Characterising the Specrtal Signal of Discharge Zones at the Margin of the Great Artesian Basin

Diffuse discharge along the southwest margin of the Great Artesian Basin is thought to be an important, but currently poorly understood, component of the Basin’s water budget. Remote sensing data are being investigated as a tool for mapping the salt and soil moisture variations associated with diffuse discharge zones, in order to assist in scaling up of field based discharge measurements to regional diffuse discharge estimates. The discharge zones have been segregated into four conceptual discharge scenarios which relate subsurface processes and variation in discharge rates, with expected observable surface characteristics. A handheld soil moisture probe (“Hydraprobe”) was used to collect surface soil moisture and conductivity data from the top 5 cm soil layer to ground-truth a number of processing methods applied to the satellite data. The Hydraprobe data were analysed to investigate the relationship between soil moisture and salinity at the ground surface. Hydraprobe data showed areas non-saline areas exhibited a uniformly low soil moisture content, consistent with the conceptual model of the discharge zones. Average field soil moisture and salinity data generally correlated well with landforms. Landsat TIR data showed a correlation with field data over the discharge area. This response was inconsistent between satellite images and it is presently unclear what the response in TIR data relates to. The Landsat Band7/Band1 salt ratio showed the strongest correlation to landforms characterised by salt at the surface and results suggest the salt ratio will be useful in distinguishing discharge zones. Introduction The Great Artesian Basin (GAB) is Australia’s largest groundwater system, and one of the largest in the world, underlying approximately 22% of the continent (Habermehl, 1980). The GAB is the only reliable source of water for agriculture and industry in parts of arid and semi arid central and eastern Australia. Discharge from the basin is thought to be concentrated along its south west margin, where the aquifer is either outcropping or brought close to the surface by folding and faulting. In these areas higher rates of water are lost from the GAB than in deeper aquifer settings. In shallow settings the vertical leakage feeds local water tables, and is then lost to evaporation.