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Western
Australia is Australia's largest state in area, covering
the western third of the mainland, and is bordered by
South Australia and the Northern Territory. It is, after
the Sakha Republic in Russia, the second largest subnational
entity (statoid) in the world. The capital city, Perth,
is one of the most isolated cities of its size on the
planet, being closer to Jakarta (3,007 km) than to Sydney
(3,284 km).
West Australians are often colloquially
referred to as sandgropers due to the sandy soils around
Perth
The bulk of Western Australia consists
of the extremely old Yilgarn craton and Pilbara craton
which merged with the Deccan of India, Madagascar and
the Karoo and Zimbabwe cratons of South Africa, in the
Archean Eon to form Ur, one of the oldest Supercontinents
on Earth (3,200-3,000 million years ago). Because the
only mountain-building since then has been of the Stirling
Range with the rifting from Antarctica, the land is extremely
eroded and ancient, with no part of the State today above
1245 metres AHD (at Mount Meharry in the Hamersley Range
of the Pilbara region). Most parts of the State form a
low plateau with an average elevation of about 400 metres
(1200 feet), very low relief, and no surface runoff. This
descends relatively sharply to the coastal plains, in
some cases forming a sharp escarpment (as with the Darling
Range/Darling Scarp near Perth).
The extreme age of the landscape has
meant that the soils are remarkably infertile and frequently
laterised. Even soils derived from granitic bedrock contain
an order of magnitude less available phosphorus and only
half as much nitrogen as soils in comparable climates
in other continents. Soils derived from extensive sandplains
or ironstone are even less fertile, being even more devoid
of soluble phosphate and also deficient in zinc, copper,
molybdenum and sometimes potassium and calcium.
The infertility of most of the soils
has required heavy inputs of chemical fertilisers, particularly
superphosphate, insecticides and herbicides, which, with
the ensuing damage to invertebrate and bacterial populations,
and compaction of soils through heavy machinery and hoved
mammals has done great damage to the fragile soils. The
massive clearing of the land has not only damaged habitats
for native flora and fauna, making the South West region
of the state that with the greatest percentage of flora
and fauna rare, threatened or endangered in Australia,
and one of the biodiversity "hot spots" in the
world, it has also led to major problems with dryland
salinity and the loss of fresh water. (See Southwest Australia)
The southwest coastal area is relatively
temperate and was originally heavily forested, including
large stands of the karri, one of the tallest trees in
the world. This agricultural region of Western Australia
is in the top nine terrestrial habitats for terrestrial
biodiversity with a higher proportion of endemic species
than most other equivalent regions, and thanks to the
offshore Leeuwin Current, numbers in the top six regions
for marine biodiversity, containing the most southerly
coral reefs in the world. Annual rainfall varies from
300 millimetres (12 inches) at the edge of the wheatbelt
to 1400 millimetres (55 inches) in the wettest areas near
Northcliffe, but in the months of November to March evaporation
exceeds rainfall and it is generally very dry. Plants
must be adapted to this as well as the extreme poverty
of all soils. Climate change is producing a major reduction
in rainfall, with a greater number of rainfall events
in the summer months. (see Southwest corner of Western
Australia)
The central four-fifths of the State
is semi-arid or desert, and is lightly inhabited with
the only significant activity being mining. Annual rainfall
here averages about 200 to 250 millimetres (8 to 10 inches)
but is very erratic because most of it is produced in
torrential falls by cyclones in the summer months that
are often unreliable.
An exception to this is the northern
tropical regions. The Kimberley has an extremely hot monsoonal
climate with average annual rainfall ranging from 500
to 1500 millimetres (20 to 60 inches), but there is a
very long almost rainless season from April to November.
Almost all (85%) of the State's runoff occurs in the Kimberley,
but because it occurs in violent floods and the insurmountable
poverty of the generally shallow soils, the only development
has taken place along the Ord River with an ambitious
scheme that has only recently begun to pay off.
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