To further develop the technology of biodrainage, the following issues will need to be considered:
There is wide variation in reported tree water use values. Transpiration rates depend on climatic conditions, type and age of tree species, size of plantation, density of tree plantations (spacing), soil moisture regime, etc. This makes it difficult to select accurate design criteria for biodrainage tree plantings. Reporting of tree water use values as multiple of Class A pan would take care of climatic variations, provided the pan conditions, including site maintenance, were maintained at appropriate design standards.
Density of tree plantings for biodrainage should aim at maximum evapotranspiration per unit area. As maximum water use is achieved at canopy closure, high-density plantings should be aimed for at establishment, followed by thinning to minimize competition for water and light. Site-specific research is needed to develop management systems that optimize transpiration rates, biomass production and economic cost/returns.
On the basis of available data in literature and without reliable local experimental data, it should be reasonable to use the value of Class A pan for tree water use under low-salinity conditions (ECe<4 dS/m) where there is no constraint to water supply at the tree roots.
There is a school of thought that states that the potential transpiration rate of plantations per unit area is independent of tree species. If found valid, this would provide greater flexibility in the choice of most suitable and acceptable species of trees for biodrainage. This is a subject that requires further research.
Further research is required on the mineral absorption by trees and salt-tolerant crops and the soil salt-balance.
The measurement of mineral content in plant tissues needs to be standardized to facilitate comparison of experimental results.
The growth of trees and salt-tolerant crops in relation to increasing salt levels in the case of biodrainage systems, affecting the evapotrans-pirative capacity, requires detailed assessment and should be explicitly related to tree species' variety, respectively salt tolerance. See also: Tree species research.
Below a biodrain, viz. trees, salt may accumulate and redistribute in the soil profile and also in the groundwater. This process is not well known and needs more study.
A critical review is needed of the results of all relevant reported experiments, identifying errors in experimental procedures, measurements, analysis and computations. One of the reasons for the wide variations in the reported parameters could be the result of these experimental errors.
An important reason for the divergent results in literature, especially in the tree water use experiments, is the wide range of experimental methods that have been used. Sometimes the measurement technique has a direct impact on the conditions of tree growth, e.g. the use of ventilated chambers can result in extremely high tree water use values and lysimeter studies often suffer from the fact that only relatively small trees can be measured and/or that trees are not surrounded by a large enough area of similar-sized trees to make their growing conditions representative of field conditions. Methods such as the heat pulse, ventilated chambers, energy balance, deuterium (a naturally occurring isotope of hydrogen) tracing, porometry, the 'left-behind salt' method, etc. measure tree water use indirectly and can result in large experimental errors if applied inappropriately. The development of a manual describing the applications and shortcomings of the different techniques would overcome much of the problems associated with these issues.
Because of the costs associated with large-scale field experiments, replication in experimental design is often insufficient and interpretation of experimental data is rarely based on sound statistical information.
To facilitate comparison of results standard experimental procedures are required.
Tree water use information is only available for a limited number of tree species. Some promising species have been overlooked, e.g. Prosopis cineraria, a deep-rooted species that grows well in desert areas under non-irrigated conditions. The leaves provide palatable fodder to cattle and the tree is very acceptable to farmers as they improve farm yields. Under natural conditions, Prosopis cineraria trees are widely spaced. When grown on farms often planting densities of about 40-50 stems/ha are used. Prosopis' capacity for biomass production and water use under conditions of abundant water supply in the rootzone has not been investigated. Potential growth and transpiration rates of Prosopis and other desert trees could be expected to be high but this will need to be tested.
Biomass produce varies greatly depending on species and growing conditions. The first objective of the trees used for biodrainage is to obtain highest evapotranspiration, however, biomass production is an important secondary objective. It is not only the volume of biomass but also its kind that determines the choice of the plantation. Tree produce includes timber, fuelwood, fruits, fibre, oil, and different degrees of protection against wind erosion, some measure reclamation of alkali soils. The relation between the two objectives requires investigation.
Timber yields of trees grown under saline conditions can be reasonably high; e.g. E. occidentalis grown with 9 dS/m irrigation water on a saline site in northern Victoria, Australia, showed annual growth increments of nearly 17 m3 during its fourth year of saline irrigation. However, because of poor tree shape, the timber was not suitable for anything but fuelwood. Breeding efforts to improve form and timber quality have traditionally focused on high yielding species in more benign high-rainfall environments. There is much scope for improvement in the breeding of tree species for high-salinity environments.
The different demands placed on the choice of tree species for biodrainage resulting from climate and altitude, viz. the location in the river basin, needs further assessment as well as the impact large-scale biodrainage systems may have on the river basin as a whole, wherein irrigation and drainage projects are established or planned.
The issue of salt accumulation and the longevity of deep-rooted tree plantings in shallow water table conditions continue to be a topic of discussion. A mechanism for salt removal from the rootzone of deep-rooted plants is critical for the long-term survival of plants growing under these conditions. This mechanism could be either 'conventional' drainage or 'natural' deep drainage.
Modelling of salt dynamics under tree plantations is important to provide field practitioners with guidelines on establishment and management. Sound data sets (soil salinity, soil water content, water table levels, piezometric levels, etc.) based on proven measurement techniques are important to validate these models and to test hypotheses on sustainability. This often requires long-term investigation programmes. A standardized approach is required to allow inter-site comparison.
The generally positive environmental consequences of biodrainage systems lack recognition and should be paid greater attention and disseminated to a wide audience of planners, designers, and project implementers.
Effectiveness of trees as biodrainage system depends often on socio-economic conditions: collaboration of the local rural farming families is adamant to its function and knowledge must be generated on these aspects.
