Irrigation
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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.
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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
Ditch (furrow) irrigation
The plants are grown in somewhat raised beds or listed rows, and the water is distributed throughout the field via canals, unlined ditches, or furrows, between the rows or beds. Depending on economic and physical factors such as the size of the field, the types of technology available, and the cost of manpower, the ditches can be dug with hand tools, turned with a plow pulled by an animal or tractor, or precisely fashioned using laser-guided instruments. Water can be transported to the furrows via 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 are generally used to 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 shunt 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 U.S., 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
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 the fields are small and access to them may be steep and narrow (so it's hard 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
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, while higher pressure sprinklers that rotate are usually called rotors. Rotors are usually 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 lower initial costs are a factor.
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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).
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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
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- 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 being 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.
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" (CATHOLIC ENCYCLOPEDIA: Egypt: I. GENERAL DESCRIPTION: Flora and Agriculture (http://www.newadvent.org/cathen/05329b.htm)).
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.