Chandra A. Madramootoo
McGill University, Quebec, Canada
Need for artificial drainage
Agricultural, environmental and socio-economic benefits of drainage
Types of drainage systems
Environmental impact assessment
Water quality issues
Drainage water management and disposal options
Planning and designing drainage systems to protect and enhance water quality
There are several concerns about the sustainability of irrigation and drainage projects, and there are water quality problems related to the disposal of drainage water. There are also problems with land degradation due to irrigation induced salinity and waterlogging. There have been instances where saline or high nutrient drainage water has damaged aquatic ecosystems. Drainage continues to be a vital and necessary component of agricultural production systems. In order to enhance the net benefits of drainage systems, more attention will need to be given to the water quality impacts of drainage water disposal. This document identifies potential problems and management options in the development, production, treatment and disposal of agricultural drainage water.
Excess water in the crop root zone soil is injurious to plant growth. Crop yields are drastically reduced on poorly drained soils, and, in cases of prolonged waterlogging, plants eventually die due to a lack of oxygen in the root zone. Sources of excess soil water that result in high water tables include: high precipitation in humid regions; surplus irrigation water and canal seepage in the irrigated lands; and artesian pressure. Waterlogging in irrigated regions may result in excess soil salinity, i.e., the accumulation of salts in the plant root zone. Artificial drainage is essential on poorly drained agricultural fields to provide optimum air and salt environments in the root zone. Drainage is regarded as an important water management practice, and as a component of efficient crop production systems. World food supply and the productivity of existing agricultural lands can only be maintained and enhanced if drainage improvements are undertaken on cropland currently affected by excess water and high water tables.
Drainage (both surface and subsurface) is not simply the conversion of wetlands, but the improvement of naturally inadequately drained cropland. It is complementary to irrigation and is viewed as an essential component of irrigated agriculture. The objective is to increase production efficiency, crop yields and profitability on naturally poorly drained agricultural lands.
The primary benefits of drainage go beyond the control of excess soil water and accumulation of excess salts in the crop root zone (Fausey et al., 1987). The coincident environmental and socio- economic benefits associated with disease vector control and public health must be fully recognized. One of the major environmental benefits of drainage is its positive impact on improving the health of humans, plants and farm animals. Drainage of wet, swampy areas has led to a reduction in mosquito breeding sites in all parts of the world. The effect has been a drop in the incidence and prevalence of important water related and mosquito transmitted diseases, e.g., malaria, yellow fever and filariasis. Furthermore, drainage of stagnant water can eliminate foot-rot in large animals and, to a certain extent, the breeding environment of aquatic and semi-aquatic snails, which are the intermediate host of schistosomiasis. Drainage also reduces or eliminates mildew infections and various root rots of plants. The overall impact of improved drainage has been an improvement in hygienic conditions, in the health sector and in the productivity of human beings. By growing high value food crops in well-drained soils, the health, nutrition and economic status of rural populations can be improved. There are also increased opportunities for employment, as new industries may develop in prosperous areas.
Where drainage is used to reclaim salinized and waterlogged lands, it is an environmentally beneficial practice, because the land is returned to its full productive potential. The adaptation of subsurface drainage systems to serve as sub-irrigation or controlled drainage systems leads to other benefits, i.e., the reduction of nitrate pollution.
The field-scale benefits of drainage can be summarized as follows:
i. Drainage promotes beneficial soil bacteria activity and improves soil tilth.ii. There is less surface runoff and soil erosion on drained land.
iii. Improved field machine trafficability reduces soil structural damage. Soil compaction is reduced and less energy is required for field machine operations. Drainage also allows for more timely field operations. Consequently, the growing season can be lengthened and crops can achieve full maturity.
iv. Crop yields are increased because of improved water management and uptake of plant nutrients.
v. Higher value crops can be planted, and there is flexibility to introduce new and improved cropping systems.
vi. In general, land value and productivity are increased.
vii. Farm income is increased and income variability reduced.
viii. Drainage maintains favourable salt and air environments in the crop root zone.
Surface drainage
Subsurface drainage
Secondary drainage treatments
Agricultural land drainage usually consists of surface or subsurface systems, or a combination of both. At the field scale, subsurface drain pipes and field ditches normally exit to an open main or collector drain. At the regional level, the latter then empties into a river or its tributaries. In some instances, depending on the character of the hydrological basin, main drains may dispose of drainage water to an evaporation pond, to a wetland, or to a saline agriculture/ agriculture-forestry system. A schematic of drainage system components and several options for drainage water disposal within a watershed are shown in Figure 1. Not all of these management options will necessarily be used in a single watershed.
FIGURE 1 Drainage water disposal options within a watershed
Surface drainage is often achieved by land forming and smoothing to remove isolated depressions, or by constructing parallel ditches. Ditches and furrow bottoms are gently graded and discharge into main drains at the field boundary. Although the ditches or furrows are intended primarily to convey excess surface runoff, there is some seepage through the soil to the ditches, depending on the water table position. This could be regarded as a form of shallow subsurface drainage. Surface drainage is especially important in humid regions on flat lands with limited hydraulic gradients to nearby rivers or other disposal points. There is also a need for good surface drainage in semi-arid regions which are affected by monsoons.
Surface drainage alone is seldom sufficient to remove excess water from the crop root zone. Deep ditches or subsurface pipe drainage systems enable a more rapid water table drawdown. The downstream ends of the laterals are normally connected to a collector drain. The required diameter of the pipe collectors increases with the area drained. Drain spacing is usually dependent on soil hydraulic conductivity and a design drainage rate coefficient. Depending on topography, land formation and proximity of a water receiving body, the collector may outlet by gravity to an open main drain or into a sump. In the latter case, the discharge is then pumped to another drain, or ultimately to a lake or stream.
In some flatter parts of eastern Canada, the eastern and mid-western United States, and parts of Europe, subsurface pipe drains are also used for sub-irrigation. In this case, in dry periods, surface water is introduced into the drain pipe system from an external source, and the water table is raised. Moisture then moves upward by capillary action to the root zone. Sub-irrigation is regarded as a highly energy and water efficient method of irrigation. In another process, known as controlled drainage, elevated water tables can be maintained with a control structure on the collector pipe.
Horizontal subsurface drainage systems are used in irrigated arid and semi-arid regions to reclaim saline and waterlogged lands, and to maintain favourable long-term salt and water balances in the crop root zone. Salinity and waterlogging are caused by a build up of the water table due to deep percolation of normal excess water and canal seepage. Buried pipe drains are generally installed deeper in arid regions than in humid regions in order to control salinity. Water in excess of plant evapotranspiration (ET) needs is always unavoidably applied during irrigation. This additional quantity of water applied is known as the leaching fraction. Naturally occurring as well as applied salts are then leached from the root zone by this water, and removed from the field via the pipe drains. Deeper drain installation ensures that salts do not rise too rapidly to the soil surface due to capillary action. Drainage also prevents waterlogging of the root zone. The amount of irrigation water to be removed is generally less in arid than in humid regions. Vertical drainage by means of tube-wells is also used to control waterlogging and salinity in some parts of the world, e.g., India, Pakistan and central Asian republics. The primary purposes of tube-wells are the same as those of horizontal drains, and at the same time to extract groundwater for irrigation. As a result of pumping, the water table is lowered, and salinization due to capillarity is minimized. This situation is ideal where the groundwater is not very brackish or saline, and is therefore suitable for irrigation. In areas where the groundwater is highly saline, the pumped water may be too saline for irrigation, unless mixed with fresher or less saline water. Where the groundwater is too saline for crop production, it must be disposed of. Drainage does not have a direct impact on groundwater quality. It only serves to collect and transport excess water.
Methods of improving the internal drainage of low permeability soils include: subsoiling, deep tillage, mole drainage, and biological practices, viz., cropping with deep rooted legumes (e.g., alfalfa) and crop rotations. In some parts of the world, deep rooted trees are used to lower the water table. There are usually no water quality hazards associated with these supplemental drainage practices.
Many institutions require an environmental impact assessment (EIA) prior to construction activities associated with new projects or the rehabilitation of existing projects. The objectives are to identify potential adverse environmental effects, the magnitude of these effects, and to develop mitigative measures. Positive benefits are also identified. In cases where the adverse impacts far outweigh the expected benefits, the project may be completely redesigned or suspended. This is also the case where mitigative measures would be either too costly or technically not feasible. The EIA should be conducted in the earliest stages of decision making, when crucial decisions are still being deliberated.
According to Ochs and Bishay (1992), environmental effects can be classified as:
i. direct and indirect, or first order and higher order. These are chain effects which are felt throughout, and possibly downstream of a catchment.ii. secondary. The primary activity of a drainage project may be extended to include secondary activities.
iii. synergistic. These effects include an increased threat to the survival of certain species of wildlife that are under pressure in several ways as a result of the same project.
The ICID has developed an 'Environmental Checklist to Identify Environmental Impacts of Irrigation, Drainage and Flood Control Projects' (ICID, 1993). The World Bank has prepared an Environmental Assessment Sourcebook (World Bank, 1991). FAO has produced a paper on the steps in the EIA process and the major environmental impacts of irrigation and drainage projects (FAO, 1995). The purpose of these documents is to enable detailed environmental impact assessments of irrigation and drainage projects to be conducted. The ICID checklist provides a comprehensive list of environmental parameters which must be evaluated in an EIA. Table 1 shows the ICID checklist of possible environmental impacts of irrigation, drainage and flood control projects.
Ochs and Bishay (1992) listed the main steps in the EIA process as:
i. Scoping. This is a public process, involving the participation of all parties. It results in specific guidelines for inclusion in the environmental impact study (EIS). (EIS refers to the document, whereas EIA refers to the entire process).ii. Drawing up the EIS. Potential adverse and beneficial impacts are identified. Proposed actions with alternatives are indicated and mitigative measures are presented.
iii. Submitting the EIS for public review.
iv. Receiving advice from the reviewing agency.
v. Accepting the EIS.
vi. Choosing project components. The decision-maker chooses the project option to be implemented and the mitigative measures.
vii. Implementing the project.
viii. Monitoring. During and after project implementation, the actual environmental impacts are monitored and compared with the EIS predictions. This is useful for improving future predictions and project designs.
Matrices and checklists are some of the tools most commonly used in an EIA to assess the magnitude or relative weight of both positive and adverse environmental effects. It is hoped that the criteria on drainage water quality provided in the publication will further assist in the evaluation of EIA of irrigation and drainage projects.
The installation of drainage systems may result in changes to the associated ecosystem. These changes may be either beneficial or adverse. The positive environmental benefits were listed earlier in this chapter. However, there are potential adverse water quality impacts associated with drainage. The concentrations of salts, nutrients and other crop-related chemicals in drainage discharge vary with time and discharge rate. The use of fertilizers and pesticides in intensive agricultural production has sometimes led to damage to downstream aquatic ecosystems (FAO, 1996). Drainage planners therefore need to analyse effluent for nutrients and pesticides. The nutrients of most concern are N and P. In addition, from time to time, natural trace elements from the soil itself may be harmful to the ecosystem. Effluent laden with N and P stimulates eutrophication in receiving water bodies. In addition to agricultural chemicals and trace elements, drainage water from irrigated areas frequently contains salts. The impact of salts on downstream users needs to be evaluated. Some soils are abundant in trace elements, and these could leach to the drainage system. Small amounts of trace elements such as As, Cd, Hg, Pb, B, Cr and Se are harmful to aquatic species because of biological magnification. The environmental consequences of disposing of drainage water from California's irrigated San Joaquin Valley into the 470 ha Kesterson Reservoir (a closed basin) are well known. The reservoir was a waterfowl habitat, and concentrating Se in the drainage water caused fish species to disappear, and resulted in deformities in waterbird embryos. The failure to construct an adequate outfall drain to the sea, the use of a waterfowl habitat for drainage water disposal, and the lack of proper water quality monitoring were the major reasons for these negative environmental impacts. Care must be taken to ensure that the disposal of drainage water does not interfere with the ecosystem's aquatic and terrestrial species. Concern has been expressed about damage by agricultural drainage water to estuarine fisheries in some countries.
TABLE 1 ICID checklist of possible environmental impacts of irrigation, drainage and flood control projects
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Site: |
Date: |
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Hydrology |
1-1 Low flow regime |
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1-2 Flood regime |
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1-3 Operation of dams |
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1-4 Fall of water table |
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1-5 Rise of water table |
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Pollution |
2-1 Solute dispersion |
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2-2 Toxic substances |
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2-3 Organic pollution |
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2-4 Anaerobic effects |
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2-5 Gas emissions |
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Soils |
3-1 Soil salinity |
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3-2 Soil properties |
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3-3 Saline groundwater |
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3-4 Saline drainage |
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3-5 Saline intrusion |
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Sediments |
4-1 Local erosion |
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4-2 Hinterland effect |
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4-3 River morphology |
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4-4 Channel structures |
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4-5 Sedimentation |
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4-6 Estuary erosion |
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Ecology |
5-1 Project lands |
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5-2 Water bodies |
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5-3 Surrounding area |
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5-4 Valleys and shores |
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5-5 Wetlands and plains |
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5-6 Rare species |
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5-7 Animal migration |
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5-8 Natural industry |
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Socio-economic |
6-1 Population change |
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6-2 Income and amenity |
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6-3 Human migration |
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6-4 Resettlement |
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6-5 Women's role |
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6-6 Minority groups |
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6-7 Sites of value |
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6-8 Regional effects |
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6-9 User involvement |
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6-10 Recreation |
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Health |
7-1 Water and sanitation |
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7-2 Habitation |
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7-3 Health services |
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7-4 Nutrition |
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7-5 Relocation effect |
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7-6 Disease ecology |
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7-7 Disease hosts |
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7-8 Disease control |
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7-9 Other hazards |
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Imbalances |
8-1 Pests and weeds |
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8-2 Animal diseases |
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8-3 Aquatic weeds |
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8-4 Structural damage |
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8-5 Animal imbalances |
On-farm source control
Re-use of drainage water
Disposal and management of drainage water in closed basins
Water table management
Drainage water can be disposed directly to open surface water bodies, e.g., rivers, lakes, outfall drains, seas or oceans. Wetlands, evaporation ponds and solar evaporators are used as discharge points where there is no direct outlet to one of the open surface water bodies mentioned above. This is generally the situation in hydrologically closed drainage basins. Some re-use of drainage water for irrigation is another management option, where sequential irrigation of increasingly salt-tolerant plants, ending with Salicornia, occurs. Drainage water can also be disposed of by injection to the deep groundwater.
If drainage water is disposed of to large, open surface water systems with significant dilution or assimilative capacity, then water quality problems are minimized. However, water quality problems may develop with repeated re-use, disposal in closed basins and injection to deep wells. In this last case, the concern is that if the drainage water contains sufficient amounts of salts, nitrates, bacteria and trace elements, the groundwater could become contaminated. The problem may be especially severe if the aquifer is also used for drinking water supplies downslope.
The most efficient method of minimizing environmental problems is to implement source control practices at the farm level. In irrigated areas, this can be achieved by improved irrigation water management. Higher irrigation efficiencies and lined irrigation conveyance structures will reduce the amount of drainage water which needs to be removed. Furthermore, timely and efficient applications of fertilizers and pesticides, used only when necessary, will reduce chemical leaching. Research by Madramootoo et al. (1995) has shown that intercropping corn with a legume or ryegrass reduces the amount of nitrates found in drainage water.
In many regions where irrigation water is scarce, drainage water is used to meet crop water requirements. Re-use is only sustainable if the drainage water is of sufficiently good quality. Some of the water quality concerns about drainage water re-use with plants of increasing salt tolerance are that: the effluent may be high in salt content (in irrigated lands); the drainage water can be contaminated with trace elements, toxic organic substances, industrial waste and municipal waste in open main drains. Contaminated drainage water could lead to various problems including: impairment of soil physical and chemical properties, water related health problems, and possible contamination of food products.
Ultimate disposal of drainage water to a river or sea is not always possible. Closed drainage basins present a unique environmental or water quality challenge. In such situations, evaporation ponds may be an appropriate means for disposing of drainage water. However, these ponds may eventually lead to other environmental problems. For example, toxic substances could accumulate in the ponds. Furthermore, in arid climates, as pure water evaporates from the pond, the concentration of the remaining water approaches that of brine. The health of waterfowl, fish and other aquatic biota which use the pond could be negatively affected. Other environmental problems associated with evaporation ponds include: use of the pond to collect wastewater from homes; human health problems caused by consuming water from the ponds; and the need to ultimately dispose of the accumulated concentrated chemicals in the ponds. In some cases, dry toxic materials may be spread by the wind. Furthermore, these ponds could become habitats for snails and mosquitoes, thereby causing malaria and schistosomiasis epidemics. In addition, if not properly managed, new waterlogged and saline areas will develop adjacent to the ponds.
There is now increasing interest in the utilization of natural and constructed wetlands to manage drainage water. Wetlands are particularly effective for removing sediment, N and P. Plant, soil and hydrologic parameters interact in a complex way to filter and trap pollutants, and to recycle nutrients. Certain tree and plant species have the potential to absorb pollutants. Residence time, flow rate, hydraulic roughness and wetland size and shape are some of the factors which influence treatment efficiency. The water supply to the wetland must be sufficient to provide an excess to discharge and prevent salt accumulation.
In areas where soils, geologic and hydrologic conditions do not permit constructed wetlands, the saline agriculture/agriculture-forestry system may be appropriate for the disposal of drainage water. The operating principle is to successively re-use saline drainage water to irrigate crops and trees of increasing salt tolerance, and to discharge the final much reduced volume of water into a solar evaporator for salt crystallization. The depth of water ponded in the evaporator is regulated to match the daily evaporation rate. The goal is to make the crystallization pond unattractive to flora and fauna. Another challenge of evaporation ponds is that evaporation is reduced as the ponds become more concentrated. This necessitates the use of additional land to maintain disposal (evaporation) capacity.
While there are instances where ecosystems have been damaged by poor quality drainage water, it is also possible to use subsurface drainage systems beneficially to improve water quality. By managing the water table, through controlled drainage or sub-irrigation, nitrate concentrations in drainage effluent can be significantly reduced. Downstream flood flows can also be reduced. Water table management could therefore be viewed as a best management practice. Skaggs and Gilliam (1981) and Madramootoo et al. (1993) have shown that sub-irrigation and controlled drainage not only increase crop yields, but also enhance denitrification. Water table management is best suited to flat lands, and can be achieved by a simple modification to the outlet of subsurface drainage systems.
It is important to recognize that while technologies may be available to minimize water quality impacts from drainage effluent, institutional mechanisms must also be put in place to ensure that the technologies can be implemented. Policies and programmes, including legal and monitoring aspects, require an institutional framework. These are discussed in detail in Chapter 8. Chapter 2 covers the physical, chemical and biological constituents of drainage water which are of environmental concern. Drainage water management options are also included in Chapter 2. Drainage water re-use for various purposes is discussed in Chapter 4. Systems for treating drainage water are presented in Chapter 5, and possibilities for the disposal of drainage water are discussed in Chapter 6. Water table management systems are covered in detail in Chapter 3. Health issues related to drainage water management are discussed in Chapter 7.
It is recognized that, in some instances, there may be insufficient data in the project design and planning stages to enable precise conclusions to be drawn about possible environmental impacts. For this reason, most chapters contain sections on environmental monitoring, so that project planners and designers can elicit the information required for proper decision making. For more detailed information on water quality monitoring, the reader is referred to the United States Environmental Protection Agency (USEPA, 1982).
