17.1.1  Geographic extent

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About 30 million hectares of mainland Australia are underlain by salt, including both arid and semi-arid rangelands plus arable lands with naturally occurring saline subsoil. As something of an indictment of management practices over the past century, at least four million hectares are suffering from human-induced (secondary) salinisation. This includes large scalded areas where overgrazing has led to erosion of topsoil and exposure of saline subsoil, plus about 1.2 million hectares subject to secondary salinisation where saline groundwater emerges as surface seeps on previously arable land. Irrigation salinity due to saline irrigation water and/or inadequate drainage adds another 500 000 ha to our national total.

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Figure 17.1 Secondary salinisation is a consequence of comprehensive land clearing, and in southwest Western Australia cleared areas have increased linearly between 1955 and 1975 with a subsequent decline in rate of expansion. By contrast, salt-affected areas have increased curvilinearly and with a faster rate of expansion subsequent to 1975. (Note the different scales: area cleared is in millions of hectares whereas area salt affected is in thousands of hectares) (A smoothed representation or data from George 1990)

Most dryland salinity is evident in wheat-growing and sheep-grazing zones of Western Australia and South Australia which occupy the 400–600 mm rainfall region. Here extensive land clearing with subsequent cropping and grazing has increased net accessions of rain to groundwater. Local hydrologic cycles have been disturbed to such an extent that rising water tables (commonly highly saline) have broken out on lower slopes to form saline seeps (hence ‘secondary salinisation’). Salt-sensitive plants soon die in these discharge zones, and if recharge areas are left uncorrected saline seeps expand upslope, leading to further land degradation. Secondary salin-isation is an insidious process and continues long after native vegetation is cleared. For example, land cleared for agriculture in southwest Western Australia has increased in area more or less linearly between 1950 and 1990 but at a decreasing rate of expansion (Figure 17.1). By contrast, the corresponding area of salt-affected land has increased curvilinearly, and with an increasing rate of expansion. Australia wide, secondary salinisation is spreading at a compounding rate of about 5% per year (SCAV 1982). By 2020, somewhere between 10% and 25% of previously arable land could be out of production.

Turning to a more geographically confined issue in eastern Australia, salinisation of irrigated land in the Murray–Darling basin derives from salt left behind by intrusions of seawater that commenced at least 65 million years ago. The Murray River now flows across sediments that are underlain by highly saline groundwater, and even prior to European settlement Murray River water was naturally saline at times of low flow. Extensive development of irrigation schemes in the Murray–Darling basin over the past 100 years or so has led to a dangerous rise in groundwater over large areas where capillary rise of salt in fine-textured soils can put irrigation cropping at risk. Regional projects for subsurface drainage and groundwater pumping schemes are restoring a new hydrologic balance within some parts of the Murray–Darling basin, so that long-term viability of irrigation cropping and grazing enterprises is now assured in such areas. Stable forms of agriculture will continue to call for salt-tolerant cultivars, while long-term horticulture necessitates salt-excluding rootstocks. Reclamation of degraded land is possible where saline groundwater can be safely disposed of, and salt-affected soils restored to an arable state by leaching out profile salt. Soil ameliorants such as gypsum are commonly applied as part of this leaching process to displace Na+ ions adsorbed by clays during salinisation.

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