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Chapter 4: Major impacts of irrigation and drainage projects


Hydrology
Water and air quality
Soil properties and safety erects
Erosion and sedimentation
Biological and ecological change
Socio-economic impacts
Ecological imbalances
Human health


When considering impacts, two perspectives must be taken into account, those of:

• the project on the environment, and
• external factors on the project (externalities).

In the detailed sections below, many of the impacts described are most extreme in the case of new irrigated areas. However, rehabilitation and changes resulting from alterations to the operating infrastructure, for example, will also have environmental impacts that may not at first be anticipated. The intensification of agriculture can lead to groundwater pollution related to the increased use of pesticides and fertilizers. Improved efficiency may significantly reduce return flows which are often utilized downstream by other irrigation schemes or wildlife habitats. Similarly, upstream developments are likely to impact on an irrigation scheme either in the form of reduced water availability (surface or groundwater) or reduced water quality.

Different types of irrigation will have different impacts and it should not be assumed that modern methods will have fewer impacts: they may significantly increase energy consumption and lead to social problems due to reduced employment in agriculture. Impacts will also vary according to the stage of implementation. For example, during the construction period there may be specific health and other social risks due to an influx of migrant workers living in temporary and unsanitary accommodation. Later, once the project has been operating for several years, cumulative impacts may begin to present serious environmental constraints to project sustainability. Such issues must be predicted by the EIA and mitigation measures prepared.

The most common problems of, and threats to, irrigation schemes are listed in Table 5, together with potential mitigation measures. Irrigation is defined as much, if not more, by farmers and managers as by the physical infrastructure; the 'hardware'. Its sustainable operation is just as dependent on the 'soft' environment: education, institutional building, legal structures and external support services. These are all powerful tools to ensure sustainability in conjunction with well-designed and well-managed hardware and Table 5 indicates that many of the mitigation measures are 'soft'.

The sections below describe the most common environmental impacts associated with irrigation schemes. Under each item, both positive and negative impacts are briefly described and the most usual mitigating measures outlined. The opportunity to identify positive impacts and to propose measures to enhance such impacts should not be neglected. The structure of the chapter generally follows that of the ICID Environmental Check-list and is divided into eight major sections. As a slight deviation from the Check-list, human health has been included, in order to present the human health dimensions of the environmental impacts.

TABLE 5 Main problems resulting in the non-sustainability of irrigation and drainage schemes and appropriate mitigation measures

Problem

Mitigation measures

Degradation of irrigated land:

- Improve I & D operation to match demand both 'how much & when'.

Salinization


- Provide drainage including disposal of water to evaporation ponds or the sea if quality of river flow adversely affected by drainage water.

Alkalization


- Maintain channels to prevent seepage, and reduce inefficiencies resulting from siltation and weeds. Allow for access to channels for maintenance in design.

Waterlogging


- Provide water for leaching as a specific operation.

Soil acidification


- Set-up or adjust irrigation management infrastructure to ensure sufficient income to maintain both the irrigation and drainage systems.


- Analyse soils and monitor changes so that potential problems can be managed.

Reduced socio-economic conditions:

- Manage I & D to prevent disease spread.

Increased incidence of water related disease


- Educate about causes of disease.

Increased inequity


- Improve health facilities.

Weaker community infrastructure


- Allow sufficient time and money for extensive public participation to ensure that plans are optimal, that all sections of affected society are considered and that local institutions are in place to sustain irrigated agriculture, particularly in respect of land and water rights.


- Consider markets, financial services and agricultural extension in conjunction with proposed irrigation and drainage changes.


- Ensure that agricultural intensification does not preclude other economic or subsistence activity, such as household vegetables, fodder or growing trees for firewood.


- Provide short-term support and/or skills for an alternative livelihood if irrigation removes existing livelihood

Poor water quality:

- Define and enforce return water quality levels (including monitoring).

Reduction in irrigation water quality


- Control industrial development.

Water quality problems for downstream users caused by irrigation return flow quality


- Designate land for saline water disposal; build separate disposal channels.


- Educate for pesticide or sewage contamination dangers.


- Monitor irrigation water quality

Ecological degradation:

- Define ecological requirements.

Reduced big-diversity in project area


- Operate dams to suit downstream requirements and encourage wildlife around reservoirs (see Sections 4.1.3 and 4.5).

Damage to downstream ecosystems due to reduced water quantity and quality


- Designate land (in law and supported by protection institutions) for flood plains; wetlands; watersheds; drainage water disposal; river corridors.

Ground water depletion:

- Define and enforce abstraction regulations.

Dry drinking & irrigation wells


- Monitor ground water levels.

Saline intrusion at coasts


- Adjust abstraction charges.

Reduced base flow/wetlands



Hydrology


Low flow regime
Flood regime
Operation of dams
Fall of water table
Rise of water table


This section is concerned with the consequences of impacts resulting from a change in the flow regime of rivers, or a change in the movement of the water table, through the seasons. The consumptive nature of irrigation means that some change to the local hydrological regime will occur when new schemes are constructed and, to a lesser extent, when old schemes are rehabilitated. The ecology and uses of a river will have developed as a consequence of the existing regime and may not be able to adapt easily to major changes. It is also important to recognize the interrelationship between river flows and the water table. During high flow periods, recharge tends to occur through the river bed whereas groundwater often contributes to low flows. Figure 3 is a conceptual diagram of flow through a river-supplied irrigation scheme. Figure 4 illustrates the links between surface and groundwater.

Low flow regime

Changes to the low flow regime may have significant negative impacts on downstream users, whether they abstract water (irrigation schemes, drinking supplies) or use the river for transportation or hydropower. Minimum demands from both existing and potential future users need to be clearly identified and assessed in relation to current and future low flows. The quality of low flows is also important. Return flows are likely to have significant quantities of pollutants. Low flows need to be high enough to ensure sufficient dilution of pollutants discharged from irrigation schemes and other sources such as industry and urban areas. A reduction in the natural river flow together with a discharge of lower quality drainage water can have severe negative impacts on downstream users, including irrigation schemes.

Habitats both within and alongside rivers are particularly rich, often supporting a high diversity of species. Large changes to low flows (20%) will alter micro-habitats of which wetlands are a special case. It is particularly important to identify any endangered species and determine the impact of any changes on their survival. Such species are often endangered because of their restrictive ecological requirements. An example is the Senegal river downstream of the Manantali Dam where the extent of wetlands has been considerably reduced, fisheries have declined and recession irrigation has all but disappeared.

The ecology of estuaries is sensitive to the salinity of the water which may be determined by the low flows. Saline intrusion into the estuary will also affect drinking water supplies and fish catches. It may also create breeding places for anopheline vectors of malaria that breed in brackish water.

The operation of dams offers excellent opportunities to mitigate the potential negative impacts of changes to low flows.

FIGURE 3 Conceptual diagram of the irrigation return flow system for a given reach of a river system (Utah State University Foundation, 1969)

FIGURE 4 The interrelationship between surface water and groundwater

Flood regime

Uncontrolled floods cause tremendous damage and flood control is therefore often an added social and environmental benefit of reservoirs built to supply irrigation water. However, flood protection works, although achieving their purpose locally, increase flooding downstream, which needs to be taken into account.

Radically altered flood regimes may also have negative impacts. Any disruption to flood recession agriculture needs to be studied as it is often highly productive but may have low visibility due to the migratory nature of the farmers practicing it. Flood waters are important for fisheries both in rivers and particularly in estuaries. Floods trigger spawning and migration and carry nutrients to coastal waters. Controlled floods may result in a reduction of groundwater recharge via flood plains and a loss of seasonal or permanent wetlands. Finally, changes to the river morphology may result because of changes to the sediment carrying capacity of the flood waters. This may be either a positive or negative impact.

As with low flows, the operation of dams offers excellent opportunities to mitigate the potential negative impacts of changes to flood flows. The designation of flood plains may also be a useful measure that allows groundwater recharge and reduces peak discharges downstream. This is one of the positive functions of many areas of wetland.

It is important that new irrigation infrastructure does not adversely effect the natural drainage pattern, thus causing localized flooding.

Operation of dams

The manner in which dams are operated has a significant impact on the river downstream. There is a range of measures that can be undertaken to reduce adverse environmental impacts caused by changing the hydrological regime that need not necessarily reduce the efficacy of the dam in terms of its main functions, namely irrigation, flood protection and hydropower. Multi-purpose reservoirs offer enormous scope for minimizing adverse impacts. In the case of modifying low flows, identifying downstream demands to determine minimum compensatory flows, both for the natural and human environment, is the key requirement and such demands need to be allowed for at the design stage. The ability to mimic natural flooding may require modifications to traditional dam offtake facilities. In particular, passing flood flows early in the season to enable timely recession agriculture may have the added advantage of passing flows carrying high sediment loads.

A number of disease hazards are associated with dams some of which can be minimized, others eliminated by careful operation. They include malaria, schistosomiasis and river blindness; this is discussed more fully in the section Human health.

Rooted aquatic weeds along the shore (or in shallow reservoirs) can be partially controlled by alternate desiccation and drowning. In some parts of the world local communities are willing to de-weed reservoirs and use the weeds as animal fodder.

Fall of water table

A possible advantage of reducing the water table level prior to the rainy season is that it may increase the potential for groundwater recharge. Lowering the water table by the provision of drainage to irrigation schemes with high water tables brings benefits to agriculture.

Lowering the groundwater table by only a few metres adversely affects existing users of groundwater whether it is required for drinking water for humans and animals or to sustain plant life (particularly wetlands), especially at dry times of the year. Springs are fed by groundwater and will finally dry up if the level falls. Similarly low flows in rivers will be reduced. Any changing availability of groundwater for drinking water supply needs to be assessed in terms of the economics of viable alternatives. Poor people may be disproportionately disadvantaged. They may also be forced to use sources of water that carry health risks, particularly guinea worm infection and schistosomiasis. In parts of Asia there are indications that lowering the ground level may favour the sandfly which may be vectors for diseases such as visceral leishmaniasis.

Saline intrusion along the coast is a problem associated with a falling groundwater level with severe environmental and economic consequences.

A continued reduction in the water table level (groundwater mining), apart from deleting an important resource, may lead to significant land subsidence with consequent damage to structures and difficulties in operating hydraulic structures for flood defence, drainage and irrigation. Todd (1980) gives an example of a drop in ground level of over 3 m associated with a 60 m drop in groundwater level over a period of 50 years in the Central Valley, California. Vulnerable areas are those with compressible strata, such as clays and some fine-grained sediments. Any structural change in the soil is often irreversible. The ground level can fall with a lowering of the water table if the soils are organic. Peats shrink and compact significantly on draining, with consequent lowering of the ground level by several metres.

Particular care is needed in the drainage of tropical coastal swamp regions as the FeSO4 soils can become severely acidic resulting in the formation of "cat-clays".

A number of negative consequences of a falling water table are irreversible and difficult to compensate for, eg salt water intrusion and land subsidence, and therefore groundwater abstraction needs controlling either by licensing, other legal interventions or economic disincentives. Over-exploitation of groundwater, or groundwater mining, will have severe consequences, both environmental and economic, and should be given particular importance in any EIA.

Rise of water table

In the long-term, one of the most frequent problems of irrigation schemes is the rise in the local water-table (waterlogging). Low irrigation efficiencies (as low as 20 to 30% in some areas) are one of the main causes of rise of water table. Poor water distribution systems, poor main system management and archaic in-field irrigation practices are the main reason. The ICID recommendation to increase field application efficiency to even 50% could significantly reduce the rise in the groundwater. The groundwater level rise can be spectacularly fast in flat areas where the water table has a low hydraulic gradient. The critical water table depth is between 1.5 and 2 m depending on soil characteristics, the potential evapotranspiration rate and the root depth of the vegetation/crops. Groundwater rising under capillary action will evaporate, leaving salts in the soil. The problem is of particular concern in arid and semi-arid areas with major salinity problems. A high water table also makes the soil difficult to work.

FIGURE 5 Causes and impacts of reduced water quality in a river system

Good irrigation management, closely matching irrigation demands and supply, can reduce seepage and increase irrigation efficiency, thereby reducing the groundwater recharge. The provision of drainage will alleviate the problem locally but may create problems if the disposal water is of a poor quality. Apart from measures to improve water management, two options to reduce seepage are to line canals in highly permeable areas and to design the irrigation infrastructure to reduce wastage. Waterlogging also implies increased health risks in many parts of the world.

Water and air quality


Solute dispersion
Toxic substances
Agrochemical pollution
Anaerobic effects
Gas emissions


In general the purer the water, the more valuable and useful it is for riverine ecology and for abstractions to meet human demands such as irrigation, drinking and industry. Conversely, the more polluted the water, the more expensive it is to treat to satisfactory levels. The causes and impacts of reduced water quality are illustrated in Figure 5. Tables 6, 7 and 8 are generalized water quality standards for irrigation, drinking and fresh-water fisheries. As soil salinity levels rise above plant tolerance levels, both crops and natural vegetation are affected. This leads to disruption of natural food chains and the loss of agricultural production. The critical problem of salinity is covered in the section Soil properties and salinity effects.

Solute dispersion

The changing hydrological regime associated with irrigation schemes may alter the capacity of the environment to assimilate water soluble pollution. In particular, reductions in low flows result in increased pollutant concentrations already discharged into the water course either from point sources, such as industry, irrigation drains and urban areas, or from non-point sources, such as agrochemicals leaking into groundwater and soil erosion. Reduced flood flows may remove beneficial flushing, and reservoirs may cause further concentration of pollutants. Where low flows increase, for example as a result of hydropower releases, the effect on solute dispersion is likely to be beneficial, particularly if the solutes are not highly soluble and tend to move with sediments.


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