The causes of dryland salinity are in principle well understood. The replacement of perennial deep-rooted native vegetation by shallow-rooted annual crops or pastures results in wetter subsoils and accompanying larger deep drainage beyond the reach of shallow roots, leading eventually to rising water tables. If the ground water is saline, which it commonly is in semiarid environments, salt scalds appear when the water tables reach the soil surface. Even if the water tables are only brackish rather than saline the surface can become saline owing to the salt concentrating as water evaporates.
The essential difference, hydrologically, between native perennial vegetation and annual crops is that the perennials can use substantial amounts of water throughout the whole year. In a Mediterranean environment, for example, with its winter wet season, there is little difference during the winter in water use by annual or perennial vegetation. Rainfall that substantially exceeds evaporation during the winter may penetrate deeply into the subsoil under both types of vegetation. But during the summer, when the crops have been harvested, the perennials, enabled by their deep roots, can use water that may have penetrated well beyond the rooting depth of the crops during the winter.
Thus, mitigation of dryland salinity in cropping lands requires control of drainage beyond the reach of the crop roots. There is no single solution. There is, however, a range of options that farmers can select from, including growing longer-season crops, which tend to have deeper roots, and various techniques for incorporating some deep-rooted perennial species into cropping systems to tap the water in the deep subsoil that may have accumulated during a wet season(Black et al., 1981).
Phase farming is one effective way of incorporating perennials into a cropping system. It involves the tactical rotation of herbaceous perennial pasture, such as alfalfa (lucerne) which can be grazed or harvested for hay, with a series of annual crops. The perennial pasture dries the subsoil below the roots of annual crops, thereby creating a buffer zone in which water and nutrients that leak below the crops can be held for a few seasons, remaining largely accessible to the roots of the next phase of deep rooted perennials. A special issue of the Australian Journal of Agricultural Research (vol 52(2), 2001) contains several papers that discuss phase farming. One general conclusion from these is that the size of the buffer created by herbaceous perennials varies, reflecting soil type and local climate as well as species, from about 50 to 150mm. The larger values were due to lucerne and were typically achieved within two years of establishment. Phalaris was less effective than lucerne in developing the buffer. Danthonia and Eragrostis, were less effective still. With deep drainage under crops averaging about 30 to 50 mm per year, such buffers can deal with one to five years of drainage below the roots of crops.
Strips of woody perennials can also help, though their effectiveness is limited by the maximum lateral movement of water through unsaturated soil to their roots, which is typically no more than about 1 m. Even though surface roots may spread from the tree for a distance several times the spread of the canopy (and thereby reduce yields of adjacent crop), the deeper roots typically do not spread so far and the strips may do little more than control deep drainage only in a strip of soil little larger than the width of the canopy (Stirzaker et al., 2002). If the roots of such perennials can tap the groundwater, though, the lateral flow towards them can be large enough for them to take up a lot of water that might otherwise flow to lower points in the landscape, providing that the water is not too saline. The appropriate proportion of woody perennials to herbaceous species depends on the competitive interactions at the interface between trees and crop or pasture as well as on the ability of trees to extract groundwater and considerations of both help determine the area of cropland that needs to be sacrificed.
Another possibility is to identify, using georeferenced yield monitors, areas giving consistently poor yield. Such areas, which are especially prone to deep drainage, can be excluded from cropping, and put under either permanent perennial pasture or trees.
Determining the right mix of these various options, both in time and space, is hampered by the difficulty of estimating what the deep drainage is and how it may vary with season and treatment. Few long-term measurements are available. The development of a cheap drainage meter (Hutchinson et al., 2001) may help overcome this deficiency and help farmers find out what is happening on their own farms. Augmenting such measurements with reliable simulation models, of both phase farming and agroforestry, will also help. Essential input for such models includes the effective water-holding capacity of the soil as a function of depth and the effective depths of rooting of both the perennials and the crops. That the depths of rooting can depend markedly on season and cropping history is a considerable challenge. A reliable model can help estimate the impact of seasonal variability and management decisions on deep drainage by running the model through several decades of rainfall records; it should also be able to guide those management decisions, at least if the troublesome groundwater system is local. Where an outbreak of dryland salinity is a long way from where the accessions to groundwater are occurring, hydrologic models and measurements are needed, and although these can handle general flows of groundwater, they are as yet not able to guide specific actions on farms.
If these various options for reducing deep drainage are effective in lowering water tables so that any salt scalds dry out, there is still the problem that the salt remains in the root zone. Further rehabilitation requires a succession of plants, starting with halophytes, with can take up the water and thereby create space for rain to wash the salt deeper into the soil profile. Salt tolerant crops may then be able to grow there, and enable further leaching of the salt. If the soil has become sodic, chemical amelioration (say, with gypsum) may also be necessary.
Another possibility is to identify, using georeferenced yield monitors, areas giving consistently poor yield. Such areas, which are especially prone to deep drainage, can be excluded from cropping, and put under either permanent perennial pasture or trees.
Determining the right mix of these various options, both in time and space, is hampered by the difficulty of estimating what the deep drainage is and how it may vary with season and treatment. Few long-term measurements are available. The development of a cheap drainage meter (Hutchinson et al., 2001) may help overcome this deficiency and help farmers find out what is happening on their own farms. Augmenting such measurements with reliable simulation models, of both phase farming and agroforestry, will also help. Essential input for such models includes the effective water-holding capacity of the soil as a function of depth and the effective depths of rooting of both the perennials and the crops. That the depths of rooting can depend markedly on season and cropping history is a considerable challenge. A reliable model can help estimate the impact of seasonal variability and management decisions on deep drainage by running the model through several decades of rainfall records; it should also be able to guide those management decisions, at least if the troublesome groundwater system is local. Where an outbreak of dryland salinity is a long way from where the accessions to groundwater are occurring, hydrologic models and measurements are needed, and although these can handle general flows of groundwater, they are as yet not able to guide specific actions on farms.
If these various options for reducing deep drainage are effective in lowering water tables so that any salt scalds dry out, there is still the problem that the salt remains in the root zone. Further rehabilitation requires a succession of plants, starting with halophytes, with can take up the water and thereby create space for rain to wash the salt deeper into the soil profile. Salt tolerant crops may then be able to grow there, and enable further leaching of the salt. If the soil has become sodic, chemical amelioration (say, with gypsum) may also be necessary.
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