Mexico near Zempoala and Hidalgo and at the city of Querétaro; in Peru at Cuzco; and in Brazil at Rio de Janeiro.

Another Iberian introduction may have been filtration galleries, also called minas, ganats, foggaras, or puquois. Such galleries are lines of vertical wells linked at their bottoms by slightly sloping tunnels with the main water outlet at the tunnel's mouth. The largest and best-studied Spanish water mina is in Madrid. It was begun in the early thirteenth century under Christian control. Spanish engineers built several filtration gallery systems in Mexico, including those at Tehuacán and Puebla. Dozens of others were constructed in the Andes during the viceregal period, but many scholars believe that the systems in the Nasca drainage were built in the first millennium a.d. by people of the indigenous Nasca culture.

Latin American agriculture remains dependent upon irrigation. Ambitious, high-tech projects have become almost impossible to finance in the wake of the 1980s debt crisis. However, in remote areas farmers continue to employ the agricultural technology of their pre-Columbian and Spanish ancestors.

A major change since the late 1980s has been the privatization of water resource management. The World Bank and other critics have accused Latin American governments of mismanaging water resources. Yet, many groups in Ecuador, Peru, and Bolivia have successfully protested water privatization schemes, as exacerbating social and economic inequality. A vigorous debate in the early twenty-first century continues over whether water and irrigation projects should be public or private or if a third, moderate, approach can be achieved.

See alsoAgriculture; Potato.

BIBLIOGRAPHY

For an excellent survey of ancient Mexican hydraulic technology see William E. Doolittle, Canal Irrigation in Prehistoric Mexico (1990). A study of the important prehistoric, colonial, and modern Tehuacán system can be found in Kjell I. Enge and Scott Whiteford, The Keepers of Water and Earth: Mexican Rural Social Organization and Irrigation (1989). A good general work on the central Andes is Michael E. Moseley, The Incas and Their Ancestors: The Archaeology of Peru (1992). A beautifully illustrated pioneering study of ancient Andean waterworks is Paul Kosok, Life, Land, and Water in Ancient Peru (1965). An influential article on the social organization governing canals is Patricia J. Netherly, "The Management of Late Andean Irrigation Systems on the North Coast of Peru," in American Antiquity 49, no. 2 (1984): 227-254. Sunken fields have been described in Ana María Soldi, Chacras excavadas en el desierto (1979). A general discussion of raised fields can be found in Clark L. Erickson, "Prehistoric Landscape Management in the Andean Highlands: Raised Field Agriculture and Its Environmental Impact," in Population and Environment 13, no. 4 (1992): 285-300. For a controversial study of Andean filtration galleries see Monica Barnes and David Fleming, "Filtration-Gallery Irrigation in the Spanish New World," in Latin American Antiquity 2, no. 1 (1991): 48-68; an opposing point of view is Ronald I. Dorn et al., "New Approach to the Radiocarbon Dating of Rock Varnish, with Examples from Drylands," in Annals of the Association of American Geographers 82 (1992): 136-151. A solid book on highland irrigation, with emphasis on present practice, is William P. Mitchell and David Guillet, Irrigation at High Altitudes: The Social Organization of Water Control Systems in the Andes (1994).

Additional Bibliography

Aboites, Luis. El agua de la nación: Una historia política de México (1888–1946). Mexico City: Centro de Investigaciones y Estudios Superiores en Antropología Social, 1998.

Bennett, Vivienne, Sonia Dávila-Poblete, and Nieves Rico. Opposing Currents: The Politics of Water and Gender in Latin America. Pittsburgh, PA: University of Pittsburgh Press, 2005.

Eakin, Hallie Catherine. Weathering Risk in Rural Mexico: Climatic, Institutional, and Economic Change. Tucson: University of Arizona Press, 2006.

Gelles, Paul H. Water and Power in Highland Peru: The Cultural Politics of Irrigation and Development. New Brunswick, NJ: Rutgers University Press, 2000.

                                    Monica Barnes