The methods, benefits and drawbacks
Written by : Wondemagegnehu A. Gezahegne
Supervised by: Prof. B.J. Merkel
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Contents
1.
2.
3.
4.
5
6.
7.
8.
Abstract
Introduction
Irrigation methods
Water for irrigation
Hazards of irrigation
An example of water misuse in irrigated agriculture
Conclusion
References
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1.
ABSTRACT
Irrigation plays a major role in increasing food production. Irrigated land presently
accounts for 15 percent of the cultivated land but produces 36 percent of the world's
food (FAO 1988). The world's irrigated land was 8 million hectares in 1800, 48 million
hectares in 1900, 94 million hectares in 1950, 198 million hectares in 1970, and about
220 million hectares in 1990 (Jensen et al. 1990). About three-quarters of the irrigated
land is presently in the developing countries. In these countries, almost 60 percent of the
production of major cereals, primarily rice and wheat, is derived from irrigation.
Since higher yields are obtained with irrigated agriculture and because it is less
dependent on the vagaries of weather, it assumes special importance in this regard.
Expansion of irrigated agriculture could contribute significantly towards achieving and
stabilizing food and fiber needs. However, new water supplies for such expansion are
limited. Irrigated agriculture is already the largest consumer of developed water
resources. At the same time, drainage return from irrigated lands is one of the major
causes of waterlogging and of water pollution due to salts, nitrates, agricultural
chemicals and certain natural, potentially toxic trace elements.
2.
INTRODUCTION
While only about 15% of the World’s cultivated land is irrigated, it accounts for
34-40% of the global harvest. One of the primary objectives of agriculture is to provide
the food and fibre needs of human beings. These needs increase as the population
increases. The world population was 2.5 thousand million in 1950; 4.9 thousand million
in 1985, and 5.3 thousand million in 1990. It is expected to be 8.5 thousand million in
2025 (UN 1991). The population of the developing countries, which is presently over
three-quarters of the worlds total, accounts for about 90 percent of the expected
increase in global population. These growth rates will require an increase in agricultural
production of about 40 to 50 percent over the next thirty to forty years, in order to
maintain the present level of food intake; a 20 and 60 percent increase for developed
and developing countries, respectively.
Growth in crop production can come from increases in arable land, cropping
intensity and yield per unit area of cropped land. Irrigation is obviously significant in arid
regions where the potential water losses by evaporation and transpiration are greater
than the amount of water supplied by precipitation. But supplemental irrigation to meet
special or occasional needs is used in sub-humid and even humid areas.
Irrigation is practiced in many countries, but Asia has more than 60% of the total
irrigated land in the World. China is in the first place with about 47.5 million ha of
irrigated land, followed by India with 45.8 million ha. Table (1). Irrigation land increased
by about 43% in the 20 years from 1970 to 1990, but the increase was spotty. Some
countries such as USA showed only small increases, and some such as Japan and
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Egypt has decreases, where as India, China, the former Soviet Union, and several
smaller countries had large increases during this period.
Table 1. Irrigated land in the Major irrigating countries of the World for selected years from 1970 to 1991
in thousands of hectares.
3.
Irrigation Methods
Irrigation methods can be divided into four main types- surface, subsurface,
sprinkler, and trickle irrigation- and many subtypes. Surface irrigation is the oldest type
and still constitutes about three-fourths of all irrigation. Subsurface irrigation is limited in
its adaptation. Sprinkler irrigation can be used in any climate, is the most popular
method in humid regions, and is still expanding in use. Trickle irrigation, the newest type,
makes the most efficient use of water. Sprinkler and trickle irrigation are both generally
more efficient than surface irrigation, but they also involve more investment in
equipment.
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3.1.
Surface irrigation
Surface irrigation includes both furrow and flood types. These are distinguished
by whether the water flows in distinct lines or floods the entire surface.
a.
Furrow irrigation: Furrow irrigation uses the ridges formed by cultivation to guide
water across fields of row crops. The rows can be fed with siphon tubes or spiles, or in
groups from ditch turnouts. A serious erosion hazard occurs because the water flows
where cultivation has loosened the soil. Irrigation furrows without eroding them is difficult
on slope gradients that are steeper than 2%. Large streams of irrigation water would
erode the soil, and small streams will flow only a short distance. Uniform irrigation
requires that irrigation furrows be no longer than the distance that water flow during onefourth of the irrigation period. The maximum length of the irrigation rows, therefore,
depends on the soil infiltration rate and erodibility, the slope, and the amount(depth) of
water to be applied. Table 2 shows estimates of appropriate row lengths for various
conditions.
Table 2. Suggested maximum lengths in m of cultivated furrows for different soils,
slopes, and depths of water in cm to be applied
A recently developed method known as surge irrigation has enabled some
irrigators to improve irrigation uniformity on longer rows. Water is applied alternately to
two sets of furrows for periods of 15 to 30 minutes. Switching the water allows the wet
part of the rows to partially seal as the water soaks into the soil. Then, when the water is
turned into these rows again, a surge flows quickly across the partially sealed section
into the dry part of the rows to continue the irrigation. Surge irrigation improves efficiency
by reducing the amount of the excess water infiltrating into the upper part of the rows.
Another approach to improving furrow irrigation efficiency is by stabilizing soil
structure with soil conditioners or polymers. One such compound is Krilium. These
materials stabilize soil structure without modifying the soil fertility. The soil conditioning
approach that is most affordable, is to add a small amount of soluble soil stabilizer to the
irrigation water during the early part of an irrigation period. The stabilizer helps the soil
resist erosion and maintains an infiltration rate two to three or more times as fast as that
of the unstabilized soil (Shainberg and Levy,1994). The most effective materials
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identified for this purpose are very high molecular weight (>107 g/mole) anionic
polyacrylamides (PAMs).(formed by polymerization of acrylamides ([-CH2-CHCONH2]x) or related monomers.
Furrow irrigation can achieve about 60% efficiency in water use. The 40% is lost
by evaporation, by deep percolation in the upper ends of the rows and in the most
permeable soils, and runoff from the lower end of the rows.
b.
Flood irrigation: The three main types of flood irrigation are: basin irrigation,
border irrigation, and wild flooding. Basin irrigation is a simple method and is probably
the oldest method of all. It was practiced in Egypt more than 5000 years ago and is still
used for long-term flooding of paddy rice or for shorter periods for many other crops.
Basin irrigation requires a narrow ridge between 15 to 50cm high on all sides of each
area to be flooded. The entire basin should be as level as possible. Basins range in size
from those designed to irrigate individual trees or small areas of vegetable crops to rice
paddies that occupy several acres. Their maximum size may be limited by elevation
changes, by the area that the available water supply can cover uniformly, or by cropping
factors.
Border irrigation can be described as elongated basin with a gentle slope in the
long direction.Water supplied at the upper end flows down the length of the border as
though it were a very wide furrow. Borders range from 3 to 30m wide and must be level
across their width so the water will spread uniformly across them. Their lengths are
about the same as those of furrows on similar soils and slope gradients. Extensive land
leveling is often required because the topography must be smoother than for furrow
irrigation.
Wild flooding is used on uneven topography to irrigate pasture or hay, and
sometimes small grains. The pastures may have slope gradients as steep as 10 or 15%.
Water floods across the land from ditches on the ridges. The irrigator uses shovel to
make small furrows and ridges that guide water to any areas that would otherwise
remain dry. Wild flooding uses both water and labour inefficiently, but it irrigates land
that cannot be managed by other methods of surface irrigation.
3.2.
Subsurface irrigation
Subsurface irrigation, which is also known as subirrigation is actually a
controlled drainage system in which ditches are used. Water is removed during wet
seasons and added during dry seasons so that the water table is always at a controlled
depth. That depth might be as little as 30 cm for shallow-rooted vegetation in a coarse
sandy soil, or as great as 120 cm in some loamy soils. The surface soil should be dry
but most of the root zone should be moist. The field can even be cultivated and irrigated
at the same time.
Relatively little land is subirrigated because the required conditions such as high
permeability, shallow water table and a slope gradient less than 0.5% are not always
fulfilled.
3.3
Sprinkler Irrigation
Sprinkler irrigation is much newer than surface irrigation because the necessary
pipes, pumps and power supply were not available long ago. Advantages such as
portability, adaptability to a wide range of soil and topographic conditions with little or no
land smoothing and good control of water application have made sprinkler irrigation
popular. Efficient water application may save energy, reduce leaching of nitrates and
other nutrients, and help avoid erosion. Disadvantages include high equipment and
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operating costs, moving lines in muddy conditions and salt damage to some plants if
poor-quality water is used.
Most field sprinklers use a rotating sprinkler head. Although the sprinklers may be fixed
in permanent locations for limited areas of high-value crops, they are usually mounted
on either moving or portable lines.
Portable lines : Hand-moved sprinkler lines are conventional in many areas. They are
used on a regular schedule throughout the growing season in arid climates, but in humid
climates they are often kept in storage except during periods of drought. The application
rate should be slow enough to avoid runoff.
The number of irrigation lines needed depends on the size of the field, the irrigation
period and frequency, and the area irrigated by each line. Most sprinklers spray past the
next sprinkler position for complete coverage. Sprinkler irrigation is usually about 75%
efficient in use of water- the other 25% is lost by evaporation and by deep percolation
where the water application is heavier than average. Excess wind distorts the pattern
and reduces efficiency.
Rolling Lines: One way to make sprinkler lines easier to move is to mount them on
wheels. Some lines run through the hubs of large wheels. Other systems use small
wheels on each side of the line. Some rolling lines have long, flexible supply lines so
they can be motor-driven to roll during the irrigation period.
Center-Pivot Systems: Center-pivot irrigation systems are the most convenient and the
most expensive movable systems. Either a well or a buried main line supplies water to
the pivot point. The sprinkler line is supported at about 30-m intervals by two-wheeled,
motor-driven towers that carry the line at a height of about 2 to 3m). Either hydraulic or
electric power moves the towers in concentric circles at a rate proportional to their
distance from the pivot point.
The original center-pivot irrigation systems sprayed water upwards from the top of he
line some 2 or 3 m above the ground. This design works well except that it allows a
significant amount of water to evaporate before reaching the ground. Some of the newer
lines have drop sprinklers that extend downward from the line and spray downward, thus
achieving water efficiency up to 80%.
3.4.
Trickle Irrigation
Trickle irrigation also called drip irrigation, supplies water to individual plants
through small plastic lines. It is the newest and only one efficient enough to deliver 90%
of the irrigation water to the plant root zone. Water is supplied either continuously or so
frequently that the root zone is constantly moist.
Trickle irrigation is especially suitable for watering trees or other large plants. Much of
its use has been in orchards and vineyards, but it has also been used to irrigate many
row crops, including various vegetables and fruits. Its advantages are greatest where
areas between plants can be left dry. It has no advantage for close-growing vegetation
such as pastures, or small grain crops. Trickle irrigation in the United States, for
example, increased from 40ha in 1960 to about 600,000ha in 1991, out of the worldwide
total of about 1,770,000 ha. Nearly half of the trickle irrigation in the United States is in
California. Erosion is not a problem because there is no runoff.
A bonus with trickle irrigation is its ability to use water with a higher salt content than any
other method up to about 2500 mg/liter. The constant flow of water from the trickle
emitter towards the outer edges of the plant root zone carries the salt along with it. Salt
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concentrations become very high in the dry areas between plants but not in the active
root zone.
Trickle irrigation saves water, functions well in all but extremely coarse – or fine-textured
soils, works on almost any topography without causing erosion, and requires little labour
after it has been installed. The main problems are high equipment costs and plugging of
the line by sediment, salt encrustations, or algae. A recently developed technique is the
use of subsurface drip irrigation in which the drip system lies below the earth surface in
the immediate vicinity of the root zone. Below are the prons and cons of such a
subsurface drip irrigation system. (SDI).
Advantages
a. more efficient water use
b. Greater water application uniformity (resulting in better control of the water, nutrients,
and salts)
c. Enhanced plant growth, crop yield and quality
d. Improved fertilizer and pesticide management
e. Less runoff into streams
f. Less nutrient and chemical leaching and deep percolation
g. limited weed growth and reduced weed germination
h. decreased energy costs
i. less disease pressure due to drier and less humid crop canopies
j. field operations can occur during irrigation
k. no soil crusting due to irrigation
l. easy to automate
m. less mechanized parts
n. less irrigation system corrosion
o. Reduced weather-related application constraints (especially high winds and freezing
temperature)
p. Well suited to widely spaced crops provided sufficient soil wetting pattern is achieved
Drawbacks
a. Less developed technology than some alternative irrigation systems
b. high initial investment cost
c. may require more complex water quality management than surface drip irrigation
d. more design components (i.e. flush lines, etc.) than surface drip irrigation
e. fewer visual indicator of system operation
f. cleaning water through filtration required
g. subsurface repairs more difficult
h. rodent problems more difficult to solve
i. Roots from perennial crops may pinch dripline
j. Restricted plant root development
k. System is spatially fixed, so annual crop spacing/orientation must be carefully
matched
l. Salinity may be increased above dripline, increasing salinity for small germinating crop
m. persistent maintenance requirements
4.
Water for irrigation
The water supply for irrigation is usually more limited than the area of suitable
soil. Most of the available water in arid regions is already in use. Inadequate supplies of
water result in competition among irrigators, would-be irrigators, livestock operations,
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and other users such as cities and factories. Furthermore, streams and lakes need to
have water left in them for fish, wildlife, recreational and navigational uses. Water
shortages are becoming more and more common along with population increases.
Poster(1992) indicates that global water use now is more than three times as much as it
was in 1950. Overuse of available water causes dropping water tables, shrinking lakes,
and disappearing wetlands. Less water is left then to sustain aquatic life and for people
to use. A nation needs about 1000m³ of water per person per year to meet modern living
standards, but more than 200 million people in 26 countries now have less than that
amount available (Postel, 1992). Water for irrigation is either essential or highly
beneficial for agriculture in all 26 of these countries, but the water supply is not equal to
the need.
Water from streams and lakes is used for irrigation on both small and large
scales. Irrigation began thousands of years ago with simple diversions of stream water
onto nearby bottom-land. Large diversions are distributed to many users by canal and
ditch systems. Thus, bottomland is usually irrigated first . Higher land requires a long
canal to carry water diverted far upstream, a high dam, or a means of lifting the water.
Many lifting devices have been used, including water wheels, Archimedean screws, and
pumps. Irrigation by diversion is limited by low stream flow during dry seasons. An
upstream reservoir can make the water supply more reliable.
Rainwater has always been a favoured source of water for irrigational purposes
when possible. In other words, when there is a sufficient supply of it. Land surfaces are
sometimes treated to decrease infiltration and make more runoff water available for
irrigation and other uses. The runoff can be stored in a reservoir to supply water for
households, livestock, gardens, small fields, and wildlife. Rainwater harvest has been
practiced for thousands of years and can be used where average annual rainfall is as
low as 50 to 75 mm. Ditches were used in ancient times to harvest rainwater from
hillsides or gentle slopes where the soil permeability was slow. Newer practices include
decreasing soil permeability by treatment with sodium salts or using water-repellent
compounds such as asphalt or paraffin to resist infiltration.
Harvested rainwater may be guided directly to a field or garden and distributed through
some form of irrigation system.
Storage reservoirs are needed when collected water is to be saved for later use.
Evaporation can be a problem because the air is usually very dry in such areas. A deep
reservoir with minimum surface area helps reduce evaporation.
Groundwater also supplies reliable water for irrigation. The conditions are the
availability of Aquifers and that the groundwater recharge is not ignored in order to
avoid the depletion of the Aquifer.
5.
Hazards of irrigation
Some notable hazards occur along with the benefits associated with irrigation.
One is the economic risk from high irrigation costs, and another is the spreading of
water-related diseases. A third hazard is productivity loss caused by increased erosion
and by excess salts accumulating from improper use of water.
Irrigation expenses arise from obvious factors such as the costs of buying water or
drilling a well, purchasing additional equipment, paying for the increased labour required
to apply water to the land and utilization of fertilizers, usually needed to increase crop
yield. Another expense is caused by the constant struggle to maintain the water
distribution system and to keep it free of pests such as burrowing animals and weeds.
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Still another problem that needs consideration is the depletion of sweet water
normally used for irrigation. More than half the world's groundwater supplies are already
saline, and the proportion is increasing as demand for water outstrips supply. In practical
agricultural use, a common source of saline water is groundwater. Salinity of
groundwater can be man-induced or natural. When fresh groundwater is pumped from
aquifers that are in hydraulic connection with seawater, the change in gradients as a
result of pumping may result in a flow of salt water from the sea towards the well and
seawater intrusion may result. Groundwater could also become brackish when a well,
located close enough to saline water underlying freshwater, is pumped at a rate
sufficient to cause the salt water to be drawn into the well in an upward shaped cone, a
mechanism referred to as upconing. It has been estimated that in the USA over twothirds of the continental area are underlain by saline groundwater that could intrude on
freshwater supplies as a result of upconing. Salinity already limits crop production on
about half the world's irrigated land and there may be as much as twice as much saline
land as there is land currently under irrigation. Salts exert both general and specific
effects on plants which directly influence crop yield. Additionally, salts affect certain soil
physico-chemical properties which, in turn, may affect the suitability of the soil as a
medium for plant growth. The suitability of soils for cropping depends heavily on the
readiness with which they conduct water and air (permeability) and on aggregate
properties which control the friability of the seedbed (tilth). Poor permeability and tilth are
often major problems in irrigated lands. Contrary to saline soils, sodic soils may have
greatly reduced permeability and poorer tilth. Excess salinity within the plant root zone
reduces plant growth primarily because it increases the energy that must be expended
to acquire water from the soil of the root zone and to make the biochemical adjustments
necessary to survive under stress. This energy is diverted from the processes which
lead to growth and yield.
The suitability of a saline water for irrigation is dependent upon the conditions of use,
including crop, climate, soil, irrigation method and management practices. For the
purpose of identifying the levels of water salinities which could serve as guidelines to
determine the suitability of saline water for irrigation, it is possible to give a water
quality classification scheme.
Water class
Electrical
conductivity dS/m
Salt concentration Type of water
mg/l
Non-saline
<0.7
<500
Drinking and irrigation water
Slightly saline 0.7 – 2
500-1500
Irrigation water
Moderately
saline
2 – 10
1500-7000
Primary drainage water and
groundwater
Highly saline
10-25
7000-15 000
Secondary drainage water
and groundwater
1 5 000-35 000
Very saline groundwater
>45 000
Seawater
Very
saline
Brine
highly 25 – 45
>45
Table 3 Classification of saline waters
As seen in the figure below, only very tolerant crops (hardly any conventional crops) can
be successfully produced with waters that exceed about 10 dS/m in EC. Few generallyused irrigation waters exceed about 2 dS/m in EC. Many drainage waters, including
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shallow groundwaters underlying irrigated lands, fall in the range of 2-10 dS/m in EC.
Such waters are in ample supply in many developed irrigated lands and have good
potential for selected crop production. Reuse of second-generation drainage waters for
irrigation is also sometimes possible and useful. Such waters will generally have ECs in
the range 10-25 dS/m. But the crops that can be grown with them are atypical and much
less experience exists with this regard.
The salt tolerances of various crops are conventionally expressed (after Maas and
Hoffman 1977), in terms of relative yield (Yr), threshold salinity value (a), and percentage
decrement value per unit increase of salinity in excess of the threshold (b); where soil
salinity is expressed in terms of ECe, in dS/m), as follows:
Yr = 100 - b (ECe - a)
where Yr- is the percentage of the yield of the crop grown under saline conditions
relative to that obtained under non-saline, but otherwise comparable, conditions. This
use of ECe to express the effect of salinity on yield implies that crops respond primarily
to the osmotic potential of the soil solution.
There are thousands of salt-tolerant plants that could produce food, fuel, fodder, fiber
and other products or that could be used for environmental greening. Examples include
the following:
Common name
Maximum permissible2 ECe dS/m
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Moderately tolerant
Weeping bottlebruch
6-8
Oleander
6-8
European fan palm
6-8
Blue dracaena
6-8
Spindle tree, cv. Grandiflora 6-8
Rosemary
6-8
Aleppo pine
6-8
Sweet gum
6-8
Tolerant
Brush cherry
>83
Ceniza
>83
Natal palm
>83
Evergreen pear
>83
Bougainvillea
>83
Italian stone pine
>83
Very tolerant
White iceplant
>103
Rosea iceplant
>103
Purple iceplant
>103
Croceum iceplant
>103
Boron tolerance limits for agricultural crops (after Maas 1990)
Common name
Botanical name
Threshold1 g/m3
Moderately tolerant
Broccoli
Brassica oleracea botrytis 1.0
Pepper, red
Capsicum annuum
1.0-2.0
Pea
Pisum sativa
1.0-2.0
Carrot
Daucus carota
1.0-2.0
Radish
Raphanus sativus
1.0
Potato
Solarium tuberosum
1.0-2.0
Cucumber
Cucumis sativus
1.0-2.0
Lettuce
Lactuca sativa
1.3
2
Cabbage
2
Brassica oleracea capitata 2.0-4.0
Turnip
B. rapa
2
2.0-4.0
Bluegrass, Kentucky Poa pratensis
2.0-4.0
Barley
Hordeum vulgare
3.4
Cowpea
Vigna unguiculata
2.5
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Oats
Avena sativa
2.0-4.0
Maize
Zea mays
2.0-4.0
Cynara scolymus
2.0-4.0
Nicotiana tabacum
2.0-4.0
Brassica juncea
2.0-4.0
Melilotus indica
2.0-4.0
Cucurbita pepo
2.0-4.0
Cucumis melo
2.0-4.0
B. olearacea botrytis
4.0
Medicago sativa
4.0-6.0
Vicia benghalensis
4.0-6.0
Petroselinum crispum
4.0-6.0
Beet, red
Beta vulgaris
4.0-6.0
Sugarbeet
B. vulgaris
4.9
Tomato
Lycopersicon lycopersicum 5.7
Artichoke2
2
Tobacco
Mustard2
Clover, sweet
2
Squash
Muskmelon
2
Cauliflower
Tolerant
Alfalfa2
Vetch, purple2
Parsley
2
Very tolerant
Sorghum
Sorghum bicolor
7.4
Cotton
Gossypium hirsutum
6.0-10.0
Apium graveolens
9.8
Asparagus officinalis
10.0-15.0
2
Celery
Asparagus
2
All irrigation systems require proper maintenance. The major cause of failure in
micro-irrigation systems and SDI worldwide, for example, is clogging. The emitters in
SDI systems are small, therefore for SDI high-quality ground water should be used, but
with proper precaution and maintenance, SDI also can be used with surface water and
other, lower quality, waters.
Clogging hazards for the SDI systems, regardless of the water source, fall into
three general categories: physical, chemical and biological. Physical clogging hazards
are usually removed with screen filters. Sizing of screen filters is based on the maximum
particle size allowable by the designed SDI system, quality of the irrigation water, the
flow amount between required cleanings, and the allowable pressure drop across the
filter.
Sand media filters are usually used to filter organic materials. Particle size of the
media is selected according to the desired degree of filtration. Chlorine injection is
usually used to assure that any unfiltered biological material does not accumulate
elsewhere in the SDI system. If the microbiological load of the irrigation water is high, a
low concentration (1 to 2 ppm) of chlorine should be injected continuously. Chlorine gas
is the most effective and least expensive chlorine source for injection but is hazardous
and must be used with caution. Sodium hypochlorite (liquid bleach), is safer and easy to
obtain and use.
Two major chemical clogging hazards to SDI systems are precipitation of calcium
carbonate and formation of iron ochre (slime). Precipitation of CaCO3 can occur in one
of two ways- evaporation of water, leaving the salts behind, or change of solubility due to
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change of solution characteristics (mainly temperature and pH). Evaporation isn’t
usually a problem in SDI systems, but chemistry changes can cause CaCO3 to
precipitate. Increased pH decreases CaCO3 solubility, raising the potential for
precipitation. A water analysis can be used to determine the predisposition of the water
source to CaCO3 precipitation. If precipitation is likely to occur, acid injection is used to
lower pH and decrease the propensity for CaCO3 precipitation. An acid formulation of
nitrogen fertilizer can be used for pH control and nitrogen fertilization concurrently.
At very low concentrations, it may be possible to keep iron in solution by adding
acid to lower the pH. Other concentrations will require more treatment, however. One
hazard of iron is bacterial interaction with iron. Various bacteria can react with ferrous
(+2 charge) iron through an oxidation process. The resulting ferric (+3 charge) iron is
insoluble. The ferric iron eventually will be surrounded by filamentous bacteria, forming
the slime (gel) that clogs emitters. Chlorination is used to oxidize the ferrous iron. The
resultant ferric iron is filtered before it can reach and clog the emitters.
Some irrigation systems have never produced a marketable crop, most often
because of sodic soils. Others have failed after some decades of use, most commonly
because of erosion and sedimentation problems. Even so, some irrigation systems have
endured, and it is unfair to suggest that the failure of some means that all will fail.
6. An example of water misuse in irrigated agriculture of Sana’a Basin, Yemen
The low rainfall (200-250 mm/a) and the lack of surface water sources in the
Sana’a basin led on the past to almost total dependence on dry farming practices for
agriculture. The limited availability water led to a special land use arrangement in which
every agricultural plot was connected to an attached catchments area along hilly slopes
for rainfall harvesting. Agricultural practices were mostly of a labour-intensive nature to
enhance the moisture holding capacity of the soil. The traditional cropping pattern was
dominated by cereal crops like wheat, sorghum, and barley. Cash crops were rarely
cultivated under dry farming practices since they demand water quantities and for some
crops, such as grapes, high initial financial investment.
The introduction of drilled boreholes in the early 1970s facilitated an easy access
to a reliable water source that could be used for irrigation. As a result, many farmers
invested in deep boreholes and thus converted from dry farming practices into irrigation
agriculture.
In order to maximize the return on their investment, farmers changed the cropping
pattern in the 1970s-1980s from the low-profit cereals into high-profit cash crops. The
main reason that discouraged farmers to continue with a cropping pattern dominated by
cereals were (i) the government policy to subsidize imported grains, thus lowering the
market price of cereals, (ii) the rapid growth of Sana’a, increasing the demand for cash
crops, particularly qat, and (iii) high profits from cash crops and low pumping costs
resulting partially from subsidizing energy. Within the basin area it is estimated that, at
present, groundwater irrigated cereal cultivation covers less than 5% of the agricultural
land in the basin.
The expansion in irrigated agriculture resulted in heavy groundwater mining
throughout the basin area, leading to an annual 3-6 meters drop in groundwater level.
The Tawilah aquifer, the main productive aquifer in the basin, is also the principal water
source for the growing municipal water demand, particularly within the capital Sanaa. It
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has been estimated that the water deficit in the basin area was around 135 Mm³/a in
1990 and it is projected to reach 400 Mm³/a by the year 2010 (HWC, 1992a).
Obviously, the decision to irrigate is in the hands of farmers who, in many cases, are
illiterate and do not believe in scientific techniques to determine water requirements.
Given the slow and limited recharge of groundwater, the irrigated agriculture and thus
the livelihood of most of the rural population within the basin area, as well as the
municipal water sector are at risk, especially given the high cost of the alternative water
source. It can be argued that water in the agricultural sector is not being utilized in an
economic productive manner.
Within the Sana’a basin qat and grapes are roughly estimated to consume around
40% and 25 % of the total irrigation water consumption, respectively (HWC,1992). Even
though groundwater abstraction in effect is subsidized and thus may be considered
relatively cheap, many qat farmers in the Sana’a area use expensive pipes and plastic
tubes, rather than earthen channels, to convey groundwater from the wells to the various
and often distant plots. The reason behind these investments is not to save water as
such, but driven usually by the limited supply ( many shareholders in a single well).
The approximate groundwater use for irrigated agriculture, and the area under qat and
grapes within the Sana’a basin in 1996, are shown in the table 3 below.
Table 4. Total groundwater use for agriculture and the area under qat and grapes in the Sana’a basin area
in 1996.
As seen in the table, the total groundwater abstraction for irrigated agriculture was 205250 mm³/a in 1996., with 82-100 Mm³/a and 51-63Mm³/a for qat and grapes cultivation,
respectively, on a total area of around 5,500-8,000 ha and 5,400-7,900 ha under qat and
grapes, respectively.
7.
Conclusion
The future expansion of food production will be increasingly dependent upon
sound irrigation and water management and upon the concurrent maintenance of the
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present agricultural resources and the environment - two of the most challenging tasks
facing mankind today.
Nearly two-thirds of the increase in crop production needed in the developing countries
in the next decade must come from increases in average yields, a fifth from increases in
arable lands, and the balance from increases in cropping intensity (FAO 1988). About
two-thirds of the increase in arable lands is expected to come from the expansion of
irrigation. Thus it is concluded that the needed increases in food production in
developing countries must come primarily from existing cropland, mostly irrigated land.
The present rate of expansion in irrigation has recently slowed to less than 1 percent per
year (CAST 1988). The reasons for this slowing down in expansion rate are many.
Among them are the high costs of irrigation development and the fact that much of the
suitable land and water supplies available for irrigation have already been developed;
progressively more expensive and socio-economically less favourable areas are left for
further expansion. Water is the limiting constraint for almost 600 million hectares of
potentially suitable arable land. Also, the overall performance of many irrigation projects
has been less than expected due to inadequate operation, maintenance and inefficient
management (FAO 1990). It is not unusual to find that less than 60 percent of the water
diverted or pumped for irrigation is actually used in crop transpiration. Furthermore,
improper irrigation causes environmental and ecological problems.
Water availability for irrigation could be enhanced through judicious and proper
use of saline water and the recycling of drainage waters for irrigation. Considerable
amounts of such water are available in various places in the world, including Australia,
Egypt, India, Israel, Pakistan, the USA, and the former USSR. Waters generally
classified as unsuitable for irrigation can, in fact, be used successfully to grow crops
without long-term hazardous consequences to crops or soils, with the use of improved
farming and management practices. The development of crops with increased salt
tolerance and the adoption of new crop and water management strategies will further
enhance and facilitate the use of saline waters for irrigation and crop production, while
keeping soil salinity from becoming excessive. The reuse of drainage waters for
irrigation will also help to conserve water and to minimize the hazardous effects of
irrigation on the environment and ecology.
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8. References
•
•
•
J.D. Rhoades (1992),The use of saline water for crop production, FAO irrigation
and drainage paper 48
Monique Mainguet (1999), Aridity, droughts and human development,
Mohammed I. Al-Hamdi (2000), ‘Competition for scarce groundwater in the
Sana’a plain, Yemen’, Dissertation Thesis
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