Land Use
Investments in promoting changes in land use have a role in conjunctive water management. Initiatives that alter the catchment water balance such as revegetation or agroforestry can also alter the seepage flux between streams and aquifers. Such land use initiatives are commonly applied to reduce groundwater recharge for salinity mitigation.
Land use change and salinity
A significant challenge for Australian agriculture is the control and mitigation of the widespread problem of salinity. Adoption of European methods of farming without thorough understanding of the unique Australian landscape has caused rising groundwater levels and mobilisation of salt stores. Widespread clearing of native deep-rooted woodlands and forests and the introduction of shallow-rooted crops and pastures in large areas has led to a significant increase in dryland salinity. Application of large additional volumes of water through extensive irrigation has brought the same result - rising water tables and salinisation of soil and streams (see Salinity).
An important factor in reducing groundwater recharge is the maintenance of an adequate vegetation cover throughout a catchment, particularly on sites of potentially high recharge. The greater the amount of water that the vegetation intercepts (through transpiration etc), the more effective it will be in addressing the problem of excessive leakage to the subsurface.
Prevention and mitigation of dryland salinity in cropping lands requires control of drainage beyond the reach of the crop roots. There is a range of options that landholders 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. Significant work has been undertaken to better match management options to the nature and scale of groundwater flow systems operating in catchments (Walker et al, 2003).
Specific land use management practices may include:
- avoid clearing of native vegetation throughout the whole catchment but particularly in areas of potentially high groundwater recharge i.e. the upper parts of a catchment on hills and ridge tops with thin soils;
- reafforestation and tree planting (Keenan et al, 2004) - most desirable in areas of potential high groundwater recharge (Schofield and Bari, 1991). These areas often correspond with sites of lowest agricultural productivity. It is important that species appropriate to the site conditions are selected, although species with known high water use would be desirable;
- agroforestry - this offers landholders the opportunity to offset the cost of tree planting to address salinity problems with income from forest products. Tree plantations are recommended for land where either perennial pastures cannot be reliably established, will not persist or will be unable to provide adequate protection from groundwater recharge;
- pasture selection - use of deep-rooted perennial pasture species such as lucerne and phalaris rather than shallow-rooted annual species, as in clover-based pastures for example (Clifton and Schroder, 1996). Often, a mixture of species that have different growth habits and climatic requirements will maximise water use;
- avoid overgrazing - this may greatly reduce pasture cover, allowing increased water infiltration into the ground. The control of pests such as rabbits may also be necessary. A minimum ground cover of about 70% is considered appropriate in most areas;
- phase farming - this 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; and
- planting of strips of woody perennials - has limited effect, mainly within a strip of soil little larger than the width of the canopy.
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.
The reclamation of saline sites is also an important function, but it is not usually considered to be an effective form of long-term salinity control. Rehabilitation of such sites 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. The purpose of reclamation programs is to allow the productive use of salt-affected land, to inhibit soil erosion at the site, and to prevent the leaching of salts into adjacent waterways.
Further Information
References
Clifton C, Schroder P, 1996. The potential of upgraded perennial pastures to reduce groundwater recharge in southern Victoria. Proceedings 8th Australian Agronomy Conference
Schofield NJ, Bari MA, 1991. Valley reforestation to lower saline groundwater tables - Results from Stene Farm, Western-Australia Australian Journal of Soil Research 29(5) 635-650
Walker G, Gilfedder M, Evans R, Dyson P, Stauffacher M, 2003. Groundwater flow systems framework - Essesntial tools for planning salinity management. CSIRO [PDF 1MB]
Keenan R.J, Parsons M, Gerrand A, O'Loughlin E, Beavis S, Gunawardana D, Gavran M, Bugg A, 2004. Plantations and water use: a review prepared for the Forest and Wood Products Research and Development Corporation. Bureau of Rural Sciences, Canberra.