John Gowing
School of Agriculture, Food & Rural Development
University of Newcastle upon Tyne, Newcastle, UK
Gowing, J. 2006. A review of experience with aquaculture integration in large-scale irrigation systems. In M. Halwart & A.A. van Dam, eds. Integrated irrigation and aquaculture in West Africa: concepts, practices and potential, pp. 7–16. Rome, FAO. 181 pp.
Abstract
Irrigation systems do not only supply water to field crops but have many other productive and non-productive uses. Aquaculture is a productive, non-consumptive use of water that does not compete with irrigation. Theoretically, integration of aquaculture into irrigation systems may contribute to more efficient use of scarce freshwater resources. However, there is a need to study opportunities and constraints to integration of aquaculture in irrigation systems. Within formal, large-scale irrigation systems four functional subsystems can be distinguished: water source, delivery, use and disposal. While aquaculture may be integrated within any of these, this study focuses on canals and storage ponds within the delivery subsystem. Both floating cages and pens may be used to grow fish in these structures. Because of the great variability in conditions both within and between irrigation systems, conditions in storage structures must be evaluated carefully. Aquaculture is more demanding than irrigated crops in terms of continuity of water supply. Other points of attention are high loads of agrochemicals in return flows from agriculture fields, slow response times of water regulation in large irrigation systems, and the effect of aquaculture structures on water conveyance in the canals.
Introduction
It is generally acknowledged that since the 1960s technological advances in agriculture, collectively known as the “Green Revolution”, have provided the means for the developing world to feed its growing population. The dominant role of irrigation in promoting food security is also recognized. Irrigated agriculture in developing countries produces 40% of food and agricultural commodities and in Asia even 60% of total production. The corollary of this dependence on irrigated agriculture is that wherever irrigation is practised, it is one of the largest consumers of water. Globally 70% of all water extracted from rivers and underground aquifers is used for irrigation and in low-income countries the dominance is still greater, being 90% of total abstraction (Seckler et al., 1998). The distribution of irrigated land, however, is skewed towards a few countries and shows wide regional variations.
During the 1990s there was a dramatic shift in priorities for water resource allocation and development. Water scarcity has become a prominent issue with the result that irrigation is seen as both a profligate and a low-value user of water resources. There is great pressure to use water more effectively and often this involves reallocation of resources away from irrigation and in favour of municipal, industrial and environmental uses (Molden et al., 2001; Hamdy et al., 2003). Although this is not currently a major issue in sub-Saharan Africa, given projected population increases, it is expected that future food security depends upon rapid expansion of irrigated area and water scarcity will become a constraint (Gowing, 2003). This is leading to new perceptions of the need for proper understanding of the multiple uses of water within irrigation systems, for economic evaluation of non-irrigation uses (Meinzen-Dick and van der Hoek, 2001) and greater recognition of the linkages between water management activities and aquatic ecosystems (Bakker and Matsuno, 2001).
It is a common perception that irrigation systems supply water only to field crops, but the true picture is more complicated. Even within the agricultural sector, irrigation systems supply water not only for the main fields, but also for home garden cultivation and for livestock. Other productive uses may include fishing, harvesting of aquatic plants and animals and a variety of other enterprises such as brick-making. Important environmental functions may include water supply to trees and other permanent vegetation, which provide an amenity to the local population and support biodiversity in plants, birds and other animals. Other non-productive uses may include laundry, bathing and domestic supply. Important implications for water management and policy arising from recognition of these multiple uses include considerations of: the valuation of water in irrigation systems, how systems are managed to maximize productivity and how water is allocated to alternative uses (Meinzen-Dick and Bakker, 2001).
The focus here is on fish production within irrigation systems and, in particular, opportunities for poor people to derive a livelihood benefit from this activity. It is evident that the extensive hydraulic engineering works associated with large-scale irrigation development have had a profound negative impact on many river ecosystems, which is reflected in a dramatic loss of biodiversity (Halls et al., 1999; Petr and Mitrofanov, 1998). Where this has resulted in loss of important subsistence fisheries, the impact of this change has generally been felt disproportionately by poor people. The opportunity may exist within the newly created irrigation systems to mitigate this negative impact by promoting complementary development of fish production, but this has generally been overlooked. Important fisheries do exist in irrigation canals in some countries, e.g. China (Tapiador et al., 1977), Pakistan (Javid, 1990), Egypt (Sadek and El Din, 1988), Sudan (Coates, 1984) and Thailand (Swingle, 1972), but little consideration has been given to the replacement of lost fishery potential through systematic development of aquaculture potential. Surprisingly little research evidence exists linking fish production to irrigation, either in terms of the impact on natural fisheries or the potential created for new managed fisheries. Furthermore, the linkages between fisheries management institutions and water management institutions are generally weak.
The irrigation environment
This paper is concerned with relatively large-scale irrigation systems providing full water control. There is no universally accepted classification, but irrigation systems can be defined in terms of their physical characteristics and also of their organizational characteristics. Size of the command area alone is not the distinguishing feature, since a 500 ha scheme may be classified as a “major” or “large-scale” scheme in one country, but be regarded as “minor” or “small-scale” in another. A more useful definition of the scope of this paper is that it concerns:
All formal large-scale irrigation systems comprise four functional subsystems: water source, water delivery, water use and water disposal. Figure 1 is a representation of these subsystems and their water flow linkages. A minority of irrigation systems include pipelines instead of open channels for part of their water delivery and/or water use subsystems, but open channel systems are more common.
As the size of command area may vary, so will the capacity and size of the primary and secondary canals. Typically, a primary canal may have a 5 to 50 m bed width and 1 to 5 m depth. In most circumstances, it will be designed to operate more or less continuously throughout the irrigation season. Design velocity depends on the nature of the bed material, whether it is lined or unlined, and whether it carries clear or sediment-laden water. Secondary and tertiary canals deliver water to progressively smaller sections of the command area and have accordingly smaller bed widths and depths. They are less likely to operate continuously.
Water storage structures provide operational flexibility in that they buffer differences between supply and demand. Distributed storage within the water delivery system is not always present; where it exists it may be provided as a buffer between primary and secondary canals or between secondary and tertiary canals. Storage may also exist within the water use subsystem as on-farm storage; either as farm ponds or as paddy basins. Important differences between storage structures are the duration and depth of storage, and the frequency and rate of variation.
It is sometimes reported that irrigation systems provide only a narrow range of habitats with much less diversity than natural rivers (Redding and Midlen, 1991) and sometimes that they provide a wide range of habitats (Fernando and Halwart, 2000). Notwithstanding this seeming disagreement, which may also reflect ecoregional variations, it must be recognized that the human-made environment of a canal network differs from the natural environment of a river system in several important ways. Firstly, the flow regime is generally managed within a narrower range, but may be subject to more frequent no-flow events. Secondly, the presence of water control infrastructure creates physical barriers, which restrict habitat connectivity. Thirdly, water temperature and quality (turbidity, salinity etc.) may differ.
Figure 1. Principal components of a formal irrigation system.
Fish in irrigation systems
Most irrigation systems probably sustain capture fisheries to some extent, although the practice is generally opportunistic. Usually, fish stocks are dependent upon fish entering the canal system from the source. Some species may form self-sustaining populations, but this is limited to those systems having favourable environmental conditions (Fernando and Halwart, 2000). A degree of management of fish stocks will be required to maintain a more productive and sustainable fishery. Such management may involve restocking and introduction of new species, but there is little recorded experience of such measures being adopted in canal systems, except where they have been primarily aimed at weed control through stocking of grass carp (e.g. Armellina et al., 1999).
Aquaculture offers greater control over production and access than is the case with capture fishery and even low-cost, semi-intensive systems can produce 1500–2000 kg/ha/year, which compares favourably with estimates for production from capture fisheries in canals. Cage-based aquaculture has been widely promoted in South and Southeast Asian countries (Beveridge and Muir, 1999) as a technology which can be easily adopted by resource-poor people. Cages can be made cheaply using widely available materials, such as bamboo (for frame) and plastic containers (for floats), but availability of suitable netting can be a constraint. Cages have the advantage for landless people that ownership is required only for the cage and its contents irrespective of the ownership status of the water body provided that access is assured.
An alternative is to produce fish in larger containment structures known as pens. Like cages, their sides are human-made, but an important difference is that the base is the canal substrate itself. This allows access to benthic organisms, providing an additional food source for the fish. At the same time it makes them less suitable than floating cages for canals with widely fluctuating water levels. Pens may be created by enclosing the full width of the canal, or may be aligned along the bank and occupy only part of the canal width (Beveridge, 1996; Haylor, 1993).
In parts of China and Southeast Asia “integrated aquaculture” within the water use subsystem has existed for generations and its introduction into other countries has received considerable attention in recent years. Similarly, the potential for and constraints to harvesting and culturing fish within reservoirs of the water source subsystem are relatively well documented on the basis of experience in many countries. However, opportunities and constraints within the extensive engineered components of water delivery subsystems and water disposal subsystems have been largely neglected.
Case study of integrated aquaculture and irrigation
With this knowledge gap in mind, detailed interdisciplinary research was conducted between 1998 and 2002 at two sites in India and Sri Lanka to investigate potential for and constraints to integration of poverty-focused aquaculture within large-scale irrigation systems. Both sites lie within semi-arid environmental zones and both experience a tropical monsoon climate. Both experience water scarcity. Both are public irrigation schemes, which are managed by large bureaucratic organizations. At the time of the research, both were attempting to introduce institutional reform aimed at devolving some management responsibilities to water users.
Site 1
The Lower Bhavani Project (LBP) is located in Tamil Nadu state in southern India. Its water source is the Bhavani River, which rises in the Nilgiri Hills as a tributary of the Cauvery River. It is a typical valley-side system, comprising a 200 km contour canal serving a command area of 78500 ha. Design capacity at the head of the system is 65 m3 /s with a canal bed width of 32 m and full supply depth around 3 m. In the tail reach, canal dimensions reduce to 4.5 m width and 1 m depth. The canal is unlined for most of its length with generally rocky bed and it carries a low sediment load. The system is about 50 years old.
Since the system experiences water scarcity, a “seasonal sluice turn system” applies. In the dry season of a calendar year (from 16 December to 15 April), water is rotationally released to half the command area for raising irrigated dry crops. In the wet season (from 15 August to 15 December) water is “continuously” issued to the same zone for a rice crop. In the following year, this water allocation pattern is repeated for the other half of the command area.
Irrigation water in the LBP canal is used for livestock watering, laundry and bathing as well as irrigation. Groundwater recharge also depends to a large extent on irrigation. However, the statutes which govern the operation of the system do not recognize the water rights of other stakeholders and recent initiatives to devolve some management responsibilities to water users' organizations have involved only irrigators.
Site 2
Mahaweli System H is situated in the North Central Province in the so-called “dry zone” of Sri Lanka. It was the first system developed under the Mahaweli Ganga Development Scheme and has been operational since 1978. System H lies within the Kala Oya basin and includes 14200 ha of old irrigated areas and 28750 ha of new land developed by the project. It incorporates three main reservoirs, whose limited storage is supplemented by transfers from the Mahaweli system.
There are two cultivation seasons in System H. Maha season (October to March) corresponds to the northeast monsoon and water supply is generally adequate for the full command area to be cultivated with rice. Yala season (April to September) corresponds to the Southwest Monsoon when water supply is limited and a “Bethma” system of cultivation is adopted, in which only 50% of the total area is brought under cultivation and “other food crops” (i.e. non-rice) dominate.
The feature of System H which is of particular interest is the large number of shallow reservoirs (known locally as tanks) distributed throughout the command area. These tanks existed prior to the development of System H. Originally they received runoff from local catchments (normally primary forest), but many are now connected to the new irrigation system by feeder canals, while others receive irrigation return flow in addition to catchment runoff.
Interdisciplinary research teams conducted extended fieldwork at these two sites on three key areas of research, as follows:
Socio-economic studies combined quantitative surveys at household level with qualitative studies based on PRA techniques in sample villages to investigate:
Engineering studies explored constraints to integrating cage-based aquaculture in irrigation canals and in secondary storage structures (tanks) within the irrigation distribution networks:
Aquaculture studies began with in-depth consultations with primary stakeholders, which identified cage-based aquaculture as an appropriate technology. This led to
Lessons for integrated aquaculture in large-scale systems
Opportunities
The aim of the intervention is to improve the livelihoods of the rural poor and to promote food security through improved food supply, employment and income. We are therefore seeking to introduce and promote appropriate technologies for aquaculture within irrigation systems where these are likely to provide livelihoods benefits to poor people without unacceptable impacts on other water users. A general framework for the assessment is shown in Figure 2. In the past, poor people have been largely bypassed in aquaculture development and their specific needs require careful consideration. It is by no means certain that the introduction of small-scale aquaculture technologies will contribute to the alleviation of poverty. There may be better ways for poor people to use their limited resources. A key question is:
Having established that there may be potential demand amongst the target group, the next consideration is whether suitable mechanisms exist to introduce appropriate technologies to them. Within the wider context of agricultural development, there is frequent criticism of the traditional transfer of technology based on training and demonstrations. The alternative farmer first or participatory approach attempts to ensure the relevance of the technology by providing options and ideas and developing capacity to evaluate them and make informed decisions. A key question is:
The next requirement is to remove technical and institutional obstacles that may affect the successful adoption of appropriate technologies by the target group. This requires consideration of opportunities and constraints within the irrigation system. The aim is to identify favourable sites (niches) where the environment is suitable for aquaculture and its introduction will not have any adverse impact on the integrity of the irrigation system or on other water users. The key question is:
It is clear that the range of niches that may be available for aquaculture depends on the nature of the irrigation system and opportunities within each subsystem should be evaluated systematically. The situation can best be understood by considering the four component subsystems previously defined (Figure 1). We can then identify the possible niche opportunities for aquaculture that may exist in each subsystem, as summarized in Table 1. This paper concentrates on niches within the water delivery system.
Figure 2. Framework for assessing opportunities and constraints.
In general, there will be greatest diversity within rice systems of tropical lowlands, but some niches can probably be identified in all irrigation systems. Conditions prevailing in the different niches must be considered carefully, since they can be expected to lead to different constraints. Key differences are:
Constraints
Fish production poses a far greater challenge to system managers than irrigation in that continuity of supply must be guaranteed for the duration of the growing season. Whereas crops will suffer no yield penalty from a discontinuous supply because of buffer storage in the soil, fish will not survive any break in supply. In the case of LBP, the duration of the wet season (4 months) was sufficient to allow a short-cycle aquaculture crop, but reliability was a problem in 10 of the 12 seasons investigated (Li, Gowing and Mayilswami, 2005). Water supply reliability is a crucial constraint to integration of aquaculture and this is related to inherent difficulties in efficient operation of extensive canal systems under upstream control because:
Reliability of conditions suitable for aquaculture depends on design and operation decisions that influence continuity of supply and/or storage, such as:
Flow depth in irrigation canals is typically in the range 0 to 3 metres, while velocity is usually in the range 0.1 to 1.0 m/s. These factors may be important considerations for:
The desirable range of velocity depends upon the fish species and size. Good water exchange is essential for oxygen supply and removal of waste metabolites from fish. If velocity is too slow then this may be a problem. On the other hand, excessive velocities reduce fish growth rates and contribute to food losses. A range of 0.1 to 0.6 m/s is usually satisfactory, although the upper end of this range may create problems with anchorage of containment structures. If access depends on wading or swimming then the lower end of the range will be safer.
Where flow velocity is too high, it is possible to design the fish containment structure in such a way that velocity inside the structure is restricted (Li, Gowing and Mayilswami, 2005). The consequence of this action is that the dynamic loading on anchorages will be increased as will flow resistance. The limited documented experience of aquaculture in irrigation systems does include some cases of uncontrolled development interfering with canal performance. Careful consideration must therefore be given to:
Table 1. Niche opportunities for aquaculture in irrigation sub-systems.
| Sub-system | Niche | Aquaculture technology |
| Water source | Storage dam | Floating cages, stocked fishery |
| Open wells | Stocking | |
| Water delivery | Primary canals | Pens, cages |
| Secondary storage | Floating cages, stocked fishery | |
| Water use | Farm ponds | Stocking |
| Irrigated fields | Integrated rice-fish | |
| Water disposal | Primary drains | Pens, cages |
| Evaporation ponds | Floating cages, stocked fishery |
Any cage or pen introduces additional flow resistance and has a local effect on canal conveyance. The question is: does this represent a serious obstruction or can cages be designed and sited in such a way that they have negligible influence on canal water levels and discharge capacity? The hydraulic performance of any cage (or pen) is essentially the same. Water flowing through a mesh panel imposes a drag force on the panel, which results in a reduced velocity on the downstream side of the panel.
The desirable range of depth depends primarily on the type of fish containment structure. Pens and some cages are designed to sit on the bottom. The effective volume (and therefore stocking density) varies with flow depth and in general fixed structures are not appropriate if water depth varies. This is a particular problem if the structure is situated at the side of the canal and does not utilise the full depth. Depth variation is not important in floating cages, but access becomes difficult if depth exceeds 1.2 m and water depth should always exceed cage height.
Selection of the cage site can make a considerable difference to the influence on the canal and the recommended general approach is to regulate their installation and monitor their impact. Problems are likely to be more severe in a canal system on minimum slope as the effect of increased flow resistance will be to raise water levels upstream, which may affect performance of off-takes and/or lead to overtopping. For a canal built on a steeper alignment with drop structures at intervals to dissipate excess energy, then opportunity to install cages/pens without affecting performance will be greater. In either case, the increase in hydraulic resistance need not be any more severe than the recurrent problem of weed growth. Any site where this is known to be a particular problem and to affect canal performance should be excluded from consideration for installation of cages and/or pens.
Cages will normally be small relative to the canal width, but may be sited in mid-stream or close to the bank. Where a cage/pen occupies less than 25% of canal width and is sited close to the bank, the current will be partly deflected around the obstruction and its effect will be relatively small.
Any storage site within the irrigation system is likely to represent a potentially more favourable niche when compared to any canal site. Duration and reliability constraints are likely to be greatly reduced, thus making the enterprise less risky for target beneficiaries. At the same time, the impact on hydraulic performance is negligible, thus making the introduction of aquaculture less likely to create any problems for system managers. In this context, we are not concerned with any large reservoir that may exist within the water source subsystem. Rather we are interested in niche opportunities within any structures providing relatively short-term storage distributed throughout the water delivery subsystem. These may be:
Work within the research project was focused on secondary storage reservoirs (known locally as “tanks”), which received water from the canal system as well as rainfall runoff from a local catchment and released supplies to a distinct downstream command area. Their key characteristics were found to be (Gowing, Li and Gunawardena, 2004):
It can be assumed that any secondary storage structure will behave similarly, since its function is to buffer flow variations over a short time scale. The shallow depth of such reservoirs results in wide variations in water spread area as water level fluctuates, which introduces constraints on siting of cages/pens. Improved operating procedures can reduce this problem, but it cannot be avoided. At the same time, the turnover rate can be reduced, but retention time will always be quite short (typically 20 days) thus limiting primary productivity and therefore fishery potential.
Conclusions
It is a common perception that irrigation systems supply water only to field crops, but the true picture is more complicated. Growing recognition of the multiple uses of water within many established irrigation systems has revealed many other productive and non-productive uses. Aquaculture is a water-dependent activity, which is productive, but non-consumptive and therefore, in principle, not in competition with irrigation. However, opportunities for and constraints to its integration within irrigation systems have received little attention.
Within formal, large-scale irrigation systems, we can identify four functional subsystems: water source, water delivery, water use and wastewater disposal. Opportunities may exist to incorporate aquaculture within any of these subsystems, but this paper has focused on canals and storage ponds within the delivery system and in particular on opportunities for the introduction of appropriate technologies targeted at the needs of poor people. The aim is to identify favourable sites (niches) where the environment is suitable for aquaculture and its introduction will not have any adverse impact on the integrity of the irrigation system or on other water users.
Conditions prevailing in the different niches must be evaluated carefully, since they can be expected to lead to different constraints. Formal, large-scale systems are generally considered to provide full water control, but aquaculture poses a far greater challenge to system managers than irrigation in that continuity of supply must be guaranteed for the duration of the growing season. Whereas crops will suffer no yield penalty from a discontinuous supply because of buffer storage in the soil, fish will not survive any break in supply. The requirement for strict continuity of supply may lead to lower water use efficiency, particularly during the rainy season. Any storage site within the irrigation system is likely to represent a potentially more favourable niche when compared to any canal site, but problems still remain.
Irrigation canals provide flowing conditions, which may present fewer problems with water quality than occur within storage ponds. However, water temperature and quality (turbidity, salinity etc.) may differ greatly from those occurring in natural channels. Return flows from agricultural fields may carry high loads of agrochemicals and in some cases industrial effluents may also cause significant water quality deterioration.
Local management institutions exist or are currently being created in many large-scale irrigation systems, but representation of non-irrigation water users is generally poor. Water rights, access and charging issues therefore require careful consideration in order to promote multiple use management of irrigation infrastructure.
Acknowledgement
The author wishes to thank the UK Department for International Development (DFID) who funded part of the work reported here for the benefit of developing countries. The comments and opinions expressed herein are those of the author and are not necessarily those of DFID.
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