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Irrigation

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Agricultural Processes
A center-pivot sprinkler system is used to irrigate a soybean crop.
Irrigation is the controlled application of water for use in agricultural crop production through artificial methods when natural means are not sufficient. Irrigation is necessary in certain climate conditions such as deserts or areas with only seasonal rainfall.

Water used for irrigation purposes comes from a variety of sources from within the ground, like a well or aquifer or from lakes, reservoirs, rivers, holes, dams, draglines, or bores at the surface.

Irrigation involves transporting water from a source and applying it to crops using various means. One way is to control the flow of water through a network of canals. A large canal transports water from the source to smaller canals called laterals. The lateral ditches or furrows are used to distribute water evenly amongst crops in a field. When a crop rests at a lower elevation than the water source, gravity can be used to direct water flow. At higher elevations, water may be pumped up into the canals. If ground water, such as that from a well, is the source used for irrigation, then the water is usually pumped up to the surface into a ditch or network of pipes.

Contents

[edit] History

Around 5000 B.C., the Egyptians were cultivating crops in the fertile land running along the length of the Nile River. The river provided an available water source for flooding irrigation.[1] They were able to build a complex network of large-flat bottom basins for crop production using sluices for guiding water into the basins at peak flood times. The water was diverted in a manner that enabled it to flow successively through several basins controlled by floodgates. As flooding occurred, silt deposited along the bottom of the basin, creating a flat floor and providing rich nutrients. To irrigate, the Egyptians learned to anticipate what the height of an annual flood would be by developing a system of measuring floodwaters called “Nilometers” at certain points along the Nile River.[2]

The Sumerians of Mesopotamia were the first civilization to develop complicated systems of irrigation that have been described as “interventionist and active” compared to Egyptian irrigation systems.[3] Sumerians essentially learned how to harness and channel the waters from the mighty Tigris and Euphrates Rivers to flood surrounding fields. Silt from the bottom of the rivers mixed with soil to provide a fertile bed for growing crops but presented a risk to the safety of Sumerian homes. After flooding, the water would evaporate, leaving fields caked in a layer of salt making them useless for supplemental crops. As a result, the Sumerians had to explore other options such as building canals to redirect the water to areas they wanted it to go.[4] Canals would often be degraded by silt and have to be dredged.[5] Sumerian engineers eventually figured out a way to use the Euphrates River—which sat at a higher elevation—as the water source for irrigation, and the Tigris River as a drainage channel.[6] There is also evidence that the Sumerians built diversion dams to create reservoirs. The water from these reservoirs were transported across considerable distances over the flat countryside and used in irrigation. [7]

Another substantial development in the historical evolution of irrigation practices occurred during the rise of Persian Empire with introduction of a series of qanats (a type of underground aqueduct), known as the Kareze irrigation system. The system was built as a way of obtaining power but failed in the end. Engineers placed stone channels around streams to direct the flow of water towards the town center. Later, a complicated network of underground qanats was constructed. Qanats were built by sinking a well to the level of the water table. They then connected the well to a tunnel that was open at the surface and built on a sloping incline. Other notable ancient irrigation systems were also constructed in China, India, and Southwest Asia.[8]

[edit] Process

As a process, irrigation influences every aspect of crop growth, including: preparing the seed beds for planting, germination, root growth, the use of nutrients, the growth of the final possible crop yield, and quality of any secondary growth. A number of factors are used to determine what type of irrigation method should be applied to a crop. These include soil type, land topography, power sources, availability of water supply, source of water, size of area to be irrigated, and farm water storage capacity.[9] There are four basic types of irrigation systems:

A self-propelled overhead sprinkler system irrigates farmland in Nebraska.

[edit] Surface Irrigation

Surface irrigation occurs when water is left to run over an entire field; it accounts for 60 percent of the irrigation used in the Northern Hemisphere. Flood and furrow irrigation are the two most common methods of surface irrigation.[10] Irrigation by flooding covers the entire surface of a field. Soil walls, dikes, or levees surround the field and enclose the water inside the field. This method is also referred to as a basin method, and is suited for crops spaced closely together like rice paddies.[11] A variation of basin irrigation is known as border irrigation; it differs slightly in that the field is not completely enclosed. Water is channeled into the field through a small opening at one end. Another opening at the opposite end of the field permits water to be drained. Border irrigation is a preferred method when irrigation must be applied to crops on sloping land.[12] The use of narrow ditches called furrows is also an effective way of irrigating crops planted in rows, such as corn, potatoes, and cotton. Furrow irrigation controls the water flow through the furrows or channels dug out in rows inside the field. Seeds are planted in the ridges between each furrow. As water enters each ditch, it seeps into the ridges.[13]

[edit] Sprinkler or Overhead Irrigation

Sprinkler, or overhead, irrigation is another effective means of delivering water to crops. A pump forces the water up through a pipeline to sprinkler heads very similar to a lawn sprinkler.[14] The water is dispersed through the sprinkler heads over the crops in the form of a mist or spray. The pipes may be portable so they can be moved across the field for complete irrigation.

Variable flow irrigation is the mounting of sprinkler heads on an overhead network of aluminum pipes or stakes.
There are even self-propelled irrigation systems mounted on overhead booms with engines and wheels that operate and move over a field under their own power. Water guns are another variation of a sprinkler irrigation system.[15]

Sprinkler systems are quite adaptable and can be used on various kinds of terrain and topography. It is a particularly effective form of irrigation for covering large areas of land, and when a limited water supply presents a problem. One drawback is that sprinkler irrigation systems require large quantities of power to operate. Some sprinkler heads are therefore designed with low-pressure water heads that require less energy.[16]

[edit] Trickle and Drip Irrigation

A third type of irrigation system is trickle and drip irrigation. This method is more controlled, with water being dispersed through tiny plastic tubes with openings called emitters; these tubes are located underground. As water travels through the tubes, it trickles out very slowly with little water being used or wasted.[17] This method of irrigation can also be deployed through trickled tape, a long hose with built-in emitters.[18] Since water is closely controlled in drip and trickle operations, little water is needed or wasted during irrigation.

[edit] Subsurface Irrigation or Subirrigation

This method is accomplished by applying drip irrigation tubing underneath the ground at the root, usually five inches (13 cm) below the surface. The roots of the plant receive all of the water in this method while the ground surface stays dry.[19] Water evaporation and surface run-off are greatly minimized. Plastic mulch, a layer of organized material, can be laid over a field and crops to achieve a similar effect in reducing evaporation and protection from weed growth and frost. It has become an important component of many drip irrigation systems. The use of drip and trickle irrigation systems also permits for the fertigation of crops – fertilizing through an available irrigation system.[20] One of the drawbacks of this method is it is relatively expensive to install and maintain.

[edit] Types

The furrow system is a sequence of small and shallow ditches dug across a field used to channel water, sometimes on a steep slope. The furrows can be dug straight or curved to follow the natural contour of the land. It is used widely for row crops grown on beds in between the furrows spaced 3.3 feet (1 m) apart.[21]

A flood or border check system is a field separated into bays with parallel ridges or border checks. Water flows down a sloped field as a sheet, guided by the ridges. On steeper sloped fields, ridges may be curved or spaced close together to accommodate hilly terrain. Border systems are a common form of irrigation method used on orchards, vineyards, pastures, and grain crops.

The level basin system is used when the slope of the field is level and there are no openings at either end of the field. Water can be applied to the irrigated area at a high level. The outcome is rapid ponding inside the basin area, to a desired depth.

The center-pivot sprinkler system is a self-propelled sprinkler system that features a single pipeline mounted upon a row of portable towers suspended seven to 13 feet (2 to 4 m) above the ground. Water is pumped through a central pipe. Nozzles affixed to the pipeline distribute water in a circular fashion with each tower rotating slowly on a pivot point.

A hand-move sprinkler system is a sequence of lightweight sections of pipeline that can be moved manually for successive irrigation of smaller crop areas. A lateral pipeline is connected to a portable pipeline or a buried mainline.

The solid-set/fixed sprinkler system is a stationary sprinkler system that is fixed in place and positioned below the soil with the sprinkler heads elevated and exposed.

The traveling gun sprinkler system uses a large, high-pressure sprinkler mounted on a wheel or trailer and fed by a flexible, self-propelled rubber hose. The system applies water to crops as it travels in a lane guided by a cable.

A side-roll wheel-move system is a pipeline system with large-diameter wheels that can be rolled as a whole unit and positioned to provide successive irrigation across a field. The pipeline is typically located 3.3 feet (1 m) above the ground.[22]

The linear or lateral move system works on the same principle as a central-pivot system, except it features a lateral pipeline and towers that provide a continuous path of water across a rectangular shaped field instead of in a circular fashion.

The low-flow irrigation system, also referred to as trickle and drip irrigation, is designed with small-diameter tubes situated under the surface of the soil. Irrigation is dispersed slowly and frequently at the root level through emitters connected to a network of mainline, sub-main, and lateral line tubing.

[edit] References

  1. How Irrigation Works. How Stuff Works. 2008-11-21.
  2. Ancient Irrigation. Dept. of Geology. University of California Davis. 2008-11-21.
  3. Ancient Irrigation. Dept. of Geology. University of California Davis. 2008-11-21.
  4. Irrigation. ThinkQuest. 2008-11-21.
  5. History of Agriculture and Irrigation. Drake Marin. 2008-11-21.
  6. Ancient Irrigation. Dept. of Geology. University of California Davis. 2008-11-21.
  7. Ancient Irrigation. Dept. of Geology. University of California Davis. 2008-11-21.
  8. History of Agriculture and Irrigation. Drake Marin. 2008-11-21.
  9. Irrigation. Department of Primary Industries. State of Victoria. 2008-11-21.
  10. Irrigation. ThinkQuest. 2008-11-21.
  11. How Irrigation Works. How Stuff Works. 2008-11-21.
  12. How Irrigation Works. How Stuff Works. 2008-11-21.
  13. Irrigation. ThinkQuest. 2008-11-21.
  14. How Irrigation Works. How Stuff Works. 2008-11-21.
  15. How Irrigation Works. How Stuff Works. 2008-11-21.
  16. Irrigation. ThinkQuest. 2008-11-21.
  17. Irrigation. ThinkQuest. 2008-11-21.
  18. How Irrigation Works. How Stuff Works. 2008-11-21.
  19. How Irrigation Works. How Stuff Works. 2008-11-21.
  20. What's New with Plastic Mulches? How Stuff Works. 2008-11-21.
  21. Irrigation. Department of Primary Industries. State of Victoria. 2008-11-21.
  22. Irrigation. Department of Primary Industries. State of Victoria. 2008-11-21.