Paper 11 Small-scale irrigation in sub-Saharan Africa: a balanced view

Richard Carter

Silsoe College, Silsoe, Bedford

Summary: Small-scale or farmer-managed irrigation (SSI), is a promising vehicle for rural development. It can offer the farmer increased security of crop production, while avoiding many of the problems which have been experienced by large scale, formal irrigation projects. The experience of SSI can also provide pointers to the improved management of existing large scale projects. The conditions for successful small farmer irrigation development are set out, together with the implications for present practice. Key areas for research and development are identified, and the importance of national irrigation policy development and strategy formulation is highlighted.

Introduction

Since at least the late 1970s large-scale, formally managed smallholder irrigation has been the subject of increasing criticism by researchers and observers in the field and, to a lesser extent, by donors. The reasons for this barrage of criticism are numerous and have been well documented elsewhere.

In relation to Nigeria's massive investments in such irrigation projects the World Bank commented: "There can be no doubt that this sum [nearly 1 billion Naira], if spent judiciously on the promotion of rainfed agriculture and small irrigation schemes, would have produced vastly superior returns" (World Bank, 1979). Nigeria's generally disappointing experience with large-scale irrigation has been widely commented on in the academic literature (e.g. Adams, 1991; Carter et al., 1983; Kimmage, 1991; Palmer-Jones, 1987; Wallace, 1981). An evaluation (van Steekelenburg and Zijlstra, 1985) of a number of smallholder irrigation projects in Africa funded by the EEC found fault with many aspects of large-scale irrigation schemes, and concluded: "In irrigation projects in sub-Saharan Africa, it would appear that the larger the projects are, and the higher the level of their technology, the poorer is their performance." In Tanzania, in the course of the UNDP-funded FAO-executed project 'Institutional Support to Irrigation Development' (1986-90), criticism of capital-intensive sophisticated schemes requiring high levels of support services grew; irrigation development policy in that country now stresses low-cost improvements requiring only limited Government inputs (Chapman, 1987).

In a number of countries it has only recently been acknowledged that the development of major water resource projects (including irrigation schemes) has damaged or destroyed existing traditional irrigation systems whose economic value was grossly underestimated.

Problems with large-scale irrigation projects

Through the 1980s a reaction against the promotion of large-scale irrigation projects has been gathering pace (especially, but not exclusively in Africa). With some exceptions, national governments, donors and lending agencies are starting to recognise the management issues associated with scale and philosophy of irrigation development. Just as with environmental issues, so with the subject of small-scale irrigation, it has suddenly become respectable not only to criticise large-scale projects but also to promote small-scale alternatives. Figure 1 lists more than 20 of the factors identified by evaluators of large scale irrigation projects around the world

It is perhaps not surprising then that both the British irrigation fraternity and the wider international interest group established networks in the late 1980s to promote small-scale irrigation (SSI). In the UK the Small-Scale Irrigation Working Group comprises over 100 consultants, researchers and academics who meet (generally 30 40 on any occasion) for its regular biannual meetings. The International Irrigation Management Institute (IIMI) has an active research, publication and networking programme in Farmer Managed Irrigation Systems (FMIS). Both these groups have as their aims the understanding and promotion of a form of irrigation development which emphasises the farmer's role in management and which of necessity therefore involves small units of production. This emphasis represents a radial departure from previous practices, as is shown in Figure 1 (for definitions of terms, see Annex).

Figure 1 Reasons identified for poor performance of large-scale irrigation schemes

The current debate is constructive and helpful for a number of reasons. Firstly, because it transcends the sterile confrontations (especially between engineers and sociologists) of earlier years, in particular by attempting to build bridges between disparate disciplines; secondly, because it is producing genuinely new and promising approaches to development practice; and thirdly, because it is able to contribute ideas to the management and rehabilitation of schemes constructed in the old-fashioned 'top-down' style which, though far from ideal in conception, represent major investments which must therefore be made to work if possible.

Top-down smallholder irrigation development an obsolete approach

The inappropriateness of past approaches to irrigation development in sub-Saharan Africa is becoming more widely accepted. It is recognised that past perceptions of the role and place of irrigation in the farming system have led to unrealistic expectations about how irrigation schemes would be managed and operated, and about the benefits which would follow.

Figure 2 (from the Proposal for the Training Course 'Design for sustainable Farmer-Managed Irrigation in sub-Saharan Africa', by Wageningen Agricultural University, Institute of Irrigation Studies and Silsoe College) shows the mismatch which often occurs between the intended and the actual operation of an irrigation scheme.

Figure 2 Mismatch between intended and actual operation of an irrigation scheme

A new approach to irrigation development in sub-Saharan Africa is necessary. This approach should be:

- farmer centred
- truly inter-disciplinary
- characterised by dialogue, interaction and flexibility.

These characteristics and their implications are developed further below. They are at the core of what has come to be called Small-Scale Irrigation (SSI).

The dangers of over-reaction

As with any radical change in approach, there are dangers: the dangers of wholesale rejection of previous practices and standards, and the risks associated with unthinking commitment to the relatively unknown.

Perhaps the most common mistake among non-specialists is to ignore the complexity of smallscale irrigation development. Talk of 'small scale', 'low technology', and farmer-management' can give an impression of delightful simplicity, especially when set against the detailed and multi-disciplinary studies involved in the preparation of large irrigation schemes. On the contrary, small-scale irrigation does not escape any of the complexity of irrigation in general. Indeed because of its scale and its inherently less predictable nature, some aspects of SSI are actually more difficult to handle than the corresponding aspects of large scale schemes. To indicate the nature of this complexity Figure 3 sets out the major issues and subjects which have to be addressed in the design of any public irrigation scheme be it large or small.

In implementation, likewise, the development of small-scale irrigation may not be able to support the wide range of expertise and technical input available to large and costly enterprises. A single engineer may now need sufficient economic, agronomic and management know-how, as well as engineering skill, to combine a number of roles. This presents difficulties for current approaches to training and education, but it does have the major advantage of creating an interdisciplinary professional mentality more akin to that of the farmer than to that of the narrow subject specialist.

Thirdly, the importance of quality (in survey, design and construction especially) needs emphasising. A stress on 'low' technology and low-cost works need not, and should not, mean compromises over quality. Surveys should be accurate, designs should be competent, and construction quality should be managed to a high level, all within the constraints of budgets which reflect the real level of benefits achievable.

Lastly, in the reaction from 'top-down' approaches and in the current fashion of respect for 'indigenous technical knowledge', it is possible to have an over-romantic image of the farmer. We are right to arrive at a deep respect for the farmer's expertise in survival and risk-spreading, and in his knowledge of the land and the crops it can support; but this respect must be tempered by the recognition of the rainfed farmer's areas of ignorance. He probably has little innate understanding of on-farm water management, he may have little experience of organised group activities (such as channel maintenance), and many aspects of agricultural intensification will be new to him. Indeed, ideas of crop responses to water developed under rainfed conditions may lead to problems such as gross over-irrigation.

Production versus social development: the dilemma of smallholder irrigation projects

It is the hypothesis of this writer that there is often a fundamental incompatibility between the objectives of national governments in promoting irrigation development, and the aims and aspirations of small farmers. Government objectives may include social development (rural employment, increased incomes, improved nutrition and public services) but their primary aim is usually in terms of increased production, often of specific crops (e.g. rice, wheat, sugar), and often to substitute for imports. On the other hand, developing country farmers may be less interested in putting all their eggs in the one basket of intensified production of a specified crop or crops, and more concerned to reduce risks through diversifying their food production and income-generating activities.

If these two sets of aims can come together in a single scheme, then success may follow; however, it seems more common than not (at least in Africa) that they do not. Conventional design procedures for smallholder irrigation embody many assumptions about farmers' aims, and about their willingness to take part in this form of development. Reality has shown the error of these conventional top-down approaches.

Figure 3 Checklist of issues and subjects to be addressed in the planning and design of any public irrigation scheme

It may well be that in regions with little experience of irrigation the attempt to combine production objectives with social development objectives should be abandoned. If the aim is production, then the estate/plantation/aommercial sector is the right arena. If the aim is social or rural development, then (small-scale) farmer managed approaches may fit better. There is no good reason why smallholder irrigation projects targeted at former rainfed producers, who have complex farming systems and diversified household economies, should be expected to work.

There is a major difference between the background just painted of much of rural Africa and the situation of strong traditional rice irrigation economies in Asia; in the latter case smallholder schemes are really water supply schemes to supply an existing activity, rather than involving a radical shake-up of the farming system and rural economy. This arguably is the pattern which government irrigation agencies elsewhere should follow.

Role of irrigation in poverty alleviation and social and economic development

If then the production objective is best met through private sector commercial enterprise, what role can irrigation development play in social and rural development? Is irrigation a useful vehicle for delivering wider developmental benefits, including those of poverty alleviation and food security, or are its associated problems and costs simply too great, as Moris (1987) has suggested in referring to irrigation as a "privileged solution"?

The World Bank (Barghouti and Le Moigne, 19903 still appears to view irrigation primarily as a means of increasing production, and of course without production benefits (including both increased production and more reliable production) then no other benefits are likely to accrue. The questions are; production of what, for whom, and under whose control?

Evidence is widespread that, under the right conditions (of relative autonomy in crop selection and production, ready access to markets and attractive prices) farmers will benefit from irrigation facilities. The precise ways in which they benefit are not always easily predictable. For example, a recent programme of studies (Diemer and Huibers, 1991) in Senegal showed that successful village-level irrigation schemes were used by farmers not to increase significantly production for market (as might have been expected), but rather to reduce hot-season farming activities and to permit them to pursue other income-generating activities at this time of year: in other words, to reduce risks and take advantage of greater opportunities for diversification. In Nigeria, on the other hand, the more attractive wheat price since 1988 has stimulated private small- and large-scale (irrigated) production in a way that 40 years' of conventional smallholder irrigation development failed to do (Kimmage, 1991).

Irrigation development can provide a real contribution to rural social development in two main ways, and under a number of conditions:

· through assistance provided to existing (traditional) irrigators
· through the introduction of irrigation to former rainfed producers or to those who were not previously involved in crop production.

The first of these is the easier although it has its own pitfalls A major reason why Asian irrigation development has, on the whole, been more successful than that promoted in Africa is that the beneficiary farmers have already been practicing some form of irrigation or water management (usually for paddy production), often for generations. But in Africa too, where Governments and NGOs have sensitively addressed some of the difficulties or bottlenecks experienced by existing (traditional) irrigators, results have been promising. Examples of such interventions can be found for example in the northern Nigerian fadamas, in Burkina Faso and in Zimbabwe (Carter, 1989) as well as in Nepal, Bhutan and Thailand. Interventions need not and arguably should not be comprehensive, but may address a single issue such as water lifting, water control or water conveyance.

The more difficult situation is where the attempt is made to introduce irrigation to farmers who have no previous experience. The enormity of the step from pastoralism or rainfed production to irrigation is easily under-estimated, and it is here that many well-meaning attempts to provide assistance have come adrift. The main problems, apart from the farmers' lack of technical know-how in water management, relate to the intensification of farming that goes with irrigation (and so the need for the farmer to learn many new skills at once), and the inevitable loss of independence which accompanies most forms of irrigation activity. The argument for improving rainfed production first, before introducing irrigation, is very strong. Figure 4 lists the major ways in which this may be done. When irrigation development is further complicated by resettlement, and the consequent disruption of existing social structures and infrastructure, the task can become even more problematic.

Figure 4 Possible improvements to rainfed agriculture as a precursor or alternative to the introduction of irrigation (after van Steekelenburg and Zijlstra, 1985)

Conditions for successful SSI

In the previous section reference was made to the conditions under which irrigation development could make a real contribution to rural and socio-economic development. The experience and reflection of the last 15 or so years - the SSI era - allows us to summarise at least some of these conditions and to identify areas where further understanding is needed.

· The first condition for farmer-centred irrigation development to succeed is the coincidence of farmer aims and objectives with those of any agency (government or NGO) providing assistance. A mismatch here spells disaster. Too often government irrigation policy and strategy are couched in terms too vague to be clear on this point, and in any case insufficient time is spent in dialogue with farmers to ascertain their own aims. Much of the most successful irrigation in Africa has been developed by farmers without the intervention or interference of Government agencies; in these circumstances the conflict of objectives highlighted above has not arisen.

· The second necessary condition is farmer autonomy. Producers should either be permitted to behave as farmers, who make their own decisions on cropping calendars and who manage their own farming systems, or they should be employed as wage labourers. In either case their position is clear. In many (African) smallholder projects farmers are such in name only, when virtually all autonomy is taken from them.

· The third condition is that the technology and physical systems supplying irrigation water must be able to be operated and maintained (within farmers' own budgets) without heavy dependence on unreliable supply and service agencies. The VLOM (Village Level Operation and Organisation of Maintenance) concept of village drinking water supply (Arlosoroff et al., 1987) is highly relevant to the case of small-scale irrigation.

· The fourth condition is that agencies (government or NGO) providing technical or other assistance in irrigation should take a support role rather than a dominant part. Their role should be to assist farmers in their own development, not to impose a particular pattern of 'development' on them.

· The fifth condition, which is implicit in the first four, is best expressed by Diemer and Huibers (1991) on the basis of their work in Senegal. It is a condition which must be reflected in the attitudes and approaches of irrigation agencies, and which has important implications for the whole process of intervention. "The most important conclusion is the need to recognise that the irrigator's economic and social circumstances are just as important for viable irrigation development as the physical conditions. These circumstances have to be reflected in any irrigation design. Schemes based on desirable behaviour patterns imposed from above are almost bound to fail."

These five conditions should no longer be controversial although they are by no means universally observed in practice. There is still a major task ahead to establish these basic tenets as accepted orthodoxy. Although directions are becoming clearer, there is still much to learn about what can work and what cannot work in irrigation development. The needs for R&D in small-scale irrigation fall under three broad headings (Carter, 1991). Firstly there is a need for increased understanding of both the technology and the social organisation of traditional irrigation systems; secondly there is the need to develop improved approaches to design of new farmer-managed systems, particularly taking a lead from the excellent work of Wageningen Agricultural University (e.g. WAU, 1990); and thirdly there is the need to develop and change the approaches of irrigation professionals, institutions (both government and NGO), and funding bodies.

Implications of the SSI philosophy

Project conception and assessment of irrigation potential

The first major implication of the new approach relates to the identification of 'irrigation potential' and of specific projects. Conventional wisdom has made the simplistic equation that suitable soils plus exploitable water adds up to irrigation potential. Organisations such as FAO (Thomas, 1987) and the World Bank (Barghouti and Le Moigne, 1991) have perpetuated this myth by failing to allow for the human element. Suitable soils and available water are necessary, but by no means sufficient, conditions for irrigation development since the success of farmer irrigation depends crucially on many factors other than the accepted technical and economic feasibility criteria (Figure 5).

Of course this new realism makes the identification of irrigation potential much more difficult and far less predictable; but present approaches to this subject can at best give only extreme upper bounds to the potential for development.

Figure 5 Two paradigms of the concept 'irrigation potential'

Design and construction

The implications for scheme design have been particularly highlighted by a number of workers at Wageningen Agricultural University in the Netherlands. They have pointed out two key aspects of a new design process. Firstly, the willingness on the part of the designer to question conventional wisdom about human behaviour (in other words to identify how farmers will actually utilise and benefit from irrigation facilities provided); and secondly, the need to carry out the design process interactively, with farmers' views and wishes in relation to scheme location, site layout and other aspects of design being respected as fully as possible. These two aspects alone represent a radical departure from conventional engineering design practice.

Other aspects of design and construction, such as the use of local materials and construction skills, and selection of field layouts to suit land levelling practices which are readily available, are fairly obvious. The principle of 'design for (farmer-) management' is fundamental and this too requires professionals with field experience not merely office skills.

Operation and maintenance

The imperative of farmer control of operation and maintenance is not merely for the reasons outlined earlier in this paper. There is a pragmatic imperative too. Increasingly Governments are recognising their own inability, for financial or other resource reasons, to manage and maintain an increasing acreage of irrigated agriculture. Were it not for the widespread philosophy of disengagement, turnover or privatisation, there would be a real danger of irrigation becoming one of Moris's (1987) "privileged solutions" (strategies which seem so obviously right as to need little justification and which therefore win priority access to funding).

Institutional roles

The institutions involved in the promotion of irrigation need to evolve new roles and attitudes.

· Government agencies need to develop approaches which give greater respect to the knowledge, skills and wishes of farming communities; and irrigation professionals, particularly engineers, need to develop a more farmer-centred orientation.

· NGOs, while generally having strengths in the community-based approach of SSI, often lack rigorous technical expertise. They must be prepared either to buy in such know-how or develop it themselves.

· Donors will need to come to terms with far less cut-and-dried project formulations, since the SSI approach demands more flexible funding arrangements and timetables. Donors should require evidence of full farmer participation in proposed irrigation developments.

· Educational and training institutions should make greater efforts to bridge the divide between social sciences and the agricultural and engineering studies needed for irrigation development. Narrow subject specialisms are inappropriate in this field.

Focus for action

Four major areas emerge in which 'outsiders' (Government, donors and technical assistance) may have a role:

· Traditional Water Management and Irrigation Practices: The priority here is to identify, quantify, evaluate and understand. Too often 'modern' irrigation development has supplanted well managed existing agricultural practices which have been ignored or overlooked in the urge for progress. A true value should be placed on such 'traditional' systems of crop production.

· New Development: Here the need to develop new interactive design procedures, and the consequent requirements for training are key.

· Existing Smallholder Schemes: This area needs attention. Better provision of support services, assistance in turnover processes, and a fundamental re-think about processes of rehabilitation are needed here.

· National Irrigation Development Policy: Arguably, the fundamental area is discussed in the next section.

A key role for technical assistance: policy development and strategy formulation

The key area is that of national irrigation development policy. In any particular country or region the roles of irrigation need to be defined, and the options and priorities for intervention need to be spelled out. In most developing countries, and especially in Africa, SSI should be an important component of such policy (alongside existing large-scale smallholder schemes and the commercial sector). Formulation of strategies in pursuance of such policies can then follow. Models for such strategy formulation can be found in the work on irrigation manpower planning by Carter et al. (1986) and Carter and Mason (1988) and in that of Morris and Bishop (1989 and 1990) and Bishop (1990) on agricultural mechanisation.

It is essential that policy development and strategy formulation are carried out by the responsible national Governments, not by outsiders. Nevertheless, external technical assistance may be necessary to expose policy makers to the options and to give them the opportunities to consult widely as they develop their own approaches.

It is particularly important that younger irrigation professionals are exposed to the new approaches to irrigation development. This should be by a combination of in-country training and, where appropriate, on international training programmes.

Irrigation development in sub-saharan africa: a summary of priorities for donors

The following list suggests key ways in which donors can assist in the promotion and support of irrigation in the region:

· Continuation and communication of results of sector evaluations
· Interdisciplinary research on all types of scheme/system
· Quantification and valuation of traditional practices
· Research on alternatives to irrigation and alternative approaches to irrigation
· Assistance in policy/strategy formulation
· Co-ordination with other donors/lenders
· Support to NGOs
· Support to commercial sector
· Rehabilitation.

Annex

Definitions

Definitions adopted by Underhill (1984) and UK SSI group:

Formal irrigation: Formal irrigation is the development and management of irrigated agriculture in a structurally formal way, usually by a government body. [Such schemes] are often established with very little prior involvement from farmers or landholders; and are usually managed by a structured government organisation on behalf of the resettled smallholders.

Small-scale irrigation (SSI): Small-scale irrigation, usually on small plots, in which small farmers have the major controlling influence, and using a level of technology which the farmers can effectively operate and maintain.

[Note this SSI concept and IIMI's Farmer Managed Irrigation Systems (FMIS) are identical]

Scale: For Africa, FAO has adopted the following scheme size classes for smallholder irrigation projects: FAO 1987, quoted in Underhill (1990).

1. Very large-scale schemes: typically over 10 000 ha with full water control and under government management. Examples are the gravity schemes in the large river basins in Sudan (Gezira), Morocco (Gharb) and Egypt

2. Large-scale schemes: typically 1000 to 10 000 ha with full water control. Generally under government or commercial management, the latter usually less than 5000 ha. Examples are found in Kenya (Bura; Mwea), Tanzania (Mbarali), Somalia (Shebelli)

3. Medium-scale schemes: typically 100 to 1000 ha with full or partial water control. Government managed, government assisted cooperatives, or commercial estates

4. Small-scale schemes: typically 1 to 100 ha controlled by farmers; groups, or single farmers. Examples are: Kenya, Zimbabwe, Tanzania, Madagascar for simple river diversions, Nigeria (fadama) for shallow groundwater, and Kenya, Tanzania for pumping from lakes

Note: To these four classes of irrigation 'schemes' could be added irrigated 'garden plots' from a few square metres to perhaps 0.5 ha in size, controlled by single farmers and usually based on shallow groundwater. Examples are found all over Africa. (The term 'micro irrigation' is sometimes used for garden plots but should be avoided as the International Commission on Irrigation and Drainage recommends that this term be used for the technologies of 'localized irrigation' such as trickle and drip (see FARO 1980c).

(Note the links between scale and management)

References

ADAMS, W. M. (1991) Large-scale irrigation in Northern Nigeria: performance and ideology. Transactions of the Institution of British Geographers, 16, 287-300.

ARLOSOROFF, S., TSCHANNERL, G., GREY, D., JOURNEY, W., KARP, A., LANGENEGGER, O. and ROCHE, R. (1987) Community Water Supply: the Handpump Option. World Bank, Washington.

BARGHOUTI, S. and Le MOIGNE, G. (1990) Irrigation in sub-Saharan Africa, the Development of Public and Private Systems. Technical Paper No. 123, World Bank, Washington.

BISHOP, C. (1990) Agricultural Machinery Industrial System in Ethiopia. Vienna: UNIDO (Programme Development Support Unit).

CARTER, R. C., CARR, M. K. V. and KAY, M. G. (1983) Policies and prospects in Nigerian irrigation. Outlook on Agriculture, 12 (3), 73-76.

CARTER, R. C., KAY, M. G. and CARR, M. K. V. (1986) Manpower Planning for Irrigation in sub-Saharan Africa. Proceedings of the Institution of Civil Engineers Part 1, 80, 1279-1296.

CARTER, R. C. and MASON, D. (1988) Manpower Planning for Irrigation. FAO, Rome.

CARTER, R. C. (ed) (1989) NGO Casebook on Small-Scale Irrigation in Africa. AGL/MISC/15/89, FAO, Rome.

CARTER, R. C. (1991) The Design and Development of Small-scale Irrigation in sub-Saharan Africa. ODU Bulletin, 22: 4-6, Hydraulics Research Ltd., Wallingford.

CHAPMAN, C. (1987) Draft Project Outline for Mbuga Smallholder Irrigation Project. Working Paper 1, Institutional Support for Irrigation Development (Project URT/86/012), FAO, Rome.

DIEMER, G. and HUIBERS, F. P. (1991) Farmer-Managed Irrigation in the Senegal River Valley: Implications for the Current Design Method. End-of-Project Report, WARDA/Wageningen Agricultural University, Water Management Project. St Louis (Senegal) and Wageningen.

FAO (1987) Consultation on Irrigation in Africa. FAO, Rome
MOTES, F. L. (1982) World Bank irrigation experience. Water Resources Development 1 (1), 65 75.

KIMMAGE, K. (1991) Small-scale irrigation initiatives in Nigeria: the problems of equity and sustainability. Applied Geography, 11, 5-20.

MORRIS, J. (1987) Irrigation as a privileged solution in African development. Development Policy Review, 5, 99-123.

MORRIS, J. and BISHOP, C. (1989) An Agricultural Mechanisation Strategy for a Country in Central Africa. Case study prepared for FAO (Agricultural Engineering Services), Rome.

MORRIS, J. and BISHOP, C. (1990) Training Programme for Agricultural Mechanisation Strategy Formulation. Prepared for FAO (Agricultural Engineering Services), Rome

PALMER-JONES, R. (1987) Mismanaging the peasants: some origins of low productivity on irrigation schemes in Northern Nigeria. In: Irrigation in Africa: Problems and Problem-Solving, Adams and Grove (eds) Cambridge African Monographs 3, 98-108, University of Cambridge African Studies Centre.

THOMAS, R. (1987) Irrigation and Water Resources Potential for Africa. AGL/MISC/11/87, FAO, Rome.

UNDERHILL, H. W. (1984) Small-Scale Irrigation in Africa in the Context of Rural Development. FAO, Rome.

UNDERHILL, H. W. (1990) Small-Scale Irrigation in the Context of Rural Development. Cranfield Press.

Van STEEKELENBURG, P. N. G. and ZIJLSTRA, G. (1985) Evaluation of Irrigation Projects Sponsored by the European Economic Community, Synthesis Report. International Institute for Land Reclamation and Improvement (ILRI), Wageningen, the Netherlands.

WAU (WAGENINGEN AGRICULTURAL UNIVERSITY) (1990) Design for Sustainable Farmer-Managed Irrigation Schemes in sub-Saharan Africa. Introductions and Contributions to the International Workshop, February 5-8, 1990.

WALLACE, T. (1981) The Kano River Project: the impact of an irrigation scheme on productivity and welfare. In: Rural Development in Tropical Africa, MEYER, ROBERTS and WILLIAMS (eds), Macmillan, London.

WORLD BANK (1979) Nigeria: Agricultural Sector Review. World Bank, Washington.

Discussion

In the discussion a recent ODA/NGO seminar on popular participation projects was referred to. The difficulties of multiplying up small-scale projects was mentioned as well as problems in accounting for process type projects because firm programmes, targets and costs cannot be prepared in advance. Opinions differed as to whether donor assistance should be to small- or large-scale projects. However, emphasis should be on rehabilitation based on the tenets of SSI experience in large schemes. Donors should support NGOs and local banks for provision of credit rather than major commercial lending. It was suggested that ODA should reduce irrigation spending by 25 40% in view of the changing demographic situation but it was pointed out that some donors are increasing their rural-based activities to discourage rural migration. It was said that 35 40% of Asian agricultural land is irrigated, producing 60 70% of food grains and that this is from both small- and large-scale developments. So the distinction should be more between well- and ill-conceived projects rather than between small-and large-scale projects. The question was posed as to what countries such as India and Pakistan should do without irrigation; the carrying capacity of rainfed land in Pakistan is no more than 5% of the total population. Finally, concern was expressed at the speed with which the management of irrigation schemes is being handed over in some countries. In many cases, this necessitates a sudden switch after many years of highly centralised and top-down management to water users' groups which have hardly developed in any meaningful sense.

Paper 12 Environmental and health aspects of irrigation

P. Bolton

Overseas Development Unit, HR Wallingford

Summary: The paper shows how awareness and understanding of the relationship between human activity and environmental change have developed over the last 20 years with regard to irrigation in developing countries. The terminology of 'impacts' and of 'sustainability' are examined in relation to irrigation projects and to the wider environment, resources and societies within which they are located. From this discussion various key issues are identified concerning the environmental impact and sustainability of irrigation systems. These have implications both for policy formulation and research. The paper ends by examining the difficulties of research in such interdisciplinary subject areas using projects undertaken by the Overseas Development Unit at HR Wallingford by way of example.

The rise in environmental consciousness and conflict in irrigation and water resources development

The Overseas Development Unit (ODU) of HR Wallingford (formerly the Hydraulics Research Station) was established in 1973 to provide specialist expertise and undertake research on behalf of the Engineering Division of the ODA in problems of water resources and irrigation in developing countries. The ODU endeavours to ensure that its research programme addresses the water resources and irrigation problems considered most acute, at a particular time, by practicing engineers in developing countries and specialists and aid officials from the UK. Surprisingly the term 'environment' does not feature in the title of any ODU research project prior to 1987 (An investigation of schistosomiasis control in irrigation schemes, begun in 1983, was later given an 'environmental' title). By contrast, the current (1992/93) programme groups approximately one third of the Unit's work under a theme 'Environmental Effects'.

The deduction from this, that the environment was not an important consideration until approximately five years ago in the perception of most Third World irrigation and water resources engineers and researchers would, however, be misleading: although key terminology and concepts were absent, the processes were under active study. For example, one of the projects on which the ODU started its research 19 years ago was a study of salinity (sea water) intrusion in tropical estuaries. This phenomenon, resulting from freshwater abstraction and the regulation of river flows particularly for irrigation, has major environmental implications. The types of numerical modelling technique developed in that early project remain invaluable as part of the tool-kit used by engineers in seeking to understand, predict and manage environmental change in tropical river basins. Undoubtedly this project would legitimately be included as part of the 'Environmental Effects' theme if it were part of the ODU's current research programme. Similarly, studies of catchment erosion and soil salinisation which have featured in the ODU's research for many years are now regarded as 'environmental' issues.

On the same basis, almost every other project on which the ODU has been engaged over the last 19 years could be considered as having an important bearing on 'the environment'. This is hardly surprising when one considers that irrigation is a process which involves interactions between almost every facet of the environment: atmospheric gases, water, soils and minerals, energy, biological species and communities and human activities and relationships.

Yet vocabulary and concepts relating to 'the environment' have, as noted above, assumed importance only relatively recently in irrigation and water resources development. Unfortunately this change has arisen for largely negative reasons as a result of the publicity given to particular water resources projects which have been built without adequate consideration for their overall and long-term effects. In response to this, the techniques of Environmental Impact Assessment were developed and, in many countries, enforced through legislation in order to prevent projects from proceeding if they were likely to cause adverse environmental impacts. This inevitably created a situation of conflict between engineers and planners on the one hand and environmental specialists and 'conservationists' on the other over the implementation of new projects; disputes which led to severe delays and, in some cases, to the cancellation of projects which may have taken many man-years of planning and design. Such conflicts came as a psychological shock to many engineers who had previously believed that their's was a profession which was entirely and self-evidently serving the needs of society.

The basis for future co-operation

Situations of conflict and inertia as described above are beginning to give way to a realisation that the traditional roles of engineers and other professional groups are changing and that closer co operation is needed to address the complex issues which we face in seeking to optimise decisions concerning the future development of water resources and their impact on the environment. However, considerable progress must first be made in clarifying concepts, in developing a broader understanding and new ways of working and in evolving new technical skills. The principal needs, as I see them, are discussed below.

To develop a dearer understanding of 'the environment'

One source of confusion in considering 'environmental effects' is the different shades of usage which are given to the term. Often when examined closely, the intended meaning is restricted to effects related to wildlife and natural ecology. The use of the term 'environmental effects' to refer primarily to ecological changes accords with the increasing concern about 'conservation' in 'industrialised' countries which have already undergone substantial social and land-use changes and where the most common and far-reaching effects of current water-related activity are changes to water quality affecting wildlife. By contrast, in most tropical developing regions physical changes to the environment and changes which affect society are equally, if not more, important than the biological changes. It is, therefore, necessary to ensure that in discussions concerning 'environmental' effects the physical, biological and social aspects are each considered in full (see Figure 1).

In the remainder of this paper the combined interactions of the human, biological and physical spheres will be intended whenever the terms 'environment' or 'environmental' are used. In this usage, they are synonymous with such words as 'comprehensive', 'global', 'holistic'. This meaning, which links with the development of EIA procedures, carries the connotation 'everything we can possibly think of which might be influenced by or influence the activity under examination'. This is the meaning envisaged by the Environmental Impacts Working Group of the International Commission on Irrigation and Drainage (ICID) when it drafted a Check-list of possible environmental impacts (see Figure 2).

A further step towards clarification of terminology is suggested by the phrase "might be influenced by or influence.. A distinction can be drawn between those effects which the environment has on the project and those which the project has on the environment. The merit of stressing this distinction is that it ensures that a project planner, designer or manager is assigned the full responsibility for all factors which affect the primary productivity of the project. Effects such as sedimentation, soil salinisation, social, financial or organisational problems and agricultural weeds and pests cannot be pushed to one side as 'environmental impacts' simply because the person responsible for the project does not know how to deal with them. Such effects must be considered to ensure that a project's primary productivity will not decline with time.

In this respect, the relatively recent introduction of the term 'sustainable' development is particularly helpful. Rather than talking in teens of the impact of the environment on a project, the question "is the project sustainable?" clearly places the responsibility for ensuring that such effects are considered on the person responsible for the project. For practical purposes, this question may be sub-divided into separate questions concerning the resources on which the project depends. The first part of Table 1 lists the separate resources and illustrates how concern over impacts (e.g. salinisation) translates into questions of sustainability (in that case, sustained soil fertility). Thus, with respect to the effects of the environment on the project the term 'sustainability' provides a better framework for guiding the project official than does the term 'impacts'.

Figure 1 Examples of disciplines relating to the major components of the environment associated with irrigation

Figure 2 ICID Check-list of possible environmental impacts of irrigation, drainage and flood-control projects

Table 1 Selected items to illustrate the links between impacts and sustainability

The extent to which the project official carries responsibility for wider environmental effects depends to some extent on the institutional framework which exists. Such effects might also be viewed in terms of 'sustainability', as shown in the right-hand columns of Table 1, but it would generally be the responsibility of a higher authority such as a river basin authority or environmental protection agency, rather than the project official, to ensure that individual projects do not compromise the sustainability of these regional, or even global, resources and systems. If competent authorities do exist at this higher level, the official responsible for an individual project can treat wider environmental effects as 'impacts' of his project whose nature he/ she must attempt to specify but whose implications the higher authority has the responsibility to assess. Thus the terms of 'impacts' and 'sustainability' may both have a role to play depending on the particular context.

Unfortunately in many situations in developing countries institutional responsibilities may not be dearly defined and a project official may not be in a position to pass to others the responsibility for considering how the sustainability of wider environmental systems is affected by the project. In such cases the above attempt to clarify the terminology may not be relevant and the project official may find he/she has responsibility for impacts which are more far reaching than he/she has resources to assess.

To understand the special circumstances relating to tropical developing regions

There are substantial differences between temperate (industrialised) and tropical (developing) countries with regard to the environmental effects associated with irrigation and water resources development. It has already been mentioned that the main environmental preoccupation in 'western' countries tends to be the effect of water quality changes on wildlife. In the Third World this is only one of several major environmental changes which are occurring alongside or as a result of irrigation and water resources development. The reasons for the wide range of factors to be considered include:

· a greater intensity, seasonality and variability of rainfall leading to high rates of erosion and geomorphological activity and the need for substantial reservoir storage capacity

· higher temperatures leading to higher rates of evaporation and hence higher crop water requirements and also to greater productivity and diversity of ecological systems which in turn cause increased problems of human disease and crop pests

· higher population growth rates leading to increasing demands for agricultural land and for irrigation, domestic and industrial water

· weaker economies and a poorer population leading to problems of poverty-related disease and environmental degradation as well as intense competition for capital which results in the desire for cost savings (for example, by the omission of drainage works) which have long-term environmental repercussions

· social, political and institutional systems which are less capable of coping with the conflicts which arise in resource allocation and environmental management, conflicts which the projects themselves may generate

· relative scarcity of the technical and managerial skills needed to predict and manage environmental change.

Considering the above differences in relation to the scope of environmental effects defined in Figure 1, it is clear that a vast amount of skill, knowledge and experience would be needed to 'manage' the environmental changes associated with irrigation if it were to be undertaken with the thoroughness and confidence that is implied by those who call for improved environmental management.

Throughout the remainder of the paper I shall try to highlight practicable ways in which progress can be and is being made, albeit slowly, in the areas of environmental management with which I am most familiar. My starting point is always to consider how to assist the existing professional staff, many of whom are engineers like myself, to broaden the scope of their activities and understanding, with respect to identifying and managing more effectively environmental change, given the severe constraints and conflicting demands which characterise their situation.

To address the key issues

The central issue with regard to future irrigation development, an issue which is of growing importance for the environment, is the impact of the consumption of water for irrigation on surface water and groundwater resources and hence on other human water users and on the physical and ecological characteristics of the terrestrial areas and water bodies depending on these resources. Irrigation is man's most significant consumptive use of water with typically a depth of between half and one metre of water being evaporated from every topical irrigated field during the growing period of a single crop. During the process the sediment and dissolved solids (salts and pollutants) previously carried by the water are left behind either through deposition (resulting in sedimentation or salinisation) or by causing increased concentrations in the water which remains. Apart from the evaporated water, some of the water diverted for irrigation will percolate into the ground where it may result in a progressive rise in water-table and eventual waterlogging. These processes are, therefore, the source of several major types of environmental impact and the cause for a lack of sustainability in many individual projects. Moreover, irrigation's high consumptive use of water is likely to feature as an increasingly important cause for water shortage and hence conflict in the overall management of water resources.

For the above reasons, water-use efficiency (the productivity of a unit volume of water in terms of the amount of crop it produces) is becoming a key parameter in irrigation planning and management. In the first analysis it can be assumed that if water-use efficiency can be improved, the overall environmental impact of irrigation will be reduced. There are a number of practical ways in which possible improvements in efficiency can be sought: from the purely physical (improved canal linings and seepage reduction, software to assist the scheduling of irrigation releases, micro irrigation and sprinkler techniques); to the biological (alternative crops or varieties); and human aspects (institutional, managerial and financial factors). Work on each of these is progressing in various institutions in the UK and around the world. They are important but by themselves they are not enough to ensure reduced environment impacts. This is why I used the phrase "in the first analysis" when referring to the importance of water-use efficiency in relation to environmental effects.

Unfortunately there are other interlinking factors which are environmentally significant and must also be considered. For example, to achieve the highest possible crop yields for a given volume of water is likely to entail the use of agrochemicals. These introduce both a direct financial cost as well as an environmental cost in terms of pollution. Likewise high water-use efficiencies may be environmentally harmful if low rates of water application lead to progressive accumulation of salts in the soils and consequent loss of soil fertility. As a further example, some crops which are highly productive in terms of water use may demand high levels of farm labour or mechanisation or require post-harvest processing which have possible social and pollution implications.

Two aspects of environmental change which relate less closely than most to the efficiency of irrigation water use are the ecological impacts and the social impacts (including health impacts). In many instances, the attempt to increase the 'efficiency' of exploiting natural resources inevitably leads to a loss of species diversity and 'poorer' ecosystems. This is true in an irrigation scheme where large tracts of land are converted to monocropping and the creation of 'managed' canals and reservoirs provides only limited support for aquatic life. Modified operation of existing schemes to achieve less 'wasteful' use of water may also lead to the drying out of wetland areas which had previously been rich in wildlife. (However, in relation to human health, the opposite is generally true: for example, high water-use efficiency is likely to produce fewer water bodies suitable for colonisation by the vectors or hosts of human parasitic diseases.) In other respects, social impacts often bear little relation to the efficiency of water use: resettlement, which in many countries is one of the most contentious issues, relates to the land area under development, not the water use.

Thus, whilst high water-use efficiency is a key parameter to consider it is not sufficient to rely solely on this as a means of ensuring that adverse social and environmental changes are minimised. For this reason there is need for some caution in applying the principle of water as an 'economic good' as an environmental management tool without also introducing safeguards to avoid certain damaging changes which are not directly related to water-use efficiency. The optimum solution to complex water allocation and management decisions is unlikely to be achieved through oversimplification of the problem.

To be aware of areas of special sensitivity

The key issues in relation to environmental change associated with irrigation and water resources development are likely to vary from locality to locality and project to project. It is necessary to be aware that certain regions or certain types of project are particularly sensitive. In relation to the physical environment the four factors which lead to particular environmental sensitivity in relation to irrigation are:

· areas of low or erratic rainfall

· areas where sediment loads in rivers are high whether due to high erodibility (e.g. the loess plateau of China) or to earthquake and volcano activity

· areas in which solute loads are high especially resulting from the re-use of drainage water

· areas with problematic soils.

In the biological and human spheres the factors leading to particular sensitivity are:

· areas which have hitherto been largely undeveloped

· areas of high population density

· projects which lead to rapid or large-scale social disruption or migration.

There are, moreover, different degrees to which environmental management may be expected to control or ameliorate adverse environmental effects. A desire to avoid certain changes would be virtually unreconcilable with the development of a region for irrigation: the preservation of the ecology of wild or undeveloped areas and the preservation of rural society with its particular socio/political and economic structures may both lie in this category.

Other changes, although having the potential for serious adverse effects, also have the potential, with appropriate resources and skill, to be managed in such a way that the net positive effects far outweigh the negative. Such changes include the control of pollution, the control of human disease, the control of weeds and pests, the achievement of economic and social well-being and the management of ecology within regions which have already been 'developed'.

Two types of environmental change have potential for some amelioration through careful and well-resourced management but may nevertheless have cumulative impacts which will, in the long term, have important water resource management implications: the sedimentation of reservoirs and the salinisation of soils. Both must be studied carefully in relation to the sustainability of irrigation in certain regions and further research and strategic thinking are required to clarify the potential for managing these changes.

The role of interdisciplinary research

Methods to identify key issues

In the above context of seeking to address complex environmental management decisions the ODU, through its collaboration with the ICID Working Group on Environmental Impacts, identified the need for a systematic procedure to assist engineers and others who are unfamiliar with many of the necessary environmental disciplines to highlight key issues in relation to the planning and management of irrigation, drainage and flood-cntrol projects. The result of this initiative was the ICID Check-list which provides a comprehensive summary of the areas of environmental concern which should be considered in relation to these types of project (in this instance issues of 'sustainability' and 'impacts' were combined into a single category of effects to simplify usage). In its summary form (Figure 2) the Check-list appears to provide little practical guidance to the non-specialist user but the Working Group amplified the procedure by providing detailed descriptions, a targeted bibliography, data sheets for recording relevant information and results sheets for recording whether the user judges particular impacts to be important or whether he/she has insufficient data or knowledge to make a judgement. The relationship of the various components to each other is shown in Figure 3 and the first draft of the procedure is presented in Mock and Bolton (1991). The procedure has undergone field trials in a number of countries (including Pakistan and Sri Lanka) where the main benefits reported have been in relation to educating and involving the engineering profession in the environmental implications of their planning, design and management decisions.

Figure 3 Procedure for assessing key issues based on the ICID Check-list

Education and involvement of the non-specialist are the primary objectives of this work rather than to replace the need for specialists in the various environmental disciplines. This is well illustrated by one of the ODU's current research projects, in northern Nigeria, in which a questionnaire developed under the framework of the ICID Check-list is being used to collect information about the environmental status of some existing small irrigation schemes (see Bolton et al., 1990 and 1991). In this, and many other similar situations, there are few resources available to undertake thorough environmental studies but the people with first-hand knowledge of a particular project (the project manager, the community leaders, the village health workers) are not pooling their knowledge, nor are they trained to assess the implications of their local knowledge in terms of environmental change. The questionnaire enables an initial assessment to be made of the most important health and environmental factors so that specialist help and advice can be targeted to best effect. In other words, non-specialists are able to use the procedure to undertake an initial scoping of the situation in order to optimise the use of scarce specialist skills. The preliminary work, in which a team of scientists collected field data to validate the results obtained from the questionnaire, suggests that this approach has merit both for identifying hitherto unrecognised problems and for creating awareness and understanding amongst the project officials.

Development and testing of methods for environmental management

With reference to Figure 1, I believe that the principal areas where new initiatives and research are required are in the areas in which there is an overlap between the three broad spheres of knowledge, physical science, biological science and human science. I have deliberately highlighted these three spheres because the disciplines within them have, to some extent, developed under different sets of principles concerning the way in which new knowledge is acquired, validated and presented. They also have different understandings about the ways in which systems behave and about the extent to which future change can be predicted. I have become particularly involved at the meeting-point of physical and biological sciences. It took me some time to understand that my fundamental belief in the deterministic nature of physical systems (excluding sub-atomic phenomena) and the over-riding desire to describe their behaviour with rules which enable quantitative generalisations and predictions to be made is not embedded so deeply within the fundamental tenets of biological scientists. Their outlook is more strongly descriptive and probabilistic: the desire to prove direct causality is less strongly present.

The different approaches are highlighted when joint research is attempted in the field of environmental management. Engineers and physical scientists are accused by biologists of being too ready to ascribe causality in situations where changes may simply be coincident and of being too ready to generalise without understanding that only slight changes from one situation to another may result in major differences in the response of biological communities. Biologists, on the other hand, are accused of being too tied to scientific procedures which are observational rather than experimental and, therefore, of being too cautious in designing studies which will demonstrate how changes in the management of a physical system may affect biological productivity.

The main focus of my own research in this area has been the developing and testing of measures for the control of water-related disease in irrigation systems In particular, the ODU has recently completed the first phase of a seven-year study in Zimbabwe to develop techniques which engineers can adopt to help reduce the transmission of schistosomiasis (see Chimbari et al., 1991 a and b) The study has attracted particular attention since few previous attempts have been made to pilot test methods for the environmental management of schistosomiasis within an operational irrigation project. Measures under investigation involve physical changes to the irrigation system to discourage the colonisation of water bodies by the aquatic snails, which are the intermediate hosts of the disease (canal lining, development of free-draining irrigation structures, careful scheduling of irrigation releases, attention to the design and operation of local storage ponds and adequate drainage provision), to discourage humans from contaminating water bodies (construction of household and in-field latrines) and to discourage humans from coming into contact with infected water (careful village location and provision of adequate, safe supplies of domestic water).

To a large degree, practices which are considered good engineering are also good for reducing schistosomiasis transmission. However, as the preliminary results indicate, see Figure 4, the effectiveness of these measures is variable and difficult to predict. In two of the irrigated areas, disease prevalence appears to have been held at a low level for three years whereas in a third, where similar measures had been introduced, the prevalence has risen to a level equivalent to that in an area without control measures. The research has increased our knowledge about the effectiveness of particular control measures but has also highlighted the difficulty of controlling this disease by environmental means: deviation from the recommended control strategy by only a small amount may result in transmission levels which are as high as if no measures were used.

Of more direct relevance to the subject of this paper are some of the lessons learned concerning the overall strategy for control and the methods by which research can be undertaken In terms of strategy, the work has clearly demonstrated that to focus on planning and design alone is mistaken. Irrigation systems mature with time and biological communities change and adapt. More focus must be given to the dynamic aspects of control and, in particular, to the involvement of the local community so that they can be enabled to monitor the situation and know how to respond to changes which might occur In this way research which was previously considered to be interdisciplinary only to the extent of involving the physical and biological sciences is now seen to include an important human element as well. With regard to research method, the work has demonstrated the importance of working within the constraints of a normal operational irrigation scheme rather than in a 'laboratory'. The Zimbabwe work has shown the importance of considering financial, social and administrative constraints in seeking to develop environmental management measures suitable for wider replication.

Figure 4 Prevalence of schistosomiasis (haematabium) in adults and pre-school children, Mushandike, Zimbabwe

Conclusions

The conclusions of this paper reflect the personal interests and experience of the author rather than presenting a comprehensive review of the subject. Particular emphasis has been laid on the need to clarify terminology. The suggestion made is that the 'environment' should include all physical, biological and human components (see Figure 1), that ideally 'sustainability' is the preferred term when discussing environmental changes which affect the project or region over which the interested person or institution has responsibility and that the term 'impacts' be used to refer to changes for which another individual or organisation has responsibility. This distinction clearly has little relevance unless responsibilities are first set and agreed. The special circumstances surrounding environmental change in developing countries are discussed and point to the conclusion that techniques of environmental management developed for temperate conditions are not adequate to address the situation in tropical developing countries: new techniques must be developed and field tested. Because of the diversity of factors that must be considered some attempt must be made to pinpoint key issues. The primary consideration in many cases is to improve water-use efficiency since not only does this reduce the likelihood of conflict in water-short situations but also water-use efficiency and 'wastage' relate directly to several important aspects of environmental change. The identification of key issues must be supported by an awareness of areas of particular environmental sensitivity.

Among the large number of possible research topics which relate to this subject, two have been discussed from the author's own experience: the development of procedures to enable engineers and other professional groups who do not have specialist environmental knowledge to recognise existing or potential environmental hazards in order that specialist assistance can be sought; and the development and field testing of environmental management techniques for the control of water-related parasitic diseases. In each case the work points to the need for a reassessment of the relationship between subject disciplines, for an awareness of the financial, social and administrative constraints within which environmental management techniques will be applied and for a pragmatic recognition that, faced with the complexity of environmental relationships and systems, a small step in the right direction, albeit taken cautiously, is better than no step at all.

References

BOLTON, P., IMEVBORE, A. M. A. and FRAVAL, P. (1990) A Rapid Assessment Procedure for Identifying Environmental and Health Hazards in Irrigation Schemes. Report OD 120, HR Wallingford.

BOLTON, P., IMEVBORE, A. M. A. and FRAVAL, P. (1991) Field evaluation in northern Nigeria of a rapid assessment procedure for identifying environmental and health hazards in irrigation schemes. In: Techniques for Environmentally Sound Water Resources Development, WOOLDRIDGE, R. (ed) Pentech Press, London.

CHIMBARI, M., NDHLOVO, P. D., CHANDIWANA, S. K., CHITSIKO, R. 1., BOLTON, P. and THOMSON, A. 1. (1991a) Schistosomiasis control measures for small irrigation schemes in Zimbabwe Results from three years of monitoring at Mushandike Irrigation Scheme. Report OD 123, HR Wallingford.

CHIMBARI, M., CHITSIKO, R. 1., BOLTON, P. and THOMSON, A. 1. (1991b). Design and operation of a small irrigation project in Zimbabwe to minimise schistosomiasis transmission. In: Techniques for Environmentally Sound Water Resources Development. WOOLDRIDGE, R. (ed) Pentech Press, London.

MOCK, J. F. and BOLTON, P. (1991) Environmental effects of irrigation, drainage and flood control projects: Checklist for environmental impact indication. Report OD/TN 50, HR Wallingford.

Discussion

In discussion it was commented that the interacting role of the political economy should be highlighted further in Figure 1 and that an appropriate blend of politics economic-biosphere is necessary. A note of caution about the success of irrigation in raising agricultural production was expressed since pest and disease incidence often increases at the same time; the extended cropping season enables pests and diseases to overcome natural limitations imposed by the absence of food or host plants. The question was raised as to whether sufficient account of these factors was being taken in the planning and management of irrigation schemes.