Overview

The Upper Molonglo River is home to the last population of the endangered Green and Golden Bell Frog in the Southern Highlands of New South Wales. The frog habitat is in floodplain pools that interconnect with the river and retain water after flow events have ceased. Agricultural processes and river management have led to the continued incision of the Molonglo River channel, meaning that events that break the banks and recharge the floodplain pools are less frequent. This has the potential to dry out the frog habitat and cause the loss of the local population.

The Molonglo River is also a critical part of the ACT water supply as well as other critical industries downstream, and so the potential impacts to downstream water users needed to be accurately quantified before any remediation work could be undertaken.

The Mulloon Institute is a not-for-profit organisation that specialises in rehydration of floodplains. Catchment Simulation Solutions worked in partnership with them to undertake the project, where Catchment Simulation Solutions role was to develop the tools to test potential remediation options and ensure they weren’t having an impact on either the event scale or over a long period of time.

Challenges

One of the key challenges of water modelling in Western NSW, including Toorale, is the very flat topography which leads to slow moving floods that can last months. Evapotranspiration and infiltration can be a high proportion of the water balance and the change of wetland plants density during events can impact key calibration parameters.

Approach

Develop a calibrated hydraulic model to understand how water moves across the landscape at an event time scale
Use key data from the hydraulic model to inform the development of a Source hydrologic model to understand the flow of water over a long period of time under different management scenarios

Solutions

Catchment Simulation Solutions model set is robust, calibrated and peer reviewed
Flow through the park and downstream is now better understood and the hydrological outcomes from various inflows can be predicted
Park and water managers now have a robust tool to assess different management options to best utilise water resources

Molonglo River Rehydration Project