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Irrigation

Irrigation

] Irrigation (in agriculture) is the replacement or supplementation of rainfall with water from another source in order to grow crops. In contrast, agriculture that relies only on direct rainfall is sometimes referred to as dryland farming.

Overview

The water source for irrigation may be a nearby or distant body of liquid or frozen water such as a river, spring, lake, aquifer, well, or snowpack. Depending on the distance of the source and the seasonality of rainfall, the water may be channelled directly to the agricultural fields or stored in reservoirs or cisterns for later use. In addition, the "harvesting" of local rain that falls on the roofs of buildings or on nearby unfarmed hills and its use to supplement the rain that falls directly on farmed fields also involves irrigation. Various types of irrigation techniques differ in how the water obtained from the source is distributed within the field. In general, the goal is to supply the entire field uniformly with water, so that each plant has the amount of water it needs, neither too much nor too little.

Types of irrigation

Flood (furrow) irrigation

Ditches can be dug with hand tools, turned with a plow pulled by an animal or tractor, or precisely fashioned using laser-guided instruments depending on economic and physical factors such as the size of the field, the types of technology available, and the cost of manpower. Plants are grown in raised beds or listed rows. Water is distributed throughout the field via canals, unlined ditches, or furrows, between the rows or beds by use of rigid gated plastic or aluminum pipe, layflat plastic with holes punched at each furrow, concrete or plastic lined ditches, or unlined ditches. Where ditches are used, siphon tubes move water from the main ditch to the furrow. When pipes are used, water flow can be controlled by turning it on or off at the local source or by using automatic or manually controlled gates to transfer it from one set of ditches to another. Unless the field is small or very level, parts of it may suffer from water-logging while other parts may be too dry. Depending on heat, wind, and soil permeability, much water may be lost before it can benefit the plants. Automatic valves, also known as surge valves, can increase the efficiency of furrow irrigation because they alternately wet the furrows and allow the soil infiltration rate to slow prior to using the furrow for actual irrigation. Once common in the United States, many ditch irrigation systems have been replaced because of high labor costs and increasing demands on water resources. Furrow irrigation also has a tendency to raise the water table in some areas and cause soil salination, requiring drainage. These types of systems are still common in other parts of the world.

Terracing

Terracing is a form of irrigation in which large steps are cut into hillsides and supported by stone or concrete walls. The level parts are used as garden plots or small fields. As water flows down the hillside it is channelled to each plot (probably most often by ditch irrigation). Terracing is usually very labor-intensive, since fields are small and access to them may be steep and narrow making it difficult to mechanize the work. In addition, the walls need constant maintenance, especially in rainy climates. However, terracing does allow steep mountainsides to be used to grow plants (although it may be more cost-effective to use them only for animal pasturage).

Overhead (sprinkler) irrigation

animal pasturage]] In overhead or sprinkler irrigation, water is piped to one or more central locations within the field and distributed by overhead high-pressure sprinklers or guns or by lower-pressure sprays. A system utilizing sprinklers, sprays, or guns mounted overhead on permanently installed risers is often referred to as a solid-set irrigation system. Some sprinklers can also be hidden below ground level, if aesthetics is a concern, and pop up in response to increased water pressure. This type of system is commonly used in lawns, golf courses, cemeteries, parks, and other turf areas. Sprinklers that spray in a fixed pattern are generally called sprays or spray heads. Sprays are not usually designed to operate at pressures above 30 lbf/in² (200 kPa), due to misting problems that may develop. Higher pressure sprinklers that rotate are called rotors and are driven by a ball drive, gear drive, or impact mechanism. Rotors can be designed to rotate in a full or partial circle. Guns are similar to rotors, except that they generally operate at very high pressures of 40 to 130 lbf/in² (275 to 900 kPa) and flows of 50 to 1200 US gal/min (3 to 76 L/s), usually with nozzle diameters in the range of 0.5 to 1.9 inches (10 to 50 mm). Guns are used not only for irrigation, but also for industrial applications such as dust suppression and logging. Sprinklers may also be mounted on movable platforms connected to the water source by a hose. At the high-tech end, computerized, automatically moving wheeled systems may irrigate large areas unattended. At the low end, such as in a small greenhouse or landscape, a person may be watering each plant individually with a hose end sprinkler or even a watering can. One drawback of overhead irrigation is that much water can be lost because of high winds or evaporation, and irrigating the entire field uniformly can be difficult or tedious if the system is not properly designed. Water remaining on plants' leaves may promote fungal and other diseases. If fertilizers are included in the irrigation water, plant leaves can be burned, especially on hot, sunny days. Overhead irrigation is generally the best solution for watering lawns and golf courses, although drip irrigation is gaining in popularity in some lawn applications. (See also center pivot irrigation.) Manually assembled systems of piping that are broken down to permit tillage and harvesting are sometimes called "hand set" or "hand move pipe". These are also commonly used on athletic fields where permanently installed sprinklers or outlets are not desired or where low initial costs are a factor. harvesting

Center pivot irrigation

Central pivot irrigation is a form of overhead irrigation consisting of several segments of pipe (usually galvanized steel or aluminum) joined together and supported by trusses, mounted on wheeled towers with sprinklers positioned along its length. The system moves in a circular pattern and is fed with water from the pivot point at the center of the arc. These systems are common in parts of the United States where terrain is flat. Most center pivot systems now have drops hanging from a u-shaped pipe called a gooseneck attached at the top of the pipe with sprinkler heads that are positioned a few feet (at most) above the crop, thus limiting evaporative losses. Drops can also be used with drag hoses or bubblers that deposit the water directly on the ground between crops. The crops are planted in a circle to conform to the center pivot. This type of system is known as LEPA (Low Energy Precision Application). Low Energy Precision Application Originally, most center pivots were water powered. These were replaced by hydraulic systems (T-L) and electric motor driven systems (Lindsay, Reinke, Valley). Most systems today are driven by an electric motor mounted at each tower. Center pivot equipment can also be configured to move in a straight line, where the water is pulled from a central ditch. In this scenario, the system is called a linear move irrigation system.

Lateral move (Side roll, Wheel line) irrigation

A series of pipes, each with a wheel of about 1.5 m diameter permanently affixed to its midpoint and sprinklers along its length, are coupled together at one edge of a field. Water is supplied at one end using a large hose. After sufficient water has been applied, the hose is removed and the remaining assembly rotated either by hand or with a purpose-built mechanism, so that the sprinklers move 10m across the field. The hose is reconnected. The process is repeated until the opposite edge of the field is reached. This system is less expensive to install than a center pivot, but much more labor intensive to operate, and it is limited in the amount of water it can carry. Most systems utilize 4 or 5 inch diameter aluminum pipe. One feature of a lateral move system is that it consists of sections that can be easily disconnected. They are most often used for small or oddly-shaped fields, such as those found in hilly or mountainous regions, or in regions where labor is inexpensive.

Drip, or trickle irrigation

Low Energy Precision Application :See main article at drip irrigation Water is delivered at or near the root zone of plants, drop by drop. This type of system can be the most water-efficient method of irrigation, if managed properly, since evaporation and runoff are minimized. In modern agriculture, drip irrigation is often combined with plastic mulch, further reducing evaporation, and is also the means of delivery of fertilizer. The process is known as fertigation. Deep percolation, where water moves below the root zone, can occur if a drip system is operated for too long of a duration. Drip irrigation methods range from very high-tech and computerized to low-tech and relatively labor-intensive. Lower water pressures are usually needed than for most other types of systems, with the exception of low energy center pivot systems and surface irrigation systems, and the system can be designed for uniformity throughout a field or for precise water delivery to individual plants in a landscape containing a mix of plant species. Although it is difficult to regulate pressure on steep slopes, pressure compensating emitters are available, so the field does not have to be level. High-tech solutions involve precisely calibrated emitters located along lines of tubing that extend from a computerized set of valves. Both pressure regulation and filtration to remove particles are important. The tubes are usually black (or buried under soil or mulch) to prevent the growth of algae and to protect the polyethylene from degradation due to ultraviolet light. But drip irrigation can also be as low-tech as a porous clay vessel sunk into the soil and occasionally filled from a hose or bucket. Subsurface drip irrigation has been used successfully on lawns, but it is more expensive than a more traditional sprinkler system. Surface drip systems are not cost-effective (or esthetically pleasing) for lawns and golf courses.

Subirrigation

Used in commercial greenhouse production, usually for potted plants, water is delivered from below, absorbed upwards, and the excess collected for recycling. Typically, a solution of water and nutrients floods a container or flows through a trough for a short period of time, 10-20 minutes, and is then pumped back into a holding tank for reuse. Subirrigation requires fairly sophisticated, expensive equipment and management. Advantages are water and nutrient conservation, and labor-saving through lowered system maintenance and automation. It is similar in principle and action to subsurface drip irrigation. The same concept of subsurface flooding and drainage is also being experimented with as an outdoor subirrigation method.great system of irrigation is on indus river

How an irrigation system works

Most commercial and residential irrigation systems are "in ground" systems, which means that everything is buried in the ground. With the pipes, sprinklers, and irrigation valves being hidden, it makes for a cleaner, more presentable landscape without garden hoses or other items having to be moved around manually. The beginning of a sprinkler system is the water source. This is usually a tap into an existing (city) water line or a pump that pulls water out of a well or a pond.

History of irrigation

Evidence exists of irrigation in Mesopotamia and Egypt as far back as the 6th millennium BC. There is also evidence of ancient Egyptian pharaohs of the twelfth dynasty using the natural lake of the Fayûm as a reservoir to store surpluses of water for use during the dry seasons, as the lake swelled annually as caused by the annual flooding of the Nile. Ancient visitors reported the appearance of "an artificial excavation, as reported by classic geographers and travellers" ([http://www.newadvent.org/cathen/05329b.htm CATHOLIC ENCYCLOPEDIA: Egypt: I. GENERAL DESCRIPTION: Flora and Agriculture]). In the Zana Valley of the Andes Mountains in Peru, archaeologists found remains of 3 irrigation canals radiocarbon dated from the 4th millennium BC, the 3rd millennium BC and the 9th century. These canals are the earliest record of irrigation in the New World. Traces of a canal possibly dating from the 5th millennium BC were found under the 4th millenium canal.(Dillehay, et al., 2005) By the middle of the 20th century, the advent of diesel and electric motors led for the first time to systems that could pump groundwater out of major aquifers faster than it was recharged. This can lead to permanent loss of aquifer capacity, decreased water quality, ground subsidence, and other problems. The future of food production in such areas as the North China Plain, the Punjab, and the Great Plains of the US is threatened.

Problems in irrigation


- Competition for surface water rights.
- Depletion of underground aquifers.
- Ground subsidence (e.g. New Orleans, Louisiana)
- Buildup of toxic salts on soil surface in areas of high evaporation.

See also


- Aquifer
- Evapotranspiration
- Geohydrology
- Gezira Scheme
- Groundwater
- Irrigation in Saudi Arabia
- Paddy Field
- Qanat

References


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- PMID 16284247

External links


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- Category:Agriculture Category:Agronomy Category:Water Category:Environmental science ja:灌漑 simple:Irrigation

Agriculture

working the land in the traditional way, with horse and plough]] Agriculture is the process of producing food, feed, fiber and other desired products by the cultivation of certain plants and the raising of domesticated animals (livestock). The practice of agriculture is also known as "farming", while scientists, inventors and others devoted to improving farming methods and implements are also said to be engaged in agriculture. More people in the world are involved in agriculture as their primary economic activity than in any other, yet it only accounts for four percent of the world's GDP.

Overview

GDP, Indonesia]] Agriculture can refer to subsistence agriculture, the production of enough food to meet just the needs of the farmer/agriculturalist and his/her family. It may also refer to industrial agriculture, (often referred to as factory farming) long prevalent in "developed" nations and increasingly so elsewhere, which consists of obtaining financial income from the cultivation of land to yield produce, the commercial raising of animals (animal husbandry), or both. Agriculture is also short for the study of the practice of agriculture—more formally known as agricultural science. Agricultural students are known (sometimes derisively) as "Aggies". Increasingly, in addition to food for humans and animal feeds, agriculture produces goods such as cut flowers, ornamental and nursery plants, timber or lumber, fertilizers, animal hides, leather, industrial chemicals (starch, sugar, ethanol, alcohols and plastics), fibers (cotton, wool, hemp, and flax), fuels (methane from biomass, biodiesel) and both legal and illegal drugs (biopharmaceuticals, tobacco, marijuana, opium, cocaine). Genetically engineered plants and animals produce specialty drugs. In the Western world, the use of gene manipulation, better management of soil nutrients, and improved weed control have greatly increased yields per unit area. At the same time, the use of mechanization has decreased labour requirements. The developing world generally produces lower yields, having less of the latest science, capital, and technology base. Modern agriculture depends heavily on engineering and technology and on the biological and physical sciences. Irrigation, drainage, conservation and sanitary engineering, each of which is important in successful farming, are some of the fields requiring the specialized knowledge of agricultural engineers. Agricultural chemistry deals with other vital farming concerns, such as the application of fertilizer, insecticides (see Pest control), and fungicides, soil makeup, analysis of agricultural products, and nutritional needs of farm animals.Plant breeding and genetics contribute additionally to farm productivity. Advanced seed engineering has allowed strains of seed to become perfect in every farming situation. Seeds can now germinate faster and adapt to shorter growing seasons in different climates. Present-day seed can resist the spraying of pesticides that kill all green-leaf plants. Hydroponics, a method of soilless gardening in which plants are grown in chemical nutrient solutions, may help meet the need for greater food production as the world's population increases. The packing, processing, and marketing of agricultural products are closely related activities also influenced by science. Methods of quick-freezing and dehydration have increased the markets for farm products (see Food preservation; Meat packing industry). Mechanization, the outstanding characteristic of late 19th and 20th century agricultural evolution, has eased much of the backbreaking toil of the farmer. More significantly, mechanization has enormously increased farm efficiency and productivity (see Agricultural machinery). Animals, including horses, mules, oxen, camels, llamas, alpacas, and dogs; however, are still used to cultivate fields, harvest crops and transport farm products to markets in many parts of the world. Airplanes, helicopters, trucks and tractors are used in agriculture for seeding, spraying operations for insect and disease control, Aerial topdressing, transporting perishable products, and fighting forest fires. Radio and television disseminate vital weather reports and other information such as market reports that concern farmers. Computers have become an essential tool for farm management. Aerial topdressing] According to the National Academy of Engineering in the US, agricultural mechanization is one of the 20 greatest engineering achievements of the 20th century. Early in the century, it took one American farmer to produce food for 2.5 people, where today, due to engineering technology (also, plant breeding and agrichemicals), a single farmer can feed over 130 people [http://www.greatachievements.org/greatachievements/ga_7_2.html]. This comes at a cost, however, of large amounts of energy input, from unsustainable, mostly fossil fuel, sources. Animal husbandry means breeding and raising animals for meat or to harvest animal products (like milk, eggs, or wool) on a continual basis. In recent years some aspects of industrial intensive agriculture have been the subject of increasing discussion. The widening sphere of influence held by large seed and chemical companies, meat packers and food processors has been a source of concern both within the farming community and for the general public. There has been increased activity of some people against some farming practices, raising chickens for food being one example. Another issue is the type of feedgiven to some animals that can cause Bovine Spongiform Encephalopathy in cattle. There has also been concern because of the disastrous effect that intensive agriculture has on the environment. In the US, for example, fertilizer has been running off into the Mississippi for years and has caused a dead spot in the Gulf of Mexico, where the Mississippi empties. Intensive agriculture also depletes the fertility of the land over time and the end effect is that which happened in the Middle East, were some of the most fertile farmland in the world was turned into a desert by intensive agriculture. The patent protection given to companies that develop new types of seed using genetic engineering has allowed seed to be licensed to farmers in much the same way that computer software is licensed to users. This has changed the balance of power in favor of the seed companies, allowing them to dictate terms and conditions previously unheard of. Some argue these companies are guilty of biopiracy. Soil conservation and nutrient management have been important concerns since the 1950s, with the best farmers taking a stewardship role with the land they operate. However, increasing contamination of waterways and wetlands by nutrients like nitrogen and phosphorus are of concern in many countries. Increasing consumer awareness of agricultural issues has led to the rise of community-supported agriculture, local food movement, slow food, and commercial organic farming, though these yet remain fledgling industries.

History

organic farming Archaeobotanists have traced the selection and cultivation of specific food plant characteristics, such as a semi-tough rachis and larger seeds, to just after the Younger Dryas (about 9,500 BC) in the early Holocene in the Levant region of the Fertile Crescent. Limited anthropological and archaeological evidence both indicate a grain-grinding culture farming along the Nile in the 10th millennium BC using the world's earliest known type of sickle blades. There is even earlier evidence for conscious cultivation and seasonal harvest: grains of rye with domestic traits have been recovered from Epi-Palaeolithic (10,000+ BC) contexts at Abu Hureyra in Syria, but this appears to be a localised phenomenon resulting from cultivation of stands of wild rye, rather than a definitive step towards domestication. It is not until ca. 8,500 BC, in middle-Eastern cultures referred to as Pre-Pottery Neolithic B (PPNB), where there is the first definite evidence for the emergence of a widespread subsistence economy that was dependent on domesticated plants and animals. In these contexts lie the origins of the eight so-called founder crops of agriculture: firstly emmer wheat, einkorn wheat, then hulled barley, pea, lentil, bitter vetch, chick pea and flax. These eight crops occur more or less simultaneously on PPNB sites in this region, although the consensus is that wheat was the first to be sown and harvested on a significant scale. There are many sites that date to between ca. 8,500 BC and 7,500 BC where the systematic farming of these crops contributed the major part of the inhabitants' diet. From the Fertile Crescent agriculture spread eastwards to Central Asia and westwards into Cyprus, Anatolia and, by 7,000 BC, Greece. Farming, principally of emmer and einkorn, reached northwestern Europe via southeastern and central Europe by ca. 4,800 BC (see, among others, Price, D. [ed.] 2000. Europe's First Farmers. Cambridge University Press; Harris, D. [ed.] 1996 The Origins and Spread of Agriculture in Eurasia. UCL Press). Europeing an alfalfa field]] The reasons for the earliest introduction of farming may have included climate change, but possibly there were also social reasons (e.g. accumulation of food surplus for competitive gift-giving). Most certainly there was a gradual transition from hunter-gatherer to agricultural economies after a lengthy period when some crops were deliberately planted and other foods were gathered from the wild. Although localised climate change is the favoured explanation for the origins of agriculture in the Levant, the fact that farming was 'invented' at least three times, possibly more, suggests that social reasons may have been instrumental. In addition to emergence of farming in the Fertile Crescent, agriculture appeared by at least 6,800 BC in East Asia (rice) and, later, in Central and South America (maize, squash). Small scale agriculture also likely arose independently in early Neolithic contexts in India (rice) and Southeast Asia (taro). Southeast Asia. Baked clay. Field Museum]] Full dependency on domestic crops and animals (i.e. when wild resources contributed a nutritionally insignificant component to the diet) was not until the Bronze Age. If the operative definition of agriculture includes large scale intensive cultivation of land, mono-cropping, organised irrigation, and use of a specialized labour force, the title "inventors of agriculture" would fall to the Sumerians, starting ca. 5,500 BC. Intensive farming allows a much greater density of population than can be supported by hunting and gathering and allows for the accumulation of excess product to keep for winter use or to sell for profit. The ability of farmers to feed large numbers of people whose activities have nothing to do with material production was the crucial factor in the rise of standing armies. The agriculturalism of the Sumerians allowed them to embark on an unprecedented territorial expansion, making them the first empire builders. Not long after, the Egyptians, powered by effective farming of the Nile valley, achieved a population density from which enough warriors could be drawn for a territorial expansion more than tripling the Sumerian empire in area. The invention of a three field system of crop rotation during the Middle Ages vastly improved agricultural efficiency. After 1492 the world's agricultural patterns were shuffled in the widespread exchange of plants and animals known as the Columbian Exchange. Crops and animals that were previously only known in the Old World were now transplanted to the New and vice versa. Perhaps most notably, the tomato became a favorite in European cuisine, while certain wheat strains quickly took to western hemisphere soils and became a dietary staple even for native North, Central and South Americans. By the early 1800s agricultural practices, particularly careful selection of hardy strains and cultivars, had so improved that yield per land unit was many times that seen in the Middle Ages and before, especially in the largely virgin lands of North and South America. With the rapid rise of mechanization in the 20th century, especially in the form of the tractor, the demanding tasks of sowing, harvesting and threshing could be performed with a speed and on a scale barely imaginable before. These advances have led to efficiencies enabling certain modern farms in the United States, Argentina, Israel, Germany and a few other nations to output volumes of high quality produce per land unit at what may be the practical limit.

Crops

Seed Testing

Seeds are tested for various qualities to ensure a high quality harvest, and to limit or prevent the spread of undesirable and invasive species. Seed test types Descriptions of various tests done on seed Seed related databases ISTA, the International Seed Testing Association, maintains a list of links to Seed Organizations worldwide:
- http://www.seedtest.org/en/content---1--1014--329.html

World production of major crops in 2004

In millions of metric tons, based on FAO estimates[http://faostat.fao.org/faostat/form?collection=Production.Crops.Primary&Domain=Production&servlet=1&hasbulk=0&version=ext&language=EN]: By crop types :Cereals 2,264 :Vegetables and melons 866 :Roots and Tubers 715 :Milk 619 :Fruit 503 :Meat 259 :Oilcrops 133 :Fish 130 (2001 estimate) :Eggs 63 :Pulses 60 :Vegetable Fiber 30 By individual crops :Sugar Cane 1,324 :Maize 721 :Wheat 627 :Rice 605 :Potatoes 328 :Sugar Beet 249 :Soybean 204 :Oil Palm Fruit 162 :Barley 154 :Tomato 120

Crop improvement

Tomato Tomato
- See main article on Plant breeding Domestication of plants is done in order to increase yield, improve disease resistance and drought tolerance, ease harvest and to improve the taste and nutritional value and many other characteristics. Centuries of careful selection and breeding have had enormous effects on the characteristics of crop plants. Plant breeders use greenhouses and other techniques to get as many as three generations of plants per year so that they can make improvements all the more quickly. Plant selection and breeding in the 1920s and '30s improved pasture (grasses and clover) in New Zealand. Extensive radiation mutagenesis efforts (i.e. primitive genetic engineering) during the 1950s produced the modern commercial varieties of grains such as wheat, corn and barley. For example, average yields of corn (maize) in the USA have increased from around 2.5 tons per hectare (40 bushels per acre) in 1900 to about 9.4 t/ha (150 bushels per acre) in 2001, primarily due to improvements in genetics. Similarly, worldwide average wheat yields have increased from less than 1 t/ha in 1900 to more than 2.5 t/ha in 1990. South American average wheat yields are around 2 t/ha, African under 1 t/ha, Egypt and Arabia up to 3.5 to 4 t/ha with irrigation. In contrast, the average wheat yield in countries such as France is over 8 t/ha. Higher yields are due to improvements in genetics, as well as use of intensive farming techniques (use of fertilizers, chemical pest control, growth control to avoid lodging). [Conversion note: 1 bushel of wheat = 60 pounds (lb) ≈ 27.215 kg. 1 bushel of corn = 56 pounds ≈ 25.401 kg] In industrialized agriculture, crop "improvement" has often reduced nutritional and other qualities of food plants to serve the interests of producers. After mechanical tomato-harvesters were developed in the early 1960s, agricultural scientists bred tomatoes that were harder and less nutritious (Friedland and Barton 1975). In fact, a major longitudinal study of nutrient levels in numerous vegetables showed significant declines in the last 50 years; garden vegetables in the U.S. today contain on average 38 percent less vitamin B2 and 15 percent less vitamin C (Davis and Riordan 2004). Very recently, genetic engineering has begun to be employed in some parts of the world to speed up the selection and breeding process. The most widely used modification is a herbicide resistance gene that allows plants to tolerate exposure to glyphosate, which is used to control weeds in the crop. A less frequently used but more controversial modification causes the plant to produce a toxin to reduce damage from insects (c.f. Starlink). There are specialty producers who raise less common types of livestock or plants. Aquaculture, the farming of fish, shrimp, and algae, is closely associated with agriculture. Apiculture, the culture of bees, traditionally for honey—increasingly for crop pollination. See also : botany, List of domesticated plants, List of vegetables, List of herbs, List of fruit

Environmental problems

Agriculture may often cause environmental problems because it changes natural environments and produces harmful by-products. Some of the negative effects are:
- Nitrogen and phosphorus surplus in rivers and lakes.
- Detrimental effects of herbicides, fungicides, insecticides, and other biocides.
- Conversion of natural ecosystems of all types into arable land.
- Consolidation of diverse biomass into a few species.
- Erosion
- Depletion of minerals in the soil
- Particulate matter, including ammonia and ammonium off-gasing from animal waste contributing to air pollution
- Weeds - feral plants and animals
- Odor from agricultural waste
- Soil salination in dry areas.

Policy

Agricultural policy focuses on the goals and methods of agricultural production. At the policy level, common goals of agriculture include:
- Food safety: Ensuring that the food supply is free of contamination.
- Food security: Ensuring that the food supply meets the population's needs.
- Food quality: Ensuring that the food supply is of a consistent and known quality.
- Conservation
- Environmental impact
- Economic stability

Methods

There are various methods of agricultural production:
- aeroponics
- aerial topdressing
- agricultural machinery
- animal husbandry
- aquaculture
- beekeeping
- crop rotation
- Concentrated Animal Feeding Operation (CAFO, factory farming)
- composting
- dairy farming
- detasseling
- domestication
- fencing
- fertilizers
- greenhouse
- harvest
- heliciculture
- hybrid seed
- hydroponics
- Integrated Pest Management (IPM)
- irrigation
- livestock
- market gardening
- monoculture
- no-till farming
- organic farming
- plant breeding
- pollination management
- precision farming
- ranching
- season extension
- seed saving
- shepherding
- subsistence farming
- succession planting
- sustainable agriculture
- terracing
- vegetable farming
- tillage
- weed control

References


- Wells, Spencer: The Journey of Man : A Genetic Odyssey. Princeton University Press, 2003. ISBN: 069111532X
- Crosby, Alfred W.: The Columbian Exchange : Biological and Cultural Consequences of 1492. Praeger Publishers, 2003 (30th Anniversary Edition). ISBN: 0275980731
- Collinson, M. (editor): A History of Farming Systems Research. CABI Publishing, 2000. ISBN: 0851994059
- Davis, Donald R., and Hugh D. Riordan (2004) Changes in USDA Food Composition Data for 43 Garden Crops, 1950 to 1999. Journal of the American College of Nutrition, Vol. 23, No. 6, 669-682.
- Friedland, William H. and Amy Barton (1975) Destalking the Wily Tomato: A Case Study of Social Consequences in California Agricultural Research. Univ. California at Sta. Cruz, Research Monograph 15.·

See also


- Agricultural and Food Research Council, UK
- Agricultural education
- Agricultural science
- Agricultural sciences basic topics
- Arid-zone agriculture
- Barnyard
- Community-supported agriculture
- International agricultural research
- Family farm hog pen
- Farm equipment
- Land Allocation Decision Support System
- List of domesticated animals
- List of subsistence techniques
- List of sustainable agriculture topics
- Permaculture
- Timeline of agriculture and food technology.
- USA agriculture

External links


- [http://www.fao.org www.fao.org] — Food and Agriculture Organization of the United Nations World Agricultural Information Centre
  - [http://www.fao.org/waicent/portal/statistics_en.asp www.fao.org] — The UN Statistical Databases
  - [http://www.fao.org/ag/ FAO Agriculture Department] and its [http://www.fao.org/docrep/006/y5160e/y5160e00.HTM State of Food and Agriculture 2003-2004] with a focus on the impact of biotechnology
  - [http://www.greenfacts.org/gmo/index.htm GM Crops in Agriculture] – A summary for non-specialists of the above FAO report by GreenFacts.
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- [http://imperium.lenin.ru/~kaledin/tmp/agricltr.txt Agriculture: Demon Engine of Civilization] by John Zerzan
- [http://www.livinghistoryfarm.org/index.html History of farming in Nebraska, USA]

Specific countries


- [http://www.agr.gc.ca/ www.agr.gc.ca] — Agriculture & Agri-Food Canada
- [http://www.nationalpak.com www.nationalpak.com] — Agriculture of Pakistan
- [http://www.nationalacademies.org/agriculture/ www.nationalacademies.org] — Agriculture at the United States National Academies
- [http://www.usda.gov/ www.usda.gov] — United States Department of Agriculture
  - [http://www.fas.usda.gov/currwmt.html Current World Production, Market and Trade Reports] from the Foreign Agricultural Service
  - [http://www.ers.usda.gov/ USDA's main source of economic information and research] from the Economic Research Service
  - [http://www.ars.usda.gov/ In-house Research Arm] from the Agricultural Research Service
  - [http://www.nal.usda.gov/ National Agricultural Library]
- [http://www.trader-china.com/Agriculture/index.html Agriculture Directory] ko:농업 ja:農業 nb:Landbruk simple:Agriculture

Rain

: For other uses see Rain (disambiguation). Rain is a form of precipitation, other forms of which include snow, sleet, hail, and dew. Rain forms when separate drops of water fall to the Earth's surface from clouds. Not all rain reaches the surface, however; some evaporates while falling through dry air. When none of it reaches the ground, it is a precipitation called virga.

Rain in nature

Rain plays a major role in the hydrologic cycle in which [http://wiktionary.org/wiki/moisture moisture] from the oceans evaporates, condenses into clouds, precipitates back to earth, and eventually returns to the ocean via streams and rivers to repeat the cycle again. There is also a small amount of water vapor that respires from plants and evaporates to join other water molecules in condensing into clouds. The amount of rainfall is measured using a rain gauge. It is expressed as the depth of water that collects on a flat surface, and can be measured to the nearest 0.25 mm or 0.01 in. It is sometimes expressed in litres per square metre (1 L/m² = 1 mm). Falling raindrops are often depicted in cartoons or anime as "tear-shaped", round at the bottom and narrowing towards the top, but this is incorrect (only drops of water dripping from some sources are tear-shaped at the moment of formation). Small raindrops are nearly spherical. Larger ones become increasingly flattened, like hamburger buns; very large ones are shaped like parachutes. [http://www.ems.psu.edu/~fraser/Bad/BadRain.html] On average, raindrops are 1 to 2 mm in diameter. The biggest raindrops on Earth were recorded over Brazil and the Marshall Islands in 2004 - some of them were as large as 10 mm. The large size is explained by condensation on large smoke particles or by collisions between drops in small regions with particularly high content of liquid water. Generally, rain has a pH slightly under 6. This is because atmospheric carbon dioxide dissolves in the droplet to form minute quantities of carbonic acid, which then partially dissociates, lowering the pH. In some desert areas, airborne dust contains enough calcium carbonate to counter the natural acidity of precipitation, and rainfall can be neutral or even alkaline. Rain below pH 5.6 is considered acid rain. Rain is said to be heavier immediately after a bolt of lightning. The cause of this phenomenon is traceable to the bipolar aspect of the water molecule. The intense electric and magnetic field generated by a lightning bolt forces many of the water molecules in the air surrounding the stroke to line up. These molecules then spontaneously create localized chains of water (similar to nylon or other 'poly' molecules). These chains then form water droplets when the electric/magnetic field is removed. These drops then fall as intensified rain.

Culture

lightning Cultural attitudes towards rain differ across the world. In the largely temperate Western world, rain traditionally has a sad and negative connotation — reflected in children's rhymes like Rain Rain Go Away — in contrast to the bright and happy sun. In dry places such as India and the Middle East, the rain is greeted with euphoria. Several cultures have developed means of dealing with rain and have developed numerous protection devices such as umbrellas and raincoats, and diversion devices such as gutters and storm drains that lead rains to sewers. Many people also prefer to stay inside on rainy days, especially in tropical climates where rain is usually accompanied by thunderstorms or rain is extremely heavy (monsoon). Rain may be collected for drinking water since rainwater is pure, or used as greywater. Excessive rain, particularly after a dry period has hardened the soil so that it cannot absorb water, can cause floods. Many people find the scent smelt during and immediately after rain especially pleasant or distinctive. The source of this smell is petrichor, an oil produced by plants, then absorbed by rocks and soil, and later released into the air during rainfall.

See also


- Acid Rain
- Climate
- Cloud
- Raining animals
- Water cycle
- Water resources
- Weather Category:Precipitation ko:비 ms:Hujan ja:雨 simple:Rain th:ฝน

Dryland farming

Dryland farming is an agricultural technique for cultivating land which receives little rainfall. Dryland farming is used in the Great Plains region of North American and in other grain growing regions such as the steppes of Eurasia, for example in Ukraine and southern Russia as well as Argentina. Winter wheat is the typical crop although skilled dryland farmers sometimes grow corn, beans or even watermelons. Successful dryland farming is possible with as little as 15 inches of precipitation a year, but much more successful with 20 inches or more. In marginal regions, a farmer should be financially able to survive occasional crop failures, perhaps of several years running. A soil which absorbs and holds moisture is helpful as is the practice of leaving stubble standing in the field to catch blowing snow..

River

:For the Second World War frigate class, see River class frigate. For the state of Nigeria, see Rivers State. MyScene.]] A river is a large natural waterway. It is a specific term in the vernacular for large streams, stream being the umbrella term used in the scientific community for all flowing natural waterways. In the vernacular, stream may be used to refer to smaller streams, as may creek, run, fork, etc. Passage via a river or stream is the usual way rainfall on land finds its way to the ocean or other large body of water such as a lake. A river consists of several basic parts, originating from headwaters or a spring at the source, that flow into the main stream. Smaller side streams that join the river are tributaries. Water flow is normally confined to a channel, with a bottom or bed between banks. The lower end of a river is its base level, commonly called its mouth, a river typically widens at its end and forms what is known as a river delta or estuary.

Topography

estuary.]]A river conducts water by constantly flowing perpendicular to the elevation curve of its bed, thereby converting the positional energy of the water into kinetic energy. Where a river flows over relatively flat areas, the river will meander: start to form loops and snake through the plain by eroding the river banks. Loops that are formed are sometimes cut off, forming a shorter river channel and leaving a remnant, oxbow lake. Rivers that carry large amounts of sediment develop conspicuous deltas at their mouths. Rivers whose mouths are in saline tidal waters may form estuaries. There are 4 main types of rivers. These types are:
- Youthful river - a river with a steep gradient that has very few tributaries and flows quickly. Its channels erode deeper rather than wider.
- Mature river - a river with a gradient that is less steep than those of youthful rivers and flows more slowly than youthful rivers. A mature river is fed by many tributaries and has more discharge than a youthful river. Its channels erode wider rather than deeper.
- Old river - a river with a low gradient and low erosive energy. Old rivers are characterized by flood plains.
- Rejuvenated river - a river with a gradient that is raised by the earth's movement. Where a river descends quickly over sloped topography, rapids with whitewater or even waterfalls occur. Rapids are often used for recreational purposes (see Whitewater kayaking). Waterfalls are sometimes used as sources of energy, via watermills and hydroelectric plants. Rivers begin at their source in higher ground, either rising from a spring, forming from glacial meltwater, flowing from a body of water such as a lake, or simply from damp, boggy places where the soil is waterlogged. They end at their base level where they flow into a larger body of water, the sea, a lake, or as a tributary to another (usually larger) river. In arid areas rivers sometimes end by losing water to evaporation and percolation into dry, porous material such as sand, soil, or pervious rock. The area drained by a river and its tributaries is called its watershed or catchment basin. (Watershed is also used however to mean a boundary between catchment basins.) Starting at the mouth of the river and following it upstream as it branches again and again the resulting river network forms a dendritic (tree-like) structure that is an example of a natural random fractal.

Biology

The flora and fauna of rivers are much different from those of the ocean because the water is fresh (non-salty). Living things in a river must be adapted to the current of the moving water.

Pollution

Human pollution of rivers is common, and very few rivers in the world today are clean of man-made substances. The most common pollutant is sewage piped into rivers, but chemical pollution is also common, and industrial accidents (and/or negligence) account for much of the destruction of riparian biomes. Heated water dumped into rivers by power plants and factories also affects river life.

Navigation

The Rhine is the busiest river in the world for transport ships. Inland vessels use the river to reach the major cities in Germany, Eastern France and Switzerland to transport bulk goods, liquids, containers AND passengers into the hinterland of the Port of Rotterdam and the ports of Amsterdam and Antwerp. Many millions of tons of goods are transported upstream yearly from these three sea ports to the industries near Nijmegen, Duisburg, Düsseldorf, Neuss, Köln, Koblenz, Mainz, Mannheim, Karlsruhe, Strasbourg, Colmar, Mulhouse and Basel. The lower part of the river is navigable for the largest inland vessels (up to 135 meters long and 17 meters wide) with an available depth of more than 2,50 even at the lower water levels. The further upstream, the more depth restrictions: at low water periods draught of ships is often limited to 1,90 m. for the stretch around Bingen (between the mouths of the Mosel and the Main). Upstream from Karlsruhe the Rhine is the border between France and Germany. The French have canalized the river by means of a series of hydropower dams and double ship locks, thus ensuring a year round navigable depth of 3.50 meters. (Source: NoorderSoft Waterways Database)

Dams

In places where the elevation changes of a river are great, dams for hydroelectric plants and other purposes are often built. This disrupts the natural flow of the river, and creates a lake behind the dam. Often the building of dams affects the whole of the river, even the part above the dam, as migrating fish are hindered (see fish ladder), waterflow is no longer bounded by seasonal changes and sediment flow is blocked. Dams are useful in many ways, such as providing HEP, acting as regulator of river flow so as to regulate the occurrence of flooding, which is especially important to wet-rice agriculture, and also to improve navigation and transport on the river. Often, dams such as Hoover Dam along Colorado River become famous tourist attractions. However, critics of dams, especially 'Green' advocates, argue that dams remove upper-river biodiversity such as through deforestation and forced migration of rural villages and indigenous tribes. Furthermore, trapping of river sediments behind the dams lead to salination and loss of nutrients for down-water fish. It also raises concern of eathquakes due to instablity of incompetent dams which have to support thousands of tonnes of sediments behind them. One very famous, and problematic, dam is the Aswan High Dam in the Nile.

Flooding

Flooding is a natural part of a river's cycles. Human activity, however, has upset the natural way flooding occurs by walling off rivers and straightening their courses. Removal of bogs, swamps and other wetlands in order to produce farmland has reduced the absorption zones for excess water and made floods into sudden disasters rather than gradual increases in water flow. In ancient Egypt, life was made possible through the floods of the Nile and the accompanying silt and sediment which enriched the fields with fresh nutrients. Nowadays, since people have built on these floodplains, floods are disasters, causing untold property loss each year. Human interference in the form of deforestation can also worsen conditions. The removal of vegetation leads to a reduction in Interception (vegetation stopping precipitation) and the 'weakening' of soil since plant roots no longer hold it together. As a result there is a reduced Infiltration capacity (how much water the soil can hold) and greater infiltration (precipitation going into the ground). This leads to faster soil saturation and therefore greater overland flow (also known as surface run off) and therefore, there are flash floods as the lag time decrease.

Logjams

Logjams are barriers within rivers, created by dead and uprooted trees. Over time, the obstruction prevents further logs to bypass, resulting in the creation of new network channels. According to author David R. Montgomery in his book, King of Fish, a logjam also causes water to buildup within a small space, forming peaceful pools within the main channel for young salmon to live within. The existence of these deep pools along with the complex web of channels creates an ideal salmon habitat. Today, many believe that the rebuilding of salmon runs is contingent upon reproducing the same environment shaped by logjams. As a result, many scientists have attempted to recreate artificial logjams. Marc Duboiski and Mike Ramsey of the Salmon Recovery Funding board staff, George Pess of the National Marine Fisheries Service, and Kevin Bauersfeld of Washington Department of Fish and Wildlife have prepared the Report to the Salmon Recovery Funding Board On the Engineered Log Jam (ELJ) Workshop ([http://iac.wa.gov/Documents/SRFB/Log_Jam_Report.pdf#search='log%20jams%20and%20salmon']), with the hope of mimicking natural logjams. Report to the Salmon Recovery Funding Board On the Engineered Log Jam (ELJ) Workshop."]]

Management

In its natural state a river may be inconvenient to man in a variety of ways. Rivers in inhabited areas have therefore been managed or controlled to make them more useful and less disruptive to human activity.
- The river channel may be dredged to make it deeper for navigation or to prevent flooding.
- Dams (see above) or weirs may be built to control the flow, store water, or extract energy.
- Levees may be built to prevent flooding.
- Sluice gates provide a means of controlling flow and adjusting river levels.
- floodways may be added to draw off excess river water in times of flood.
- Canals connect rivers to one another for water transfer or navigation.
- River courses may be modified to improve navigation, or straightened to increase the flow rate. River management is an ongoing activity as rivers tend to 'undo' the modifications made by man. Dredged channels silt up, sluice mechanisms deteriorate with age, levees and dams may suffer seepage or catastrophic failure.

River lists

(See also :Category:Lists of rivers.)

The world's ten longest rivers

It is difficult to measure the length of a river, mainly because rivers have a fractal property, which means that the more precise the measure, the longer the river will seem. Also, it's hard to state exactly where a river begins or ends, as very often, upstream, rivers are formed by seasonal streams, swamps, or changing lakes. This is an average measurement. # Nile (6,690 km) # Amazon (6,400 km) # Yangtze (Chang Jiang) (6,380 km) # Mississippi-Missouri (6,270 km) # Ob-Irtysh (5,570 km) # Huang He (Yellow) (5,464 km) # Amur (4,410 km) # Congo (4,380 km or 4,670 km). (The source of this river is disputed.) # Lena (4,260 km) # Mackenzie (4,240 km) For a longer list see Longest rivers. This also gives more information on measuring river lengths.

Well-known rivers (in alphabetic order)


- Aa - multiple rivers in Europe
- Amazon - largest river in the world
- American
- Amu Darya
- Amur - principal river of eastern Siberia
- Arkansas - major tributary of Mississippi River
- Arno - river through Florence
- Arvandrud (Shatt al-Arab) the large border river between Iran and Iraq.
- Brahmaputra - principal river in North East India & Tibet
- Chao Phraya - principal river of Thailand
- Colorado (Argentina)
- Colorado (U.S.) - principal river of American West
- Columbia - principal river of Pacific Northwest
- Congo - principal river of central Africa
- Danube - principal river of central and southeastern Europe
- De La Plata - the widest river in the world. South America
- Ebro - river in northwest Spain
- Elbe - major German river, Hamburg is situated on it
- Euphrates - twin principal river of Mesopotamia(Iraq)
- Ganges - principal river of India
- Han-gang - river of Seoul
- Helmand River - Principle river of (Afghanistan)
- Hari Rud (Afghanistan)
- Huang He (Yellow) - principal river of China
- Hudson - principal river of New York
- Indus - principal river of Pakistan
- Jordan - principal river of Israel
- Karun - principal (navigable) river of southern Iran.
- Kaveri - principal river of South India
- Lena - principal river of northeastern Siberia
- Mackenzie - longest river in Canada
- Magdalena - principal river of Colombia
- Main - river in Germany
- Mekong - principal river of Southeast Asia
- Mersey - river on which sits the English city of Liverpool
- Meuse - principal river of the southern provinces of the Netherlands and eastern Belgium.
- Mississippi - principal river of central United States
- Missouri - principal river of the Great Plains
- Murray - principal river of southeastern Australia
- Niger - principal river of west Africa
- Nile - Possibly the longest river in the world (or second after the Amazon)
- Ob - large river of Siberia
- Odra - major river in Eastern Europe
- Ohio - largest river between Mississippi and Appalachians
- Orinoco - principal river of Venezuela
- Parana - major South American river
- Paraguay - principal tributary of Parana river and major South American river in Brazil, Bolivia, Paraguay and Argentina
- Po - principal river of Italy
- Potomac River - principal river of the District of Columbia in the United States
- Rhine - principal river of northwestern Europe
- Rhône - principal river of southern France
- Rio Grande - border between United States and Mexico
- Saint Lawrence - drains Great Lakes
- Seine - river of Paris
- Segura- in southeast Spain
- Severn- longest river in Great Britain
- Shinano-gawa - longest river in Japan
- Snake - largest tributary to the Columbia river in Washington
- Tajo - largest river in the Iberian Peninsula
- Tay - largest river in Scotland
- Thames - river of London
- Tiber - river of Rome
- Tigris - twin principal river of Mesopotamia(Iraq)
- Tonegawa - largest river in Japan
- Vistula - principal river of Poland
- Volga - principal river of Russia
- Yangtze (Chang Jiang) - longest river in China
- Yenisei - large river of Siberia
- Yukon - principal river of Alaska and Yukon Territory
- Zambezi - principal river of southeastern Africa

Other lists


- List of waterways
- List of rivers by continent
  - List of rivers of Europe
    - Rivers of the United Kingdom
  - List of rivers of Asia
  - List of rivers of Africa
  - List of rivers of Australia
  - List of rivers of New Zealand
  - List of rivers of the Americas
  - List of rivers of Oceania
- List of river name etymologies

Rivers in myth and fiction

Real rivers


- The Thames in Edward Rutherfurd's London.
- The Thames in Jerome K. Jerome's Three Men in a Boat.
- The Thames and the Congo in Joseph Conrad's Heart of Darkness.
- The Mississippi in Mark Twain's Huckleberry Finn.
- The River Liffey through Dublin in James Joyce's Finnegans Wake.

Mythological rivers


- In Greek mythology, the Acheron, Cocytus, Phlegethon, Lethe and Styx (the five rivers of Hades); and the Eridanus.
- The Alph, an underground river imagined by various mystics and mentioned in Coleridge's poem Kubla Khan.
- The Sambation river stops flowing every Saturday.

Fictional rivers


- River Ankh traversing the city of Ankh-Morpork in Terry Pratchett's Discworld series.
- Chocolate river in Willy Wonka and the Chocolate Factory.
- River Djel in the country of Djelibeybi in Terry Pratchett's Discworld series.
- The River in the Riverworld novels of Philip José Farmer.
- Rivers of Middle-earth in various works of J. R. R. Tolkien.

See also


- Aquaduct
- Canal
- Drought
- Water dispute

Crossings

Rivers may be crossed by:
- bridges
- ferries
- fords
- tunnels.

Transport


- barge
- riverboat
- sailing
- towpath

External links


- [http://www.srbc.net/about.htm Management: River Basin Commissions]. Category:Bodies of water Category:Geomorphology zh-min-nan:Hô ja:川 ko:강 ms:Sungai simple:River th:แม่น้ำ

Spring (water)

A spring is a point where groundwater flows out of the ground, and is thus where the aquifer surface meets the ground surface. Dependant upon the constancy of the water source (rainfall or snowmelt that infiltrates the earth), a spring may be ephemeral (intermittent), perennial (continuous) or artesian. When it leaves the ground it may form into a pool or a stream. Minerals become dissolved in the water as it moves through the underground rocks. This gives the water flavour, and even carbon dioxide bubbles, depending upon the nature of the geology. This is why spring water is often bottled and sold as mineral water, although the term is often the subject of deceptive advertising.

Classification

Springs are often classified by the volume of the water they discharge. The largest springs are called "first-magnitude," defined as springs that discharge water at a rate of at least 100 cubic feet per second (2800 L/s). The scale for spring flow is as follows:
- 1st Magnitude - > 100 ft³/s (2800 L/s)
- 2nd Magnitude – 10 to 100 ft³/s (280 to 2800 L/s)
- 3rd Magnitude – 1 to 10 ft³/s (28 to 280 L/s)
- 4th Magnitude - 100 US gal/min (gallons per minute) to 1 ft³/s or 448 US gal/min (6.3 to 28 L/s)
- 5th Magnitude - 10 to 100 gal/min (0.63 to 6.3 L/s)
- 6th Magnitude - 1 to 10 gal/min (63 to 630 mL/s)
- 7th Magnitude - 1 pint to 1 gal/min (8 to 63 mL/s)
- 8th Magnitude - Less than 1 pint/min (8 mL/s)
- 0 Magnitude – no flow (sites of past/historic flow)

See also


- Pond
- Chalk stream
- Hot spring
- Geyser
- Seep
- Water cycle
- Well

External links


- [http://www.tampadiving.com/sections/diving/cave_systems.asp Florida Caves & Caverns]
- [http://web.archive.org/web/20021121180111/http://www.rps.psu.edu/0201/water.html "The Science of Springs" (via Wayback Machine)]
- [http://www.srwmd.state.fl.us/water+data/springs/what+is+a+spring.htm "What Is A Spring?"]
- [http://www.tfn.net/springs/Springbook/FirstMagnitude.htm List of First-Magnitude springs in Florida] Category:Geology Category:Hydrology Category:Drinking water ko:샘 ja:湧水

Aquifer

An aquifer is an underground layer of water-bearing permeable rock, or permeable mixtures of unconsolidated materials (gravel, sand, silt, or clay) (see also groundwater). Some productive aquifers are in fractured rock (carbonate rock, basalt, or sandstone). The study of water flow in aquifers and the characterization of aquifers is called hydrogeology.

Aquifer classification

hydrogeology This diagram indicates typical flow directions in a cross-sectional view of a simple confined/unconfined aquifer system (two aquifers with one aquitard between them, surrounded by aquiclude) which is in contact with a stream (typical in humid regions). The water table and unsaturated zone are also illustrated.

Saturated versus unsaturated

Groundwater can be found at nearly every point in the earth's shallow subsurface, to some degree; although aquifers do not necessarily contain fresh water. The earth's crust can be divided into two regions: the saturated zone (e.g., aquifers, aquitards, etc.) and the unsaturated zone (also called the vadose zone). Saturated means the pressure head of the water is greater than atmospheric pressure (it has a gauge pressure > 0). The definition of the water table is surface where the pressure head is equal to atmospheric pressure (where gauge pressure = 0). Unsaturated conditions occur above the water table where the pressure head is negative (absolute pressure can never be negative, but gauge pressure can) and the water which incompletely fills the pores of the aquifer material is under suction. The water content in the unsaturated zone is held in place by surface adhesive forces and it rises above the water table (the zero gauge pressure isobar) by capillary action to saturate a small zone above the phreatic surface (the capillary fringe) at less than atmospheric pressure. This is termed tension saturation and is not the same as saturation on a water content basis. Water content in a capillary fringe decreases with increasing distance from the phreatic surface. The capillary head depends on soil pore size. In sandy soils with larger pores the head will be less than in clayey soils with very small pores. The normal capillary rise in a clayey soil is less than six feet but can range between 5 and 100 feet. [http://www.ces.ncsu.edu/plymouth/programs/vepras.html] The capillary rise of water in a small diameter tube is this same physical process. The water table is the level to which water will rise in a large diameter pipe (e.g. a well) which goes down into the aquifer, and is open to the atmosphere.

Aquifers versus aquitards

Aquifers are typically saturated regions of the subsurface which produce an economically feasible quantity of water to a well or spring (e.g., sand and gravel or fractured bedrock often make good aquifer materials). Aquitards (sometimes, if completely impermeable, called an aquiclude or aquifuge) are saturated regions, which due to lower hydraulic conductivity, do not yield a sustainable amount of water in a economic fashion (e.g., clay, silt or fresh bedrock often form aquitards). Economically feasible is a relative term; for example, an aquifer that is quite adequate for local domestic use, as in a rural area, might be considered an inadequate aquitard for industrial, mining, or urban water supply. In non-mountainous areas (or near rivers in mountainous areas), the main aquifers are typically unconsolidated alluvium. They are typically composed of mostly horizontal layers of materials deposited by water processes (rivers and streams), which in cross-section (looking at a two-dimensional slice of the aquifer) appear to be layers of alternating coarse and fine materials. Coarse materials, due to the high energy needed to move them, tend to be found nearer the source (mountain fronts or rivers), while the fine-grained material will make it farther from the source (to the flatter parts of the basin or overbank areas - sometimes called the pressure area). Since there are less fine-grained deposits near the source, this is a place where aquifers are often unconfined (sometimes called the forebay area), or in hydraulic communication with the land surface.

Confined versus unconfined

There are two end members in the spectrum of types of aquifers; confined and unconfined (with semi-confined being in between). "Unconfined aquifers" are sometimes also called water table or phreatic aquifers, because their upper boundary is the water table or phreatic surface. Typically (but not always) the shallowest aquifer at a given location is unconfined, meaning it does not have a confining layer (an aquitard or aquiclude) between it and the surface. Unconfined aquifers usually receive recharge water directly from the surface, from precipitation or from a body of surface water (e.g., a river, stream, or lake) which is in hydraulic connection with it. "Confined" aquifers have the water table above their upper boundary (an aquitard or aquiclude), and are typically found below unconfined aquifers. A "perched aquifier" occurs when the porous, water-bearing segment of rock is located atop a layer of non-porous rock. If the distinction between confined and unconfined is not clear geologically (it is not known if a clear confining layer exists, or the geology is more complex, i.e., fractured bedrock), the value of storativity returned from an aquifer test can be used to determine it (although aquifer tests in unconfined aquifers should be interpreted differently than confined ones). Confined aquifers have very low storativity values (much less than 0.01, and as little as 10-5), which means that the aquifer is storing water using the mechanisms of aquifer matrix expansion and the compressibility of water, which typically are both quite small quantities. Unconfined aquifers have storativities (typically then called specific yield) greater than 0.01 (1% of bulk volume); they release water from storage by the mechanism of actually draining the pores of the aquifer, releasing relatively large amounts of water (up to the drainable porosity of the aquifer material, or the minimum volumetric water content).

Human dependence on groundwater

Most land areas on Earth have some form of aquifer underlying them, sometimes at significant depths. Fresh water aquifers, especially those with limited recharge by meteoric water, can be over-exploited and, depending on the local hydrogeology, may draw in non-potable water or saltwater (saltwater intrusion) from hydraulically connected aquifers or surface water bodies. This can be a serious problem especially in coastal areas and other areas where aquifer pumping is excessive. Aquifers are critically important in human habitation and agriculture. Deep aquifers in arid areas have long been water sources for irrigation (see Ogallala below). Many villages and even large cities draw their water supply from wells in aquifers. Some aquifers are "riparian aquifers". These are related to rivers, fluvial deposits, or unconsolidated deposits along river corridors, and are usually rapidly replenished by infiltration of surface water. Some municipal well fields are specifically designed to take advantage of induced infiltration of surface (usually river) water, leaving them potentially vulnerable to water quality problems in the surface water body (chemical spills, petroleum spills, and bacteriological problems). Aquifers that provide sustainable fresh groundwater to urban areas and for agricultural irrigation are typically close to the ground surface (within a couple of hundred meters) and have some recharge by fresh water. This recharge is typically from rivers or meteoric water (precipitation) that percolate into the aquifer through overlying unsaturated materials.

Subsidence

In unconsolidated aquifers, groundwater is produced from pore spaces between particles of gravel, sand, and silt. If the aquifer is confined by low-permeability layers, the reduced water pressure in the sand and gravel causes slow drainage of water from the adjoining confining layers. If these confining layers are composed of compressible silt or clay, the loss of water to the aquifer reduces the water pressure in the confining layer, causing it to compress due to the weight of overlying geologic materials. In severe cases, this compression can be observed on the ground surface as subsidence. Unfortunately, much of the subsidence due to groundwater extraction is permanent (elastic rebound is small).

Examples

An example of a significant and sustainable carbonate aquifer is the Edwards Aquifer [http://www.edwardsaquifer.org/] in central Texas. This carbonate aquifer has historically been providing high-quality water for nearly 2 million people and, even today, is completely full because of tremendous recharge from a number of area streams, rivers and lakes. The primary risk to this resource is human development over the recharge areas. One of the largest aquifers in the world is the Guarani Aquifer, with 1.2 million km² of area, from central Brazil to northern Argentina. Aquifer depletion is a global problem, and is especially critical in northern Africa; see the Great Manmade River project of Libya for an example. However, new methods of groundwater management such as artificial recharge and injection of surface waters during seasonal wet periods has extended the life of many freshwater aquifers, especially in the United States. The Ogallala Aquifer of the central United States is one of the world's great aquifers, but in places it is being rapidly depleted for growing municipal use, and continuing agricultural use. This huge aquifer, which underlies portions of eight states, contain primarily fossil water from the time of the last glaciation. Annual recharge, in the more arid portions of the aquifer, is estimated to total only about ten percent of annual withdrawals. The Mahomet Aquifer supplies water to some 800,000 people in central Illinois and contains approximately four trillion US gallons (15 km³) of water. The Mahomet Aquifer Consortium [http://www.mahometaquiferconsortium.org/] was formed in 1998 to study the aquifer with hopes of ensuring the water supply and reducing potential user conflicts. The Great Artesian Basin is one of the largest groundwater aquifers in the world. It plays a large part in water supplies for remote parts of South Australia.

See also


- Water table
- Groundwater
- Fresh water
- Spring
- List of aquifers Category:Civil engineering Category:Water Category:Hydrology

Cistern

: For cisterns in neuroanatomy, see cistern (neuroanatomy). A cistern (Middle English cisterne, from Latin cisterna, from cista, box, from Greek kistê, basket) is a receptacle for holding liquids, usually water. Often cisterns are built to catch and store rainwater. They range in capacity from a few litres to thousands of cubic metres (effectively covered reservoirs).

Creating and using cisterns

Cisterns are built by digging a hole in the ground to form a tank, with a single opening in the top to allow access. The walls of a cistern must be watertight in order to retain moisture. In the early 1900s cisterns were often made with a cement floor and dirt walls that had been coated in plaster. Modern-day cisterns may also be made from above-ground tanks, made of plastic. Cisterns usually have a lid covering their openings to prevent dirt, animals, insects, and other things from getting into the water. Cisterns are commonly used in areas where water is scarce, either because it is rare or because it has been depleted due to heavy use. Early on the water was used for many purposes, including cooking, irrigation, and washing. Present day cisterns are often only used for irrigation, due to concerns over water quality. Cisterns today can also be outfitted with filters or other purification methods when the water is meant for consumption. Many greenhouses use cisterns to help meet their water needs, especially in the USA. Some countries, such as Bermuda and the U.S. Virgin Islands, have laws that require rainwater harvesting systems to be built alongside any new construction, and cisterns can be used in these cases. Other countries, such as Japan and Germany, also offer financial incentives or tax credit for installing cisterns.

Rain barrel

A Rain Barrel is a container (usually plastic 55-gallon food grade barrels) used to catch rain water to be used during times of drought or to reduce water consumption from city water supplies. The water is not usually intended for drinking but instead to be used for gardening or lawns.

More history of the word

The word "cistern" is the translation of the Hebrew word bor, a receptacle for water that has been brought to it. This is distinguished from beer, which denotes a place where water rises on the spot, like a fountain. (Jer. 2:13; Prov. 5:15; Isa. 36:16) Cisterns are frequently mentioned in Scripture. The scarcity of springs in Palestine made it necessary to collect rain-water in reservoirs and cisterns (Num. 21:22). Empty cisterns were sometimes used as prisons (Jer. 38:6; Lam. 3:53; Ps. 40:2; 69:15). The "pit" into which Joseph was cast (Gen. 37:24) was a beer or dry well. There are numerous remains of ancient cisterns in all parts of Palestine.

Famous cisterns


- Basilica Cistern in Istanbul, Turkey
- Cistern in Silves, Portugal
- Cistern in El Jadida, Morocco

External links


- [http://www.rain-barrel.net/rain-barrels.html Difference between rain barrels and harvesting systems]
- [http://www.cbf.org/site/DocServer/rain_barrel_guide-web.pdf?docID=2868 Build Your Own Rain Barrel]
- [http://www.ne-design.net/ Rain Barrels made from Recycled Plastic]

Tractor

A tractor (from Latin trahere "to pull") is a device intended for drawing, towing or pulling something which cannot propel itself and, often, powering it too. Most commonly the word is used to describe a vehicle intended for such a task on some other vehicle or object. In Britain the word "tractor" usually means "farm tractor", and using "tractor" to mean other types of vehicles is known of in the vehicle trade but unfamiliar to much of the general public.

Farm tractor

Britain The most common use of the term tractor is for the vehicles used on farms. The farm tractor is used for pulling agricultural machinery or trailers, for ploughing, harrowing and similar tasks. The classic farm tractor is a simple open vehicle with two very large driving wheels on an axle below and slightly behind a single seat (the seat and steering wheel consequently are in the center) and the engine in front of the driver with two steerable wheels below the engine compartment. This basic design has remained unchanged for a number of years, but now enclosed cabs are available for many models of farm tractor. There are usually four foot-pedals, for the operator, on the floor of a tractor. The pedal on the left is the clutch. The operator presses on this pedal to disengage the transmission for either shifting gears or stopping the tractor. Two of the pedals on the right are the brakes. The left brake pedal stops the left rear wheel and the right brake pedal does the same with the right side. This independent left and right wheel braking augments the steerin