Major faults and the development of dryland salinity in the western wheatbelt of Western Australia

Dryland salinity poses a major threat to agricultural production in the whe atbelt of Western Australia and much time and effort is expended on underst anding the mechanisms which cause it and on developing techniques to halt o r reverse its development. Whilst the location of much dryland salinity can be explained by its topographic position, a significant proportion of it c annot. This study investigated the hypothesis that major faults in the Yilg arn Craton represented in aeromagnetic data by intense curvilinear ions exp lained the location of areas of dryland salinity not explained by topograph y. Moreover, the causal mechanisms that might underpin a spatial relationsh ip between major faults and dryland salinity were sought. In one fourth order catchment, nearly 85% of the salinity that was not expl ained topographically was within 2km of the centre line of a major fault, t he remaining 15% being in the other 12km of the catchment. Three groups of similar third order catchments in the western wheatbelt of Western Australi a a-ere also investigated; in each case the catchment that was underlain by a major fault had dryland salinity an order of magnitude more than the unf aulted catchment(s). This evidence demonstrates a strong spatial associatio n between major faults and the development of dryland salinity. Other evide nce suggests that the underlying mechanism is hydraulic conductivity 5.2 to 2.9 times higher inside the fault zone compared to outside it and shows th at geomorphology, salt store, regolith thickness, and degree of clearing ar e not the underlying mechanisms. In one of the groups of catchments, it has been calculated that an amount of recharge, significant in relation to rec harge from rainfall, was entering from an adjacent catchment along a major fault. The paper concludes that geological features such as major faults affect th e development of dryland salinity; in the wheatbelt of Western Australia be cause of permeability differences in the regolith and therefore computer mo dels of salinity risk need to take these differences into account. Techniqu es need to be developed to map, quickly and relatively cheaply, the geology -related permeability differences over wide areas of the landscape.