Salinity - Management - Tasmania
Tasmania
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.
What is the scale of the groundwater systems and how can they be managed?
The groundwater flow systems for Tasmania are present in the table below. Within the >300mm rainfall zone, 68,000 hectares in local, 24,000 hectares in iate and 700 hectares in regional flow systems are coincident with regions within which there are areas with a high hazard of dryland salinity.
| Groundwater Flow System Type | Area (ha) at risk in 2050 | Perecntage of total risk area (%) |
|---|---|---|
| Intermediate flow systems in sedimentary sequences in large valleys | 173 | 0 |
| Local flow systems in fractured or weathered rocks or colluvial fans | 29,703 | 32 |
| Intermediate flow systems in fractured rocks | 23,746 | 26 |
| Local flow systems in fine grained unconsolidated sediments | 116 | 0 |
| Regional flow systems in permeable alluvial sediments | 443 | 0 |
| Regional flow systems in fractured basaltic rocks and layered sedimentary rocks | 270 | 0 |
| Intermediate and local flow systems in fractured basaltic rocks and layered sedimentary rocks | 38,528 | 41 |
* Area within >300mm rainfall zone
Government responses to dryland salinity as at the year 2000
Government responses
As a result of the Audit-funded initiative to assess the extent and impacts of dryland salinity in Tasmania, the Environment/Resources Heads of Agencies Group has endorsed:
- the development of a state salinity management strategy; and
- for Tasmania to become a full member of the National Dryland Salinity Program.
Further information
- Tasmania 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
- Tasmania Department of Primary Industries, Water and Environment
- 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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