Lake Warden Case Study Catchment, Western Australia
Lake Warden catchment comprises four sub-catchments that feed the Lake Warden wetlands in southern Western Australia. It is dominated by annual pasture, with the remainder being cropped (approximately 33%) and perennial pasture, remnant vegetation and farm forestry (approximately 10% in total). With its low to medium permeabilities and low to moderate groundwater gradients, the catchment has a low to moderate ability to move groundwater.
It is estimated that 7.5% or 122 500 ha of cleared agricultural land in this catchment is affected by dryland salinity.
Figure 27.Distribution of regional groundwater flow systems in alluvial sediments.
Results of groundwater investigations and modelling
The results of the groundwater investigation suggest that groundwater movements through the catchment are slow, and therefore response times to extensive changes in land use are also expected to occur very slowly.
- If current land use is maintained, watertables will reach the surface in most of the lower parts of the catchment within 40 years.
- A recharge reduction of 50% would not decrease the ultimate expanse of the area at risk of shallow watertables when compared with the status quo scenario.
- A recharge reduction of 90% would be necessary to stabilise rising groundwater level trends by 2020 and reverse the trends by 2050.
Implications
In Lake Warden area, recharge to the confined aquifer is taking place in well-defined areas of the upper parts of the catchment. Controlling recharge in these areas will manage the rise in artesian heads and groundwater discharge in the long term. On the other hand, recharge to the unconfined aquifers that takes place over most of the catchment will be very hard to manage.
Managing the water levels in the numerous lakes in the area through drainage needs to be investigated to assess its viability in controlling the water levels in the shallow aquifers.
A full technical report is available on the Audit's Australian Natural Resources Atlas.
Figure 28.Lake Warden (WA): change of area at different recharge reduction rates - based on current recharge rate (confined aquifer - upper catchment).
Table 24.Lake Warden (WA): change of area at different recharge reduction rates - based on current recharge rate (confined aquifer - upper catchment).
Recharge reduction |
||||
Year |
No change (%) |
50% |
75% |
90% |
2000 |
2 |
2 |
2 |
2 |
2020 |
9 |
4 |
3 |
3 |
2050 |
26 |
12 |
6 |
4 |
2100 |
40 |
23 |
11 |
5 |
Table 25.Lake Warden (WA): change of area at different recharge reduction rates - based on current recharge rate (unconfined aquifers).
Recharge reduction |
||||
No change |
50% |
75% |
90% |
|
Year |
(%) |
(%) |
(%) |
(%) |
2000 |
2 |
2 |
2 |
2 |
2020 |
27 |
6 |
4 |
4 |
2050 |
45 |
33 |
7 |
4 |
2100 |
48 |
38 |
27 |
6 |
Figure 29.Lake Warden (WA): change of area at different recharge reduction rates - based on current recharge rate (unconfined aquifers).
CAPACITY TO CHANGE - Lake Warden case study of dryland salinity and watertable control
Lake Warden catchment, situated near the coastal town of Esperance on the south east coast of Western Australia. Salinity impacts within the Study Area appear mainly on farm land and around important wetlands at Esperance, as well as other low lying waterbodies. At present 8 per cent of the catchment is salinised but this is expanding rapidly and CSIRO believes that, without management, the extent will increase to 45 per cent of the catchment over the next 50 years.
Background
The analysis compared the benefits and costs associated with salinity control in Lake Warden catchment being one of four contrasting case studies (Kamarooka, Upper Billabong, Wanilla). 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 Lake Warden catchment (W.A.), agricultural lands comprising about
2 per cent of the catchment are presently severely salinised, and salinisation
around wetlands and other low lying waterbodies comprises another 6 per cent
of the catchment. CSIRO modelling predicts that, with a continuation of current
land use and agricultural practice, there will be a (dramatic) expansion in
the areas of salinisation on agricultural lands from the current 2 per cent
to 45 per cent of the catchment by 2050. This represents an unusually high
proportion of one catchment being affected.
- CSIRO modelling shows that a 50 per cent reduction in recharge could be
achieved by replacing all annual pastures with kikuyu and replacing 50 per
cent of cropped land also with kikuyu. This would lead to an area of salinisation
on agricultural lands of 33 per cent by 2050. The change in land use would
result in a slight increase in farm incomes, relative to the status quo. That
is, the shift towards perennial pastures would be marginally profitable for
landholders.
- CSIRO modelling shows that a 75 per cent reduction in recharge could be
achieved by replacing all annual pastures with kikuyu, replacing 67 per cent
of cropped land with trees and replacing the remaining cropped land with a
rotation based on phase farming with lucerne. This would lead to an area of
salinisation on agricultural lands of 4 per cent of the catchment by 2020
and 7 per cent by 2050. The change in land use would lead to a 25 per cent
reduction in agricultural incomes. This is because of the poor returns from
trees throughout most of the catchment.
- CSIRO modelling shows that a 90 per cent reduction in recharge could be
achieved by replacing all annual pastures with trees and by replacing 90 per
cent of cropped land with trees. This would lead to an area of salinisation
on agricultural lands of 4 per cent of the catchment by 2020 and 4 per cent
by 2050. With trees planted across almost the total catchment, agricultural
incomes would be almost eliminated. This is because even though some areas
of the coastal strip of the Lake Warden catchment are well suited to commercial
tree production because of the higher rainfall, yields would not generally
be high enough for profitable tree enterprises for most of the (drier) catchment
(see Section 3.2.2 of project report).
- Officers of CALM W.A. are presently evaluating whether the environmental
benefits for the Ramsar wetlands near Esperance could be achieved by diverting
saline flows away from wetlands in the lower catchment; that is, using an
engineering approach, possibly at much lower costs than that represented by
the loss of incomes for landholders if catchment-scale tree planting were
adopted.
- There is much research, extension and implementation of salinity control
measures on farms proceeding at present in this severely affected catchment.
Many landholders appear to be working closely with officers of Agriculture
W.A. and CALM, and salinity management measures are being implemented at a
farm scale, and are being adopted by landholders because of farm-scale benefits.
The returns from perennial pastures, as evaluated here for an average season,
indicate that the switch would be marginally profitable.
- We can take some comfort from the observation that landholders in Lake Warden
are adopting salinity management measures and innovation in the face of a
drastic expansion of salinisation. An optimistic interpretation would be that
it indicates that landholders will adopt substantial levels of salinity control
when the problem warrants it and when they are fortunate enough to be in a
catchment where technical options are available.
- In the Lake Warden there is a rapidly expanding shallow watertable. The local farm community has the capacity to increase the area of dryland lucerne to reduce the recharge of watertables. Anecdotal evidence is that this is well underway. However, whilst this will reduce recharge, in the long run it will delay but not reduce the ultimate impacts of dryland salinity on farm land. Delaying the ultimate expression of salinity will reduce the social and economic impact of salinity by providing a far longer time for structural adjustment. Even in the short to medium term it is unlikely changes to existing farming systems will save the downstream wetlands of Lake Warden. A landscape change option to save these wetlands based upon extensive reforestation would impose social and economic costs on the local community far greater than the costs of unmitigated salinity.
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:
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- Assessment of Salinity Management Options for Lake Warden Catchments, Esperance WA: Groundwater and Copr Water Balance modelling in PDF format (2.7 MB)
- Report Appendices - Assessment of Salinity Management Options for Lake Warden Catchments, Esperance WA: Groundwater and Copr Water Balance modelling in PDF format (1.7 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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