Newsletter and Technical Publications
<Sourcebook of Alternative Technologies for
in Small Island Developing States>
PART B - ALTERNATIVE TECHNOLOGIES
1. TECHNOLOGIES GENERALLY APPLICABLE TO ISLAND STATES
1.4 Water Conservation
1.4.4 Irrigation Systems
Globally, some 70% to 80% of the total freshwater consumed is used for
agricultural irrigation. However, this is not the case in most SIDS, as
there is normally adequate rainfall for growing food crops (e.g., taro,
cassava, yams, bananas, vegetables coconuts, and breadfruit) without the
need for supplemental water. Irrigation is used mainly for agro-industrial
cropping (i.e., for growing sugar cane and rice) and for limited export
cropping (i.e., for growing squash, pumpkins, and pawpaws or papayas).
Irrigation schemes on small islands, where they exist however, tend to be
relatively small: in general, the very small islands, such as coral
atolls, do not have the water resources required to sustain irrigated
agriculture, although there may be some potential for irrigated
agriculture on the larger small islands. The principal users of water for
irrigation in SIDS are tourist resorts and golf courses. On some islands,
golf courses are irrigated with treated wastewater (see Part C, Case
There are four general types of irrigation systems; namely, surface
irrigation; subsurface irrigation; sprinkler irrigation; and, drip or
microspray irrigation (including, for example, LEPA or low-energy
precision applications). The selection of an irrigation system depends
upon water availability, topography, soil characteristics, crop
requirements, cost, and cultural practices. As an example, surface
irrigation would not be appropriate for fruit crops but would be for rice,
whereas drip or microspray irrigation would not be appropriate for rice
but would be for fruit crops. Likewise, if the crop to be irrigated is
located adjacent to a stream or similar water source, and water
abstractions would not adversely affect downstream users, surface
irrigation might be used in preference to other, perhaps more
water-efficient and more costly, methods of irrigation.
Irrigation water quality does not need to be of as high quality as that
for domestic water supplies, especially when the irrigation water is used
in surface and sprinkler systems. Therefore, reuse of wastewater in the
irrigation of golf courses and park gardens may be possible.
Water is either diverted or pumped from a river or stream onto the area
to be irrigated. There are three common surface irrigation methods;
namely, flood irrigation, where water enters the area uncontrolled and
minimal land preparation is required (Figure 15); border irrigation where
water enters the area as a controlled sheet of water, resulting in less
water loss than flood irrigation, and earth borders must be constructed to
achieve controlled sheet flows (Figure 16); furrow irrigation, where water
is directed into shallow channels that can be constructed using ordinary
farm machinery (Figure 17).
Figure 15. Flood irrigation method (ESCAP, 1989).
Water is directed to the subsoil (crop root zone) in the area to be
irrigated. The water is used to artificially control the groundwater
table, and is normally delivered through perforated pipes buried in the
ground. In some Latin American countries, porous clay pots are buried in
the ground and filled with water that slowly seeps into the subsoil to
moisten the roots of crops. Of the irrigation methods, subsurface
irrigation is used the least.
Figure 16. Border Irrigation (ESCAP, 1989).
Water is applied to the areas to be irrigated in a form that mimics the
action of rain. This is achieved by distributing the water through
pipelines under pressure to various types of sprinkler heads that spray
the water from above onto the crop and land. Sprinkler irrigation systems
can be fixed in place, portable, semi-portable, or mobile. Sprinkler
nozzle types and numbers are selected depending on designed application
rates and wetting patterns (Figure 17).
Figure 17. Example of Sprinkler Irrigation (ESCAP, 1989).
Water is applied to each plant through one or more emitters and/or
microsprayers located at, or just above, ground level (up to 300 mm
above). The system suits areas of high temperatures and limited water
resources. This system allows for the accurate application of water with
minimal loss due to evaporation, poor distribution and seepage, or
over-watering. Due to the small diameter of the emitter openings,
filtration of the water is normally required to reduce potential blockages
in these systems (Figure 18).
Figure 18. Drip or Microspray Irrigation System (ESCAP,
Extent of Use
Irrigation in SIDS is not extensive. Where it is used, it is mainly used
for agro-industrial crops (e.g., sugar cane), export crops, market grade
crops, or for tourism (e.g., golf courses). In some of the small islands
of Indonesia, vegetables are grown in black polythene bags and are
watered, or irrigated, by hand. In Fiji, over 1 000 ha, used for the
cultivation of rice, is equipped with surface irrigation facilities, and
approximately 300 ha of market-garden, for the production of produce, is
equipped with sprinkler irrigation facilities. In one river catchment, on
Rarotonga in the Cook Islands, water is diverted onto terraced
compartments for the growing of "swamp" taro. This irrigation
system has been in operation for hundreds of years. On Guam, approximately
3.5% (1 million m3/year) of the water consumed from the public
water supply system is used for irrigation. In Mauritius, 15 000 ha is
irrigated using surface, sprinkler and drip methods of irrigation, while,
in Barbados, sugar cane is irrigated.
Operation and Maintenance
The success of any irrigation system depends on the timely application
of water to satisfy crop-water requirements. This timing is influenced by
such factors as rainfall, crop, and soil moisture content. An irrigation
schedule (i.e., a timetable stating when to apply water and when to stop)
should be developed for each irrigation system to maximise crop production
with minimal water wastage due to over-watering.
Each irrigation system has at least three components that must be
maintained for trouble-free operation; namely, the headworks, conveyance
system, and on-farm systems. The headworks are any intakes, dams,
reservoirs, pumps, wells, and boreholes required to abstract water from
the source (i.e., river or groundwater) for delivery to the conveyance
system. The conveyance system includes canals, channels, and pipelines
that convey water from the source to the on-farm systems.
The on-farm systems could be surface, subsurface, spray or drip
irrigation systems, and include any subsystems required for (booster)
pumping, filtration, flow control, distribution, turn out, and support.
All irrigation systems require routine maintenance to maximise crop
production and minimise water losses.
Level of Involvement
Generally, surface irrigation methods require less technical skill to
operate and maintain than sprinkler and LEPA methods. LEPA methods require
the highest skill levels. Generally, the more effort put into managing
irrigation systems, the higher the level of crop production and the more
optimal the level of water use.
The capital cost of the headworks and conveyance systems for irrigation
systems are very site-specific. However, on-farm costs, while also
site-specific to a certain extent, do have typical ranges as shown in
Table 4. Operation and maintenance costs also vary depending on gravity-
or pumped operation of the system, specific crop-water requirements,
rainfall, and soil moisture deficit.
TABLE 4. Relative Costs of Various Types of Irrigation Systems Used
|Type of Irrigation System
||$100 - $1 000
||$1 000 - $3 000
||$2 000 - $5 000
|Drip irrigation (LEPA)
||$3 000 - $8 000
Effectiveness of the Technology
Compared to the other irrigation systems, surface irrigation results in
the highest water losses due to surface runoff and deep percolation within
the area irrigated. Much of the water supplied in this manner is not
utilised by the crop being irrigated. Other losses often occur as a result
of water transmission through open channels, due to seepage and
evaporation. Spray irrigation systems are generally more efficient users
of water but less efficient than drip or microspray systems, which are the
most efficient types of irrigation system, with application-rate
efficiencies of over 90%. Drip or microspray irrigation systems also use
less energy than conventional sprinkler systems. Surface irrigation is the
least efficient irrigation method, drip irrigation the most efficient, and
sprinkler irrigation somewhere in between.
Surface irrigation should only be used where there is a sustainable
water source available, and where irrigation abstractions do not adversely
affect the environment and downstream users. While this technique is
widely used in continental applications, it is less widely used in SIDS.
Subsurface irrigation is the least used technology, globally, but could be
used in SIDS if conditions are right. However, sprinkler irrigation
systems, while generally using less water then surface methods, tend to be
more costly and labour-intensive (depending on the specific method used).
The LEPA method is the most suitable from a water-usage point of view but
the cost could be prohibitive to individual farmers. LEPA would be
suitable for large-scale production of high value crops (for example,
Surface irrigation is the least expensive form of irrigation, requiring
few on-farm works and having low-skill, technical operation and
maintenance requirements. Subsurface irrigation, while more complex,
offers the advantage of delivering water directly to the crop root zone.
Sprinkler irrigation systems offer a more efficient use of water and can
be used on a larger variety of crops. Drip irrigation, or LEPA irrigation
systems, provide the most efficient use of water and is best suited for
use in water-short areas.
Surface irrigation results in inefficient use of water resources, due to
the magnitude of losses by evaporation, infiltration below the root zone,
and runoff. While subsurface irrigation delivers water to the root zones,
buried, on-farm pipes create difficulties in addressing water application
problems. Sprinkler irrigation systems have high initial costs and may
require the use of high-pressure supply systems. Similarly, drip or LEPA
irrigation systems have high initial capital costs, and require an
high-level of technical knowledge to operate and maintain the systems.
No cultural inhibitors to the use of these technologies have been
Further Development of the Technology
The most efficient system, the LEPA irrigation system, is also the most
costly. Thus, a desired development would be lower cost systems that would
encourage their wider use.
ESCAP [Economic and Social Commission for Asia and the Pacific] 1989.
Guidelines for the Preparation of National Master Water Plans.
United Nations Water Resources Series No. 65.
FAO [Food and Agriculture Organization of the United Nations] 1971. Integrated
Farm Water Management. FAO Irrigation and Drainage Paper No. 19, FAO,
FAO [Food and Agriculture Organization of the United Nations] 1971. Design
Criteria for Basin Irrigation System. FAO Irrigation and Drainage
Paper No. 3, FAO, Rome.
FAO [Food and Agriculture Organization of the United Nations] 1973. Trickle
Irrigation. FAO Irrigation and Drainage Paper No. 14, FAO, Rome.
FAO [Food and Agriculture Organization of the United Nations] 1977. Crop
Water Requirements. FAO Irrigation and Drainage Paper No. 24
(Revised), FAO, Rome.
FAO [Food and Agriculture Organization of the United Nations] 1982. Organization,
Operation and Maintenance of Irrigation Schemes. FAO Irrigation and
Drainage Paper, FAO, Rome.
FAO [Food and Agriculture Organization of the United Nations] 1982. Mechanized
Sprinkler Irrigation. FAO Irrigation and Drainage Paper No. 35, FAO,
FAO [Food and Agriculture Organization of the United Nations] 1984. Irrigation
Practices and Water Management. FAO Irrigation and Drainage Paper No.
1 (Revised), FAO, Rome.
Israelsen, O.W. and V.E. Hansen 1967. Irrigation Principles and
Practices. John Wiley and Sons, New York.
Jensen, M.E. 1983. Design and Operation of Farm Irrigation Systems.
American Society of Agricultural Engineering, St. Joseph, Michigan.
UNESCO [United Nations Education, Scientific and Cultural Organization]
1994. Efficient Water Use. UNESCO, Paris.
UNDP [United Nations Development Programme] 1973. Design of Low Head
Hydraulic Structures. UNDP, New York.