Australia: The Land Where Time Began
The western Australian plateau has chains of salt lakes occurring along palaeodrainage channels from the Tertiary that haven't flowed since that time. Groundwater discharge has resulted in a chain of large playa lakes along the centre of the Amadeus Basin. They stretch from Lake Hopkins in Western Australia to the Finke River in the Northern Territory via Lake Neale, Lake Amadeus and a number of smaller lakes, along the inward flowing palaeodrainage systems.
The catchment in this valley covers 90 000 km2, from the south, the Petermann and Musgrave Ranges, and from the north, the George Gill Range. All the water from these ranges flood-out and soak into the barren sandy soil and evaporate before reaching any lakes. The extremely high evaporation rate means that any water reaching the lakes evaporates quickly, so water can't accumulate. Population centres such as Alice Springs and the Uluru tourist complex depend on subterranean aquifers. Because the aquifers are not recharged, or at best very little recharge takes place, because the evaporation rate in the area of the catchment is 3000 mm/year, and the rainfall is only 250 mm/year. The result is that the all the water drawn to supply the inhabitants of the area is palaeowater, water that accumulated under much wetter climatic conditions.
As well as the finite nature of the underground water, it also contains a number of chemicals, the exact composition varying with the rock types that it drawn fro, it contains high salt and nitrate levels. The nitrates are produced by the soil micro-organisms such as cyanobacteria that are pat of the 'biological duricrust' in the soil profile.
A characteristic of Lake Frome that has been discovered is that huge amounts of water evaporates from the lake, even when it appears to be a dry salt pan. The water table comes very close to the surface and it has been found that 460 million cubic metres of water from the water table evaporates from ground surface each year. It is believed that the same may occur at other salt lakes at which the water table is a short distance below the surface.
In all arid and semi-arid basins, with rainfall less than 250 mm/year, the high evaporation rate produces dryland salination of the soils. In 1982 it was estimated that 4.3 % of the Australian continent has been salinised. Of this area, 13 % - 4.2 million Ha - has been caused by the land use practices during the first 200 years of European occupation. The natural soil salinity is called primary salinity, and has resulted from the combination of geology and the drying climate.
The mechanisms of the natural hydrological cycles have not yet been completely understood, and it is possible changes in the hydrological cycles may be responsible for increasing areas of salinity. Groundwater responses can lag well behind climate fluctuations, it has been suggested that dryland salinisation may still be increasing by natural processes, but they are being exacerbated by human activity.
The Murray Basin Mallee, and small salt lakes, saltpans and scalds are examples of primary salinity resulting from inward-draining groundwater. The only outflow of the Murray Basin is via the Murray River to the sea. Beginning about 500 000 BP, when Lake Bungunnia dried up, arid conditions changed the region into a groundwater discharge zone. The clearing of Mallee land has caused secondary salinisation.
There are 2 types of secondary salinisation in Australia, dryland salinisation and irrigation-related salinisation. In areas with an arid or semi-arid climate, and undulating country with a shallow water table - much of Australia - are prone to secondary salinisation. The water balance in these areas is fragile - recharge versus discharge - is controlled by the amount of rainfall, amount of surface and sub-surface water, and of course the rate of evaporation and transpiration. Under these conditions the amount of vegetation cover is very significant, the clearing of trees often leading to degradation of the environment.
Subsurface water evaporates much more efficiently from bare soil, greatly concentrating salts in the subsurface water and the soil. The rate of this process is greatly increased after a wet season, the rising groundwater picking up salts from the subsoils and weathering rocks, concentrating them in the lower parts of the landscape.
A major cause of secondary salinisation is the agricultural practices used for the past 200 years. Clearing of deep-rooted native trees for the growing of shallow-rooted crops has allowed the water table to rise towards the surface in many areas bringing with it salts in aquifers and or salt deposited in layers of the soil profile in times of inundation of the land by the sea. The result has been the salinisation of once-productive agricultural land and increasingly salt levels in the rivers draining these agricultural lands. The introduction of large-scale irrigation in many areas has had a dramatic effect on the salinity problem in the affected areas, raising the salt levels in the soil and associated rivers at an increasing rate. The situation has been made worse by modifying river systems to cater for the increased water requirements of the irrigated land. Rivers that used to take any salt entering them to the sea have had their flow rates drastically reduced by diverting water for irrigation and damming them, that reduces the flow rates still further. A side effect of dams is that large areas of standing water evaporate vast amounts of water from their greatly increased surface area, especially in the hot Australian summer.
The Murray Basin is a case in point. It has been estimated that beneath this basin is 100 billion tons of salt. The Murray Basin is not the only place with a salinity problem. Most agricultural areas of Australia are affected to some degree, even the highlands and Riverine Plain. All irrigation areas are affected. The Murray Basin is very susceptible because of its topography. It is a shallow basin, with sedimentary layers that become saturated, and its drainage system is inward-flowing, away from the coast, for much of its length, concentrating the salts even further as it flows through country of varying degrees of aridity where evaporation is very high. The only external drainage of the system is the main Murray channel from Morgan to the sea.
The Murray-Darling Basin covers a 7th of the continent. It supports 1/4 of Australia's cattle, beef and dairy, 1/2 of the sheep and 1/2 the cropland of the whole country. The cause of a lot of the problem is that the Basin contains 3/4 of the country's irrigated crops. At the current rate of rise of the water table under this area it has been estimated that within 50 years it will be totally useless for agriculture because of salinisation.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|