Re-evaluating the margin of the South Australian grain belt in a changing climate

The drier inland edge of the Australian grain belt, where wheat cropping transitions into extensive grazing, is an important economic and ecological margin. This margin is determined primarily by climate rather than topography or soil type. In South Australia a line drawn by the Surveyor General George Goyder in 1865 between land that was reliable for wheat growth and land that was unreliable has become an icon of land use planning. In this study we compare Goyder's Line with maps of pre-European vegetation, climate variables such as temperature, rainfall, ratio of precipitation to evaporation (P:E ratio), current land use as measured by the edge of grain farming in 2008 and crop simulation on a transect. Although Goyder's Line is a reasonable representation of the margin of reliable cropping in some regions, it is internally inconsistent and care should be used if interpreting it as an isopleth of crop production risk. A better fit to the South Australian and Australian grain belt was the ratio between rainfall (P) during the growing season (April to October) and potential evaporation (E) of 0.26. The surprisingly close fit of this ratio with much of the Australian grain belt suggests a climatically determined hard edge to the cropping zone. The paper explores how the 0.26 P:E ratio may shift under projected changes in climate using 2 global climate models, one 'dry' and one 'wet'. We observe that the speed of change in km decade ^-1 will be greater where the edge of grain farming is on extensive plains than in those areas where it is topographically determined.