Salinity - Management - Western Australia
Western Australia
Introduction
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 (see figure below).
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.
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.
Northern Western Australia
Treating the cause of salinity through recharge reduction may be effective in reversing salinisation in only a few responsive groundwater systems. Once the salinisation process is under way it is extremely difficult to slow, halt or reverse in order to protect water and land resources. Prevention is a far better investment than any attempt at control or management.
Northern Australia presents opportunities to avoid the dryland salinity problems of temperate Australia. Broad-scale clearing without recognition of salt stores and the resulting change in water balance is a recipe for problems, whether it is in 20 or 100 years. Wise management now to protect the landscape and prevent dryland salinity will prove far more cost-effective than any attempts to solve the problem once it occurs.
While salinity analysis has focused on southern Australia, sound scientific evidence (Bui et al. 1996, Williams et al. 1997, Bui 2000, Gordon et al. 2000, Gunn1967, Shaw et al. 1994) shows that all the factors that contribute to salinity hazard also exist over large areas of the semi-arid zones of northern Australia. Two factors that must be present for a salinity hazard to exist after clearing or change in vegetation cover are :
- presence of stored salt in the soil, regolith or groundwater systems, and
- an increase in the water draining beneath the root zone following tree clearing or vegetation change.
What is the scale of the groundwater systems and how can they be managed?
Groundwater trends
Groundwater trends are dominated by rising or stable (see figure below). No systems have significant falling trends. Rising trends occur over the majority of agricultural areas with only a few exceptions. Rates of rise are highly variable and have not been presented but range from less than 5cm/year to greater than 50cm/year.
Areas with 'No trend' have watertables that have reached the near surface, oscillate on a seasonal basis, or have deeper watertables that show no trend. No trend was recorded for several zones: in particular, zone 255 (Western Darling Zone), where the major land use in the zone is forest, some of the coastal systems and for an area to the north east of Esperance.
Where seasonal fluctuations occur in deeper systems and there is no long-term trend the area may have reached a local equilibrium.
Regional mapping of groundwater trends is shown in the figure below, and in the accompanying map sheet titled 'Groundwater trend for soil-landscape systems'.
More detailed information is available in the Western Australia Dryland Assessment 2000 Report
Groundwater trend for soil-landscape systems
Groundwater Flow Systems in Western Australia
The groundwater flow systems for Western Australia are present in the table below. Within the >300mm rainfall zone, 6,784,000 hectares in local, 630,000 hectares in intermediate and 1,359,000 hectares in regional flow systems are coincident with regions within which there are areas with a high risk of dryland salinity.
| Groundwater Flow System Type | Area (ha) at risk in 2050 | Percentage of total risk area (%) |
|---|---|---|
| Local and intermediate flow systems in deeply weathered rocks | 6,781,856 | 77 |
| Intermediate flow systems in sedimentary sequences in large valleys | 567,844 | 6 |
| Local flow systems in fractured or weathered rocks or colluvial fans | 1,975 | 0 |
| Intermediate flow systems in fractured rocks | 33,844 | 0 |
| Local flow systems in fine grained unconsolidated sediments | 219 | 0 |
| Regional flow systems in permeable alluvial sediments | 467,975 | 5 |
| Regional flow systems within fractured basaltic rocks and layered sedimentary rocks | 891,450 | 10 |
| Intermediate and local flow systems in fractured basaltic rocks and layered sedimentary rocks | 28,206 | 0 |
* Area within >300mm rainfall zone
Government responses to dryland salinity as at the year 2000
Government responses
In recognition of the magnitude of the salinity threat to agriculture, rural towns and the environment, the Western Australian Government released its first Salinity Action Plan in 1996 (Government of Western Australia 1996). Recently, the State Salinity Council reviewed the plan and developed a strategy that places greater emphasis on community-based programs. Goals of the strategy are:
- To reduce the rate of degradation of agricultural and public land, and where practical, recover, rehabilitate or manage salt-affected land.
- To protect and restore key water resources to ensure salinity levels are kept to a level that permits safe potable water supplies in perpetuity.
- To protect and restore high value wetlands and natural vegetation, and maintain natural (biological and physical) diversity within the region.
- To provide communities with the capacity to address salinity issues and to manage the changes brought about by salinity.
- To protect infrastructure affected by salinity.
The strategy gives priority to managing recharge and discharge, and ensuring a partnership approach between government, science and the community.
One of the major investments in salinity management in Western Australia is the Land Monitor Project. This project is a Natural Heritage Trust and Western Australian State Government initiative to map and monitor the extent of salinity through satellite imagery at the farm and catchment scale. The project aims to provide information about land condition-specifically salinity and the status of remnant vegetation-for the whole of the south-western agricultural region of Western Australia. It is a collaborative project involving Agriculture WA, CSIRO, Conservation and Land Management, Department of Land Administration, Waters and Rivers Commission, and the Department of Environmental Protection and Main Roads Western Australia.
Further information
- Western Australia Dryland Salinity Assessment 2000 report
- Australian Dryland Salinity Assessment 2000 report
- National Technical Overview Report of the State-based dryland salinity assessments
- Australian Groundwater Flow Systems Report
- Agriculture WA
- National Dryland Salinity Program
- National Action Plan for Salinity and Water Quality
Link to Map maker to make a map using this information.
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