Factors which affect the operation of the project itself within its assumed economic life

Environmental effects which reach beyond the project boundaries or operate over larger time scales than those commonly used in project planning

Changes to the environment brought about by the project which destroy unique habitats or threaten endangered species.

By identifying and recognising the effects associated with each of these three categories, it is hoped that engineers and planners may see more clearly the areas for which they are responsible and the ways in which they can make a positive contribution to environmental matters. The categories are considered in detail below.

1.   Environmental changes at project level

A project which cannot provide the desired benefits within its economic life because some ‘environmental’ influence has impaired its performance, is flawed in its design. Many such influences are now recognised as factors which must be considered at the design phase but, because of their complexity, the techniques by which they may be controlled are not yet fully understood. There is a need for further research to provide quantitative information about these regional effects, including :

1. Soil erosion and reservoir sedimentation

   The real increase in cultivated land areas in the tropics over the last forty years has also resulted in a steady increase in the quantities of soil being worn away from the exposed land surface (soil erosion) and washed into the river systems. This sediment is then trapped behind dams constructed to provide water for hydro-power, irrigation and domestic consumption. It has been estimated that in many reservoirs in the Asian region, sediment inflows are increasing at a rate of about 50% per decade. Because the effects of such changes over the life of projects have been ignored, long-term sedimentation rates calculated during the feasibility and design stages have been grossly underestimated (by factors of up to five or 6 times) with the result that the irrigation schemes being supplied are unable to achieve their full potential. Further research is needed if field data collected at discrete sites, and over short periods, are to be used to accurately predict sediment yield.

2. Sediment effects on irrigation canals and structures

The accumulation of sediment in an irrigation system will reduce the ability of the canals to supply sufficient water to the fields and may lead either to the eventual abandonment of the scheme or to the need for a costly programme of regular sediment removal. This occurs most frequently in schemes where the water is taken from a sediment-laden natural river. It is possible to design structures which can reduce the amount of sediment being taken into a canal system (sediment exclusion) or remove the sediment once it has entered a canal (sediment extraction).

3. Drainage

Excess water (water logging) in the fields of an irrigation scheme is a common cause of lost production and results from a failure to provide an adequate drainage system, inadequate maintenance of that drainage system or poor control over water application to the fields. For land to be classed as "water logged" the soil need only to be saturated, there does not necessarily have to be standing water on the surface. However, the drainage water need not be considered to be lost from the scheme since, by returning it to the distributary channels, it may be re-applied to fields at the tail end.

4. Soil salination

All naturally occurring water carries a range of dissolved salts and chemicals and their impact on the environment will usually be minimal. However, if an irrigation scheme is poorly managed and the ground is allowed to become waterlogged, evaporation will take place from the ground surface, leaving the dissolved salts behind. The process of evaporation causes more water to be drawn up through the soil and, as this continues, the concentration of salts will also gradually increase. In many irrigated areas this has resulted in reduced crop yields and has even caused some land to go out of production.

5. Pests and Weeds

The performance of many irrigation schemes is impaired by a proliferation of crop pests and weeds which was not anticipated by the designers.

Aquatic vegetation can have considerable detrimental impacts on the running of irrigation and drainage systems. The ability of channels to convey the design discharge is reduced by increases in channel roughness and resistance. Canals are restricted in size by marginal weed growth, and storage dams and flood protection ponds can be severely reduced in capacity. Areas of canal and reservoir siltation, though not usually caused by vegetation, are rapidly colonised by plants which makes cleaning a more laborious and expensive task Vegetation also provides habitats for the vectors of several tropical diseases such as schistosomiasis and malaria.

Possible control measures may be broadly categorised as mechanical (removal of vegetation by cutters or diggers), chemical (regular or intermittent releases of herbicide) or biological (eg ducks or grass-eating fish). However, considerable care must be taken because if used incorrectly, these measures will at least be costly and ineffective; in the worst case they could do positive harm to the environment. As in many areas of impact, the appropriate action can only be taken if engineers and planners work with experts of other professional disciplines.

6. Water Quality

The quality of irrigation water has an important influence on crop yields and will, to a large extent, depend on the scheme location. Water released from large reservoirs may be very cool or have had oxygen removed through eutrophication. An irrigation intake located near to the mouth of a river will take water of varying salinity depending on the state of the tide and river discharge. Schemes located near to major conurbations may receive domestic effluent to augment the available water supply, either as a deliberate policy or by taking contaminated water through a river offtake. Unless this is practised with care, it may post health hazards both to the irrigators and, with some crops, to the consumers.

The re-use of drainage water for irrigation increases the salt load applied to the soil and/or groundwater reservoirs. Careful management is required to prevent salinity building up to a level which causes yield depression. Re-cycling drainage water also creates a risk of spreading crop disease between areas and speeding up the re-infestation of fumigated land.

2.   Environmental changes of regional significance

A project may be performing well under the terms of its own narrowly defined economic objectives but it may be causing widespread and long-term damage to the health of the local population or to the surrounding environment. In the past, there was seldom any explicit requirement for scheme operators to take account of the effects of their actions on other users. Nowadays, many countries have introduced legislation making environmental impact statements (EIS’s) mandatory for new projects.

One of the primary responsibilities of central government should be to ensure that its natural resources are used in such a way that they bring the maximum benefit to the population as a whole. Engineers and scientists may help authorities by developing techniques to predict the effects of proposed irrigation projects and measure the impact of existing ones. There is considerable scope for further research and for the establishment of close interdisciplinary co-operation. The following are some areas of environmental concern within this category.

1. Sedimentation and land degradation

Many of the project specific effects described above take place over such long time scales that their impact may be negligible within the prescribed ‘economic life’ of an individual project. Over the longer time scale, however, changes to surface and sub-surface hydrology and to the transport of salts and sediments brought about by irrigation development in a region may have a considerable impact on the environment and the economy. For example, sediment extracted from irrigation canals and returned to the natural river often cannot be transported in the reduced flows of the river. Its deposition in the river channel may cause a possible increase in the flood hazard or it may cause the low-flow channel to move away from an irrigation or water supply offtake. Conversely, a reduction in sediment carried by a river may lead to serious erosion which could undermine river banks and structures such as bridges immediately downstream. The change may even lead to erosion of the delta and coastline.

2. Health

Irrigation may improve the general health of the population by increasing levels of income and providing a secure food supply, but it frequently also causes an increase in the prevalence of water-related diseases by increasing water contact by the human population and by providing suitable habitats for animals that transmit the parasites – the disease vectors. The population affected by such diseases may include many who have no direct involvement in the project although in some cases the projects themselves may be disrupted due to the debilitating effects on the working population. Of particular concern are such diseases as schistosomiasis (frequently called bilharzia) and the various mosquito-borne diseases, for example, malaria, encephalitis and filariasis (elephantiasis).

The prediction and control of adverse health impacts from irrigation schemes require close inter-disciplinary co-operation since complex interactions between the physical, biological and social systems must be considered. It is particularly important for engineers and planners to become more aware of potential problems and to know what expertise is available so that possible adverse effects can be identified at an early stage for appropriate action to be taken to avoid or at least control the disease. In the past, the most common methods of control attempted have been through the treatment of infected people or the chemical control of disease vectors. Engineering or environmental control measures, directed towards reducing the population of disease vectors and changing patterns of human water contact are now receiving more attention since they offer the possibility of sustainable protection with the dangers and cost of continued use of chemicals and drugs.

3. Water quality

Irrigation can influence the quality of surface water and groundwater. It modifies the natural hydrology, thereby affecting the pollutant carrying capacity of natural water courses, and it acts as a source of pollutants itself. Pollutants leaving an irrigation scheme include leached salts, fertilisers, pesticides and human effluent.

4. Hinterland development

An increase in human effluent is only one aspect of the environmental impact of a new centre of population such as that created by irrigation development. Probably the most important effect is that of deforestation which results from the search for building materials and fuel, and from the free grazing of livestock. Another possible cause of environmental degradation is the opening of new roads to provide a transport route for supplies to be brought into the area and for produce to be taken to market. Although none of these are specific to irrigation development they are frequently acute in cases where irrigation has enable human settlement to occur in previously unpopulated areas.

5. Socio economic impacts

New projects, where for irrigation or not, may also have far-reaching effects on the local economy and society which may not be wholly beneficial. New opportunities for irrigated farming may have been won at the cost of displacing or disrupting traditional communities who will possibly gain little or no benefit from the project. Land ownership, employment status, family size and income, seasonal labour availability, organisational and leadership structures within the project, local skills, educational standards and cultural practices are all factors which should be considered by project planners. Increasingly, attempts are being made to involve the local population in the planning and management of new projects to avoid the hardships and conflicts which have been caused by ignoring these factors.

3.   Habitat and species conservation

There are strong arguments which go beyond those normally included in an economic analysis, for the conservation of unique natural habitats and rare species of flora and fauna. Such issues cannot solely be the responsibility of national governments, especially where they are faced with serious economic problems. Global solutions must be found where the interests of conservation do not coincide with national interests.

In practice, national or regional benefits can sometimes arise from measures which are initially seen as purely conservationist. Habitats most endangered by agricultural developments (including irrigation) are natural floodplains and coastal swamps (deltas). Frequently these play an important role in determining the hydrological and sedimentary regimes of a river system. With a better understanding of these regimes the full impact of habitat modifications can be assessed. In some cases a different type of development may be shown to be more suitable for a given region’ for example, building a sustainable fishing industry based on established techniques.

The survival of certain riverine species is closely associated with hydraulic factors such as turbidity (the "cloudyness" of the water). Engineers in collaboration with biologists can make a contribution to their survival by considering different designs of hydraulic structure and impoundments and by assessing the effects of different operating practices on the river downstream. In addition, mathematical models can be used to study the hydrological changes which will result from river regulation and the ways in which they will affect the survival of natural wetlands and floodplain areas.