While all reasonable care has been taken in the preparation of the information contained in this document, the Chief Executive Officer of the Department of Agriculture and Food and its officers and the State of Western Australia assume no responsibility for any errors or omissions it may contain, regardless or caused by negligence or otherwise, or for any loss howsoever caused, arising out of any reliance on, or use or disclosure of, this information or any part thereof. Dr. Richard George from the Department of Agriculture and Food in Bunbury attended the first workshop and contributed valuable local insight into the watershed's groundwater processes and salinity risk. Arjen Ryder of the Albany Department of Agriculture and Food undertook the technical review of the report.
The report was edited by Bill Russell of the Department of Agriculture and Food in Bunbury. The project is an initiative of the South West Catchments Council, jointly funded by the Australian Government and the Government of Western Australia under the National Salinity and Water Quality Action Plan. The Department of Agriculture and Food, Western Australia (DAFWA) was commissioned by the South West Catchments Council (SWCC) to set resource condition targets for soil salinity and natural vegetation in that part of the South West Natural Resource.
The Ministry of Agriculture and Food (Keipert et al. in preparation) developed the procedure. included two half-day workshops that combined the latest scientific data and simple models with local knowledge of salinity and its management to set long-term salinity and native vegetation goals. Modeling results were presented and the impacts of different management options were discussed. The group considered these options and then finalized the following resource status goals for the Lake Towerrinning watershed.
Landowners estimated that 12 percent of the catchment is currently affected by salinity and the full risk by 2028 was estimated to be 20 to 25 percent of the catchment.).
- Lake Towerrinning catchment
- Workshop aims
- Current salinity—local view
- Local aspirations
Find out how landowners perceive the salinity risk to the watershed and their management preferences (ie incorporating landowners' views on the likely future extent of salinity on their property and in their watershed). Present watershed information on current salinity impacts, future trends, and an estimate of the likely impact of two levels of salinity management efforts. Provide an assessment of the likely impact of salinity management options favored by landowners.
However, concerns were still expressed about salinity expanding in the lower reaches of the catchment where the valley floors are flat and wide, as well as along some creeks. At the first workshop, landowners' aspirations for the extent of future salinity in their catchment were explored using a continuum (Figure 2).
CURRENT SALINITY IMPACTS AND FUTURE TRENDS
- Groundwater trends
- Current salinity impacts
- Valley floor hazards
- Lake Towerrinning water quality
- Predicted impact of recharge reduction strategies
The Land Monitor Project used high-resolution digital elevation data and remotely sensed vegetation health data to map salt-affected land and produce an estimate of the maximum possible future extent of salinity in the region agricultural southwest (McFarlane et al. 2004). Land Monitor's estimate of current salinity has limitations that may affect the accuracy of the information. The reported accuracy of the Land Monitor salinity map for the eastern area of the Collie-Pemberton Landsat scene, within which Lake Towerrinning is located, was 99 percent for bare saline land, but only 70 percent for slightly saline land ( Evans, 2001).
At Workshop 1, landowners agreed that the Land Monitor method underestimated the extent of current salinity, but also pointed out that some salinity had only occurred since 1998 and therefore could not be detected during the Land Monitor project. The fact that Land Monitor used digital elevation as a basis for determining low-lying areas that might be affected meant that it was unable to identify salt outcrops and other small saline areas present on the upper slopes of the watershed. These rates of spread of salt-affected land cannot be used as a direct indication of the likely rate of spread in the Lake Towerrinning catchment because the county is a land management area as opposed to a catchment.
Valley floor hazard, as shown in Land Monitor (2001) information for lowland areas, indicates areas at greatest risk of clogging, flooding, shallow groundwater and salinity (Figure 4). This is indicated as the area of the valley floor within a specified elevation of the main stream line. Areas in the 0-2.0m class are almost certainly an overestimated salinity hazard for the Towerrinning catchment.
The landowners received an update on the water quality in Lake Towerrinning, which is considered an important regional asset by the watershed landowners and the wider local community. Lake Towerrinning water quality data is shown in Figure 5, which shows that since the diversion structure was commissioned in 1993, the electrical conductivity (EC) of the lake water has remained below 2500 mS/m on most observation dates. Flowtube is a simple two-dimensional model that simulates the position of the groundwater level over time along a groundwater streamline, either along a slope or along the main drainage line of the watershed.
A limitation of this type of model is that the proportions of the catchment with shallow groundwater for different scenarios must be estimated from the length of the flow line saturated. However, because the model simulates the position of the water table through time, an estimate at the end of the 20-year time frame required for this exercise is possible. The Land Monitor estimate of 28 percent of the catchment potentially experiencing shallow groundwater was used as a base case, because the landowners' observations are not included in the 20-year salt risk estimate at this stage.
SALINITY MANAGEMENT OPTIONS
Scenario 1—Deep drainage
Scenario 2—Perennial pasture and surface water control
ASSETS AND TARGETS
Assets at risk to salinity
Lake Towerrinning catchment targets
FUTURE OPTIONS TO MANAGE SALINITY AND NATIVE VEGETATION
CONCLUSION AND RECOMMENDATIONS
Raper GP, Kowald JM, Percy HM, Smith MS, Hu S (in prep.) Groundwater trends and salinity risk in the medium to low rainfall zones of the Southwest Natural Resource Management Region. South West NRM Regional Assessment Team (2006) Hillman and Narrogin Zones (Blackwood Zones 8 and 9) Rapid Catchment Assessment.
Workshop dates and attendees
Future methods of managing salinity in the
Soil-landscape units of the Lake Towerrinning catchment