Department of Sustainability, Environment, Water, Population and Communities, November 2012
Action is being taken to address Basin river constraints to allow for improved environmental watering outcomes while ensuring there are no unacceptable third party impacts.
When combined with the operation of the Sustainable Diversion Limit Adjustment Mechanism under the Basin Plan, the relaxation or removal of constraints has potential to support environmental outcomes equivalent to a 3,200 gigalitre (GL) reduction in consumptive water use across the Basin.
The MDBA will prepare a management strategy to address key constraints, in consultation with Basin States and the community, within 12 months of the Basin Plan being made.
What are constraints?
A capacity constraint prevents larger volumes of water flowing through river systems for environmental purposes.
Environmental flow delivery constraints limit the environmental flows that can be released from Basin storages and delivered to key wetlands and floodplains.
In the Murray-Darling Basin, constraints which limit the environmental outcomes which can be achieved with the water recovered for the environment, and fall into a number of different categories, including:
- Physical obstacles, such as limited outlet capacities on some major water storages. For example, the outlet capacity of the Lake Victoria water storage, on the River Murray near the NSW-SA border, limits the rate of water releases to a maximum flow of 8,000 Ml/d. This constraint impacts the ability to deliver water recovered for the environment to supplement mid-to-high river flows and achieve target environmental outcomes in the Lower Murray floodplains.
- Constraints also arise from the operating rules for the rivers in the Basin designed to protect infrastructure and private property from inundation. The types of private assets and infrastructure currently protected from inundation may include crops and pastures, farm sheds, irrigation infrastructure, pumping houses, roads, river crossings and bridges.
- In some instances constraints may arise from operating rules for rivers.
How are constraints reflected in the proposed Basin Plan?
The sustainable diversion limits (SDLs) set under the Basin Plan reflect the Murray-Darling Basin Authority's judgement on what constitutes an environmentally sustainable level of water take. The Authority has taken into account current environmental and hydrologic science, social and economic knowledge, and constraints that limit capacity to deliver environmental water.
In part, a 2,750 GL Basin Plan recovery target was adopted because river operating constraints in the Murray-Darling Basin meant that only limited additional environmental benefit could be achieved by acquiring more water (beyond 2,750 GL) for the environment.
The ability to deliver water to mid-and high-level floodplain habitats in the regulated rivers of the Basin, particularly the Lower Murray floodplains, is limited by constraints. This means that increases in the volume of water recovered for the environment will not necessarily achieve better environmental outcomes for these habitats.
The MDBA modelling of a 3,200 GL/y environmental water scenario assuming key constraints were removed or relaxed, found 17 out of 18 targets for the River Murray could be achieved, compared to 11 of 18 with the Basin Plan recovery of 2,750 GL under current constraints.
How can constraints be overcome?
Through the use of easements or other mutually-agreeable arrangements with private landholders, impacts associated with overbank flows and associated inundation of private land can be addressed in ways that enable larger releases of environmental water and enhanced environmental outcomes.
Technical assessments to investigate ways of overcoming physical constraints are also being undertaken. Potential projects include engineering works to increase the release capacities of dams, raising bridges to improve access during high flows, or relocating private assets such as farm sheds and pumps.
Community consultation will be a central element in the development of any proposals to address constraints.
Under the Basin Plan the MDBA will develop, in consultation with Basin States a Constraints Management Strategy within 12 months of the Basin Plan being made. The strategy will form the basis for a constraints work program to be developed jointly by Basin jurisdictions.
The Australian Government has committed funding towards the removal or relaxation of key constraints. This funding is included in the $1.77 billion which is being provided to fund removal of constraints and recovery of an additional 450GL of water. These funds will be administered through a special account with special appropriation.