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Newsletter and Technical Publications

<Sourcebook of Alternative Technologies for Freshwater Augumentation
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 Studies).

Technical Description

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.

Surface Irrigation

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

Figure 15. Flood irrigation method (ESCAP, 1989).

Sub-surface Irrigation

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

Figure 16. Border Irrigation (ESCAP, 1989).

Sprinkler Irrigation

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

Figure 17. Example of Sprinkler Irrigation (ESCAP, 1989).

Drip Irrigation

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

Figure 18. Drip or Microspray Irrigation System (ESCAP, 1989).

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.

Costs

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 on SIDS.

Type of Irrigation System Cost ($/ha)
Surface irrigation $100 - $1 000
Subsurface irrigation $1 000 - $3 000
Sprinkler irrigation $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.

Suitability

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, export crops).

Advantages

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.

Disadvantages

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.

Cultural Acceptability

No cultural inhibitors to the use of these technologies have been identified.

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.

Information Sources

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, Rome.

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, Rome.

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.

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