The status of biodrainage related research is presented in this chapter by country of origin.
Much of the information in the first section of this chapter is based on papers presented in a special issue of the International Journal for Agricultural Water Management, which focused on Australian work in this area (Thorburn, 1999).
Areas with shallow saline water tables in Australia have been rapidly expanding in response to widespread agricultural development over the past 100 years (Robertson, 1996). In response to this alarming trend, Regional Management Plans have been developed over the past 10-20 years for many areas around the country, both for dryland and irrigated areas.
The processes of interaction between vegetation and soil water are difficult to quantify; soils are not uniform, water fluxes are often small, vegetation is often perennial and trends in soil and plant parameters are affected by seasonal variability. In addition, work in this area covers a range of disciplines including agronomy, hydrology, soil science and forestry, however communication between the specialists working in these fields is often lacking.
Much of the work described in this section is related to trees, either in natural stands or in plantations. A few examples of other crops such as lucerne (Medicago sativa) and saltbush (Atriplex spp.) are presented. Research will be discussed under the headings 'dryland' and 'irrigation'.
Dryland scenarios
Dryland salinity, resulting from over-clearing, has only been recognized in Australia since the late seventies. Average accessions to the water table in wetter parts of Australia (rainfall > 750 mm/year) have increased from <5 mm/year to >20 mm/year following clearing; in drier regions deep drainage increased from <0.1 mm/year to >10 mm/year (George et al., 1997). In the Murray-Darling Basin about 0.5 million km2 of native vegetation had been cleared for agricultural development by 1989, equal to the removal of about 12-15 thousand million trees (Dryland Salinity Management Working Group, 1993). In Western Australia the removal of about 90 percent of the perennial, deep-rooted native forests and woodlands over an area of some 20 million ha and its replacement with predominantly shallow-rooted annual crops and pastures has resulted in salinization of about 1.8 million ha of previously productive cropping land (Ferdowsian et al., 1996). At the current rate of expansion, about 2-3 times this area can be expected to become salt affected unless counter-action is taken to restore the hydrological recharge-discharge balance (George et al., 1997).
Because of the large areas involved, the high costs of drainage engineering works and the relatively low returns from the agricultural production systems, management of the clearing-induced salinity problems is based on agronomic measures. The objective is to move to a plant water use scenario that more closely approximates that of the pre-clearance situation (Dryland Salinity Management Working Group, 1993). This could involve a range of plants, including deep-rooted permanent pastures, crops and trees.
The question of 'where to plant trees for control of dryland salinity' is addressed by Stirzaker et al. (1999). The paper presents simple rules and analytical expressions to optimize the number and location of trees required to control rising water tables on relatively flat cropping or pasture land.
Deep water table (often recharge areas)
George et al. (1999) analysed data from some 80 sites in the medium-rainfall (550-700 mm/year) southwest of Western Australia, for which data were collated to assess the impact of trees on water tables. They distinguished between discharge and recharge areas with the latter, being higher in the landscape, generally having deeper water tables. Only two statistically valid relationships were found: (1) significant lowering of groundwater tables were highly correlated to the area of revegetation and (2) only the proportion planted (%) was necessary as an explanatory variable for water table response. Both 'proportion planted' and 'age of the trees' were needed to explain the rate of water table response. The paper also concluded that trees are best planted in recharge areas for long-term hydrological benefits and that extensive plantings, covering as much as 70-80 percent of the catchment is needed to achieve significant water table reductions.
Stirzaker et al. (1999) discuss the scenario where the water table is below the depth of the tree roots, and distinguish 'competition zones' and 'capture zones' between tree plantings and surrounding crops or pastures. In the competition zone, tree roots and crop roots compete for moisture; in the capture zone the trees provide deep drainage to the surrounding crops without competing directly for moisture. The absolute maximum width of the capture zone depends on the tree root depth and the texture of the soil. In coarse sands capture zones would be less than 1 m wide, in clay soils they could be up to 3 m wide. The paper quotes Zohar (1985) who measured roots up to 20 m from the trunk of individual Eucalypt trees. Based on this figure, about ten trees per ha would potentially be enough to intercept recharge to the water table, assuming that tree water use was high enough.
Sharma (1984) found monthly evapotranspira-tion rates up to 3 x Class A pan in Eucalypt-dominated forest in Western Australia during rainy winter periods. He argued that these high values were the result of canopy interception and direct evaporation from the leaf surface. During the hot, dry summer months, much lower evapotranspiration rates of 0.1-0.5 x Class A pan were found, reflecting the low water availability during that period.
Shallow water table (often discharge areas)
Shallow water tables commonly are associated with discharge areas, they may be also perched shallow water tables. This is the case in recharge areas where they are often shallow and relatively fresh. George et al. (1999) reported that recharge sites planted over perched local aquifers had a better response than those over regional systems, the latter generally having deeper water tables. Results of statistical analyses of the effect of trees on water tables at 46 sites in discharge areas were highly variable. A significant correlation between the proportion of the revegetated catchment percentage and water table response was observed. For every 10 percent increase in planted area, the water table was lowered by about 0.4 m. However, the planted area at most sites was less than 30 percent of the catchment, so the trees had only a relatively small and localized effect.
George et al. (1999) also reported that sites with low groundwater salinity showed the greatest water table response to tree planting in discharge areas. Thorburn (1997) who hypothesized that trees are more effective at lowering water tables in lower salinity environments supports this. The study concluded that in discharge areas water table control by tree planting might only be a medium-term solution and that it may be more appropriate to plant for reasons such as reducing the visual impact of salinity and the risk of erosion.
Stirzaker et al. (1999) describe the use of tree belts grown over shallow water tables using the Dupuit-Forchheimer theory as developed for engineering open-drains (Kirkham, D. 1967). They state that for tree belt spacing intervals of 40 m (considered to be the minimum for cropping situations), drainage values of up to 100 mm/year could be accommodated for soils with a saturated conductivity of > 5 mm/day. For well-structured clay subsoils and deep sands they calculated that 400 m spacings would be feasible. Wide tree belts would be needed to keep the water table at safe levels halfway between the belts. The problem of salt build-up in the capillary fringe above the water table is also discussed in the paper.
The use of Old Man Saltbush (Atriplex nummularia) to control shallow saline groundwater tables was described by Slavich et al. (1999). Saltbush has been widely planted on salt affected land in (semi)-arid southeast Australia. The study indicated that the transpiration rate of saltbush was very low (< 0.3 mm/day) relative to the recharge rate throughout the monitoring period. Up to half the transpiration during the driest time of the year (March) was derived from groundwater. The authors concluded that saltbush plantations are likely to have a negligible hydrological impact. However, the ability of saltbush to provide soil cover and produce fodder on severely salt-affected land makes it an important crop in the management of discharge areas.
Cramer et al. (1999) describe three field studies on the ability of Casuarina glauca and Eucalyptus camaldulensis to use shallow saline groundwater. They used naturally occurring isotope signatures of soil water, groundwater and tree xylem and sapflow measurements to determine tree water sources. The studies concluded that C. glauca had a greater impact on groundwater discharge than E. camaldulensis, planted at similar densities. C. glauca sourced much of its water from the saturated groundwater zone deeper in the profile while E. camaldulensis relied on the shallower unsaturated zone (soil water), the former thus showing a greater potential to consume saline groundwater.
Morris and Collopy (1999) compared E. camaldulensis and C. cunninghamiana for their water use under shallow saline water table conditions, using a water balance monitoring approach. Their study site was in northern Victoria, Australia, with an average annual rainfall of about 480 mm and average Class A pan evaporation of 1 350 mm/yr. Evaporation exceeds rainfall in the area over nine months of the year between September and May. Similar to Cramer et al. (1999), they found that the Casuarina genus was better adapted to the saline site conditions than the Eucalyptus genus, however this depends on the species. The salt dynamics measured at the site were complex; soil solution salinity was measured using soil salinity sensors (Soil Moisture Equipment Corporation, Santa Barbara, United States) at four depths down the profile (1, 2, 3 and 4 m) at two locations. The measurements showed large fluctuations with soil solution salinity in the rootzone, rising or falling by 10 dS/m over a period of several months. This suggested that rapid re-distribution of salt was taking place in the clay soils. However, the authors suggest that the soil salinity sensor data might have to be interpreted with some care as the sensors might only measure salinities in a certain pore-size range and wetting and drying cycles preferentially occur in the larger pores. The authors also note the need for long-term monitoring under plantations to quantify (often slow) long-term trends.
The use of lucerne (Medicago sativa) as a biodrainage crop was investigated by Zang et al. (1999). Lucerne is a deep-rooted, relatively salt tolerant perennial that is believed to be able to both reduce recharge and use shallow groundwater. They used lysimeters to investigate capillary upflow from shallow water tables and the associated processes of salt accumulation, water use and growth response. Stable isotope techniques were used to determine the components of upflow resulting from the lucerne plant cover. In the presence of a shallow (1 m below surface) and saline (EC 16 dS/m) water table, lucerne did not appear to derive much of its water directly from the water table, preferring to use fresher water stored higher up in the soil profile. The authors highlighted the prediction that over time soil salinities will start to build up, plant water use will reduce, water tables will start to rise and salts will start to move up the profile to the surface. A combination of engineering and biological management approaches was mentioned as needed to make the system sustainable over the long term.
The ability of saltbush (Atriplex spp.) to provide water table control was investigated by Barrett-Lennard and Malcolm (1999). They looked at the soil profiles beneath stands of saltbushes, growing on a shallow (0.5 - 1.2 m) saline water table in a plant spacing trial conducted in Western Australia at a site with average annual rainfall of 330 mm. They found a substantial increase in soil chloride concentration beneath the plantings over the two-year trial period. The increases were proportional to the saltbush 'leaf density' (weight per unit soil surface area) and inversely proportional to the initial concentration of chloride in the soil. They argue that the increases in soil and water table salinity under the plantation were a result of the use of groundwater by the plants. They also suggest that the lower water table could provide opportunities for shallow-rooted legumes to be established in the saltbush stands.
Irrigation scenarios
During the 1980s, when salinity problems became more prevalent in the eastern irrigation areas of Victoria, the management plans developed to contain the problem were based mainly on engineering approaches, such as groundwater pumping, surface drainage, channel sealing and system management improvement (irrigation efficiencies).
In response to suggestions that vegetation could play an important role in the management of shallow water tables, a symposium in 1991 discussed the role trees could play in the management of the salinity problem in irrigation areas. Two views were presented: (1) trees could be used to manage the shallow water table/salinity problem (Alexandra, 1991) and (2) trees can lower water tables, to be long-term sustainable they have to be provided with a salt balance mechanism (drainage) to manage salt accumulation in their rootzone (Heuperman, 1991). Although initially received with some scepticism by the forestry industry, the latter view is now widely accepted and the management of the salt accumulation process is becoming an issue of increasing interest (and concern).
One of the earliest documented observations of water table lowering beneath a tree plantation was recorded by Heuperman et al. (1984). A water table drawdown of 2-4 m was measured in a seven-year old non-irrigated planting surrounded by irrigated land with a shallow (2-3 m) water table. The authors state that 'it might be expected that salt would have accumulated in the rootzone of the trees due to their use of the saline groundwater. However, the salinity profile measured to a depth of 6 m was similar inside and outside the plantation. The plantation is relatively young and as the cumulative amount of groundwater increases, the soil salinity profile may change. The salinity profile measured in the same plantation in 1993 shows that this accumulation process had taken place (Heuperman, 1999). Salts had accumulated in the top of the water table and the capillary fringe above the water table.
Silberstein et al. (1999) modeled the effect of soil moisture and solute conditions on long-term tree growth and water use. They concluded that the largest water table impact of the tree plantation occurred about ten years after establishment, after which time the water table began to rise and salt started to accumulate.
The issue of “commercial and environmental values of farm forestry in the Murray-Darling Basin irrigation areas” was discussed at a workshop in New South Wales, Australia in July 1999 (George 2000). The conclusions of the workshop session on the efficacy of non-irrigated plantations for salinity control state that on the positive side “trees have the potential of providing an alternative 'stand-by' crop which can use excess irrigation water when it becomes available”. On the negative side the summary chapter states, “the question of salt accumulation under growing trees is an area that requires major work, both at the theoretical and the field level. There is a concurrence amongst scientists that because trees usually exclude sodium and chloride ions from the transpiration stream, that these become concentrated in the soil. Limited evidence supports the contentions that the resulting salt accumulation then acts to inhibit growth of the trees. Future work can and should concentrate on quantifying this effect and determining the site requirements to either avoid this or to somehow 'leach' the material from the soil”.
The use of trees to intercept seepage losses from irrigation supply channels was first discussed in Australia by Sonogan and Patto (1985) and summarized by Webster (1984). The work involved the determination of (1) the most suitable tree species for ease of establishment, (2) growth rates in saline/waterlogged conditions and (3) assessment of impact on the water table. From the tested tree species, Sargent's mallet (Eucalyptus sargentii) performed best from the point of view of survival and growth. E. occidentalis (swamp yate) and E. kondininensis (stocking gum) also performed well. Tall wheat grass (Thinopyrum elongatum) established well on all sites within a two-year period; it was grown in mixed stands with lucerne (Medicago sativa), rye grass (Lolium perenne) and puccinellia (Puccinellia ciliata). The authors conclude that both the trees and the tall wheat grass appear to have effectively lowered water tables in strips of about 50 m alongside channels in a relatively short period (three years) following planting. Detailed case studies on channel seepage interception are presented in Chapter 5.
Summary of the Australian research
Vegetation has been used successfully in Australia to lower water tables, either indirectly by reducing recharge and/or directly by extracting water from the saturated zone underneath the plants.
In recharge areas, where plants rely on water inputs from the top (rainfall or irrigation) and have no direct access to shallow groundwater, this concept is sound and long-term sustainable. However, in discharge areas with shallow water tables, salt accumulation processes in the rootzone and the water table underlying the vegetation will have to be managed to achieve long-term sustainability. This will often involve some form of engineering drainage input to remove salts from the rootzone. This concept is now widely accepted in Australia.
A lysimeter study on the use of biodrainage to control waterlogging and secondary salinization in an irrigated (semi)-arid environment was described by Chhabra and Thakur (1998). Their experiments were conducted in 1.2 m diameter, 2.5 m deep lysimeters with constant water table depths (1, 1.5 and 2 m from the surface) and groundwater salinity (0.4, 3, 6, 9 and 12 dS/m). The authors use the term 'biodrain' to describe the plant water use in the lysimeters. Eucalyptus tereticornis and bamboo (Bambusa arundinacea) were planted in the lysimeters. The amount of water 'bio-drained' by Eucalypts and the bamboo at the given water table depths and salinities was monitored over four years by daily measurement of the water needed to maintain water table levels. Very high 'biodrainage' values of up to 5.5 m per year were measured for the Eucalypts (third year, 0.4 dS/m groundwater salinity and 1.5 m water table depth) and 4.2 m per year for the bamboo (fourth year, 0.4 dS/m groundwater salinity and 1.5 mm water table depth). 'Biodrainage' values at the higher groundwater salinities were lower (e.g. 3.2 m per year for eucalyptus at EC 12 dS/m and 1.5 m water table after three years and 2.2 m per year for bamboo at EC 12 dS/m and 1.5 m water table after four years). This is still much higher than mentioned in other literature. The authors also found that topsoil salinities were kept low by the trees but salt was re-distributed to the top of the saturated zone and the capillary fringe above the permanent water table level. This confirms findings by Heuperman (1999) in a field study beneath a eucalyptus plantation.
In a study of the desert area of Rajasthan annual tree water use by plantations with a density of 1 900 trees/ha (Eucalyptus and Acacia species) was estimated as 3 446 mm, which is about 1.4 Class A pan (see Ch. 5.2).
Shah et al.(2000) provide an overview of the salinity and waterlogging problems faced by the irrigation industry in Pakistan and the mechanisms developed to manage these problems through Salinity Control and Reclamation Projects. Most of these projects have been based on engineering inputs such as tubewells, surface drains and horizontal pipe-drains.
The authors quote Qureshi and Lennard (1998) with regard to three potential approaches to the management of saline, sodic and waterlogged soils: (1) the engineering, (2) the reclamation and (3) the saline agriculture approach. Both the reclamation and saline agriculture approach include the use of plants to improve soil conditions and to produce economic returns. The authors report on recent attempts to implement the results of saline agriculture research through biosaline field technology. The Joint Satiana Pilot Project “Extending the use of trees and forage shrubs for the productive use of salt-affected land in Pakistan” consists of 37 villages and covers 66 000 acres of which more than half are affected by waterlogging and salinity, mainly resulting from channel seepage. Saline agricultural practices have now been applied to part of this area.
Bhutta and Chaudhry (2000) discuss the consumptive water use of phreatophitic trees and bushes used for biodrainage purposes. They quote results from the western USA (ranging from 650 mm/yr for poplar to up to 2 800 mm/yr for tamarix) and present evidence from research completed in Pakistan, showing high potential water use of 174 litres/day for 2.5-month old seedlings of Tamarix articulata; no explanation was given on the measuring techniques used to arrive at this information. Water use measured in 14 tree plantations in Australia, Pakistan and Thailand, covering a range of species and site conditions, was reported to range between 300 and 2 100 mm/yr (NIAB 1997).
In a study by Roitzsh and Marsur (Bhutta and Chaudhry, 2000) average daily luxury water consumption (potential ET) by individual trees was reported as 129 litres for Acacia arabica, 137 litres for Eucalyptus tereticornis, 143 litres for E. camaldulensis, 153 litres for Morus alba, 156 litres for Dalbergia sissoo and 174 litres for Tamarix articulata. Annual water use by three to five year old Acacia nilotica was 1 248 mm on a severely saline site and 2 225 mm on a mildly saline site at Tando Jam in the Sind province. The same report presents values of daily consumptive use of water of individual trees as 45 litres for three-year-old poplars, 60 litres for tamarix and 143 litres for full-grown eucalyptus trees.
Work in the USA has focused on production functions of trees (mainly Eucalypts) grown under saline irrigation conditions and in the presence of trace elements (selenium, molybdenum and boron) (i.e. Oster et al. 1999a; Grattan et al. 1996). This research does not focus on the ability of trees to lower water tables but rather investigates evapotranspiration and tree growth under different salinity regimes. Trees using drainage water with an EC of 8.5 dS/m and a SAR of 33 reduced ET by about 50 percent (Oster et al. 1999a).
Estimated annual consumptive use of water in the southwestern USA is reported as 770-1 350 mm for willow, 650-2 350 mm for poplar, 1 680 mm for alder and 2 200-2 800 mm for Tamarix (Bhutta and Chaudhry, 2000).
