Salinity and the discharge of salts from catchments in Australia

Discharge of soluble salts from catchments following clearing of native vegetation for dryland agriculture is a serious environmental and economic problem affecting soil and water resources in Australia. The fundamental challenges are: To identify areas of soil at risk of becoming saline. To relate soil and water salinity risk to options for management of land in the area that contributes to the risk, and thereby contribute to the evaluation of management options. These challenges are faced in an environment where native vegetation has roots that extend to depths of order 10 m in soil profiles that are formed by in situ weathering of granitic rocks and dolerite dykes to depths of about 20 m. The profiles typically contain 1-100 kg m(-2) of salt (primarily sodium chloride) in solution in the pore water. The distribution of soluble salts, and the movement of water within most of the unsaturated zone of these soils results from a combination of matrix flow and flow through remnant root channels and larger-scale structures with geologic origins. Recognized options for management of salinity risk, or to reduce existing areas of saline soil, are revegetation of part of the cleared land with alternative species, pumping to lower the watertable in selected areas, and construction of ditch drains for control of surface water and shallow groundwater. All options are constrained by the economics of dryland farming, and pumping or drainage is further constrained by possible environmental impacts of disposal of saline water. Application of soil physics/hydrology to salinity in Australia has contributed to understanding, but generally it has proven to be inadequate to aid the development of effective management strategies. A classic approach to soil water movement at the primary catchment scale (areas of order 10(6) m2 or more) will always be limited by errors of measurement at each site within the catchment, those arising from the method of estimation of soil characteristics between measurement sites, and those arising from the method of integration to predict whole-catchment behaviour. The cost and effort of such an approach, and the errors that must eventuate, should be compared with the costs and errors of alternatives for characterization at a whole catchment scale.