3.3 Climate and rainfall in the Murray–Darling Basin
Historically the climate of the Murray–Darling Basin varies year to year. Climate change science has demonstrated there is likely to be an increased variability in the future, resulting in more frequent and extreme floods and droughts than in the past (MDBA 2010). Consequently the river storages and use of environmental water are managed to mitigate these varying river flows.
Between 1996 and 2010 the Murray–Darling Basin was in a drought characterised by below average rainfall in autumn and winter and very few wet periods. This drought has been significantly drier than the Federation (mid 1890s to early 1900s) and the World War II drought (1940s).
In spring 2010 widespread, above-average rainfall across the Murray–Darling Basin alleviated the short-term dry conditions. This rainfall has been associated with the breakdown of the 2009–10 El Niño and the development of a moderate to strong La Niña event in 2010–12.
3.3.1 Local climatic conditions
The Lower Lakes, Coorong and Murray Mouth icon site has a Mediterranean climate with warm dry summers and cool wet winters. The mean annual rainfall is 467.6 mm. The mean monthly maximum rainfall is in June (61.7 mm). The expected mean maximum daily temperature is highest in February at 26.4oC, lowest in July 14.9oC, and has an annual mean of 20.8oC (Figure : Monthly climate summary statistics for the Meningie area within the LLCMM Icon Site. Bureau of Meteorology 2010 Meningie (number 024518). The minimum daily temperature is at its maximum in January and February at 13.9oC, minimum in July at 6.7oC, and has an annual daily mean of 10.2oC Figure .
Figure : Monthly climate summary statistics for the Meningie area within the LLCMM Icon Site. Bureau of Meteorology 2010 Meningie (number 024518)
3.4 Baseline condition
3.4.1 Antecedent hydrological conditions
River regulation, water extraction and the recent extreme drought have reduced the total volume of water available to the river and Coorong Estuary, and have significantly altered the natural pattern of remaining flows. These changes have resulted in a substantial decline in the health of the river.
The operation of the barrages has significantly changed the hydrological regime of the Lower Lakes and the Coorong. Lake levels are now more elevated and static than under natural conditions. Historically, the target lake level was 0.75 m AHD but this varied slightly between 0.6 m and 0.85 m AHD annually. The lake level was surcharged to 0.85 m AHD in early spring to allow for evaporation losses during summer. When the lake level exceeds 0.83 m AHD, freshwater spillage may occur near the barrages. Prior to the drought, the lake level had to be maintained above 0.6 m AHD to enable flood irrigation in the Lower Murray Reclaimed Irrigation Areas below Lock 1 (Mannum–Wellington) (MDBC 2007).
Maintaining elevated lake levels has affected the geomorphology of the lakes, including:
• prograded shorelines in sheltered areas
• accelerated shoreline erosion in exposed localities
• accelerated rates of sedimentation in the lakes
• changed the character of the sediments deposited.
This has implications for littoral plants and wildlife communities, infrastructure damage, turbidity and nutrient levels in the lakes (MDBC 2007).
Since river regulation, barrage releases have been insufficient to counteract the incoming tidal flow and have resulted in congestion of the channels inside the estuary, altering the hydrological conditions in the icon site. Congestion of the mouth has allowed coarse marine sands to smother productive mudflats and reduce the area of suitable habitat for aquatic plants in the estuary and the North Coorong Lagoon. These cumulative impacts have reduced habitat and adversely affected the food chain (Paton 1997).
Sand build-up reached a critical level in mid-2002 following the longest ever period of barrage closure (630) days. This led the South Australian Government to undertake a sand dredging program at the Murray Mouth from 2002 until 2010. While sand dredging is an effective tool to maintain an open Murray Mouth during low flow periods, environmental flows are essential for this to occur naturally and achieve the icon site plan’s key target to maintain an ‘open Murray Mouth’, with adequate tidal variations to meet the needs of the Coorong ecosystem (DWLBC 2007).
3.4.2. Past management actions/activities
Since the development of the previous LLCMM Icon Site Environmental Water Management Plan in 2006 (MDBC 2006a), a number of environmental water delivery projects and the development of water delivery infrastructure have been completed within the icon site. Table : Summary of past management actions and activities in the Lower Lakes, Coorong and Murray Mouth icon site provides a summary of some of these key achievements (greater detail can be found in Appendix D).
Table : Summary of past management actions and activities in the Lower Lakes, Coorong and Murray Mouth icon site
Threatened fish refuge protection
9 ML of TLM water was delivered over summer 2009–10 to Boggy Creek on Hindmarsh Island, a refuge site for an isolated population of Murray hardyhead, an IUCN, EPBC and state listed species. Delivery of this water allowed this population to be sustained through the drought, which allowed the re-colonisation to Lake Alexandrina when drought conditions eased.
250 ML of TLM water was delivered in spring 2009 to Narrung wetland, a fringing wetland on the shore of Lake Alexandrina, to protect the seedbank of Ruppia tuberosa. The seedbank increased by 64% as a result of this watering.
In 2010–11 a total of 392 GL of environmental water from TLM, CEWH, Victorian Environmental Water Return Flows and the South Australian Drought Allocation Framework was added to the large unregulated flow event. Barrage releases were enhanced and Coorong salinities were reduced.
The Living Murray Environmental Works and Measures Program
Several fishways have been installed on the barrages separating Lake Alexandrina from the Murray Mouth Estuary. These have been designed to pass a wide range of species. This program is funded by the Murray–Darling Basin Authority(MDBC 2006b).
A variety of investigations on fish movement, reproduction and recruitment, specifically concerned with the barriers posed by the barrages, have been undertaken by SARDI Aquatic Sciences. These studies have documented the reproductive behaviour, movement and recruitment ecology of the fishes of the icon site, as well as the challenges posed to these species by the loss in connectivity imposed by the barrages and other regulating structures which have disconnected the aquatic environments in the icon site. The findings of these studies are helping to inform biologically sensitive operation of the barrages in the icon site.
Telemetered surface water monitoring stations
This project extended the range of the telemetered surface water monitoring systems present in the Lower Lakes into 11 sites in the estuarine reaches of the North Lagoon of the Coorong.
The extension of the system into the Coorong allows for the hydrological and physical effects of barrage operations to be viewed as they happen in the estuarine environment, better informing barrage operations.
The current extent of the surface water monitoring system can be viewed on-line :-
This project involved a prioritisation process where 15 wetland sites identified within the LLCMM were ranked based on predicted improvements in ecological condition for 12 ecological values. A number of sites were also identified for which the proposed works would contribute significantly to ecological values of the entire LLCMM region. The sites which ranked highly included Hindmarsh Island, Dunn’s Lagoon, Tolderol and Milang, leading to the development of management and operational plans, including recommendations for on-ground works to support key biological assets.
DEWNR drought emergency actions
Acid sulphate soil remediation
The primary threat during the drought condition within the Lower Lakes was the presence and potential for exposure of acid sulfate soils as a result of declining water levels. A series of emergency measures were put in place to prevent, mitigate, and control soil acidification. These measures included construction of regulators (bunds) at Clayton and the Narrung Narrows; pumping to water to maintain lake levels above acid sulfate soil trigger thresholds; and the application of limestone in Currency Creek, Finniss River and the Goolwa Channel;
Bioremediation and revegetation
Several thousand hectares of exposed lakebed sediments were aerial, machine and hand seeded in order to add carbon into lake beds to facilitate the consumption of acidity through sulfate reduction. This program also stabilized soils preventing soil erosion. This program has now moved into the ecological restoration phase where over 1.1 million plants have been planted during 2009–10 and over 500,000 plants planted during 2011.
The threat of local extinction of threatened fish species from the Lower Lakes due to drought lead to a variety of conservation measures being employed. DENR project managed threatened fish breeding programs through the drought in order to ensure species in the future can be returned to the Lakes. Environmental water was deliver to refuge sites of threatened species as well as a captive breeding program being undertaken to ensure the survival of Murray hardyhead, Yarra pygmy perch and southern pygmy perch.
Facilitating passage for diadromous fish
The Goolwa boat lock was used to allow movement of the diadromous Congolli between Lake Alexandrina and the Coorong in August 2010. This intervention was undertaken to overcome the inoperability of the barrage fishways caused by low lake levels. Recent fish monitoring has recorded abundant juvenile congolli, indicating successful reproduction as a result of utilising existing infrastructure.