Australia has mainly an arid or semiarid climate. Only 22% of the country has rainfall in excess of 600 mm per annum, confined to coastal areas to the north, east, southeast, and far southwest of the country (http://www.bom. gov.au/climate/ahead/soirain.shtml). Australia also has high year-to-year and decade-to-decade variation in rainfall due, in part, to the influence of the El Niño/Southern Oscillation (ENSO) phenomenon (http://www.bom .gov.au/climate/ahead/soirain.shtml). The Interdecadal Pacific Oscillation (IPO) also contributes to the rainfall variability at annual and decadal scales and modulates ENSO impacts on rainfall (Power et al., 1999).
The current geographic boundaries of agricultural production (figure 29.1) were reached in the late 19th century, and the entire agricultural region has experienced drought, in some form, over the past 100 years. Protracted dry periods occurred during the period from late 1890s to 1902 in eastern Australia, during the mid to late 1920s and 1930s over most of the continent, during the 1940s in eastern Australia, during the 1960s over central and eastern Australia, and during 1991-95 in parts of central and northeastern Australia. During these low rainfall periods, not every year was extremely dry, but rainfall in most years was below the long-term median, and there were often runs of severe drought years. Many, but not all, droughts were associated with El Niño events (http://www.bom.gov.au/ climate/ahead/soirain.shtml; chapter 3). Droughts have been more prevalent when the IPO was positive, that is, from 1896 to 1909, from 1922 to 1945, and from 1979 to 1998. During the periods when the IPO was negative, La Niña events have tended to be associated with wet conditions. During these times, Australia has largely been drought free.
Despite Australia's arid or semiarid climate, agriculture (including cropping and grazing) is practiced on 60% of Australia's land area of approximately 463 million ha (figure 29.1). Extensive grazing by beef cattle and sheep occupies approximately 90% of the agricultural land. The remaining agricultural land is equally distributed between intensively grazed, sown pastures and crops. Wheat is Australia's major crop, having the broadest geographic range (figure 29.1), and it contributes an average of 25% of total agricultural production. Only a small proportion (0.5%) of Australia's sown pastures and crops are irrigated, but these regions are nonetheless important in terms of their contribution to the value of agricultural production.
Despite the high variation in annual rainfall, cropping and grazing are geared toward years with normal rainfall, with some contingency for drought years as discussed later in this chapter. For this reason, the entire agricultural region (figure 29.1) is drought prone. In some cases, expectations of climate are biased by years with above-average rainfall, leaving a particular enterprise even more drought prone. Sensitivity to drought is therefore dependent more on an individual's experience, expectations, and management than on geographic location.
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