Encyclopedia
Irrigation is the replacement or supplementation of
rainfall with water from another source in order to grow crops or plants. 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 lake 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 irrigation
Ditches, furrows, and basins may 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, listed rows, drilled or planted into flat basins. Water may be distributed throughout the field via
canals, unlined ditches, or furrows, between the rows or beds by use of rigid gated
plastic or
aluminum water 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 surface irrigation systems have been replaced because of high labor costs and increasing demands on
water resources. Surface 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.
Overhead 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 , 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 and flows of 50 to 1200 US gal/min , usually with nozzle diameters in the range of 0.5 to 1.9 inches . 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. Automatically moving wheeled systems known as
travelers may irrigate large areas such as small farms, sports fields, and cemeteries unattended. Some of these utilize a length of polyethylene tubing wound on a steel drum. As the traveler is pulled across the field, the hose is wound onto the drum by water pressure or a powered engine. Other travelers use a flat rubber hose that is dragged along behind. At the low tech 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.
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.
Center pivot irrigation
Center pivot irrigation is a form of overhead irrigation consisting of several segments of pipe 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 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 .
Originally, most center pivots were water powered. These were replaced by hydraulic systems and electric motor driven systems . Most systems today are driven by an electric motor mounted low on each span. This drives a reduction gearbox and transverse driveshafts transmit power to another reduction gearbox mounted behind each wheel. Reinke sprinklers have strobe lights when running. Valleys have regular lights. Reinke sprinklers are kind of red covered. Most new sprinklers are Zimmatics with drops.
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 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
- 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 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 for lawns and
golf courses.
Subirrigation
Subirrigation also sometimes called
seepage irrigation has been used for many years in field crops in areas with high water tables. It is a method of artificially raising the water table to allow the soil to be moistened from below the plants' root zone.
Subirrigation is also 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 in greenhouses 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.
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 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" .
Developed in ancient
Persia the
Qanat is among the oldest known irrigation methods developed and still used today. 'Qanats are constructed as a series of well-like vertical shafts, connected by gently sloping tunnels.' meaning that the receiving populus was always lower than the source, the source being higher and connected to these Qanats which had many exit points for the water at villages and pastures.
Irrigation Works of Ancient Sri Lanka were one of the most complex irrigation systems of the ancient world, the sinhalese managed to build major irrigation schemes to support the agriculture which thrived at the time. The sinhalese civilization is responsible for the invention of the valve pit which remains unchanged to-date. Highly complex use of trigonometry and other engineering aspects such as soil mechanics, built environment had been used for the construction of these schemes.King Parakrama Bahu had been responsible for the construction or the restoration of 165 dams, 3910 canals, 163 major tanks and 2376 minor tanks, all in a reign of 33 years.
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 millennium canal.
The
Indus Valley Civilization in
Pakistan and
North India also had an early
canal irrigation system.
In ancient
China the
Dujiangyan Irrigation Systemwas built in 250 BC which irrigated a large area and it still supplies with water nowadays.
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
- Buildup of toxic salts on soil surface in areas of high evaporation. This requires leaching to remove these salts and a method of drainage to carry the salts away.
- Overirrigation because of poor distribution uniformity or management wastes water, chemicals, and may lead to water pollution.
See also
References
External links
- 19th century Irrigation in India
