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Title:Assessment of environmental water requirements for the proposed Basin Plan: Riverland-Chowilla Floodplain
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Murray region
Assessment of the Riverland-Chowilla Floodplain environmental water requirements
1.Introduction
The Water Act 2007 (Cwlth) established the Murray‐Darling Basin Authority (MDBA) and tasked it with the preparation of a Basin Plan to provide for the integrated management of the Basin’s water resources. One of the key requirements of the Basin Plan is to establish environmentally sustainable limits on the quantities of surface water that may be taken for consumptive use, termed Sustainable Diversion Limits (SDLs).SDLsare the maximum long‐term annual average volumes of water that can be taken from the Basin and they must represent an Environmentally SustainableLevel of Take (ESLT).
The method used to determine the ESLT is described in detail within ‘The proposed “environmentally sustainable level of take” for surface water of the Murray-Darling Basin: Method and Outcomes,’ (MDBA2011). A summary of the main steps undertaken to determine the ESLT is presented in Figure 1. The assessment of environmental water requirements including specification of site-specific flow indicators at a subset of hydrologic indicator sites (Step 3 of the overall ESLT method) is the focus of this document.
The work described herein is the MDBA’s current understanding of the environmental water requirements of Riverland-Chowilla Floodplain. It is not expected that the environmental water requirements assessments will remain static, rather it is intended that they will evolve over time in response to new knowledge or implementation of environmental watering actions. Within this context, feedback is sought on the material presented within this document whether that be as part of the formal draft Basin Plan consultation phase or during the environmental watering implementation phase within the framework of the Environmental Watering Plan.
1.1.Method to determine site-specific flow indicators
Assessment of environmental water requirements for different elements of the flow regime using the hydrologic indicator site approach is one of the key lines of evidence that has informed the proposed SDLs.Effort focussed on regions and parts of the flow regime with greatest sensitivity to the scale of reduction in diversions necessary to achieve environmental objectives, an ESLT and a healthy working Basin.
Within the overall framework of the ESLT method(Figure 1) the MDBA used an iterative process to assess environmental water requirements and develop site-specific flow indicators.
The hydrologic indicator site approach usesdetailed eco-hydrological assessment of environmental water requirementsfor a subset of the key environmental assets and key ecosystem functions across the Basin. Effort focused on high flow (freshes, bankfull flows and overbank flows) requirements reflecting the prioritisation of efforton parts of the flow regimethat are most sensitive to the determination of the ESLT and SDLs.Riverland-Chowilla Floodplain is one of the key environmental assets where a detailed assessment of environmental water requirements was undertaken.
Figure 1: Outline of method used to determine an Environmentally Sustainable Level of Take (Source: MDBA 2011).
Detailed environmental water requirement assessments lead to the specification of site-specific flow indicatorsto achieve site-specific ecological targets. Flow indicators were expressed at a hydrologic indicator site or sites. Environmental water requirements specified at hydrologic indicator sites are intended to represent the broader environmental flow needs of river valleys or reaches and thus the needs of a broader suite of ecological assets and functions.
This report provides a description of the detailed eco-hydrological assessment of environmental water requirements for the Riverland-Chowilla Floodplain including information supporting the development of site-specific flow indicators for the site (with reference to flows measured on the River Murray at the South Australian border). More information on how the site-specific flow indicators for Riverland-Chowilla Floodplain were used within the Basin-wide modelling process to inform the ESLT (i.e. Step 5 and 6 in Figure 1) can be found in the report ‘Hydrologic modelling to inform the proposed Basin Plan: Methods and results’ (MDBA 2012).
A description of the detailed eco-hydrological assessments of environmental water requirements for other indicator sites are described in other documents in the series ‘Assessment of environmental water requirements for the proposed Basin Plan’.
1.2.Scope and purpose forsetting site-specific flow indicators
The MDBA’s assessment of environmental water requirements and associated site-specific flow indicators at hydrologic indicator sites has been used to inform the development of SDLs. This enables the MDBA to estimate the amount of water that will be required by the environment over the long-term to achieve a healthy working Basinthrough the use of hydrological models. Accordingly, site-specific flow indicators are not intended to stipulate future use ofenvironmental water. MDBA expects that the body of work undertaken to establish these site-specific flow indicators willprovide valuable input to environmental watering but this watering will be a flexible and adaptiveprocess guided by the framework of the Environmental Watering Plan and natural eco-hydrological cues. It will be up to the managers of environmental water, such as the Commonwealth Environmental Water Holder, State Government agencies, and local communities to decide how best to use the available environmental water during any one year to achieve environmental outcomes.
2.Site location and extent
The Riverland-Chowilla Floodplain hydrologic indicator site comprises the Riverland Ramsar site and The Living Murray Chowilla Floodplain and Lindsay–Wallpolla Islands icon site. The Riverland Ramsar site was listed in 1987 covering an area of 30,615ha that includes the South Australian portion of the Chowilla Floodplain, as well as adjacent wetland and floodplain areas downstream to Renmark (Figure2). It includes many wetlands and wetland complexes, such as the RalRal Creek anabranch (including Lake Merriti) and theWoolenook Bend wetland complex.
The Living Murray Chowilla Floodplain and Lindsay–Wallpolla Islands icon site covers 43,856ha and comprises four main areas of floodplain. The Chowilla Floodplain covers 17,700ha, of which 74% is in South Australia (noting that the entire Riverland Ramsar site is not included within the boundary of The Living Murray icon site)and 26% is in New South Wales. The other main floodplain components of the icon site are in Victoria. Moving upstream, they are: Lindsay Island (15,000ha), Mulcra Island (2,156ha) and Wallpolla Island (9,000ha).
The extent of the indicator site has been defined using a number of data sources. The Wetlands GIS of the Murray–Darling Basin Series2.0 (Kingsford, Thomas& Knowles 1999) and Directory of Important Wetlands in Australia dataset were used to determine the internal extent of the site (Department of the Environment, Water, Heritage and the Arts 2001). The downstream extent was determined using the Ramsar Wetlands in Australia dataset; and The Living Murray indicative icon site boundaries were used to include the Wallpolla extent. Spatial data used in Figure 2are listed in Appendix A. Issues with mapping of the boundary of the Riverland Ramsar site have been identifiedbut are yet to be resolved and may result in minor future modifications to the indicator site boundary.
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Figure 2Location and extent of the Riverland-Chowilla Floodplain hydrologic indicator site comprising the Riverland Ramsar site and The Living Murray Chowilla Floodplain and Lindsay–Wallpolla Islands icon site. Flow indicators are specified at River Murray South Australian border.
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3.Ecological Values
The Riverland-Chowilla Floodplain hydrologic indicator site consists of a mosaic of anabranch creeks, wetlands, lagoons, lakes and floodplains. The floodplains are dominated by red gum woodland, lignum shrublands, black box woodlands, grasslands and some localised areas of denser river red gum forest. The indicator site’s vegetation communities have varying locations and extents within the floodplain, with corresponding variations in inundation thresholds (MDBC 2006; Newall et al. 2009; Cale 2009).
The Riverland-Chowilla Floodplain is important as critical habitat for both nomadic and migratory waterbirds during times of drought in central and eastern Australia, including as stop-over habitat for a number of migratory bird species listed under the Japan–Australia, China–Australia and Republic of Korea–Australia migratory bird agreements (Newall et al. 2009).
The anabranch environments present within the Riverland-Chowilla Floodplain site support significant populations of native fish. In particular, the site provides valuable habitat for Murray cod (Macullochellapeeliipeelii) populations, allowing different sized Murray cod to exploit different habitats (Zampattiet al. 2008; Newall et al. 2009; Zampattiet al. 2011). This attribute is relatively rare in the post-regulation River Murray, is largely restricted to this and other Ramsar sites, and represents, to some degree, remnants of main channel environments before regulation. Other animals such as the southern bell frog is supported by the habitats and flow regime in the Riverland Ramsar site.
These ecosystems support important species and habitats that are listed in international agreements and include vulnerable and endangered species. Appendix B provides a summary of the conservationally significant species recorded at the floodplain.
The ecological values of the Riverland-Chowilla Floodplain are reflected in MDBA’s assessment against the criteria used by the to identify key environmental assets within the Basin. The MDBA established five criteria to identify assets based on international agreements and broad alignment with the National Framework and Guidance for Describing the Ecological Character of Australian Ramsar Wetlands (Department of the Environment, Water, Heritage and the Arts 2008) and the draft criteria for identifying High Conservation Value Aquatic Ecosystems (SKM 2007).
Based on theecological values identified at Riverland-Chowilla Floodplain, the site meets all five key environmental asset criteria(Table 1).
Table 1Assessment of the Riverland-Chowilla Floodplain against MDBA key environmental asset criteria.
Criterion / Ecological values that support the criterion- The water-dependent ecosystem is formally recognised in international agreements or, with environmental watering, is capable of supporting species listed in those agreements
- The water-dependent ecosystem is natural or near-natural, rare or unique
- The water-dependent ecosystem provides vital habitat
- Water-dependent ecosystem that supports Commonwealth, State or Territorylisted threatened species or communities
A significant population of the nationally listed southern bell frog is supported by the Riverland Ramsar site which provides the combined habitat of permanent waters with still to slowflowing areas and nearby forests (Newall et al. 2009).
- The water-dependent ecosystem supports, or with environmental watering is capable of supporting, significant biodiversity
4.Hydrology
The Riverland-Chowilla Floodplain hydrologic indicator site is located downstream of the Murray–Darling junction and therefore receives flows from both the Murray and Darling Rivers. The largest flows occur when both rivers are in flood. At low flows, the hydrology of the site is governed by the operation of locks 5, 6, 7, 8 and 9. These maintain elevated pools in the River Murray, and permanent water in some anabranch creeks and wetlands.
There are localised variations in inundation thresholds across the South Australian, New South Wales and Victorian portions of the Riverland-Chowilla Floodplain hydrologic indicator site (MDBC 2006), although for the purposes ofassessing environmental water requirements there is considerable overlap and consistency. The extent and depth of floodplain inundation is determined by flood magnitude and changes in elevation across the floodplain (Cale 2009). Flows of 40,000ML/d to 60,000ML/d are required to overtop the river banks and inundate wetlands and floodplains, with significant floodplain inundation at the upper end of this range (MDBC 2006). About half the Chowilla Floodplain is inundated at flows of 80,000ML/d and flows in excess of 100,000ML/d inundate extensive areas of the entire site (MDBC 2006). Newall et al. (2009) mapped inundation thresholds for the Riverland Ramsar component of the hydrological indicator site and these are consistent with the inundation thresholds identified above.
River regulation has significantly reduced the magnitude of flows, particularly medium-sized floods (Cale 2009). Average annual flow to South Australia has also been significantly reduced with a 52% reduction under current water resource development conditions compared to withoutdevelopment conditions (CSIRO 2008). Flows of 80,000ML/d, which inundate about 50% of Chowilla Floodplain, previously occurred once every two years on average, but now occur once every eight years (Cale2009). CSIRO (2008) found that as a result of water resource development, the average period between beneficial spring-summer floods has more than tripled (from 2.4 to 9.3 years). Similarly, the maximum period between events under current conditions is more than five times the maximum period experienced under without development conditions (from 5.7 to 28.7 years). Flood volumes have also been greatly reduced, such that the average annual flood volumeis now less than half of the volume compared to withoutdevelopment conditions (from 2431 to 947 GL).
5.Determining the site-specific flow indicators for the Riverland-Chowilla Floodplain
5.1Setting site-specific ecological targets
The objective setting framework used to determine the ESLT is outlined in the report ‘The proposed “environmentally sustainable level of take” for surface water of the Murray-Darling Basin: Method and Outcomes’(MDBA 2011).In summary, the MDBA developeda set of Basin-wide environmental objectives and ecological targets, which were then applied at a finer scale to develop site-specific objectives for individual key environmental assets. Using these site-specific objectives, ecological targets that relate specifically to the Riverland-Chowilla Floodplain were developed (Table 2). Information underpinning site-specific ecological targets is shown in Table 2.
Table 2Site-specific ecological targetsfor the Riverland-Chowilla Floodplain.
Site-specific ecological targets / Justification of targets- Provide a flow regime which ensures the current extent of native vegetation of the riparian, floodplain and wetland communities is sustained in a healthy, dynamic and resilient condition
- Provide a flow regime which supports the habitat requirements of waterbirds and is conducive to successful breeding of colonial nesting waterbirds
- Provide a flow regime which supports recruitment opportunities for a range of native aquatic species (e.g. fish, frogs, turtles, invertebrates)
- Provide a flow regime which supports key ecosystem functions, particularly those related to connectivity between the river and the floodplain.
The Riverland-Chowilla Floodplain hydrologic indicator site supports a variety of flood dependent vegetation communities. Riverland Ramsar site ecological character description specifies a long-term limit of acceptable change as no loss of more than 20% of any wetland or vegetation type over the site as a whole within any 10-year period, and no loss of more than 20% of any habitat type over the site as a whole. This highlights some inconsistencies with ecological character descriptions developed for other Ramsar-listed indicator sites along the River Murray (e.g. Barmah and Gunbower forests, Hattah Lakes) which specifies that any change in the distribution or area of vegetation communities would signal a change in ecological character of the site.
Ecological targets for the Riverland-Chowilla floodplain propose to ensure the current extent of native vegetation communities is sustained in recognition that it is rare as only one of a few areas of the lower River Murray Floodplain not affected by irrigation, preserving much of its natural character (Newall et al. 2009) and to ensure it is representative of the broader environmental water requirements of the lower Murray.
The Riverland-Chowilla Floodplain is an important area for waterbird breeding and migratory birds (MDBC2006; Newall et al. 2009). The Riverland Ramsar site ecological character description specifies no net reduction in waterbird breeding numbers or reduction in waterbird populations (particularly migratory) over any rolling 10-year period.
The site supports important habitat and species that are listed in international agreements including vulnerable and endangered species such as Murray cod (Maccullochellapeeliipeelii) and Murray hardyhead (Craterocephalusfluviatilis). Achieving the targets for floodplain wetlands and waterbirds will ensure inundation of breeding and feeding habitats considered key for a range of fish, amphibian and water-dependent reptile and invertebrate species.
Key ecosystem functions support fish, birds and invertebrates through habitat maintenance, energy transfer and facilitating connections between rivers and floodplains. Overbank flows supply the floodplains with nutrients and sediments from the river, accelerate the breakdown of organic matter and supply water to disconnected wetlands, billabongs and oxbow lakes. As the floodwaters recede, the floodplains provide the main river channel with organic matter.
The hydrological connection between watercourses and their associated floodplain provides for the exchange of carbon and nutrients (Thoms 2003). The connections are considered essential for the functioning and integrity of floodplain-river ecosystems.
Site-specific ecological targets formed the basis of an assessment of environmental water requirements and the subsequent determination of site-specific flow indicators for the Riverland-Chowilla Floodplain, as described below.