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Australian National University (2001)

Dryland salinity in the south-east region, New South Wales

Wagner, Rex

Titre : Dryland salinity in the south-east region, New South Wales

Auteur : Wagner, Rex

Université de soutenance : Australian National University

Grade : M.Res.Env.S.) 2001

Description
Dryland salinity is claimed to be potentially the nation’s greatest environmental problem. In NSW, it occurs mainly in the upland areas in the south-east of the State. This region is part of the Murray-Darling Basin and has the potential to contribute to its worsening salinity. This thesis examines the nature of its occurrence in this area, and the implications for its remediation. There are two opposing models of the causes of dryland salinity. The first, a more long held view, is that salinisation is localised, restricted to particular soils and landforms, restricted in its spread, episodic in its development, and responsive to mitigation measures within its own local catchment or recharge area. A more recent view is that the problem is one of regional dimension, driven by rising regional groundwater systems, progressive in its spread, and having the potential to degrade large areas with serious off-site consequences, principally as a result of increasing stream salinity. Here, the genesis of the problem is seen as the widespread clearing of timber in regional recharge areas following European settlement. The chief remedy proposed is to restore tree cover ; with up to 50 percent of the higher rainfall zone in the Murray-Darling Basin being replanted to trees. The opposing models differ greatly in their conception of the magnitude of the problem, and its potential for expansion. They present widely different management and policy options for its mitigation. The social and economic impacts of any programs proposed similarly differ. This thesis uses aerial photography, site inspections, and farmer interviews to trace the development of dryland salinity in the south-east region of NSW. The findings are compared with the expectations from the two models. It was found that dryland salinity across the region was not a recent phenomenon. One site developed before 1900 and there were episodic periods of expansion in the 1930s and 1950s to 1960s, with most sites reaching their maximum extent by the early 1970s. Most sites occurred as isolated outbreaks, associated with local landscape features and soil types. Their development appeared to be closely related to changes in rainfall patterns, to past land use and management of the affected sites themselves, and their local recharge areas. There was no evidence of continued expansion in the last two decades, and little evidence of any progressive spread, either spatially or temporally, as projected from regional watertable modeling. However the conditions that predispose lands to dryland salinity still exist, and there could be an extension of the problem in the future. In a limited number of cases, rehabilitation of former saline areas has been achieved at the local property level. Here the landholders had broader objectives than solely controlling salinity. Their primary objectives were to develop more productive and sustainable farming systems. The improved practices introduced also overcame other associated land degradation problems. These measures, at a local recharge level, were effective within a reasonably short time. Although long term regional programs may be needed, the study showed that many of the incidences of local salinity development appear to be associated with local factors. Successful treatment at a local property level appears to confirm this. Such treatments have the potential to reduce salt flushes from farm lands to streams. The success of these treatments is not consistent with the regional groundwater model. Tree planting on the scale proposed from this model, may not be necessary ; and the development and adoption of more sustainable farming systems widely across landscapes may largely redress the situation. These systems in themselves would include more trees. This approach would not require the huge capital investment necessary with tree or engineering solutions. The necessary treatments would be more within the reach of individual landholders, and would be much less disruptive to the existing agricultural infrastructure and community life than the massive tree planting programs proposed, and may ensure a better allocation of resources overall. More support at the local community level should also be achievable. Dryland salinity is a symptom as well as the result of unsustainable land use and management. It is concluded that reduction in the hazard does not depend on a singular approach such as tree planting, but principally on the development and adoption of more sustainable and productive farming systems. Technology is being developed that could provide the basis for such systems. Means also must be developed to foster their wider adoption.

Mots clés : Soil salinization Australia New South Wales • Soils, Salts in Australia New South Wales

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