National Wetlands Update September 2012
Issue No. 21, September 2012
Toolibin Lake inundation trial
Jennifer Higbid and Paul Drake, Western Australian Department of Environment and Conservation
The inundation trial site at Toolibin Lake. (Jennifer
Higbid, Western Australian Department of Environment
The BioRisk project in Western Australia is a great example of collaboration and science informing wetland management in an agricultural zone.
Toolibin Lake is a Ramsar-listed ephemeral wetland located 200 kilometres south-east of Perth. It is being studied as part of the BioRisk project, a partnership between the Western Australian Department of Environment and Conservation, the Future Farm Industries Cooperative Research Centre and the University of Western Australia.
Toolibin Lake is one of the last remaining fresh-to-brackish wetlands in the south-west agricultural zone. The lake bed vegetation is a nationally listed endangered ecological community. In the 1990s, a recovery plan was implemented to address threats to the lake from altered hydrology, particularly secondary salinity.
The Toolibin Lake recovery plan has guided various management actions, including revegetation and engineering works. Long-term monitoring indicates that these actions have largely halted the decline of the lake bed vegetation since around 2006; however, there has not been wholesale recovery and this prompted an investigation into plant- and hydrological-processes.
The water requirements and tolerances of the two dominant trees on the lake bed – sheoak (Casuarina obesa) and paperbark (Melaleuca strobophylla) – are being studied to help understand the connection between growth and water use by the trees and hydrology.
Below-average rainfall and a lack of natural inflow events meant that understanding of the interactions between surface water, soil properties and plants had to be derived from models. To test these models and understand the dynamics of salt and water movement during inundation, a small flooding trial on the lake bed was conducted.
Sap flow sensors in a Casuarina obesa tree.
(Jennifer Higbid, Western Australian Department of
Environment and Conservation)
During March 2011, a rectangular bund (15 metres long, by 10 metres wide, by 0.5 metres high) was constructed on the lake bed. The bund contained more than 100 trees, both sheoaks and paperbarks, with some trees fitted with sensors to measure tree girth and sap flow. Sensors were also deployed to measure water and soil parameters. Over two weeks, 100 000 litres of fresh water was applied to the bund and a water depth of more than 30 centimetres was maintained for seven days.
The trial confirmed the different rooting depths of the trees and demonstrated that they can rapidly increase water use following flooding, particularly the sheoak. The discovery of different root zone niches used by the two species (sheoak roots are shallower than the paperbark's) helps to explain the sequence of tree deaths that has occurred and the ecological requirements that management must consider.
It also showed that surface and groundwater rapidly connect, which is provoking additional research to better guide groundwater management.
For further information visit Toolibin Lake .