Wanilla Case Study Catchment, South Australia
The Wanilla catchment is in the southern part of the Eyre Peninsula in South Australia. Some 80% of the catchment is used for cropping with pasture rotations. With its low permeabilities and groundwater gradients, the catchment has very limited ability to move groundwater. This local to intermediate groundwater flow system is widespread in Western Australia (wheat belt) and in South Australia (Eyre Peninsula).
About 8% of the catchment is salinised and land salinisation has been part of the Eyre Peninsula landscape for some considerable time. Descriptive names such as `Salt Creek' and `Salt Swamp' (just to the north-west of the study area) are included on a 1903 survey map. Some parts are also likely to have been groundwater seepage areas before widespread clearing of native vegetation in the early 1950s.
Figure 23.Distribution of intermediate groundwater flow systems in deeply weathered rocks.
Results of groundwater investigations and modelling
The results of the groundwater investigation suggest that to a large extent, effects of land use change to pasture and cropping in the Wanilla catchment have already occurred. Groundwater levels are close to the soil surface over much of the catchment. Even immediate major changes in land use will result in only very minor reductions in the watertable over the next 20 years because of the inability of groundwater to move easily through the system.
- If the current land-use is maintained, in 20 years the area of shallow watertables will expand from 8% to 15% of the catchment and not increase significantly beyond that.
- If recharge in the catchment is reduced by 50%, the affected area will increase to around 12% of the catchment within 20 years, but not significantly beyond that point.
- Reduction in recharge of between 50% and 90%, will prevent the further increase in the area affected by shallow watertables (and salinity).
Implications
- The main aquifer is weathered bedrock rock resulting in low permeabilities and storage coefficients. Traditional engineering solutions such as groundwater pumping are difficult to implement, as demonstrated by pump tests in the Popes catchments (eastern uplands of Wanilla) and other locations on the Eyre Peninsula. Surface drainage is already being used at Wanilla because the drainage system is naturally in place and any saline seepage can be discharged to the sea.
- Given there is little scope for expansion of salinity in the cropped plains, living with the extent of salinity is the likely management option in this catchment.
A full technical report is available on the Audit's Australian Natural Resource Atlas.
Figure 24and Table 22.Wanilla (South Australia): change of area at risk in response to different recharge reduction rates - based on current recharge rate.
|
Recharge Reduction |
|||
|
Year |
No change (%) |
50% |
90% |
|
2000 |
8 |
8 |
8 |
|
2020 |
15 |
12 |
8 |
|
2050 |
16 |
12 |
8 |
|
2100 |
16 |
12 |
7 |
CAPACITY TO CHANGE - Wanilla case study of dryland salinity and watertable control
Wanilla catchment, a small basin of about 17,000 hectares, situated about 40 km to the north west of Port Lincoln on the lower Eyre Peninsular, South Australia. Salinity impacts within the Study Area appear mainly along strips of farmland which are contiguous with the natural drainage lines. At present 8 per cent of the catchment is salinised and CSIRO believes that, without management, this will increase to 17 per cent over the next 50 years.
Background
The analysis compared the benefits and costs associated with salinity control in Wanilla catchment being one of four contrasting case studies Kamarooka, Lake Warden, Upper Billabong). The approach adopted was to take estimates of the physical scale of impacts for each type of damage caused by dryland salinity (e.g. area of agricultural enterprises, number of stream diverters, kilometres of roads affected, number of species affected), and to apply damage functions for each of those types of impact. Data describing the physical scale of impacts have been captured using mainly GIS layers which describe the location of dryland salinity in each case study catchment. The damage functions developed for the purposes of quantifying the economic impacts of dryland salinity are for: agriculture and commercial forestry; roads and rail; urban centres; water users; and environmental values.
Key findings
For the Wanilla catchment (S.A.), it was found that impacts from salinity for landholders in the catchment would continue to increase, even with a 50 per cent reduction in recharge. CSIRO modelling proposed that a 50 per cent reduction in recharge could be achieved by a catchment-wide treatment comprising the abandonment of the current agricultural use for the upper catchment (approximately 40 per cent of the total Wanilla catchment), and replacing it with trees. For the remainder of the catchment, perennial pastures would be established in place of the present annual pastures in cropping rotations.
Trees grow very poorly in the Wanilla catchment and estimated costs of implementing catchment-scale recharge control would exceed greatly the benefits. While salinity is reducing agricultural incomes by about 12 per cent, the implementation of catchment-wide treatments might eliminate all returns for landholders (see Section 3.1.1 of project report). Therefore, a lack of profitable treatment options represents the major barrier to implementation of such catchment-wide treatment. The poor growth of trees and lucerne in the Wanilla catchment means that straightforward economic sense will ensure such catchment-wide treatment is not implemented.
In the Wanilla catchment it was concluded that the only practical option available to the local community is to learn to live with salinity. Whilst this might contain some pain, it is less painful than the alternative of recharge control.
Lessons learnt from all salinity case studies:
- There is no simple broadly applicable paradigm with which to conceive our
responses to salinity.
- Expectations of farm based change leading to salinity control need to be
tempered.
- Broad scale reforestation proposals will often be poor investments from
an economic and social perspective.
- A lack of profitable technically feasible options is the major constraint
to the capacity to control salinity.
- The major issue of "capacity for change" is the capacity of our
community to make informed decisions about investment in salinity control.
- We need to re-engineer our integrated catchment management structures to
operate within an adaptive management framework.
- Investment in salinity control should be based upon a triage model.
- A "works on the ground now" imperative should be tempered by a
"least regrets" investment approach.
- Landscape change must be seen as a multi-generational challenge.
Further Information
the technical reports:
Please Note: PDF files are in Adobe Acrobat Version 4.0 format. You will need a copy of an Acrobat Reader in order to view them. Blind and visually impaired users can view.pdf files using a tool available on-line from Adobe Systems that converts the.pdf files on the fly to HTML.
- Assessment of Salinity Management Options for Kamarooka, Victoria: Groundwater and Crop Water Balance Modelling in PDF format (11.9 MB)
- Capacity to change - Case studies of dryland salinity and watertable control by Mike Read. PDF format (1.2 MB)
- Capacity to change - Case studies of dryland salinity and watertable control - APPENDICES by Mike Read. PDF format (1.9 MB)
- Structural Change in Australian Agriculture: Implications for Natural Resource Management: Salinity case studies by Neil Barr. PDF format (1.3 MB)
Table of Contents for the Australian Dryland Salinity Assessment 2000
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