The consequences of irrigated agriculture are often that the unavoidable deep percolation losses of irrigation cause groundwater tables to rise. Natural or internal drainage can no longer cope with the concentrated human activities that constitute irrigated agriculture. This causes waterlogging of the rootzone, which inevitably leads to yield reduction.
Waterlogging is a widespread problem in non-irrigated areas, where low-lying depressions serve as discharge areas, and on irrigated lands. In arid and semi-arid climates, in addition to waterlogging the major problem associated with irrigation, in the absence of drainage, is salinization. Salinization is another factor that causes land degradation and consequently yield reduction. One measure to combat these effects is to install a subsurface drainage system that keeps the water table down and allows for leaching of salts from the rootzone.
Conventional solutions to combat waterlogging and salinity are vertical and horizontal subsurface drainage systems consisting of respectively pumped tubewells or horizontal buried pipes and deep open drains. Subsurface drainage and drain water evacuation systems accomplish the following:
1. Dewatering i.e. inducing water flow through the soil towards the subsurface drain (tubewell, pipe or open drain).
2. Transportation of the drain water through the lateral, or field drain, to collector drains and thereafter to main drains.
3. Often pump lifting the drain water to higher elevations in the evacuation system and/or further conveyance by gravity outflow to disposal sites.
4. Final disposal at selected sites (e.g. by evaporation ponds).
5. Salinity control: dewatering of the rootzone and leaching of salts happens at the same time.
Biological systems make use of the evapotranspirative power of plants, especially of trees, to lower groundwater tables. Functions 1, 2, 3 and 4 are performed together. Salinity control, function No. 5, is more difficult to achieve and this requires additional means in the long-term. However, biodrainage systems may delay the salinization process.
Conventional drainage systems have performed adequately, but lack of financing often impedes their installation. The disposal of the poor-quality effluent generated by conventional drainage systems may cause problems. Where drainage effluent is reused for irrigation, salts are redistributed in the landscape. Where effluent is disposed into river systems, pollution of natural water results.
Low cost technology such as biodrainage could be an alternative providing several advantages as the negative side effects of conventional drainage systems are reduced and, as they require less investment, may find quicker application. Biological systems provide for such an alternative, although the availability of land is a decisive factor in the eventual establishment of biodrainage systems. However, in most cases, in developing countries water scarcity is the predominant feature and not land scarcity.
Apart from being a financially attractive alternative, there are many other advantages for rural livelihoods using biodrainage systems. They are environmentally friendly, provide fuelwood, timber, fruits, shade and shelter, function as windbreaks and yield organic matter for fertilizer. In addition, they contribute to the enhancement of biodiversity, as flora and fauna flourish, air pollution is diminished and they contribute to carbon sequestration.
Applicability of biodrainage systems is not restricted to the simple substitution of a pipe drain or tubewell. Canal leakage always occurs in irrigation projects. Tree plantations have effectively drained the ponds formed alongside canal embankments, as compared with large areas that have been inundated and became saline. A further application of tree plantations is that they can effectively drain natural inundated depressions, or areas where effluent is produced by conventional drainage techniques where water is disposed of in evaporation ponds.
The use of biodrainage systems in recharge areas seems to be widely accepted as a sustainable management option. In discharge areas with shallow groundwater tables, deep-rooted plant-based biodrainage systems are often associated with salt accumulation. Under these situations, combining conventional drainage with biodrainage would be the most appropriate design option.
This publication describes the processes underlying biodrainage, the principles of planning and design and a range of other associated issues such as biodiversity and socio-economics. Described are drylands, non-irrigated situations, where biodrainage can be used to (partially) restore the hydrological balance caused by inappropriate land development. The use of biodrainage in irrigated areas is explained where the system is used to protect irrigated land against the hazards of waterlogging and salinization. Case studies covering a range of scenarios in different countries are also presented.
Much research has been completed, more is required. Not all questions have been answered concerning the precise design of biodrainage systems, even in those areas where biodrainage systems have proven adequacy in integrated water management of irrigation and drainage schemes. Examples from several countries have been documented in this paper, where vegetation, especially trees and salt-tolerant plants, has been used to achieve environmentally safe and effective drainage and disposal systems.
Conventional drainage design includes many safety factors to compensate for the possible malfunctioning of the drainage system or compensation for spatial variability in soils, selection of the next largest available diameter pipe sizes, partly used cost-saving area reduction factors and over-design related to rainfall frequencies. It is fair that biodrainage systems should be allowed such safety mechanisms in the design and their implementation be initiated.
Drainage engineers should no longer ignore the opportunities that biodrainage systems can offer. When planning for projects the agricultural sector increasingly feels pressure from other users of the environment. For example it is becoming increasingly unacceptable to set aside land exclusively for routinely designed irrigation and drainage projects. This illustrates the possible advantages of biodrainage systems.
