Operations Update 22 April 2021 - Lower Darling
Inflow Forecast – Current Operations
WaterNSW issued an update on forecast system inflows yesterday, the update can be found at: https://waterinsights.waternsw.com.au/16001-barwon-darling-unregulated-river/updates
WaterNSW will provide at least weekly updates of forecast at the above website and more regularly if the change in forecast necessitates more frequent updates.
Based on the assessment completed 20 April, inflow forecast to the Menindee Lakes system has increased to be between 650 - 850 GL, this is inclusive of inflow already recorded. Inflows to date have now totalled approximately 150GL – meaning there is an additional 500 – 700 GL yet to arrive.
Lake Wetherell is currently holding 124 GL and Lake Pamamaroo - 289 GL; total storage volume is currently 413 GL.
At present, Pamamaroo inlet is fully open, allowing inflows to fill Wetherell and Pamamaroo together, minimising the surface area of the stored volume and losses, whilst allowing fish passage between the lakes.
Releases are being made from Wetherell outlet to meet the minimum release requirements at Weir 32 only.
Lake Menindee Inlet has been opened today to commence transfers. Transfers will commence at low rates – approximately 3,000 ML/day initially but will be increased over the next week with all inflow being transferred to Lake Menindee.
WaterNSW will monitor inflows and storage levels with the aim of maximising water in Lake Wetherell and Lake Pamamaroo at the time when inflow cease.
Forecast Storage Volumes – Operations Impacts
It is likely, based on inflow forecast that total storage volume will exceed 640 GL in early May. This will result in the water within the Menindee Lakes system becoming a shared resource as per the Murray-Darling Basin Agreement, until such time as the total storage volume next falls to 480 GL.
This means that water stored in Menindee is shared between Victoria and NSW and can be released to meet demands within the Murray, at the request of the Murray-Darling Basin Authority River Operations.
WaterNSW is engaging with the MDBA regarding planned releases – rates and timing – to ensure that when 480 GL is reached as much water is held in Lakes Wetherell and Pamamaroo as possible. This will need to be balanced against the demand requirements in the Murray.
WaterNSW ABN 21 147 934 787 169 Macquarie Street Parramatta NSW 2150 PO Box 398, Parramatta NSW 2124 T 1300 662 077 E Customer.Helpdesk@waternsw.com.au waternsw.com.au
Environmental water managers are considering options – both short term and medium term – for additional releases to provide outcomes for fish and vegetation however these have not yet been determined.
Lower Darling intervalley trade can resume once the total storage volume reaches 640 GL.
Background to the Decision to Open Menindee Inlet
Prior to the decision being taken to open Menindee Inlet, consideration was given to other options for management inflows and the Menindee Lakes Storage.
Assessment of the risks, benefits and opportunities included consultation with customers and community via the River Operations Stakeholder Consultation Committee; the Barkandji Native Title holders lead by the Department; other NSW and Commonwealth Agencies responsible for water and natural resource management.
The following options for management were considered:
Option 1: Standard Operations Practice – Capture, store and release – Volume excess to the Capacity of Lakes Wetherell and Pamamaroo be diverted into Lakes Menindee and Cawndilla
Benefits:
Lakes Menindee and Cawndilla have not received water since 2016, the environmental and cultural watering needs are considered critical. A minimum volume of 60 – 70 GL in Lake Menindee for at least 3 months would provide important environmental productivity outcomes for fish, invertebrates; and for 5 months would provide waterbird outcomes if nesting occurs. These are environmental watering requirements as specified in the Long- term watering plan for the Lower Darling system.
If follow up rainfall occurs, it provides a sound base to build further supplies within the Menindee Lakes system.
Inflows to Cawndilla may provide the opportunity for releases to the Darling Anabranch, an action that has not occurred since 2017.
Risks:
Based on experience from 2016, wetting the bed of Lake Menindee and Cawndilla may use up to 200 GL. Dead storage (water that can’t be released to the Lower Darling) is an additional 70 GL in Lake Menindee and around 200 GL in Lake Cawndilla. At the time of taking the decision volume forecast for diversion through Lake Menindee inlet was around 150 – 350 GL, providing a small amount of surface water in the lakes –
WaterNSW ABN 21 147 934 787 169 Macquarie Street Parramatta NSW 2150 PO Box 398, Parramatta NSW 2124 T 1300 662 077 E [email protected] waternsw.com.au
estimated to be 70 – 150 GL. As forecast inflows have now increased, so to has the volume potentially available for diversion through Lake Menindee inlet regulator, with the prospect of an increased surface volume of 300 – 500 GL, but most remains dead storage – but is likely to provide the additional opportunity of release to the Anabranch from Cawndilla Outlet.
Only filling the dead storage may result in stranded fish in Lake Menindee and potentially Cawndilla.
Diversion of water to Lake Menindee will likely result in the total storage volume exceeding 640 GL, triggering the shared resources with the Murray valley as per the Murray-Darling Basin Agreement. This permits the release of water to assist with meeting Murray demands at the request of the Murray-Darling Basin Authority – acting on behalf of the states and in accordance with the agreement. With limited accessible volume in Lakes Menindee and Cawndilla, releases from Lake Wetherell and Pamamaroo are likely to be required to meet these demands. Without further inflow, the Lower Darling may move back to drought management in a short period of time.
Option 2: Transfer volume in excess of water able to be captured in Lakes Wetherell and Pamamaroo to the Murray – capturing water in Lake Victoria and assisting NSW Murray Resource.
Benefits:
Releases could be shaped to provide a large pulse providing inundation of wetlands that have not received water since 2012.
Build on the successful breeding event of golden perch in the Lower Darling that occurred in spring 2020.
Limits volume in the Menindee Lakes system to below 640 GL, extending the duration of releases for the Lower Darling if drought conditions were to return following the current inflow event.
Risks:
Missed opportunity to divert water to Lake Menindee and Cawndilla and meet environmental watering requirements of these important habitats. Also, potential missed opportunity for Darling Anabranch watering if inflows are sufficient.
Limited resource benefit to NSW Murray users. Water transferred to Lake Victoria increases the risk of ‘spill’. In addition, other areas of the Murray catchment are wet following high Autumn rainfall totals, specifically the Murrumbidgee valley where Burrinjuck and Blowering
WaterNSW ABN 21 147 934 787 169 Macquarie Street Parramatta NSW 2150 PO Box 398, Parramatta NSW 2124 T 1300 662 077 E [email protected] waternsw.com.au
dams are both above 80% capacity. As such, Lake Victoria is likely to fill without the need for release from the Menindee Lakes system.
Option 3: Hybrid Option – Release 30GL of water into the Lower Darling and capture the remaining volume in the Menindee Lakes system, likely diversion to Lake Menindee
Benefits:
Opportunity to provide some outcomes for the Lower Darling environment, while minimising the risk to resources – expected that 30 GL could be captured in Lake Victoria meeting NSW commitments under the Murray-Darling Basin Agreement, regarding to management of Lake Victoria; and provide some outcomes for Menindee Lake in terms of wetting.
Risks:
Limits potential outcomes for both Lower Darling environment and Lakes environment.
If inflows are on the lower end, likely unable to provide releases to the Anabranch and may in a worse case not provide inflows to Lake Cawndilla.
More information Please visit our website for more information on the drought at waternsw.com.au/drought. For more information on this event, phone us on 1300 662 077 or email us at [email protected].
Authorised by
Adrian Langdon Executive Manager, System Operation and Asset Maintenance
WaterNSW ABN 21 147 934 787 169 Macquarie Street Parramatta NSW 2150 PO Box 398, Parramatta NSW 2124 T 1300 662 077 E [email protected] waternsw.com.au