All agriculture crops need water to grow. Natural precipitation does not always meet the full plant water requirements and, wherever possible, irrigation is introduced to overcome this problem. FAO (1989) reported that 15.4 percent of the 1 474 Mha of cultivated land in 1987 was irri-gated. This relatively small area produced one-third of the world’s food supplies. Average agriculture produce from a unit of irrigated area is more than two times that from average rainfed land. The World Food Summit (1996) estimated that 60 percent of the extra food required to sustain the world in the future must come from irrigated agriculture.
Irrigation of agricultural land has a long and well-documented history. Irrigation will, without doubt, play an important role in keeping the future world population supplied with their food, fibre, bio-energy and bio-industrial feedstock needs. Irrigation however has negative impacts on what is called our ‘natural resource base’, which often include shallow water tables. Waterlogging and increases in soil and groundwater salinity are associated with a lack of drainage.
Changes in land use, and especially irrigation development (which is one of the most drastic land use changes conceivable), nearly always upset the natural hydrological balance. In dryland agriculture, the introduced plants and crops rarely have the same rooting depth and annual evaporative potential as the natural vegetation they replace. In the case of irrigation, the component of applied water that is not used by the plants further adds to the water entering the water table. The hydrological changes caused by land use modification lead to changes in the salt-balance. Under rainfed conditions this often results in a lateral redistribution of salts in the landscape; examples of this will be discussed in section 3.4, Salt balance. Under irrigated conditions, the extra salts imported via irrigation water have to be removed from the rootzone to avoid long-term accumulation; this process is often referred to as ‘leaching’.
In the past, drainage has often been neglected. It is now widely accepted that it is essential to any irrigation system design. The history of the Assyrian civilization in Mesopotamia presents an example of the earliest reported case where a whole population was forced to abandon a region because of rising groundwater tables and salinity (Jacobsen and Adams, 1958). Other examples are quoted in Ghassemi et al. (1995), pp. 2-3 and Ritzema (1994), pp. 24-26.
Presently about one-third of the world’s irrigated area faces the threat of waterlogging. It is estimated that 60 Mha is already waterlogged and 20 Mha salt affected. About 30 Mha of land has been provided with subsurface drainage systems. For example in western Europe, agricultural intensification has led to the reclamation of more than 50 percent of waterlogged areas through the use of subsurface drainage measures. The proportion of drained land is largest in Europe and North America (20-35 percent of total cultivated land), moderate in Asia, Australia and South America (5-10 percent) and lowest in Africa (0-3 percent).
Recently, some of the detrimental impacts of drainage on the environment have been recognized. In some circles “drainage” has become a “dirty word” and its implementation has been restricted, or even prohibited, especially in environmentally sensitive wetland regions. The very high annual rate of installation of subsurface drainage of the 1980s (300 000 ha/year) has fallen to about 150 000 ha/year during the 1990s (Lesaffre and Zimmer, 1995).
The range of drainage techniques presently employed to manage the hydrological balance in agricultural areas has been described in this chapter, which includes some alternative approaches to engineering-based drainage designs.
The conventional engineering-based techniques most commonly used to drain excess water from land are: surface drainage, horizontal subsurface drainage and vertical subsurface drainage.
Surface drainage is described by the American Society of Agricultural Engineers as ‘the removal of excess water from the soil surface in time to prevent damage to crops and to keep water from ponding on the surface’ (ASAE 1979). The term surface drainage applies to situations where overland flow is the major component of the excess-water movement to major drains or natural streams. The technique normally involves the excavation of open trenches/drains. It could also include the construction of broad-based ridges or beds, as grassed waterways, with the water being discharged through the depressions between ridges. Surface drainage is most commonly applied on heavier soils where infiltration is slow and excess rainfall cannot percolate freely through the soil profile to the water table. The technique has also been applied in more permeable soils to de-water areas having a shallow groundwater table; under those conditions it should be considered as part of the category below. It is the most important drainage technique in the humid and subhumid zones.
Horizontal subsurface drainage involves the removal of water from below the surface. The field drains can either be open ditches, or more commonly a network of pipes installed horizontally below the ground surface. These pipes used to be manu-factured of clay tiles, with the water entering the pipes through the leaky joints (thus the term tile drains). In 1968 flexible corrugated plastic drainage pipe was introduced and this product is now widely used around the world. In spite of the different material used, the term tile drains is still in common use.
Mole drains are unlined circular channels installed at depth in the soil profile; they function similarly to tile drains. The technique can be applied in heavy soils as an alternative to surface drainage. In these soils the very close drain spacing needed to achieve water table control would make tile drainage excessively expensive. Mole drains are most commonly used for the control of perched water tables. The technique is described by Nicholson (1942) for the United Kingdom and by Hudson et al. (1962) and Bowler (1980) for conditions in New Zealand. Ritzema (1994; pp. 913-927) presents a good overview of the principles and applications.
Horizontal subsurface drainage has been found to be an effective technique. It controls the rise of groundwater tables and enables productive agriculture. Drawbacks are that it is relatively expensive to install, operate and maintain. Also the disposal of drainage water that can contain high concentrations of pollutants (nutrients and/or toxic elements such as boron) can create problems.
Vertical subsurface drainage involves the removal of groundwater through pumped boreholes or tubewells, either in single or multiple-well configurations. The common problem with this technique is that deeper, often more saline water can be mobilized which can cause disposal problems. Also, as the water is commonly used for irrigation rather than disposal, salt is recycled through the soil profile and inevitably groundwater salinities will increase over time.
Low-yielding, large diameter open wells, or skimming wells, explore lenses of fresh water overlying deeper, more saline groundwater. The system has been applied in the Indo-Dutch Operational Research Project on Hydrological Studies. The final report of the project (Agarwal and Roest, 1996) presents information on the concept and lists a number of research papers.
All the above-mentioned conventional drainage techniques require disposal of drainage effluent, management of which has become an important issue around the world. Where the drainage effluent is of a reasonable quality, it is commonly re-used, if necessary after blending with good-quality surface supplies. However, after extended periods of irrigation (in some cases more than 100 years), soil salinities in areas with arid climates have often approached levels that require salt export to maintain production. Commonly drainage effluent has been disposed of into rivers. This practice is progressively becoming problematic as drained nutrients, salts and residues of agro-chemicals affect water quality, because downstream users (both irrigators and urban/industrial populations) rely on these rivers for water supplies. In addition, environmental considerations associated with river health are now receiving more attention.
Problems associated with effluent disposal are widespread. The salinity of most inland seas is known to increase over time because of the continuing inflow of saline drainage water. In California’s Imperial Valley, drainage water from irrigated lands is discharged into the Salton Sea, whose salinity is on the increase. Discharge of drainage water from irrigated lands in the San Joaquin Valley in California into the Kesterton Reservoir has resulted in problems of selenium toxicity in the biota (Cervinka et al. 1999).
The Aral Sea Basin today faces a crisis similar to the one that destroyed the Mesopotamian civilization 4 000 years ago, as the discharge of polluted and saline drainage effluent into the river systems has reached hazardous levels. Similarly the Indus basin in Pakistan, various river systems in India and the Murray-Darling Basin Catchment in Australia are suffering the consequences of river water pollution as a result of the discharge of polluted drainage effluent from irrigation.
Where irrigation areas are in closed basins, without an outflow to rivers or the sea, disposal offers even greater challenges to sustainability. In this case various techniques such as evaporation ponds, solar evaporators, solar ponds and salt harvesting could provide a solution to the disposal problem.
In many countries disposal into rivers is restricted as this creates major ecological problems. Biodrainage systems combined with the above-mentioned techniques should be envisaged to deal with the effluent from the irrigated and drained areas.
Vertical drainage with reuse of the extracted groundwater for irrigation is effective where the groundwater is of good quality and easily accessible (well-developed aquifers). However, this approach does not remove salts from the region. The long-term sustainability of vertical drainage without drainage disposal for salt-balance is therefore questionable.
Horizontal drainage also has a proven record, as it controls the rise in the groundwater table and enables productive agriculture. However, it is relatively expensive to install, operate and maintain. Another serious drawback is the issue of drainage effluent disposal that can pollute surface water bodies, especially where a direct outlet to the sea is not available. Water quality usually restricts the use for irrigation. Even the disposal to evaporation ponds can create environmental problems.
The limitations and shortcomings of the conventional drainage techniques call for alternative approaches to help keep agriculture sustainable over the long term. Alternative techniques must be effective, affordable, socially acceptable and environmentally friendly and not cause degradation of natural land and water resources. Biodrainage is one of these alternative options. The absence of effluent makes the system attractive. However, for biodrainage systems to be long-term sustainable, careful consideration is required of the salt-balance under the biodrainage crops. This issue will be discussed in detail in section 3.4.
The term biodrainage is relatively new, although the use of vegetation to dry out soil profiles has been known for a long time. The first documented use of the term biodrainage can be attributed to Gafni (1994). Prior to that date Heuperman (1992) used the term bio pumping to describe the use of trees for water table control. Another term relating to the “bio” aspect of soil water removal is biodisposal, which refers to the use of plants for final disposal of excess drainage water (Denecke, 2000, IPTRID/FAO; personal communication). In this publication all these biotechnologies are considered under the common heading of Biodrainage.
In response to the increased interest in bio-drainage, a special session on the topic was organized at the Eighth Drainage Workshop of the International Commission on Irrigation and Drainage (ICID) in January/February 2000 in New Delhi, India. The six papers presented by Australia, India and Pakistan are in chapter 4 of this publication.
The need for drainage is not restricted to irrigation areas. In rainfed areas without irrigation, water (and salt) balances, disturbed by land use changes, often need to be managed to minimize negative environmental impacts. As the land use in these areas is often less intensive than in those using irrigation, economic considerations prevent the adoption of expensive engineering inputs. This fact makes the biodrainage approach especially attractive for the management of drainage problems.
