Salt and the Australian landscape
An already salty land
Australia is an ancient and flat continent that has been stable through enormous periods of geological time. Over the millennia, its land surfaces and rocks have eroded, mobilised and accumulated sediments and salts (Beckmann 1983, Holmes 1971, Isbell et al. 1983, Simpson & Herczeg 1994). Some of the salts in this landscape are released from weathering rocks (particularly marine sediments), but most are carried from the surrounding oceans in rain to be deposited in the soils, surface water and groundwater.
Salt stores have developed because there is little capacity to drain the continent of salt and water. Many clues to the saltiness of Australian landscapes can be found in accounts of the Australian bush from early European settlers and explorers. For example, Wood, a railway engineer from Western Australia, documented dryland salinity in south-west Western Australia and on the Eyre Peninsula (Wood 1924).
Salt is distributed widely across the semi-arid and arid landscapes of Australia. It occurs in patchy, complex patterns that reflect remnant features of the climate and geological events that formed the continent. These salt stores stretch in a huge arc from northern Australia, south by the Great Dividing Range, then broadening and sweeping south-west across the Murray Darling Basin to take in the Riverina and Mallee regions of New South Wales, Victoria and South Australia. In Western Australia, massive amounts of salt are stored in an arc that sweeps south and east across the semi-arid and arid landscapes of south-western Australia (Holmes 1971).
Figure 14.Landscape in equilibrium: 'water in' equals 'water out'.
Figure 15.Landscape out of equilibrium: `water in' is greater than `water out'.
Recharge and rising salts
Australia's natural salinity has been exacerbated by changes in land use since European settlement. Native vegetation has been replaced with crops and pastures with shallower roots and different seasonal growth patterns, affecting the rate and amount of water use in the landscape. Water `leaking' beneath the root zone and entering internal drainage and groundwater systems (known as `recharge') has increased so that it now exceeds the capacity of the system to discharge additional water to rivers and streams. Since more water is entering the system than is leaving it, the watertable rises (Figures 14 & 15) bringing dissolved salts with it.
Deep drainage beyond the root zone does not always end up in groundwater; rather it may move laterally through the soils into surface streams. Recharge into groundwater systems can also occur from the base and banks of the streams.
Increase in recharge beneath the root zone connects salt stored in the landscape to land surfaces, and intersects with rivers and streams causing dryland salinity on land and increased salt loads in rivers. The amount of recharge into a groundwater system depends on the climate; geology and topography; depth, water storage capacity and permeability of soils, and subsoil; and land use.
Under Australia's cropping and pasture land use patterns, excess recharge will percolate into the groundwater system at a higher rate (up to ten times) than under the natural native vegetation.
The initial recharge response may occur over relatively short time scales (30 to 50 years) or over much longer time scales (upwards of hundreds of years).
Salinisation can occur in situations controlled by local processes such as shallow groundwater on a hillslope stretching over less than a kilometre, where seepage zones develop as the slope flattens near the stream. Or salinisation can occur in extensive situations where processes operate over large areas such as regional groundwater basins stretching over hundreds of kilometres, where the salt emerges on the lower parts of the basin and the floodplains.
Changes in land use and land management practices that restore water balance provide the main opportunities for intervention and remediation.
A continuing and increasing problem?
Australia has a continuing and increasing dryland salinity problem because:
- it takes time to implement the scale of land use changes necessary to alter the water balance;
- in some regions farmers do not have the practical and viable options to implement the recommended management options; and
- even where management options are implemented, there are time lags before groundwater systems show responses to the changes.
Water balance
As the groundwater system fills and eventually reaches a new equilibrium, the amount of water entering the landscape as recharge and the amount of water leaving as discharge is balanced. However there is a time lag between when changes in land use or improvement in water balance occurs and evidence of a response. It will take decades to reverse the water rise in most groundwater systems (Figure 16).
Re-establishing the water balance requires farming systems with similar water use to that of deep-rooted native vegetation. Designing and implementing such farming systems is a major challenge.
Figure 16.Time response characteristics of the groundwater flow systems.
Recharge processes are generally faster than discharge processes. If it takes 30 to 50 years for our fastest groundwater system to fill with water, then it is reasonable to expect that it might take at least 30 to 50 years for it to empty back to where it was. If the system takes 100 years or more to fill, we can again expect at least a similar amount of time to establish the original equilibrium. This is an important issue for management as the degree of recharge reduction and the time taken have important consequences on land use options during any adjustment period, and the degree of change sought. Beneficial effects of land use options may well occur before the system has returned to an equilibrium.
Salt balance
As more water moves through an aquifer, more salt is mobilised. Very long periods of time are needed for catchment salt stores to be reduced to the point where the amount entering the system equals the amount leaving the system, that is, to achieve a salt balance. The net amount of salt that exits a catchment via stream flow indicates the time it will take for the catchment to flush its store of salt, when compared with the total mass of salt stored in that catchment. In some of the more responsive groundwater flow systems, the net output of salt may take about 150 years to flush from the system. In larger catchments (e.g. the Murray groundwater basin), it may take as much as 15 000 years. This means that although management may lower the watertable and allow productive use of land, there may be ongoing salt inflow to streams via groundwater.
This makes managing stream salinity very difficult. It is very important to prevent the interception of groundwater with salt stores in regions where we still have this opportunity.
The reality
The substantial lag times for catchments to come back into water balance and change salt mobilisation mean that it is inevitable that dryland salinity will be a feature of many Australian landscapes for some time. This is true even with widespread adoption of innovative land uses that manage to turn off the recharge tap and re-establish water balance. Ultimately the decisions on the measures to be taken will be influenced by the value of the threatened assets, the capacity to manipulate the environmental processes, the economic feasibility and social acceptance of the proposed actions.
Table of Contents for the Australian Dryland Salinity Assessment 2000
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