The southwest coastal area has a Mediterranean climate 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. Thanks to the offshore Leeuwin Current the area numbers in the top six regions for marine biodiversity, containing the most southerly coral reefs in the world.
Average annual rainfall varies from 300 millimetres (12 in) at the edge of the Wheat belt region to 1,400 millimetres (55 in) 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. A major reduction in winter rainfall has been observed since the mid-1970s, with a greater number of extreme rainfall events in the summer months.
The central four-fifths of the state is semiarid or desert and is lightly inhabited with the only significant activity being mining. Annual rainfall averages 200–250 millimetres (8–10 in), most of which occurs in sporadic torrential falls related to cyclone events in summer months.
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 1,500 millimetres (20–60 in), but there is a very long almost rainless season from April to November. Eighty-five percent of the state’s runoff occurs in the Kimberley, but because it occurs in violent floods and because of the insurmountable poverty of the generally shallow soils, the only development has taken place along the Ord River.
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