Salinity hazard and risk mapping of point source salinisation using proximally sensed electromagnetic instruments

In many irrigated areas in the Murray-Darling Basin, New South Wales, Australia, point-source salinisation has occurred in isolated cases. In the lower Macquarie valley, near Trangie and Warren, the cause appears to be the mobilisation of salts from perched water tables and as a function of excessive deep drainage from water storages and conveyance channels. In order to understand these hazard factors and understand the risk of further salinisation, the spatial distribution of the various factors need to be mapped. Two methods have previously been employed and include the use of GIS hazard layer (e.g. geology, topography) analysis or airborne electromagnetic (EM) mapping. The former uses qualitative data which is often unreliable, whilst the latter has large start-up costs. In this study, various sources of proximally sensed data (i.e., EM38, EM34) are first used to develop stepwise-multiple linear regression (MLR) models to map the spatial distribution of vadose-zone (0-7 m) clay and salt stores. Using a salt-water balance model (SaLF), deep drainage (mm/year) is estimated. The model has an exponential form and uses ancillary data as predictors. The predicted clay, salt store and deep drainage hazard maps are further employed to develop a map of overall salinity risk using Boolean logic and indicator kriging. Results suggest that the areas exhibiting the largest conditional probability of salinity risk correspond to permeable soil types associated with the Trangie Cowal, Contemporary Macquarie and Backplain of Old Alluvium Pedoderm. It was also noted that the predicted salinity risk was consistent with where salinisation had occurred in both Trangie and Warren districts. The methodology indicated where a more strategic approach needs to be implemented when considering options to improve irrigation efficiencies and water delivery southeast of Warren and Trangie. It also showed where more detailed investigations could be undertaken to better understand the stratigraphic nature of the landscape and the mobilisation of the salts therein.