Northern Australia’s fascinating wetness

Since at least the middle of the twentieth century, Australians have enviously eyed the wetness of our northern tropical regions — with some degree of frustration! Recent years are no different, with both the Federal Government and the Opposition looking at improving national water and food security via use of northern land and water resources.

Unlike the Murray-Darling Basin (MDB) drainage division, there are abundant water resources in the two huge drainage divisions that span the north of Western Australia, Northern Territory and Queensland.  Consider this: whereas the average annual outflow from the MDB is around 12,000 GL, the two northernmost drainage divisions (known as the ‘Timor Sea’ and ‘Gulf of Carpentaria’ divisions) average annual outflows of about 90,000 GL each. Those regions have mean annual rainfall of 800-900 mm across their range, and a lot more in sub-catchments closer to the coast, so you’d expect that the north would be an ideal area for food production.

Yet most irrigated agriculture is practised within the MDB, not the north. Other than the, only partial successful, Ord River irrigation scheme and parts or the Reef coast of northern Queensland, food production in the north is largely via pastoral grazing, focusing (e.g. in the Daly River catchment) on live-export of cattle, and supplies of feedstuffs to support that industry.

There are a number of reasons, and they need to be carefully evaluated before Australia goes down the track of developing new dams and irrigation lands in that region.

First, the apparently plentiful rain, which falls mostly (94–95%) in the summer 2–3 months only, and mainly near the coast, is relatively unpredictable. It can be as little as 300 mm or as much as 1800 mm annually and is counterbalanced by annual average evaporation of 1900–2000 mm.

An excellent reason to build dams to hold the flow, you might say, but unless there is a reason to use the water, there is little reason to build a dam, particularly considering the ecological issues Australia has experienced from damming rivers in the southern regions.

A second reason for little existing northern agriculture is the poor soils. In general they are unsuitable for agriculture (as opposed to pastoral grazing). They have very poor water-holding capability or are highly erodible, as has been identified by CSIRO’s surveys for the very purpose of assessing the north’s productive potential.

Before we invest in developing food production in the north, surely we will look at the lessons of the past in the south? One of those has taught us to regard potential salinity with caution. The salinity hazard of the northern regions may be rated in the lower part of the scale, but there is still much salt naturally stored in the soil profile, and CSIRO reports warn that significant rises in the water-table in those soils are liable to recreate the salinisation headaches we have seen in the WA and the MDB.

Ecologically, the north is a kind of Aladdin’s Cave of biodiversity, both in inland rivers and in the coastal waters fed by large rivers. There are numerous wetlands – the Directory of Important Wetlands in Australia lists 106 wetlands in northern Australia, 8 of which are Ramsar Wetlands of International Importance (the Ord River floodplain and several in Kakadu National Park, for instance). Another report says there are 1243 wetlands in the catchment of the Daly River alone.

The rivers also are rich in species, with the Daly River supporting 48 species of fish and more turtle species than any other river in Australia. Floods in the wet season drive fish spawning in the coastal waters, of Barramundi for instance which are harvested at a rate of hundreds of tonnes per year, both by recreational fishers and by Indigenous peoples.  Harvesting that water upstream for irrigation  may drive economic production in-land, but will almost certainly impact on the economy of coastal fisheries.  On purely economic grounds, trading off one food source against another may not be a wise choice, let alone the potential social, cultural and ecological impacts.

Clearly there are sound economic reasons — in fisheries and tourism for instance — as well as important Indigenous cultural reasons and ecological reasons to bring all available integrated thinking to bear before any development of the north’s water resources is agreed.  In coming decades, it may be that Australia must develop, at least some of, the land and water resources of the North – both to meet its own food needs and those of the rest of the world.

But let’s approach this challenge with our eyes open, and our minds fully engaged.  It seems perfectly possible that otherwise Australia may gain little but lose immeasurably.

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