Western Pastoral (Province 7)
This province is bounded by the Great Australian Bight and Province 4 in the south, the State border in the west, the Flinders Ranges (Province 6) in the east and the silcrete tablelands of the breakaway country of Province 8 in the north.
It has a warm to hot desert climate with short, cool to cold winters and low, unreliable rainfall showing no distinct seasonal pattern. With the exception of the area near the Gawler Ranges in the south-east, there is little variation in climate within the province due to lack of major orographic controls and to limited maritime influences. The highest mean annual rainfall of from 250 mm to 350 mm occurs in the south and south-east whilst the remainder of the province has a mean annual rainfall of 200 mm or less, especially in the north where it declines to less than 150 mm. Temperatures vary from cool to cold in winter to warm to hot in summer, Diurnal and seasonal ranges are high and increase towards the north. Mean annual evaporation increases from about 2400 mm in the south to over 3500 mm in the north-east. The dominant vegetation types in the province are mulga and myall woodlands, open mallee scrub and low saltbush and bluebush shrublands. Mallee occurs mainly on the dunefields of the Great Victoria Desert, but extends south to the Bight and south-east into the Gawler Ranges. Bluebush-saltbush vegetation is dominant on the Nullarbor Plain, whilst saltbush dominates the vegetation on the Arcoona and Woomera plateaux and the uplands west of Port Augusta. Open woodlands occur in the areas not covered by mallee, bluebush or saltbush, and are found in a variety of topographic situations, from hills and escarpment slopes to watercourses and depressions. Woodland dominated by myall occurs in the Gawler Ranges and nearby, whilst north of the trans-continental railway mulga becomes more prominent.
The eastern half of the province, where the soils are generally deep, more finely textured and therefore have greater water-holding capacity, is almost entirely under pastoral leases. Extensive livestock grazing is the major land use and grazing pressure has changed the original composition of the vegetation to various degrees. There are also a few leases along the southern edge of the Nullarbor, and, on average, the equivalent of approximately 500 000 sheep are grazed in the province. Grazing property size and livestock densities tend to vary with rainfall. The smallest properties, of less than 1000 km2, occur in the south-east and east in association with livestock densities of one sheep unit per 5-10 ha; whilst the largest, in the north, average almost 3000 km2 and carry one sheep unit per 15-20 ha. (NB. The South Australian Pastoral Board uses a conversion of 5 sheep per livestock unit compared to the 8 sheep per unit in the agricultural districts.)The western half of the province which is dominated by shallow soils with low water-holding capacity, has little pastoral industry and its settlement is confined to servicing the transcontinental rail and road links. It appears unlikely that pastoral leases will be extended into either the high country where the deep loose sands make access difficult, or the limestone plains. There are conservation reserves in Regions 5 and 6.
Six environmental regions have been recognized in this province:
Gawler Uplands (Environmental Region 7.1)
This region is the Gawler and Simmens Uplands which are low ranges with summits representing old land surfaces. The predominant landforms are low, flat-topped to broadly rounded hills extending without a notable break in slope into alluvial plains.
Torrens Depression (Environmental Region 7.2)
This region is the Lake Torrens depression. It consists mainly of a vast salt lake and associated gypsum dunes. There is also a sandy alluvial plain with numerous partly connected lakes and pans and an extensive field of longitudinal red sand dunes, mainly fixed by vegetation.
Central Salt Lakes and Plateaux (Environmental Region 7.3)
This region consists of the Arcoona and Woomera plateaux on flat sandstones and quartzites with a surface cover of gibbers, and the depositional plains to the south which are mostly covered with aeolian sand sheets and dunes. The large salt lakes, Gairdner, Everard, Harris, Macfarland and Island Lagoon, which are partly connected, are also part of this region.
Kingoonya Plains and Dunes (Environmental Region 7.4)
This region comprises the plains in the north-east of the province. They occupy a transitional position between quartzite plateaux to the south and the duricrusted tablelands of Province 8 to the north. They are largely depositional in origin with isolated rocky rises. Red sand dunes are common.
Great Victoria Desert (Environmental Region 7.5)
This region is the south-easterly extension of the Great Victoria Desert, most of which is in Province 8. It consists of parallel dune ridges over a vast erosional plain on a variety of rock types. Low outcrops of granite or volcanics form isolated, inselbergs or clusters of tors within the dune system. The dunes consists largely or red sand derived from weathered granitic rocks of the Western Australian shield, but there is a gradual colour change towards the south where white sands derived from the coast predominate. The system exhibits great regularity: individual dunes are 15-30 m high and may extend without a break for over 100 km.
Nullarbor (Environmental Region 7.6)
This region consists largely of the Nullarbor Plain. It is a vast, featureless structural plain. Erosional processes have modified the surface form little since its emergence from the sea in the late Tertiary. However, subsurface erosion has led to the development of dolines and caves systems. A spectacular cliffed coastline terminates the plain in the south. East of the Nullarbor Plain is a more undulating plain on calcarenite, with sandy beaches and cliffed headlands along its coastal margin.
