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The method of irrigation profoundly affects the vulnerability of the land to erosion. Because irrigated land is wetter, it is less able to absorb rainfall and runoff will therefore be higher. Field size, stream size (drop size), slope and field layout are all difficult to change and all significantly affect erosion rates. Careful design can avoid the occurrence of erosion problems. Agricultural practices affect soil structure and therefore the soil's erosivity, or the ease with which particles are dislodged. In general land-forming for irrigation, such as land-levelling and the construction of field bunds, tends to reduce erosion.
Archaic in-field water management practices involving poor cut and fill operations through watercourse embankments can result in serious local erosion at the head end of the irrigated field and in sedimentation at the mid or tail-end locations of the field. The micro-topography of a field will thus be disturbed. Unavoidably, this effect creates disproportionate water distribution over the irrigated field. In addition it might create disputes between water users. Improved water management practices related to surface irrigation methods (for example by using gates, siphons, checks) can reduce such hazards.
Irrigation infrastructure needs to be designed to ensure that localized erosion, eg gully formation, does not occur. Construction activities generally expose soil to erosion. Following the completion of construction work, vegetation should be established around structures so that bare soil is not exposed to erosive forces.
The development of irrigation schemes in developing countries is often associated with an increase in intensity of human activity in areas surrounding the scheme. This may be due to people moving into the area as a result of the increased economic activity or may be carried out by farmers and their families who are directly engaged in irrigation activities. In either case typical activities are: more intensive rain fed agriculture; an increase in the number of livestock; and, greater use of forests, particularly for fuel wood. All these activities are liable to increase erosion in the area by decreasing vegetative cover which will have a detrimental effect on the local fertility and ecology as well as contribute to sediment related problems.
Clearing higher non-irrigated parts of the catchment can result in a rising downstream water table. In areas where the groundwater is saline the higher recharge may cause higher salinity levels in the rivers and cause pressure levels in the lower irrigated areas to rise thus impeding leaching. This can be prevented by planting deeper rooting crops and trees in the higher lands. This phenomenon has been observed in South-eastern Australia.
Mitigating actions can be put in place relatively easily with forethought as to problems that might arise. For example, allowance should be made for livestock, fuel wood or vegetable gardens within the layout of an irrigation scheme. Alternatively, protection of vulnerable areas maybe necessary.
The capacity and shape of a river results from its flow, the river bed and bank material, and the sediment carried by the flow. A fast flowing river has more energy and is able to carry higher sediment loads (both more and larger particles) than a slow moving river. Hence, sediments settle out in reservoirs and in deltas where the flow velocity decreases. A river is said to be in regime when the amount of sediment carried by the flow is constant so that the flow is not erosive nor is sediment being deposited. The regime condition changes through the year with changing flows.
Reductions in low flows and flood flows may significantly alter the river morphology, reducing the capacity to transport sediment and thereby causing a build up of sediments in slower moving reaches and possibly a shrinking of the main channel. Increasing flows will have the reverse effect. Where the sediment balance changes over a short distance, perhaps due to a reservoir or the flushing of a sediment control structure, major changes to the local river morphology are likely to occur. The release of clear water from reservoirs may result in scour and a general lowering of the bed level immediately downstream of the dam, the reverse of the effect that might be expected with a general reduction in flows.
Changes to the river morphology may effect downstream uses, in particular navigation and abstraction for drinking, industry and irrigation. The river ecology may also be adversely effected.
The susceptibility of channel structures to damage is strongly related to changes in channel morphology and changes in sediment regime. Increased suspended sediment will cause problems at intake structures in the form of siltation as well as pump and filtration operation. Abstraction structures may become clogged with sediment or left some distance from the water. Degradation of the river bed is likely to threaten the structural integrity of hydraulic structures (intakes, headworks, flood protection etc.) and bridges. The construction of new structures impacts on nearby structures by changing local flow conditions.
Irrigation schemes can fail if the sediment load of the water supply is higher than the capacity of the irrigation canals to transport sediment. Sediment excluders/extractors at the headworks can mitigate this effect to some extent. Sedimentation from within the scheme itself can also be a problem, for example, wind-blown soil filling canals. Canal desilting is an extremely costly element of irrigation maintenance and design measures should minimize sediment entry. Reservoir siltation shortens the active life of the reservoir and must be given careful consideration at the design stage. The increases in erosion due to the economic activity prompted by the reservoir and its access roads needs to be taken into account. Upstream erosion prevention, particularly within the project catchment is an important consideration of an EIA. However, this may not be sufficient to significantly reduce reservoir sedimentation, especially in view of the time delay between soil conservation activities and a reduction in river sediment loads.
Changes to the morphology of river estuaries can result from increased erosion or sedimentation. Areas of mangrove may be threatened by changes to the estuary morphology and special studies may be required to determine any adverse impacts. Navigation and fishing may also be adversely affected.
Biological and ecological change
Valleys and shores
Wetlands and plains
This section focuses on the ecological changes brought about by the project. The most obvious ones are a consequence of the change of land use and water use in the project area but effects on the land around the project and on aquatic ecosystems that share the catchment are likely. Biological diversity, areas of special scientific interest, animal migration and natural industry are important study areas. The overall habitat as well as individual groups (mammals, birds, fish, reptiles, insects etc.) and species need to be considered. Rare and endangered species are often highly adapted to habitats with very narrow ranges of environmental gradients. Such habitats may not be of obvious economic value to man, eg arid areas, and therefore current knowledge of the biota may be poor and a special study may be required. Local knowledge is particularly important as the range of species may be very local. Thienemann's rules are useful in thinking about the ecology of the effected areas:
The greater the diversity of conditions in a locality, the larger the number of species in a biological community.
The more conditions in a locality deviate from the normal, and thus from the optimum for most species, the smaller the number of species and the greater the biomass of each.
The longer a locality has been in a stable condition, the richer its biological community. (Petermann 1993).
The nature of irrigation, ie providing water to water-short land, will radically change both the agricultural and natural ecology in the project area. The creation of compensation areas or habitat enhancement outside the project area may be useful mitigation measures where the natural habitat change is assessed as detrimental. In order to predict the likely significant effects that irrigation projects have on human interests, low intensity, pre-project use of the study area needs to be assessed, such as seasonal grazing, recreation, hunting for wild meat or bee keeping and the use of the vegetation for fuel, building, medicine etc.
The creation of reservoirs and channels provides the possibility of enhanced aquatic habitats. In particular, reservoirs and channels offer the opportunity of pisciculture and aquaculture and favourable habitats for water fowl, both permanent and migrating, but may also offer favourable habitats for disease transmitting insects and snails (see the section Human health). Bird sanctuaries and wildlife parks can be created around reservoirs.
The consumption of water for irrigated agriculture and the reduced quality of return flows is likely to adversely impact on downstream ecosystems. Reduced flows, increased salt concentrations, lower oxygen levels, higher water temperatures and increased pollution and silt loads all tend to favour vigorous, tolerant species (aquatic weeds). The demands of different ecotypes will change through the year both in quantity and quality. The needs of fowl and fish are liable to be particularly sensitive during breeding and migrating seasons: sport and commercial fish are often at risk. See Table 9 for information on water quality for freshwater fish. This table is for temperate zones and no international standards exist for tropical fish. Local standards should be studied where available. Discharges from dams can be controlled to meet ecological demands through the year and there may be scope to modify construction methods to minimize disruptions to the flow and to prevent very heavy sediment loads.
FIGURE 8 Wetland values (Source: Dungan (IUCN), 1990)
It is important to consider the biological and ecological changes that may result in areas surrounding irrigation and drainage work. Irrigation may have a positive impact, for example by settling migrant slash and burn farmers, or a negative impact, for example by raising the demand for fuel wood due to increases in the local population.
Valleys and shores
Water bodies tend to support environmentally-rich corridors and large human populations. Marked changes to the water environment, both in quantity and quality, are liable to have major impacts, both positive and negative eg by providing a food source for fish-eating mammals and birds around a new reservoir or by reducing suitable nesting sites at a riverside marsh. Downstream aquatic biota may be adversely affected by changes to the hydrology or morphology of a river system.
Wetlands and plains
The United Nations convention on Wetlands of International Importance defines wetlands as "areas of marsh, fen, peatland or water, whether natural or artificial, permanent or temporary, with water that is static or flowing, fresh, brackish or salt, including areas of marine water the depth of which at low tide does not exceed six metres". Wetlands are among the most productive ecosystems in the world. Estuaries and tidal wetlands, in particular mangroves, are important nursery areas for many species of offshore fish. Shallow waters are also, in general, rich fishing grounds. Wetlands usually support a wide range of species and are particularly important for water fowl and as staging areas for migrating birds. The other three most valuable contributions of wetlands are: as a buffer to reduce flood peaks; as a low-cost water purification system; and, as protection from coastal erosion (World Bank, 1991). Figure 8 summarizes the value of wetlands.
Mangroves need both significant fresh water recharges and sediment rich flows in order to thrive. A reduction in flow leads to an increase in the soil salinity which favours more salt-tolerant species. Mangroves trap silt, transported by flood flows, and obtain their inorganic nutrients from it. These flushing flows also serve to keep the deltaic channels open. In the Lower Indus, which now receives no fresh water for nine months of the year, the mangroves have become stunted and reduced to one, salt-tolerant species.
Seasonally flooded plains and deltas offer specialized and important habitats providing grazing for cattle and wildlife, and vital spawning grounds for many fish species. Flood flows trigger migration and breeding in a large number of species.
Irrigation can have a direct impact on wetlands by either changing the hydrological conditions or by reducing water quality in downstream areas. The often high environmental and economic value of wetlands makes their study and preservation of key importance in an EIA.
Income and amenity
Sites of value
The major purpose of irrigated agriculture is to increase agricultural production and consequently improve the economic and social well-being of the area of the project. Although irrigation schemes usually achieve this objective, they could often have been more successful in developing countries if more attention had been paid to the social and economic structure of the project area. An EIA should thus equally concentrate on ways in which positive impacts can be enhanced as on negative impacts mitigated.
Changing land-use patterns are a common cause of problems. 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 work as well as a new irrigation project. Access improvements and changes to the infrastructure are likely to require some field layout changes and a loss of some cultivated land. The 'losers' will need tailored compensation best designed with local participation. Similar problems arise as a result of changes to rights to water.
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. Consultations with and the assistance of NGOs can be particularly helpful in minimizing adverse socio-economic impacts.
Irrigation projects tend to encourage population densities to increase either because they are part of a resettlement project or because the increased prosperity of the area attracts incomers. Major changes should be anticipated and provided for at the project planning stage through, for example, sufficient infrastructure provision. Impacts resulting from changes to the demographic/ethnic composition should also be considered. Training is an important component if new skills are expected.
Income and amenity
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): who will carry out the work (both operation and maintenance); when will irrigation take place (rotation schedules); how will fair delivery be determined (communication and measurement)? Poor O&M contributes significantly to long-term salinity and water-logging problems and needs to be adequately planned at the design stage.
reduced farming flexibility. Irrigation may only be viable with high-value crops thus reducing activities such as grazing animals, operating woodlots etc.
insufficient external supports such as markets, agro-chemical inputs, extension and credit facilities
increased inequity in opportunity, often as a result of changing land-use or water use patterns. For example, owners benefit in a greater proportion than tenants or those with communal rights to land.
changing labour patterns that make labour-intensive irrigation unattractive.
Improved planning, with user involvement, has the potential to reduce if not remove the above problems for both new and rehabilitation projects. Extension services, with training and education, also offer much scope to improve the income and amenity of irrigation schemes. Farmers often choose low risk, low profit strategies rather than high risk, high profit ones.
Human migration (outside of the nomadic way of life) and displacement are commensurate with a breakdown in community infrastructure which results in a degree of social unrest and may contribute to malnutrition and an increased incidence of disease. Large, new irrigation schemes attract temporary populations both during construction and during peak periods of agricultural labour demands and provision for their accommodation needs to be anticipated. The problems of displacement during project construction or rehabilitation can usually be solved by providing short-term support.
Often the most significant social issue arising from irrigation development is resettlement of people displaced by the flooding of land and homes or the construction of canals or other works. This can be particularly disruptive to communities and, in the past, insensitive project development has caused unnecessary problems by a lack of consultation at the planning stage and inadequate compensation of the affected population. Technical ministries should seek expert assistance at an early stage. Community re-establishment often includes, for example, pilot farms, extension services and credit schemes. For more detailed information see Burbridge, 1988.
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 fulfil their household duties, eg 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. Historically, it has been men from the more settled and powerful groups that have had greatest access to the benefits and increased income from irrigated agriculture. Women, migrant 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 benefit women and children in particular. Inclusion of disadvantaged groups into the planning process maybe time-consuming, but should be considered an important aspect of EIA.
Minority groups or tribal minorities can benefit from the increased economic development of a new irrigation area. However, they are often disadvantaged by irrigation development as they are excluded from the scheme because of uncertain land rights and may be pastoralists rather than farmers. An EIA should consider the impacts on minority groups and, after consultation, appropriate rehabilitation or compensation measures should be allowed for in the project design.
Sites of value
New irrigation schemes should avoid destroying or downgrading sites of value whether that value be: aesthetic, historical, religious, mineral, palaeotological or recreational. A change in water table, associated with well-established schemes, can threaten buildings.
As with ecological impacts, the socio-economic impacts of irrigation projects will be significant outside the project area. A new project will both place demands on the region (marketing, migration, physical infrastructure) and contribute to regional development. For irrigation schemes to be economically viable, they need to complement other activities in the region and the EIA should consider the effects of any other development, such as agro-industries or new roads. Industrial and urban development may adversely affect irrigation schemes by competing for water and reducing the quality of water available. A regional planning system is essential to minimize conflicts and coordinate development.
Projects planned with the beneficiaries rather than for them have proved more sustainable and no more costly. However, they do take longer to plan and design because consultation is a lengthy process. Some countries have public participation in the planning process enshrined in law but many countries have a top down procedure only. Local consultation of all interested (not just well-organized, vocal groups) will improve the project and thus increase the potential for economic benefit and sustained operation. The process may take a particularly long-time if the mechanisms for consultation also have to be set up. Local NGOs can be helpful to government agencies in this work and should be brought into the planning process at an early stage in order to avoid later conflicts building up.
New and rehabilitation works offer the potential for improved recreational facilities, particularly around reservoirs and the EIA should highlight such potential for enhancement.
Pests and weeds
Without appropriate management measures, irrigated agriculture has the potential to create serious ecological imbalances both at the project site and in adjacent areas. Excessive clearance of natural vegetation cover in the command area, for example, can affect the microclimate and expose the soil to erosion, leading to a loss of top soil and nutrient leaching. The removal of roots and vegetation disrupts the water cycle, increasing the rate at which water enters rivers and streams, thereby changing flow regimes and increasing siltation in the downstream zone. This is often to the detriment of fisheries and aquaculture activities. The destruction of natural habitats in this manner and the creation of agricultural monocultures also impacts on the local flora and fauna reducing biodiversity. The introduction of exotic species of plant or animal may oust indigenous species or introduce disease agents which may affect plants, animals and/or man. Fertilizers and pesticides are widely applied to correct imbalances. These can percolate through the soil and/or be carried away in the drainage water polluting both groundwater and surface waters especially in the downstream zone. The nutrients in fertilizers may give rise to eutrophication of surface water bodies and promote the growth of aquatic weeds. Pesticide residues are hazardous to the health of both man and animals.
The above examples serve to illustrate, together with the range of biological and ecological changes described in the section Biological and ecological change, the wide variety of potential impacts which may arise. Many may be of relatively minor significance in their own right but they often interact to produce a cumulative effect over a prolonged period of time which can result in very significant long term changes to the local ecology. This cumulative effect may impair the long-term viability of both the project and economic activities in the surrounding area.
The following sections briefly describe three imbalances that are common problems on irrigation schemes.
Pests and weeds
Irrigated agriculture often provides improved conditions for crop diseases to develop, particularly fungal and bacterial foliage diseases. Diseases and weeds can also spread quickly via the re-use of waste-water and drainage water.
Any change to a more uniform environment on the project lands is likely to favour vigorous species adapted to a wide variety of conditions. Species, such as insects and rodents, are often regarded as pests. The preferred habitats of natural predators, such as snakes, birds and spiders, may be reduced by land use changes and by the increased use of pesticides. Local or newly imported varieties of weeds may thrive in the irrigated environment and reduce agricultural productivity.
Animals are subject to a similar range of water related diseases as humans. They may also act as reservoirs for human water-based infections and infections with water-related insect vectors, see Figure 9. The promotion of animal husbandry as a secondary, income generating activity for farmers in newly irrigated areas should be carefully evaluated for its possible environmental and health risks.
The main problems of aquatic weeds are that they reduce the storage and conveyance capacity of reservoirs, canals and drains and increase water loss through evapotranspiration. Most irrigation schemes suffer infestations of exotic species. They are difficult and expensive to control, though the use of linings, shade and intermittent drying out can compliment traditional techniques of mechanical removal, careful herbicide application and the introduction of weed eating fish and insects. The costs of removing weeds may be offset in some cases by using the debris for compost, big-gas and animal and fish food. Other problems of aquatic weeds are that they can provide a favourable and protected habitat for disease vectors such as snails and mosquitoes.
FIGURE 9 Main animal hosts of vector-borne diseases (Source Birley, 1989)
Specific risks and counter measures
This section concentrates on human health issues associated with irrigation and drainage. It refers to items of the ICID checklist which cover health and safety in their broadest sense, including for example human settlements and shelter, and nutrition. Relevant characteristics of diseases, whose transmission potential is a function of ecological parameters affected by irrigation development, are summarized for non-expert readership; health risks mentioned in connection with the environmental and socioeconomic changes are discussed with possible preventive and mitigating measures; and, opportunities to promote human health in an integrated approach to irrigation development are presented. Health is a complex subject and specialist expertise will be required when preparing an EIA. Only brief introductory comments are made here and for further information the reader is referred to the PEEM Guidelines listed in the references. Human health considerations may warrant a separate Health Impact Assessment and the Asian Development Bank have produced guidelines for this (ADB, 1992).
Irrigated agriculture contributes substantially to conditions that favour good health: food security, an improved infrastructure allowing better access to and by health services and economic progress which permits rural households a greater purchasing power for drugs and health services. On the other hand there can be significant negative impacts and two conditions need to be met to successfully deal with the potential negative impacts on human health in the context of an EIA. Firstly, relevant departments in the Ministry of Health and other appropriate health sector institutions should be involved and consulted at the earliest stages of any project. Options for institutional arrangements are described in PEEM Guideline 1, (Tiffen, 1989). For the process of impact assessment reference is made to PEEM Guideline 2 which distinguishes three categories of parameters related to: community vulnerability; environmental susceptibility; and the capacity of health services to deal with the forecast situation, (Birley, 1989). This methodology ensures a comprehensive approach, including, but not restricted to, the health sector.
The traditional classification of water-related diseases by Bradley (Feachem et al 1977) focuses on specific ecological and behavioural risk factors and these characteristics are presented in Table 9. A broad indication of the global distribution of vector-borne diseases is presented in Table 10 and for more details reference is made to WHO (1989).
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