The impact of geology on dryland salinity, and the development of revegetation strategies, in the western wheatbelt of Western Australia

Christopher John Clarke

The first objective of the research was to investigate whether major geological faults had an impact on the development of dryland salinity, and what mechanism was the cause of the phenomenon. The second objective was to investigate revegetation treatments that would prevent land salinisation, and the effect of faults on the treatments’ impact. Investigation of three groups of catchments in the western wheatbelt. Western Australia, showed ten times more dryland salinity in the catchment underlain by a major fault than in the paired unfaulted catchment(s). Geomorphometric statistics show that catchments within each group are similar. There was no correlation between the degree of clearing for agriculture and the extent of dryland salinity. Detailed investigation of one group showed that there were no differences in salt store or regolith thickness between the faulted catchment and the adjacent unfaulted catchment. However, the mean hydraulic conductivity of boreholes within the fault zone, identified from an aeromagnetic survey, was five times higher than for boreholes outside the zone. That the mechanism underlying the association between the faults and dryland salinity is higher hydraulic conductivity inside the fault zone is supported by the flat piezometric surface in the faulted valley, and the continuous base flow in the faulted catchment, whilst the other catchments only flowed in response to winter rainfall. Computer modelling showed that 40% of the cleared area of the catchments would become saline if nothing is done. Modelling of revegetation treatments showed that replacing annual pasture with deep-rooted perennial pasture, or native vegetation, prevented the onset of dryland salinity. However, tree belts (alley fanning) left 15% of the cleared area saline. With the fault 50% more tree rows were required than predicted without the fault. A way of preparing a hydraulic conductivity map for use in computer modelling by interpretation of airborne geophysical data and borehole hydraulic conductivity testing is proposed.

The impact of geology on dryland salinity, and the development of revegetation strategies, in the western wheatbelt of Western Australia

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