Shallow groundwater tables and associated salinity problems have become dominant features in agricultural areas around the world. These problems have been caused by increasing pressures on land resources caused by rising populations, especially in irrigation areas. Increasingly, large areas of land having historically deep groundwater tables are now in need of some form of water table control. In many places management strategies have been developed to address this problem. These often focus on engineering approaches such as deep open ditches, vertical drainage (groundwater pumping) or horizontal subsurface drainage.
Conventional physical drainage works require expensive capital investment, operation and maintenance. Physical drainage measures also generate drainage effluent. Disposal to rivers of the often saline, and sometimes chemically contamin-ated effluent is increasingly considered unacceptable because downstream users in the catchment rely on these river systems for their water supplies. Greenhouse emissions caused by energy-hungry pumps may also be disapproved of in a world that is becoming more aware of issues related to global warming. Any positive alternative, preferably cheaper, addition to our arsenal of drainage techniques would be extremely welcome in our fight to keep groundwater tables in our agricultural areas under control. Biodrainage, i.e. the use of vegetation to manage water fluxes in the landscape, is one such technique that has recently attracted interest in drainage and environmental management circles.
Biodrainage relies on vegetation, rather than engineering mechanisms to remove excess soil water through evapotranspiration. It is often considered attractive because it requires only an initial investment in site development (planting of a “biodrainage crop”) and (potentially) returns a benefit when the biocrop is harvested for fodder, wood or fibre. In addition, under some management scenarios, viz. certain cropping systems and slightly saline conditions, it might offer limited scope to achieve nutrient and/or salt-balance through removal of biomass, thus alleviating the problem of the disposal of polluted drainage effluent from the biodrainage crop area by reducing volumes and improving the quality of the effluent.
The concept of leaking landscapes, caused by clearing and adoption of inappropriate land use practices, has recently become topical, especially in Australia. This has resulted in increased efforts directed towards the development of agricultural systems (both rainfed and irrigated) with improved water use efficiency that minimize groundwater recharge. In addition, attention is being paid to direct management of the land affected by shallow, saline water tables. Drainage is generally neither econo-mically nor practically feasible in these areas, especially in rainfed agricultural systems. Enhancing groundwater discharge has become popular to offset increased recharge in adjoining parts of the landscape, through planting salt-tolerant trees or fodder crops. However, there are concerns about salt accumulation that might restrict the long-term viability of enhanced discharge.
The current status of biodrainage research and applications are described in this document. Much of the research information is based on Australian work presented in a special review issue of the International Journal for Agricultural Water Management. In addition work done in Asia, notably in Rajasthan, India, has also been drawn upon. Literature from other countries, including workshop proceedings and technical reports, is examined, and irrigation-based biodrainage case studies are presented.
