Irrigation-induced salinity destroys thousands of hectares of agricultural land in the Murray-Darling Basin every year. Hyperspectral imagery is being evaluated as a tool of mapping the indicators of insipient salinity so that remediation measures can be implemented. Hyperspectral imagery allows for the various materials that are present with each pixel to be individually mapped.

 

However, a major dilema is whether to use spectra measured in the field or to derive the spectra of "image-endmembers" from the data itself.

 

The test site for this work is at the Tragowel Plains Research and Development Block
(R and D Block), a facility of the Victorian Department of Agriculture.

The site is located near the village of Pyramid Hill and the town of Kerang and just a few kilometers from the New South Wales - Victorian border.

 

In the worst-affected areas succulents such as Samphire and Sea Blite are the only plants to survive, soluble salts are precipitated at the surface, primary clay minerals are destroyed and the soil becomes susceptible to erosion.

The most blantant sign of an outcropping water table is the formation of precipitated salt minerals at salt "scalds".

The salt at such features is ephemeral in nature and both extent and composition vary with the recent climate.

Salt scald spectra acquired in 1993 clearly showed the presence of gypsum (Bennett, 1998, Taylor et al, 1994) but spectra acquired in May 1999 have features characteristic of damp halite with minor amounts of a hydrated sulphate minerals such as bassanite being indicated by hydrate-related absorption features at 980, 1170 and 1740 nm.

Salt scald spectra extracted from images acquired two weeks earlier suggest the presence of polyhalite or eugsterite but these minerals were not conclusively confirmed by XRD analysis. XRD analysis of salt scald material shows the presence of polyhalite K2Ca2Mg(SO4)4.2H20, halite NaCl, Bloedite Na2Mg(SO4)2, gypsum CaSO4.2H2O and hydroglauberite, Na2Ca(SO4)2.H2O. Salt scald spectra and library spectra for polyhalite, gypsum and an other hydrated calcium sulphate, bassanite 2CaSO4.H2O are illustrated. No spectra for hydroglauberite is available to the authors at present.

 

It seems likely that the evaporite minerals present in the scalds are mostly sodium chloride (halite) with minor amounts of various sodium, potassium, calcium and magnesium sulphates. The exact composition and hydration state of these are likely to vary according to watertable height and recent climate.