Darling Water Savings: Options for Environmental Filling, No Impacts, Version 2
Commonwealth Scientific and Industrial Research Organisation: Water for a Healthy Country National Research Flagship
2011
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Executive Summary
The Australian Government and the NSW Government signed a Memorandum of Understanding (MOU) in July 2010 for the "cooperative investigation and implementation of key water reform initiatives in NSW, including Broken Hill's urban water supply and Menindee Lakes operational arrangements". This MOU required further hydrological modelling of key propositions, including "determining the water savings to be transferred to the Commonwealth, climate change scenarios, and demonstration of no directly attributable adverse impacts on the water entitlement holders at Menindee, Darling River and Murray River, and identification of any impacts on the environment of the Menindee Lakes". More specifically, the MOU made two relevant commitments:
- "protecting the local environment and heritage (including Kinchega National Park and the nationally important Menindee Lakes wetland)"; and
- "any operational changes at Menindee Lakes must be shown to have no directly attributable adverse impact on the water security of existing water entitlement holders at Menindee Lakes and the Lower Darling River, or the Murray River." (Paragraphs 7 and 8, MOU).
The Department of Sustainability, Environment, Water, Population and Communities (DSEWPaC) commissioned CSIRO to manage and report on the hydrological modelling required by the MOU. This hydrological modelling was overseen by the Hydrological Modelling Working Group (HMWG), a technical group established by the Menindee Lakes Project Joint Steering Committee. The HMWG agreed on a list of indicators to be used in this study, and reviewed and provided suggestions to be included in a final report (Podger 2010b). Further work was subsequently undertaken to supplement the HMWG report that updates results based on the latest model configuration and includes additional outputs based on discussions with Victorian and South Australian government representatives (Podger 2010c). The report was submitted to the Menindee Lakes Joint Steering Committee (MLJSC) held on 3 December 2010. The Committee agreed that further work should be carried out to:
- Explore changes in operation rules that would have a positive impact on all indicators, and
- Investigate the potential for maintaining the Lake Wetherell floodplain vegetation by reducing the frequency of surcharging the lake.
The report (Podger 2010c) details the further work that was carried out to meet these additional requirements. The report determined that the best water saving option comprises the revised proposed Basin Plan environmental filling rules, with a 185/185 NSW/MDBA operating rule1, the Lake Menindee outlet increased to 14,400 ML/day and Broken Hill's water supply secured. Although there were a number of positive impacts there were also some very minor downstream impacts, of which all were either within the error of the model or could be offset:
- NSW Murray mean November allocations are 1% less
- The percent of months that the combined storage volume of Lakes Wetherell and Pamamaroo are less than 100 GL is increased from 6 to 15%
- The drawdown of Lakes Wetherell and Pamamaroo during dry periods is greater
- Percentage of years that flows to South Australia are reduced below its entitlement under the Murray-Darling Basin Agreement is increased by 1%
- 95%ile salinity at Morgan increases from 773 EC by 17 EC (but could be offset using salinity credits owned by the Commonwealth).
This report considers additional changes that would be required to ensure that these minor impacts are removed or compensated by other means. This was achieved by removing 28 GL of licence from NSW Murray General Security.
Based on standard modelling indicators, the modelled impacts on Victoria were negligible for HRWS allocations and some slight improvements in LRWS allocations. The impacts on NSW were positive improvements for all indicators when the 28 GL reduction in water licence is taken into consideration. There are improvements in dry years for all allocations.
With respect to South Australia, generally, in the modelling runs it is assumed that the CEWH water is not used before it reaches South Australia. There are significant positive benefits for South Australia such as, on average, 20.7 GL/y and 21.2 GL/y of additional flows to both South Australia and the barrages, respectively. The average South Australian restriction is reduced from 52.8 GL/y in the Base Case to 51.7 GL/year in the Environmental Filling scenario, which is a positive impact.
The small negative impacts in salinity in South Australia could be offset by credits owned by the Commonwealth.
1 This would be a modification to the existing NSW/MDBA operational rule associated with the Menindee Lakes pursuant to clause 99 of the Murray-Darling Basin Agreement. Under the existing rule, operational control transfers between the MDBA and NS W at volumetric thresholds of 480 GL (when the volume of water in the lakes is reducing), and 640 GL (when the volume of water in the lakes is increasing).
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