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MDBA Publication No:35/12

ISBN: 978-1-922068-43-9 (online)

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Title:Assessment of environmental water requirements for the proposed Basin Plan: MidMurrumbidgee River Wetlands

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Murrumbidgee Region

Assessment of the Mid-MurrumbidgeeRiver Wetlands 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 the Mid-Murrumbidgee River Wetlands. 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.The Mid-MurrumbidgeeRiverWetlands are one of the key environmental assets where a detailed assessment of environmental water requirements was undertaken.

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.

Figure 1: Outline of method used to determine an Environmentally Sustainable Level of Take.
(Source: MDBA 2011).

This report provides a description of the detailed eco-hydrological assessment of environmental water requirements for the Mid-MurrumbidgeeRiver Wetlands including information supporting the development of site-specific flow indicators for the site (with reference to flows gauged on the Murrumbidgee Riverat Narrandera). More information on how the site-specific flow indicators for the Mid-MurrumbidgeeRiver Wetlandswere 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 for setting 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 Mid-MurrumbidgeeRiver Wetlands isformed around an assemblage of lagoons and billabongs located on the floodplain of the central Murrumbidgee River between Wagga Wagga and Carrathool (Figure 2).

The existence of extensive floodplains and remnants of a palaeo (old) river system has resulted in the formation of a large number of natural closed depressions on the mid-Murrumbidgee floodplain. Using satellite imagery of flood events between 1989 and 1997, approximately 1,600 floodplain wetlands were identified between Gundagai and Hay (Frazier 2001; Murray 2006), with most located between Wagga Wagga and Carrathool (Frazier and Page 2006).The total area of wetlands that periodically connected to the main channel between Wagga Wagga and Carrathool was around 5,000ha (Frazier and Page 2006).

The MDBAhas used the Australian Wetlands Database, as well as other scientific information, as a basis for defining the Mid-MurrumbidgeeRiver Wetlandshydrologic indicator site. The directory’s dataset location (Department of the Environment, Water, Heritage and the Arts 2001), plus the work undertaken by Frazier and Page (2006), have been used to define the longitudinal extent of the asset. The lateral extent was based on mapped inundation extents determined by Kingsford et al. (1999) (Figure2). Other spatial data used in this map is listed in Appendix A.

Figure 2Location and extent of hydrologic indicator site: Mid-MurrumbidgeeRiver Wetlands

3.Ecological values

Terrestrial vegetation of the Mid-MurrumbidgeeRiver Wetlandsis dominated by river red gum (Eucalyptus camaldulensis) which forms a continuous band along the river (Briggs et al. 1994). In the lower section of the mid-Murrumbidgee floodplain system black box woodlands (E.largiflorens) become a common feature on higher ground away from the river (Briggs et al. 1994). Lagoons and swamps occur along the river and fill from high flows. Aquatic vegetation occurs in these areas, especially when the lagoons and swamps are shallow (Briggs et al. 1994). These communities combine to form a variety of key habitats including:

  • a range of riparian and wetland vegetation communities that are critical to several fish species in the Murrumbidgee (Gilligan 2005);
  • approximately 45,000 ha of river red gum wetlands between Wagga Wagga and Hay Weir(Thornton and Briggs 1994) which have been shown to be important areas for waterbirds including breeding of colonial nesting waterbirds(Briggs et al.1994;Briggs et al. 1997; Briggs and Thornton 1999; Kingsford et al.1997); and
  • habitat for a range of species and communities listed as threatened under both Commonwealth and state legislation.

These ecosystems support important species 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 site.

The ecological values of the wetlands are reflected in MDBA’s assessment against the criteria used 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 the Mid-MurrumbidgeeRiver Wetlands, the site meets four of thekey environmental asset criteria (Table 1).

Table 1Assessment of the Mid-MurrumbidgeeRiver Wetlands against MDBA key environmental asset criteria.

Criterion / Ecological values that support the criterion
1. The water-dependent ecosystem is formally recognised in international agreements or, with environmental watering, is capable of supporting species listed in those agreements / The Mid-MurrumbidgeeRiver Wetlands are formally recognised in, or are capable of supporting species listed in the Japan–Australia Migratory Bird Agreement, the China–Australia Migratory Bird Agreement or the Republic of Korea – Australia Migratory Bird Agreement. For a full list of species listed under Commonwealth legislation that have been recorded in the Mid-MurrumbidgeeRiver Wetlands see Appendix B.
2. The water-dependent ecosystem is natural or near-natural, rare or unique / The Mid-MurrumbidgeeRiver Wetlands site contains good examples of inland river and lagoon wetlands and a selection of these are nationally important through their listing in the Australian Wetlands Database (Department of the Environment, Water, Heritage and the Arts 1998). The listing describes the wetland system as being dominated by communities of river red gum forests with marginal black box woodlands. The wetland system also contains lagoons and billabongs (Department of the Environment, Water, Heritage and the Arts 1998) containing open water habitats of aquatic plants such as spike rush (Juncusspp.andEleocharisspp.), garland lily (Calostemmapurpureum) and blanket fern (Pleurosorusrutiflolius) (CSIRO 2008).
3. The water-dependent ecosystem provides vital habitat / The wetlands and floodplain contain a range of riparian and wetland vegetation communities that provide drought refuge when wetlands and floodplains in other parts of the state are dry (Department of the Environment, Water, Heritage and the Arts 1998). They also play an important role in the functioning of river ecosystems and are critical to several fish species in the Murrumbidgee (Gilligan 2005). Appendix B provides a list of conservationally significant species and communities found throughout the Mid-MurrumbidgeeRiver Wetlands.
In a survey of hydrological changes to wetlands of the Murrumbidgee River, Thornton and Briggs (1994) identified that the Murrumbidgee River floodplain between Wagga Wagga and Hay Weir contained around 45,000 ha of river red gum wetlands.
In their assessment of significant Murray Darling Basin wetlands, Kingsford et al. (1997) identified the section of the Murrumbidgee floodplain between Wagga Waggaand the Lower Murrumbidgee Floodplain as an important area for waterbirds.
Coonancoocabil Lagoon is one of the larger wetlands in the system and is known as an important site for waterbirds (Briggs et al. 1997), frogs (Wassenset al. 2004), and supports high proportions of native fish (Baumgartner andAsmus 2009).
The Murrumbidgee River between WaggaWagga and Hay has been identified as supporting an important population of Murray cod (Maccullochellapeeliipeelii) (National Murray Cod Recovery Team 2010). This assessment was based on the scale, size and integrity of the population.
4. Water-dependent ecosystems that support Commonwealth, State or Territory listed threatened species or communities / Species and communities listed as threatened under both Commonwealth and state legislation that have been recorded at the site are in Appendix B.

4.Hydrology

The natural flow regime of the Murrumbidgee River was characterised by low average flows in summer and autumn and higher average flows in winter and spring (Page et al. 2005). Flows in the Murrumbidgee River would have naturally been quite variable, with rainfall events in the upper- and mid-catchment creating flow pulses that would have travelled down the river for most of its length (Watts 2010).

A number of dams have been built on the Murrumbidgee River and its tributaries and this river regulation has considerably altered the flow regime in terms of total volume, seasonal patterns of discharge, magnitude and frequency of floods, and frequency and duration of floodplain inundation (Watts 2010).

A comparison of observed flows with modelled natural flows for the period 1970 to 1998 shows that river regulation has significantly reduced the magnitude of the smaller, relatively frequent floods (Read 2001). Flood discharges that would typically occur every two years have been reduced by between 29% and 37% from Gundagai downstream to Hay and by about 55% at Balranald. The reduction is less for floods that would typically occur every five years, ranging from a 16% reduction at Narrandera to a 36% reduction at Balranald.

Flow regulation has also affected the frequency and duration of floodplain inundation. Read (2001) demonstrated that the duration of bankfull flow has been reduced by regulation, and has resulted in an approximate halving of the duration of floodplain inundation. This conclusion is supported by Page etal. (2005) who found that between Gundagai and Balranald, regulation had, for flood return periods of 1.25 to 5years, reduced discharges by 25–40%. The Murray–Darling Basin Sustainable Yields Project by CSIRO (2008) showed that the average period between events with peak flows greater than 26,800ML/d at Narrandera had nearly doubled as a result of water resource development. In addition, they found that the maximum period between these events had more than tripled (from 2.8 to 9.7years).

Whitten and Bennett (2000) found thatthe frequency of flows in excess of 35,000ML/d at Wagga Wagga had been reduced while the number of years with no flows greater than 35,000ML/d had increased significantly(Figure3).

Figure 4 shows that the seasonal impact of water extraction on flows greater than 35,000ML/d at Wagga Wagga is most pronounced between May and August (Whitten and Bennett 2000).

The changes to the flow regime in the central section of the Murrumbidgee River have had a significant impact on the hydrology of the Mid-MurrumbidgeeRiver Wetlands.Wetlands influenced by weir pools, irrigation storage and effluent are now almost permanently inundated, and low-lying wetlands upstream of Berembed Weir are often inundated by summer irrigation releases (Frazier etal. 2003). However, the inundation frequency has halved for wetlands between Gundagai and Hay, with river connections higher than the level of irrigation flows (Frazieretal. 2003).

Figure 3Comparison of without-development and actual flows at Wagga Wagga, 1913–96: Mid-MurrumbidgeeRiver Wetlands. (Source: reproduced from Whitten andBennett 2000).

Figure 4Seasonal comparison of without-development and actual flows of >35,000ML/d at Wagga Wagga: Mid-MurrumbidgeeRiver Wetlands. (Source: reproduced from Whitten and Bennett 2000).

5.Determining the site-specific flow indicators for the Mid-MurrumbidgeeRiver Wetlands

5.1.Setting 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 Mid-MurrumbidgeeRiverWetlandswere developed (Table 2). Information underpinning site-specific ecological targets is shown in Table 2.

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 Mid-MurrumbidgeeRiverWetlands, as described below.