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Chapter 7: Environmental considerations in irrigation development
Potential environmental impacts of irrigation development
The role of wetland and the impacts of water development projects
Regional aspects of environmental impacts and 'hot spots'
Summary of environmental impact hazard related to irrigation development
Irrigation has contributed significantly to poverty alleviation, food security, and improving the quality of life for rural populations. However, the sustainability of irrigated agriculture is being questioned, both economically and environmentally. The increased dependence on irrigation has not been without its negative environmental effects.
Inadequate attention to factors other than the technical engineering and projected economic implications of large-scale irrigation or drainage schemes in Africa has all too frequently led to great difficulties. Decisions to embark on these costly projects have often been made in the absence of sound objective assessments of their environmental and social implications. Major capital intensive water engineering schemes have been proposed without a proper evaluation of their environmental impact and without realistic assessments of the true costs and benefits that are likely to result.
The sustainability of irrigation projects depends on the taking into consideration of environmental effects as well as on the availability of funds for the maintenance of the implemented schemes. Negative environmental impacts could have a serious effect on the investments in the irrigation sector. Adequate maintenance funds should be provided to the implementing organizations to carry out both regular and emergency maintenance.
It is essential that irrigation projects be planned and managed in the context of overall river basin and regional development plans, including both the upland catchment areas and the catchment areas downstream.
This chapter reviews the most important environmental issues related to irrigation and drainage development.
Potential environmental impacts of irrigation development
Waterlogging and salinization
Water-borne and water-related diseases
Potential environmental impacts of darns and reservoirs
Socio-economic impacts irrigation schemes
Alternatives to mitigate the negative impacts of irrigation projects
The expansion and intensification of agriculture made possible by irrigation has the potential for causing: increased erosion; pollution of surface water and groundwater from agricultural biocides; deterioration of water quality; increased nutrient levels in the irrigation and drainage water resulting in algal blooms, proliferation of aquatic weeds and eutrophication in irrigation canals and downstream waterways. Poor water quality below an irrigation project may render the water unfit for other users, harm aquatic species and, because of high nutrient content, result in aquatic weed growth that obstructs waterways and has health, navigation and ecological consequences. Elimination of dry season die-back and the creation of a more humid microclimate may result in an increase of agricultural pests an plant diseases.
Large irrigation projects which impound or divert river water have the potential to cause major environmental disturbances, resulting from changes in the hydrology and limnology of river basins. Reducing the river flow changes flood plain land use and ecology and can cause salt water intrusion in the river and into the groundwater of adjacent lands. Diversion of water through irrigation further reduces the water supply for downstream users, including municipalities, industries and agriculture. A reduction in river base flow also decreases the dilution of municipal and industrial wastes added downstream, posing pollution and health hazards.
The potential direct negative environmental impacts of the use of groundwater for irrigation arise from over-extraction (withdrawing water in excess of the recharge rate). This can result in the lowering of the water table, land subsidence, decreased water quality and saltwater intrusion in coastal areas.
Upstream land uses affect the quality of water entering the irrigation area, particularly the sediment content (for example from agriculture-induced erosion) and chemical composition (for example from agricultural and industrial pollutants). Use of river water with a large sediment load may result in canal clogging.
The potential negative environmental impacts of most large irrigation projects described more in detail below include: waterlogging and salinization of soils, increased incidence of water-borne and water-related diseases, possible negative impacts of dams and reservoirs, problems of resettlement or changes in the lifestyle of local populations.
About 2 to 3 million ha are going out of production worldwide each year due to salinity problems. On irrigated land salinization is the major cause of land being lost to production and is one of the most prolific adverse environmental impacts associated with irrigation. However, very limited research has yet been conducted to quantify the economic impact of irrigation induced salinization. Quantitative measurements have generally been limited to the amount of land affected or abandoned. Estimates of the area affected have ranged from 10 to 48% of worldwide total irrigated area. Especially the arid and semi-arid areas have extensive salinity problems.
Waterlogging and salinization of soils are common problems associated with surface irrigation. Waterlogging results primarily from inadequate drainage and over-irrigation and, to a lesser extent, from seepage from canals and ditches. Waterlogging concentrates salts, drawn up from lower in the soil profile, in the plants' rooting zone. Alkalization, the build-up of sodium in soils, is a particularly detrimental form of salinization which is difficult to rectify.
Irrigation-induced salinity can arise as a result of the use of any irrigation water, irrigation of saline soils, and rising levels of saline groundwater combined with inadequate leaching. When surface water or groundwater containing mineral salts is used for irrigating crops, salts are carried out into the root zone. In the process of evapotranspiration, the salt is left behind in the soil, since the amount taken up by plants and removed at harvest is quite negligible. The more arid the region, the larger is the quantity of irrigation water and, consequently, the salts applied, and the smaller is the quantity of rainfall that is available to leach away the accumulating salts.
Excess salinity within the root zone reduces plant growth due to increasing energy that the plant must expend to acquire water from the soil. The tolerance of crops to salinity is variable: clover and rice are more sensitive to salts than barley and wheat. Comprehensive studies of farm-level effects of irrigation-induced salinity indicate that the yields of paddy and wheat are around 50% lower on the degraded soils and net incomes in salt-affected lands are around 85% lower than the unaffected land.
Irrigation-related salinity has adverse effects not only on the production areas, but also on areas and people downstream. The rivers, particularly in arid zones tend to become progressively more saline from their headwaters to their mouths. The aquifers interrelated with the river are highly saline and the salts discharged to the river system from saline aquifers adversely affect downstream water users, particularly irrigated agriculture and, in some special cases, wildlife.
Many of the soil-related problems could be minimised by installing adequate drainage systems. In Egypt, for example, the installation of drainage systems effectively reduced soil salinity. The average yield for wheat increased from 1 ton/ha before drainage to about 2.4 tons/ha. Similarly, the yield for maize increased from 2.4 tons/ha to 3.6 tons/ha after drainage infrastructure was completed. Drainage is a critical element of irrigation projects, that however still too often is poorly planned and managed. Waterlogging can also be reduced or minimized, in some cases, by using micro-irrigation which applies water more precisely and can more easily limit quantities to no more than the crops needs.
Water-borne and water-related diseases
Water-borne or water-related diseases are commonly associated with the introduction of irrigation. The diseases most directly linked with irrigation are malaria, bilharzia (schistosomiasis) and river blindness (onchocerciasis), whose vectors proliferate in the irrigation waters. Other irrigation-related health risks include those associated with increased use of agrochemicals, deterioration of water quality, and increased population pressure in the area. The reuse of wastewater for irrigation has the potential, depending on the extent of treatment, of transmitting communicable diseases. The population groups at risk include agricultural workers, consumers of crops and meat from the wastewater-irrigated fields, and people living nearby. Sprinkler irrigation poses an additional risk through the potential dispersal of pathogens through the air.
The risk that one or more of the above diseases is introduced or has an increased impact is most likely in irrigation schemes where [8]:
• soil drainage is poor, drainage canals are either absent, badly designed and/or maintained;
• rice or sugar cane is cultivated;
• night storage reservoirs are constructed;
• borrow pits are left with stagnant water;
• canals are unlined and have unchecked vegetation growth.
Malaria
Malaria is by far the most important disease, both in terms of the number of people annually infected, and whose quality of life and working capacity are reduced, and in terms of deaths. Worldwide, some 2000 million people live in areas where they are at risk of contracting malaria. The total number of people infected with malaria is estimated at 100 to 200 million with between 1 and 2 million deaths per year, with almost 90% of the cases in Africa. Drug treatment has become difficult recently because the parasite has become resistant to certain drugs that have been used for a long time in many parts of the world. Interruption of disease transmission chemicals for the control of the vector, the mosquito, has become less effective because some mosquito vector species have become resistant to previously effective insecticides and some insecticides have been banned for environmental reasons.
Bilharzia
Bilharzia is almost as widespread as malaria, but rarely causes immediate death. An estimated 200 million people are infected and the transmission occurs in some 74 countries. The infection is particularly common in children who play in water inhabited by the snail intermediate host.
Severe infection in childhood leads to long-term damage to bladder, kidneys and liver, which may cause death many years after the original infection. Infection at any age may make people feel unwell and reduce working capacity.
Bilharzia is an infection caused by parasitic worms or blood flukes of certain species of the genus Schistosoma. Adult parasites live in the blood of mammals, but their life cycle requires a phase of asexual multiplication within a fresh-water snail host. The flukes infect humans who enter their exposed skin in water, usually through swimming, bathing or wading. There exists either urinary or intestinal schistosomiasis. The type and extent of health complications associated with schistosomiasis appear to vary with species and strain of parasite and by the characteristics of the human population.
As shown in some examples below, water resources development projects may make schistosomiasis worse, and that there are serious public health consequences in many cases. Specifically, WHO stated that in areas endemic for schistosomiasis, water resources development projects should have schistosomiasis prevention and control built into programme design and implementation. Furthermore, even in cases where irrigation does not increase schistosomiasis infection rates, careful studies should be made of snail species and of existing patterns of schistosomiasis transmission. Further, a percentage of investment and operating funds should be allocated for appropriate water supply and sanitation and for health care to treat local populations for any water-related or other ailments associated with the project.
Effects of large dams and reservoirs on the prevalence of bilharzia in Africa
In Africa, in recent decades cautionary warnings have accompanied many irrigation and dam projects regarding the likely impact of increased schistosomiasis transmission. In some cases, these prophecies came true, including the Volta Lake project in Ghana in the 1960s and the Kainji Lake project in Nigeria around 1970. Most recently, research appears to confirm the prediction, that constructing a dam across the mouth of the Senegal River would lead to a surge in schistosomiasis transmission.
In part of Upper Egypt, schistosomiasis prevalence is said to have increased from 6 to 60% three years after the Aswan low dam was completed in the early 1930s. Following the construction of the Sennar dam in Sudan in 1926 and the Gezira scheme which followed, schistosomiasis spread. At the Arusha Chini irrigation project in Tanzania, research reported the prevalence to be 85 % in 1962. On the southwestern shore of Lake Volta in Ghana, prevalence is reported to have reached 90% two years after the lake was filled in 1966.
In Zambia, prevalence of schistosomiasis around Lake Kariba in 1968, 10 years after the Zambezi river was dammed, was 15% in adults and 70% in children. At Kainji Lake in Nigeria, prevalence increased from 30% to 45%.
In Ethiopia, the two Koka dams in the Awash Valley present a case of the different effects a dam has on the two different forms of schistosomiasis through the reservoir, downstream hydrology, and irrigation. It was reported that these two dams, built in 1958 and 1964, appeared to have no effect on urinary schistosomiasis transmission through their reservoirs as assessed in 1968. The dams did, however, enable intestinal schistosomiasis transmission to occur in the upper Awash Valley by changing its hydrology and introducing irrigation.
In West Africa, both urinary and intestinal forms of schistosomiasis became highly endemic in the Office du Niger area. The irrigation scheme had a mean prevalence of urinary schistosomiasis of 64.4%, compared to the river communities' 19.9%, and a mean prevalence of intestinal schistosomiasis of 53.9%, compared to the river communities' 1.9%.
Effects of small dams on the prevalence of bilharzia in Africa
Throughout the semi-arid areas of Africa, people have constructed many small earthen dams to provide irrigation water for dry season cultivation. While reports often blame these dams for spreading schistosomiasis, there is little evidence to substantiate such claims.
Small dams in the semi-arid zone of West Africa get the greatest amount of attention. Although the dams built for dry-season irrigation extended the range of Bulinus rohlfsi snails, a common vector for urinary schistosomiasis in West Africa, this did not cause any noticeable increases in prevalence of urinary schistosomiasis.
Several studies in the same Sudano-Sahel ecological zone as northern Nigeria noted that evidence linking small earthen dams to schistosomiasis was lacking. In Burkina Faso, natural seasonal ponds infested by Bulinus truncatus snails are more common and dangerous locations of infections than are artificial reservoirs created by small dams.
In Cameroon, a nation-wide survey failed to find an association between the rate of schistosomiasis and small dams. Instead, temporary ponds and snail hosts adapted to low seasonal rainfall permits intense transmission of urinary and intestinal schistosomiasis throughout northern Cameroon, regardless of dams.
Contrary to the experience in Nigeria, Burkina Faso and Cameroon, one study in northern Ghana showed small dams to be linked to schistosomiasis. Data collected during 1960-61 showed much higher prevalence of schistosomiasis in the eastern, more densely settled part of what was the Upper region of Ghana. The mean prevalence of urinary schistosomiasis in the region's eastern part was 19.8% in 15 districts without dams, 42.3% in 16 districts with dams 12 years old, and 52.0% for 6 districts with dams 3 years old. Areas in the western part of the region with few or no dams had infection rates under 10% in 6 districts, and 10 to 29% in 10 others. In contrast, prevalence was over 70% in 2 of the 3 western districts containing dams.
The control of the water-related diseases
The control of the water-related diseases can be effected in a number of ways, some of which are mutually reinforcing. Three types of measures are distinguished [8]:
• measures aimed at the pathogens: immunization, prophylactic or curative drugs;
• measures aimed at reducing vector densities or vector lifespan: chemical, biological and environmental controls;
• measures to reduce human/vector or human/pathogen contact: health education, personal protection measures and mosquito proofing of houses.
Of the above, environmental control measures are considered to be long-lasting and environmentally-sound. These include preventing or removing aquatic vegetation, lining canals with cement or plastic, regularly fluctuating water levels, periodic rapid drying of irrigation canals, preventing contamination of water bodies with faeces, supply of safe and clean drinking water, appropriate siting of housing of the farmers etc. For example, in Zimbabwe, in a communal small-holder irrigation project at Mushandike, adoption of a these measures resulted within three years in a drop of the infection rate from an initial 70 to 80% to virtually nil.
Potential environmental impacts of darns and reservoirs
The benefits of a dam project are flood control and the provision of a more reliable and higher quality water supply for irrigation, domestic and industrial use. Intensification of agriculture locally through irrigation can reduce pressure on uncleared forest lands, intact wildlife habitat and marginal agricultural land. In addition, dams create reservoir fishery and the possibilities for agricultural production on the reservoir drawdown area, which more than compensate for losses in these sectors due to the dam construction.
However, large dam projects cause irreversible environmental changes over a wide geographic area and thus have the potential for significant impacts. Criticism of such projects has grown in the last decade. Severe critics claim that because benefits from dams are outweighed by their social, environmental and economic costs, the construction of large dams is unjustifiable. In some cases, environmental and social costs can be avoided or reduced to an acceptable level by carefully assessing potential problems and implementing cost-effective corrective measures.
Damming the river and creating a lake-like environment profoundly changes the hydrology and limnology of the river system. Dramatic changes occur in the timing of flow, quality, quantity and use of water, aquatic biota, and sedimentation in the river basin. The area of influence of a dam project extends from the upper limits of the catchment of the reservoir to as far downstream as the estuary, coast and offshore zone. While there are direct environmental impacts associated with the construction of the dam (for example dust, erosion, borrow and disposal problems), the greatest impacts result from the impoundment of water, flooding of land to form the reservoir and alteration of water flow downstream. These effects have direct impacts on soils, vegetation, wildlife and wildlands, fisheries, climate and especially the human populations in the area.
Increased pressure on upland areas above the dam is a common phenomenon caused by the resettlement of people from the inundated areas and by the uncontrolled influx of newcomers into the basin catchment. On-site environmental deterioration as well as a decrease in water quality and increase in sedimentation rates in the reservoir result from clearing of forest land for agriculture, grazing pressures, use of agricultural chemicals, and tree cutting for timber or fuelwood.
The impact of dams on flooding
The function of dams and reservoirs in flood control is to reduce the peak flows entering a flood prone area. Rather than maintaining high water levels for increased head or sustained water supply for irrigation, flood control operation requires that water levels be kept drawn down deliberately prior to and during the flood season in order to maintain the capacity to store any incoming floodwater. However, flood plains may be productive environments because flooding makes them so. Flooding recharges soil moisture and replenishes the rich alluvial soils with flood deposits of silt. In arid areas flooding may be the only source of natural irrigation and soil enrichment. Reduction or elimination of flooding has the potential for impoverishing flood recession cropping, groundwater recharge, natural vegetation, wildlife and livestock population in the flood plain which are adapted to the natural flood cycles.
To maintain the productivity level of the natural systems, compensatory measures have to be taken, such as fertilization or irrigation of agricultural lands. In addition, when channelization measures reduce the frequency of flooding, the sediments entering the river systems from catchment areas upstream will be passed to the mouth of the river unless overflow areas are present downstream. Channel modification can result in a number of negative environmental impacts. Any measure that increases the velocity of flow increases the erosive capacity of the water. Although channel improvement can alleviate flooding problems in the treatment area, flood peaks are likely to increase downstream, thus simply transferring the problem elsewhere. Dikes built on the flood plain to exclude water from certain areas affect the hydrology of the area, and can have impacts on wildlife and livestock habitat and movement.
The impact of dams on fisheries and wildlife
Fishery alongside the rivers usually declines due to changes in river flow, deterioration of water quality, water temperature changes, loss of spawning grounds and barriers to fish migration. A reservoir fishery, sometimes snore productive than the previous fishery alongside the river, however, is created.
In rivers with biologically productive estuaries, both marine and estuarine fish and shellfish suffer from changes in water flow and quality. Changes in freshwater flows and thus the salinity balance in an estuary will alter species distribution and breeding patterns of fish. Changes in nutrient levels and a decrease in the quality of the river water can also have profound impacts on the productivity of an estuary. These changes can also have major effects on marine species which feed or spend part of their life cycle in the estuary, or are influenced by water quality changes in the coastal areas.
The greatest impact on wildlife will come from loss of habitat resulting from reservoir filling and land use changes in the catchment area. Migratory patterns of wildlife may be disrupted by the reservoir and associated developments. Aquatic fauna, including waterfowl, amphibians and reptiles can increase because of the reservoir.
Socio-economic impacts irrigation schemes
The objective of irrigation projects is to increase agricultural production and consequently to improve the economic and social well-being of the rural population. However, changing land use patterns may have other impacts on social and economic structure of the project area. Small plots, communal land use rights, and conflicting traditional and legal land rights all create difficulties when land is converted to irrigated agriculture. Land tenure/ownership patterns are almost certain to be disrupted by major rehabilitation works as well as a new irrigation project. Similar problems arise as a result of changes to rights to water. Increased inequity in opportunity often results from changing land use or water use patterns. For example, owners benefit in a greater proportion than tenants or those with communal rights to land. Access improvements and changes to the infrastructure are likely to require some field layout changes and a loss of some cultivated land.
Irrigation projects tend to encourage population densities to increase, either because of the increased production of the area or because they are part of a resettlement project. Impacts resulting from changes to the demographic/ethnic composition may be important and have to be considered at the project planning stage through, for example, sufficient infrastructure provision.
The most significant issue arising from large dam construction is resettlement of people displaced by the flooding of land and homes. This can be particularly disruptive to communities and insensitive project development would cause unnecessary problems by lack of inadequate compensation of the affected population. Human migration and displacement are commensurate with a breakdown in community infrastructure which results in a degree of social unrest and may contribute to malnutrition. As an example of the number of people displaced by the construction of a dam, filling of the reservoir behind the High Aswan Dam displaced 50000 to 60000 people in Egypt and some 53000 people in the Sudanese portion.
Changing land patterns and work loads resulting from the introduction or formalizing of irrigation are likely to affect men and women, ethnic groups and social classes unequally. Groups that use common land to make their living or fulfill their household duties, for example for charcoal making, hunting, grazing, collecting fuel wood, growing vegetables, etc. may be disadvantaged if that same land is taken over for irrigated agriculture or for building irrigation infrastructure. Women, migrants groups and poorer social classes have often lost access to resources and gained increased work loads. Conversely, the increased income and improved nutrition from irrigated agriculture may benefit women and children in particular.
The most common socio-economic problems reducing the income generating capacity of irrigation schemes are:
• The social organization of irrigation operation and maintenance (O&M). Poor O&M contributes significantly to long-term salinity and waterlogging problems and needs to be adequately planned at the design stage to sustain the long-term development of the schemes.
• Reduced farming flexibility. Irrigation may only be viable with high-value crops, thus reducing extensive activities such as grazing animals, operating woodlots, etc.
• Changing labour patterns that make labour-intensive irrigation unattractive.
• Insufficient external supports such as markets, agrochemical inputs, extension and credit facilities.
User participation at the planning and design stages of both new schemes and the rehabilitation of existing schemes, as well as the provision of extension, marketing and credit services, can minimize negative impacts and maximize positive ones.
Alternatives to mitigate the negative impacts of irrigation projects
Alternatives exist to mitigate adverse effects of irrigation development. Some of them are listed below:
• locating the irrigation project on the site where negative impacts are minimized;
• improving the efficiency of existing projects and restoring degraded croplands to use rather than establishing a new irrigation project;
• developing small-scale, individually-owned irrigation systems as an alternative to large-scale, publicly-owned and managed schemes;
• using sprinkler irrigation and micro-irrigation systems to decrease the risk of waterlogging, erosion and inefficient water use;
• using treated wastewater, where appropriate, to make more water available to other users;
• maintaining flood flows downstream of the dams to ensure that an adequate area is flooded each year, among other reasons, for fishery activities.
The role of wetland and the impacts of water development projects
Wetlands in the West African Sahel
The Hadejia-Nguru wetlands
Effects of the Jonglei canal on the Sudd swamps
Wetlands are wildlands of particular importance both economically and environmentally. The most important roles which wetlands perform are:
• Preservation of biological diversity: for many species of shrimp, fish and waterfowl, tidal and fresh water marshes, coastal lagoons and estuaries are of vital importance as breeding grounds as well as staging areas in their migration routes. All types of wetlands may harbour unique plants and animals.
• Production of goods: wetlands are among the most productive ecosystems in the world. Estuaries and tidal wetlands, in particular mangroves, are important nursery areas for most species of fish and shrimp which are later caught offshore. Shallow water areas are, in general, rich fishing grounds. Flood plains are important grazing areas for cattle and wildlife and vital spawning grounds for many fish species. Swamp forest may yield valuable timber.
• Production of services: wetlands can contribute to local rainfall and can be an efficient, low cost water purification system (herbaceous swamps), a recreation area (hunting, fishing, boating), a buffer against floods, and provide protection against coastal erosion by storms (mangroves).
Despite their importance, wetlands are under threat, in particular, from direct conversion of wetlands for agriculture and projects which affect the hydrology of a wetland, such as construction of dams, flood control, lowering of the aquifer drainage, and irrigation and other water supply systems.
The sections below describe some important wetlands in Africa, but the list is far from exhaustive.
Wetlands in the West African Sahel
The Senegal river waters the west of Mali, the north of Guinea, the north of Senegal and the south of Mauritania (Map 1). the main Niger river stretches through Mali, the Republic of Niger and Nigeria (Map 2). The frontiers of Niger, Nigeria, Cameroon and Chad meet in the Lake Chad (Map 3). For all of these countries, river valleys and lakes are of the utmost economic importance because of their high productivity. Mauritania, Mali, Niger and Chad, whose territories are for the most part desert, draw their agricultural resources and the greater part of their animal protein from fishing and stock-raising in the river valleys. The states along the water courses are acting in concert to increase the use of water resources and avoid the hazards of drought. It is a question of ensuring mastery over the flood waters to grow rice and other cereals, and of using surface water for irrigation. These operations inevitably tend to modify ecological conditions. In practice this will lead to significant losses of aquatic habitats and to a noticeable decrease in fish and waterfowl.
Regions such as the southern part of the Sahel, with a strongly seasonal rainfall regime and yet with sufficient rainfall to support seasonal agriculture and pastoralism, support large numbers of people. Flood plains, swamps and lakes provide a range of ecological resources and economic opportunities. Without wetlands, the drylands of the West African Sahel would be both less productive and more hazardous as a place for people to live.
In the semi-arid zone of Western Africa, patterns of rainfall and river flow are strongly seasonal. In northern Nigeria, most of the rainfall occurs in just three or four months, between June and September. During this short rainy season, precipitation exceeds evapotranspiration and runoff occurs. Savannah rivers run strongly, but start to shrink as the rains end. During the wet season rainfed agriculture is possible, and there are extensive grasslands providing relatively nutritious grazing for livestock away from the river valleys. Once the rains end, these resources also dry up, and pastoralists concentrate on the remaining wetlands in river valleys or larger wetlands such as the delta of the Senegal River the Niger Inner Delta in Mali or Lake Chad.
It is also only in these areas that agriculture can continue into the dry season. In many areas rice is planted, either in the rains before the floods arrive, or as the floods recede. Flood - recession crops such as sorghum or beans are planted as the waters recede, and farmers dig wells or use remaining pools to irrigate small gardens using buckets, shadoofs or, more recently, small pumps. At the same time, other economic activities such as fishing are also linked to the changing flood. Many fishes move laterally out of the riverbed pools into the flood plain to breed, and their offspring is caught as the water retreats. Later in the dry season, the residual pools in the riverbed are themselves fished. The high fisheries productivity of most of the seasonally inundated flood plains is fostered, at least in part, by the nutrient-rich dung left by the grazing animals during the previous dry season.
In valleys such as the Senegal or the vast flooded plains of the Niger Inner Delta, the annual cycle of farmers, herders and fishers is closely linked to the seasonal cycle of flooding. The flood plain of the Senegal stretches up to 30 km in width, and runs 600 km downstream of Bakel. It covers a total of about 1 million ha and supports farmers, pastoralists and fishing communities. Up to half a million people depend on the flood-related cropping in the 'waalo' land of the flood plain.
There is growing evidence that large-scale capital intensive water development schemes do provide neither the range of foodstuffs nor the economic return of traditional systems. Studies, comparing the efficiency per unit of water of traditional extensive systems of cultivation, grazing and fishing in the Niger Inner Delta with the intensive modern project of the Office du Niger, showed that both systems produce about the same gross profit margin, even when the running costs and management charges for the irrigation scheme are taken into account. However, the extensive system produces meat, milk, fish and rice compared to the rice-only irrigated system. More importantly, when the interest charges arising from the irrigation scheme are taken into account, the net profit from the irrigated rice turns into a loss of $0.65/100 m³ of water, whilst the extensive traditional methods benefit from the 'free services of nature' and turn in a net profit of $0.42/100 m³ at 1984/85 prices [15a].