Irrigation is the artificial application of water to
the land or soil. It is used to assist in the growing of agricultural
crops, maintenance of landscapes, and revegetation
of disturbed soils in dry areas and during periods of inadequate rainfall.
Additionally, irrigation also has a few other uses in crop production, which
include protecting plants against frost, suppressing weed growth in grain
fields and preventing soil consolidation. In contrast, agriculture
that relies only on direct rainfall is referred to as rain-fed or dryland
farming.
Irrigation systems are also used for dust
suppression, disposal of sewage, and in mining.
Irrigation is often studied together with drainage, which
is the natural or artificial removal of surface and sub-surface water from a
given area.
Irrigation has been a central feature of agriculture for
over 5000 years, and was the basis of the economy and society of numerous
societies, ranging from Asia to Arizona.
History
Archaeological investigation has identified evidence of
irrigation where the natural rainfall was insufficient to support crops.
Perennial irrigation was practiced in the Mesopotamian
plain whereby crops were regularly watered throughout the growing
season by coaxing water through a matrix of small channels formed in the
field.
Ancient Egyptians practiced Basin irrigation
using the flooding of the Nile to inundate land plots
which had been surrounded by dykes. The flood water was held until the fertile
sediment had settled before the surplus was returned to the watercourse.
There is evidence of the ancient Egyptian pharaoh Amenemhet
III in the twelfth dynasty (about 1800 BCE)
using the natural lake of the Faiyum
Oasis as a reservoir to store surpluses of water for use during the dry
seasons, the lake swelled annually from flooding of the Nile.
The Ancient Nubians developed a form of irrigation by using a waterwheel-like
device called a sakia.
Irrigation began in Nubia some time between the third and second millennium
BCE. It largely depended upon the flood waters that would flow through the Nile River
and other rivers in what is now the Sudan.
In sub-Saharan Africa irrigation reached the Niger River
region cultures and civilizations by the first or second millennium BCE and was
based on wet season flooding and water harvesting.
Terrace irrigation is evidenced in pre-Columbian
America, early Syria, India, and China. In the Zana Valley of the Andes
Mountains in Peru,
archaeologists found remains of three irrigation canals radiocarbon dated from the 4th millennium BCE, the 3rd millennium BCE and the 9th century CE. These
canals are the earliest record of irrigation in the New World.
Traces of a canal possibly dating from the 5th millennium BCE were found under the 4th
millennium canal. Sophisticated irrigation and storage systems were developed
by the Indus Valley Civilization in present-day
Pakistan and North India, including the reservoirs at Girnar in 3000 BCE
and an early canal irrigation system from circa 2600 BCE. Large scale
agriculture was practiced and an extensive network of canals was used for the
purpose of irrigation.
Ancient Persia (modern day Iran) as far back as
the 6th millennium BCE, where barley was grown in
areas where the natural rainfall was insufficient to support such a crop. The Qanats, developed in
ancient Persia
in about 800 BCE, are among the oldest known irrigation methods still in use
today. They are now found in Asia, the Middle East and North Africa. The system
comprises a network of vertical wells and gently sloping tunnels driven into
the sides of cliffs and steep hills to tap groundwater. The noria, a water wheel
with clay pots around the rim powered by the flow of the stream (or by animals
where the water source was still), was first brought into use at about this
time, by Roman settlers in North Africa. By 150 BCE the pots
were fitted with valves to allow smoother filling as they were forced into the
water.
The irrigation works of ancient Sri Lanka,
the earliest dating from about 300 BCE, in the reign of King Pandukabhaya
and under continuous development for the next thousand years, were one of the
most complex irrigation systems of the ancient world. In addition to
underground canals, the Sinhalese were the first to build completely
artificial reservoirs to store water. Due to their engineering superiority in
this sector, they were often called 'masters of irrigation'. Most of these
irrigation systems still exist undamaged up to now, in Anuradhapura
and Polonnaruwa,
because of the advanced and precise engineering. The system was extensively
restored and further extended during the reign of King Parakrama
Bahu (1153–1186 CE).
China
The oldest known hydraulic
engineers of China
were Sunshu
Ao (6th century BCE) of the Spring and Autumn Period and Ximen Bao
(5th century BCE) of the Warring States period, both of whom worked on large
irrigation projects. In the Szechwan region belonging to the State
of Qin of ancient China, the Dujiangyan Irrigation System was built
in 256 BCE to irrigate an enormous area of farmland that today still supplies
water. By the 2nd century AD, during the Han Dynasty,
the Chinese also used chain pumps that lifted water from lower elevation to
higher elevation. These were powered by manual foot pedal, hydraulic waterwheels,
or rotating mechanical wheels pulled by oxen. The water was
used for public works of providing water for urban residential
quarters and palace gardens, but mostly for irrigation of farmland
canals and channels in the fields.
Korea
In 15th century Korea, the world's first rain gauge, uryanggye (Korean:우량계), was invented
in 1441. The inventor was Jang Yeong-sil, a Korean engineer of the Joseon
Dynasty, under the active direction of the king, Sejong
the Great. It was installed in irrigation tanks as part of a nationwide
system to measure and collect rainfall for agricultural applications. With this
instrument, planners and farmers could make better use of the information
gathered in the survey.
North America
In North America, the Hohokam were the only culture to rely
on irrigation canals to water their crops, and their irrigation systems
supported the largest population in the Southwest by AD 1300. The Hohokam
constructed an assortment of simple canals combined with weirs in their
various agricultural pursuits. Between the 7th and 14th centuries, they also
built and maintained extensive irrigation networks along the lower Salt and
middle Gila rivers that rivaled the complexity of those used in the ancient
Near East, Egypt, and China. These were constructed using relatively simple
excavation tools, without the benefit of advanced engineering technologies, and
achieved drops of a few feet per mile, balancing erosion and siltation. The
Hohokam cultivated varieties of cotton, tobacco, maize, beans and squash, as
well as harvested an assortment of wild plants. Late in the Hohokam
Chronological Sequence, they also used extensive dry-farming systems, primarily
to grow agave for food and fiber. Their reliance on
agricultural strategies based on canal irrigation, vital in their less than
hospitable desert environment and arid climate, provided the basis for the
aggregation of rural populations into stable urban centers.
Present extent
In the mid 20th century, the advent of diesel and electric
motors led to systems that could pump groundwater
out of major aquifers
faster than drainage basins could refill them. 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 by this phenomenon.
At the global scale, 2,788,000 km² (689 million acres)
of fertile land was equipped with irrigation infrastructure around the year
2000. About 68% of the area equipped for irrigation is located in Asia, 17% in
the Americas, 9% in Europe, 5% in Africa and 1% in Oceania. The largest
contiguous areas of high irrigation density are found:
- In Northern India and Pakistan along the Ganges and Indus rivers
- In the Hai He, Huang He and Yangtze basins in China
- Along the Nile river in Egypt and Sudan
- In the Mississippi-Missouri river basin and in parts of California
Smaller irrigation areas are spread across almost all
populated parts of the world.
Only 8 years later in 2008, the scale of irrigated land
increased to an estimated total of 3,245,566 km², what is nearly the size
of India.
Types
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.
Surface
In surface (furrow, flood, or level
basin) irrigation systems, water moves across the surface of agricultural
lands, in order to wet it and infiltrate into the soil. Surface irrigation can
be subdivided into furrow, borderstrip or basin irrigation. It is often
called flood irrigation when the irrigation results in flooding or near
flooding of the cultivated land. Historically, this has been the most common
method of irrigating agricultural land and still is in most parts of the world.
Where water levels from the irrigation source permit, the
levels are controlled by dikes, usually plugged by soil. This is often seen in
terraced rice fields (rice paddies), where the method is used to flood or
control the level of water in each distinct field. In some cases, the water is
pumped, or lifted by human or animal power to the level of the land. The field
water efficiency of surface irrigation is typically lower than other forms of
irrigation but has the potential for efficiencies in the range of 70% - 90%
under appropriate management.
Localized
Localized irrigation is a system where water is
distributed under low pressure through a piped network, in a pre-determined
pattern, and applied as a small discharge to each plant or adjacent to it. Drip
irrigation, spray or micro-sprinkler irrigation and bubbler irrigation belong
to this category of irrigation methods.
Subsurface textile irrigation
Diagram showing the structure of an example SSTI
installation
Subsurface Textile Irrigation (SSTI) is a technology
designed specifically for subsurface irrigation in all soil textures from
desert sands to heavy clays. A typical subsurface textile irrigation system has
an impermeable base layer (usually polyethylene
or polypropylene),
a drip line running along that base, a layer of geotextile
on top of the drip line and, finally, a narrow impermeable layer on top of the
geotextile (see diagram). Unlike standard drip irrigation, the spacing of
emitters in the drip pipe is not critical as the geotextile moves the water
along the fabric up to 2m from the dripper.
Drip
Drip (or micro) irrigation, also known as trickle
irrigation, functions as its name suggests. In this system water falls drop by
drop just at the position of roots. Water is delivered at or near the root zone of plants,
drop by drop. This method can be the most water-efficient method of irrigation,if
managed properly, since evaporation and runoff are minimized. The field water
efficiency of drip irrigation is typically in the range of 80 to 90 percent
when managed correctly.
In modern agriculture, drip irrigation is often combined
with plastic
mulch, further reducing evaporation, an d 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 or if the delivery rate is too
high. Drip irrigation methods range from very high-tech and computerized to
low-tech and 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.
Sprinkler System
In sprinkler or overhead irrigation, water is piped
to one or more central locations within the field and distributed by overhead
high-pressure sprinklers or guns. A system utilizing sprinklers, sprays, or
guns mounted overhead on permanently installed risers is often referred to as a
solid-set irrigation system. 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 can also be mounted on moving platforms connected
to the water source by a hose. Automatically moving wheeled systems known as traveling
sprinklers may irrigate areas such as small farms, sports fields, parks,
pastures, and cemeteries unattended. Most of these utilize a length of
polyethylene tubing wound on a steel drum. As the tubing is wound on the drum
powered by the irrigation water or a small gas engine, the sprinkler is pulled
across the field. When the sprinkler arrives back at the reel the system shuts
off. This type of system is known to most people as a "waterreel"
traveling irrigation sprinkler and they are used extensively for dust
suppression, irrigation, and land application of waste water.
Other travelers use a flat rubber hose that is dragged along
behind while the sprinkler platform is pulled by a cable. These cable-type
travelers are definitely old technology and their use is limited in today's
modern irrigation projects.
Center pivot
Center pivot irrigation is a form of sprinkler 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 found and used
in all parts of the world and allow irrigation of all types of terrain. Newer
systems have drop sprinkler heads as shown in the image that follows.
Most center pivot systems now have drops hanging from a
u-shaped pipe attached at the top of the pipe with sprinkler head 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. Crops are often planted in a circle
to conform to the center pivot. This type of system is known as LEPA (Low Energy
Precision Application). Originally, most center pivots were water powered.
These were replaced by hydraulic systems (T-L Irrigation) and
electric motor driven systems (Reinke, Valley, Zimmatic). Many modern pivots
feature GPS devices.
Lateral move (side roll, wheel line)
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 10 m 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 (130 mm)
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.
Sub-irrigation
Subirrigation 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. Often those
systems are located on permanent grasslands in lowlands or river valleys and
combined with drainage infrastructure. A system of pumping stations, canals,
weirs and gates allows it to increase or decrease the water level in a network
of ditches and thereby control the water table.
Sub-irrigation 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. Sub-irrigation 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 basin irrigation.
Automatic, non-electric using buckets and ropes
Besides the common manual watering by bucket, an automated,
natural version of this also exists. Using plain polyester ropes combined with
a prepared ground mixture can be used to water plants from a vessel filled with
water.
The ground mixture would need to be made depending on the
plant itself, yet would mostly consist of black potting soil, vermiculite and
perlite. This system would (with certain crops) allow to save expenses as it
does not consume any electricity and only little water (unlike sprinklers,
water timers, etc.). However, it may only be used with certain crops (probably
mostly larger crops that do not need a humid environment; perhaps e.g.
paprikas).
Using water condensed from humid air
In countries where at night, humid air sweeps the
countryside, water can be obtained from the humid air by condensation
onto cold surfaces. This is for example practiced in the vineyards at Lanzarote
using stones to condense water or with various fog
collectors based on canvas or foil sheets.
In-ground irrigation
Most commercial and residential irrigation systems are
"in ground" systems, which
means that everything is buried in the ground. With the pipes,
sprinklers, emitters (drippers), 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. This does, however, create
some drawbacks in the maintenance of a completely buried system.
Most irrigation systems are divided into zones. A zone is a
single irrigation valve and one or a group of drippers or sprinklers that are
connected by pipes or tubes. Irrigation systems are divided into zones because
there is usually not enough pressure and available flow to run sprinklers for
an entire yard or sports field at once. Each zone has a solenoid
valve on it that is controlled via wire by an irrigation controller. The irrigation controller is either a
mechanical (now the "dinosaur" type) or electrical device that
signals a zone to turn on at a specific time and keeps it on for a specified
amount of time. "Smart Controller" is a recent term for a controller
that is capable of adjusting the watering time by itself in response to current
environmental conditions. The smart controller determines current conditions by
means of historic weather data for the local area, a soil moisture sensor (water potential or water
content), rain sensor, or in more sophisticated systems satellite
feed weather station, or a combination of these.
When a zone comes on, the water flows through the lateral
lines and ultimately ends up at the irrigation emitter (drip) or sprinkler heads. Many sprinklers have pipe
thread inlets on the bottom of them which allows a fitting and the pipe to be
attached to them. The sprinklers are usually installed with the top of the head
flush with the ground surface. When the water is pressurized, the head will pop
up out of the ground and water the desired area until the valve closes and
shuts off that zone. Once there is no more water pressure in the lateral line,
the sprinkler head will retract back into the ground. Emitters are generally
laid on the soil surface or buried a few inches to reduce evaporation losses.
Water sources
Sources of irrigation water can be groundwater extracted
from springs or by using wells,
surface water withdrawn from rivers, lakes or reservoirs or non-conventional sources like treated
wastewater, desalinated water or drainage water. A special form
of irrigation using surface water is spate
irrigation, also called floodwater harvesting.
In case of a flood (spate) water is diverted to normally dry river beds (wadis)
using a network of dams, gates and channels and spread over large areas. The
moisture stored in the soil will be used thereafter to grow crops. Spate
irrigation areas are in particular located in semi-arid or arid, mountainous
regions. While floodwater harvesting belongs to the accepted irrigation
methods, rainwater harvesting is usually not considered
as a form of irrigation. Rainwater harvesting is the collection of runoff water
from roofs or unused land and the concentration
Around 90% of wastewater produced globally remains
untreated, causing widespread water pollution,
especially in low-income countries. Increasingly, agriculture is using
untreated wastewater as a source of irrigation water. Cities provide lucrative
markets for fresh produce, so are attractive to farmers. However, because
agriculture has to compete for increasingly scarce water resources with
industry and municipal users (see Water scarcity below), there is often no
alternative for farmers but to use water polluted with urban waste, including
sewage, directly to water their crops. There can be significant health hazards
related to using water loaded with pathogens in this way, especially if people
eat raw vegetables that have been irrigated with the polluted water. The International Water Management
Institute has worked in India, Pakistan, Vietnam, Ghana, Ethiopia, Mexico
and other countries on various projects aimed at assessing and reducing risks
of wastewater irrigation. They advocate a 'multiple-barrier' approach to
wastewater use, where farmers are encouraged to adopt various risk-reducing
behaviours. These include ceasing irrigation a few days before harvesting to
allow pathogens to die off in the sunlight, applying water carefully so it does
not contaminate leaves likely to be eaten raw, cleaning vegetables with
disinfectant or allowing fecal sludge used in farming to dry before being used
as a human manure.The World Health Organization has developed
guidelines for safe water use.
Efficiency
Modern irrigation methods are efficient enough 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.Water use efficiency in the field can
be determined as follows:
- Field Water Efficiency (%) = (Water Transpired by Crop ÷ Water Applied to Field) x 100
Fifty years ago (as of 2010), the common perception was that
water was an infinite resource. At that time, there were fewer than half the
current number of people on the planet. People were not as wealthy as today,
consumed fewer calories and ate less meat, so less water was needed to produce
their food. They required a third of the volume of water we presently take from
rivers. Today, the competition for water resources is much more intense. This
is because there are now more than seven billion people on the planet, their
consumption of water-thirsty meat and vegetables is rising, and there is
increasing competition for water from industry, urbanisation
and biofuel crops. To avoid a global water crisis, farmers will have to strive
to increase productivity to meet growing demands for food, while industry and
cities find ways to use water more efficiently.
Successful agriculture is dependent upon farmers having
sufficient access to water. However, water scarcity is already a critical
constraint to farming in many parts of the world. With regards to agriculture,
the World
Bank targets food production and water management as an increasingly global
issue that is fostering a growing debate. Physical water scarcity is where
there is not enough water to meet all demands, including that needed for
ecosystems to function effectively. Arid regions frequently suffer from
physical water scarcity. It also occurs where water seems abundant but where
resources are over-committed. This can happen where there is overdevelopment of
hydraulic infrastructure, usually for irrigation. Symptoms of physical water scarcity
include environmental degradation and declining groundwater.
Economic scarcity, meanwhile, is caused by a lack of investment in water or
insufficient human capacity to satisfy the demand for water. Symptoms of
economic water scarcity include a lack of infrastructure, with people often
having to fetch water from rivers for domestic and agricultural uses. Some 2.8
billion people currently live in water-scarce areas.
Technical challenges
Irrigation schemes involve solving numerous engineering and
economic problems while minimizing negative environmental impact.
- Competition for surface water rights.
- Overdrafting (depletion) of underground aquifers.
- Ground subsidence (e.g. New Orleans, Louisiana)
- Underirrigation or irrigation giving only just enough water for the plant (e.g. in drip line irrigation) gives poor soil salinity control which leads to increased soil salinity with consequent buildup of toxic salts on soil surface in areas with high evaporation. This requires either leaching to remove these salts and a method of drainage to carry the salts away. When using drip lines, the leaching is best done regularly at certain intervals (with only a slight excess of water), so that the salt is flushed back under the plant's roots.
- Overirrigation because of poor distribution uniformity or management wastes water, chemicals, and may lead to water pollution.
- Deep drainage (from over-irrigation) may result in rising water tables which in some instances will lead to problems of irrigation salinity requiring watertable control by some form of subsurface land drainage.
- Irrigation with saline or high-sodium water may damage soil structure owing to the formation of alkaline soil
- Clogging of filters: It is mostly algae that clog filters, drip installations and nozzles. UV and ultrasonic method can be used for algae control in irrigation systems.
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