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Savanna Explorer > Arnhem Land > Landscapes and Climate

Landscapes and climate

By John Woinarski, Parks & Wildlife Commission of the NT

The Arnhem Land region shares many of the biological features of the Darwin-Kakadu region. The major blip on the relatively flat landscape of the north of the Northern Territory is the sandstone plateau of western Arnhem Land, which is bissected by these two regions. The biota of this plateau and escarpment has been relatively well documented in its western half (i.e. within Kakadu), but is essentially the same across the Arnhem Land border up to about the headwaters of the Mann River. Further east, into central Arnhem Land, the headwaters are far less rugged, and the sandstone biota accordingly much diminished.

Open forests

The eucalypt open forests (typically dominated by Darwin stringybark and Darwin woollybutt, Eucalyptus tetrodonta and E. miniata respectively) which form the dominant landscape element of the lowlands in the Darwin-Kakadu region also dominate extensive areas of Arnhem Land. There are some differences in floristic composition, probably mainly related to differences in soil structure (most notably with large areas of Arnhem Land forests being on either very sandy or lateritic soils) and/or fire management.

This broad vegetation map shows the similarity of vegetation in the Arnhem and Darwin/Kakadu regions divided by the sandstone country

Sorghum understorey

Annual sorghum is a far less dominant component of the understorey of these forests in Arnhem Land than in the Darwin-Kakadu region. Cypress pine Callitris intratropica is generally more prevalent, with typically better recruitment than in other areas of the Top End. Many of the animals associated with these eucalypt forests such as the brushtail possum Trichosurus vulpecula , northern brown bandicoot Isoodon macrourus , agile wallaby Macropus agilis , delicate mouse Pseudomys delicatulus , red-collared lorikeet Trichoglossus haematodus , brown honeyeater Lichmera indistincta , silver-crowned friarbird Philemon argenticeps , weebill Smicrornis brevirostris and white-bellied cuckoo-shrike Coracina papuensis -are widespread in Arnhem Land. (Click on the species name to see a list of research findings).

In contrast, some species common in eucalypt forests elsewhere in the Top End are either comparatively rare or absent from most of Arnhem Land: these include many of the finches, some raptors and the fawn antechinus Antechinus bellus.

Rivers and floodplains

East of the East Alligator River, most of the rivers of Arnhem Land are relatively small, and the floodplains relatively restricted. The Goyder/Glyde system is the most distinctive, including the vast Arafura Swamp, the most extensive melaleuca wetland in Australia. This is an important breeding site for magpie geese and other waterfowl, and the swampland and adjacent areas contain many highly restricted plant species.

Coastal areas

Coastal areas of Arnhem Land include some of the best developed sandsheet and sand-dune formations in northern Australia, especially at Cape Arnhem Peninsula and on Groote Eylandt. These support some distinctive heathlands, and some localised animal species, including the Northern Hopping-Mouse Notomys aquilo and the burrowing skinks Lerista stylis and L. carpentariae . Mangroves are also well-developed along much of the shoreline of Arnhem Land, and support a rich associated biota including the False Water-rat Xeromys myoides , Mangrove Monitor Varanus indicus , Chestnut Rail Eulabeornis castaneoventris and Mangrove Golden Whistler Pachycephala melanura .

Tropical climate: coastal factors

The Arnhem region, located in the Top End of the Northern Territory, encompasses a long stretch of the northern coastline as well as a large section of the higher plateau area. Its tropical climate, influenced by coastal factors, is characterised by hot, wet, humid summers and mild, drier winters.

Wet season

The summer months extend for a significantly longer period than in southern savanna regions where the wet may last only a few weeks. The north-west monsoons deliver much of the 800 mm to 1600 mm of the area's annual median rainfall which comes from occasional tropical cyclonic activity, tropical depressions or scattered thunderstorms. There is little variability in rainfall each year and the highest falls occur along the northern coast, diminishing to the south and inland. Thunderstorms are most prevalent where the unstable, moist, north-west winds reach the coast and may take place on more than 60 days each year reducing to less than 50 days in a south-easterly direction across the region. Onshore easterly winds blowing over the western coastline also bring rain during the wet. The chance of drought occurring is moderate to low as rainfall variability is also low.

Average termperature

The generally cloudy days of summer produce an average maximum temperature of around 33ºC. Sunny days combined with humidity in excess of 80 per cent contribute to the region's 50–200 days of climate discomfort which are fewest around the Gove Peninsula and increase in a south-westerly direction.

Dry season

After the end of the monsoons the winds tend to come from the east-north-east till the end of May before they settle into prevailing south-east trade winds. During the dry winters minimum temperatures range between 15 and 21ºC in July and are coolest inland on cloudless nights. Humidity at this time drops by only 10–20% with rain falling only occasionally. Moist conditions can linger in this region due to an onshore, easterly airflow from across the Gulf of Carpentaria that intersperses with the drier south-easterly trade winds.