Proposed Basin Plan consultation report

The underpinning science – Hydrologic modelling

Hydrologic modelling was a fundamental part of the development of the proposed Basin Plan, particularly in determining the ESLT and SDLs. MDBA used 24 well-tested river models, developed by Basin states and MDBA, linked together and adapted for the Basin Plan’s specific needs.

126.Issue

Submissions disputed or questioned how the Baseline Diversion Limit (BDL) was determined.

RESPONSE

The BDLs for surface water were estimated using best available models developed by state agencies, MDBA and CSIRO, and best available estimates for the components of use for which no detailed model exists. MDBA has published a report that details this information for each catchment²³.

The modelled component represents a baseline scenario that generally reflects the water sharing arrangements that were in place in June 2009. These arrangements include entitlements, water allocation policies, water sharing rules, operating rules and infrastructure such as dams, locks and weirs. Any water recovered under The Living Murray initiative and Water for Rivers is included as part of the baseline scenario, but water recovered under current programs is not.

Non-modelled components of the BDL estimate include water course diversions not included in the river system models (i.e. diversions from catchments upstream of storages or inflow points to the models) and interceptions (e.g. by farm dams and plantation forestry). The non-modelled water-course diversions estimates are based on information provided by state agencies for Cap reporting (for the period 1997/98 to 2009/10), and interception estimates are based on most recent available estimates of the impact of these interception activities on runoff.

Surface water BDLs are included in schedule 3 of the proposed Basin Plan as a description, and the best estimate for each component of the BDLs is included in a note. The BDL is set by a specific description; as the estimating capacity improves the quantity of water the BDL reflects will improve in its accuracy. MDBA will continue to work with Basin states and local groups to improve the accuracy of these calculations.

127.Issue

Concern was raised about the interception estimates used in the proposed Basin Plan.

RESPONSE

Interception activities of runoff dams, floodplain harvesting, and commercial plantations have been recognised as presenting a potential risk to the achievement of environmental objectives as well as to the future integrity of water access entitlements. In recognition of such risks and assessments to date, the proposed Basin Plan includes estimates of these activities under Baseline Diversion Limits (BDLs) and SDLs. Estimates in the proposed Basin Plan of interception activity take are based on two studies published by scientific groups (SKM, CSIRO & BRS 2010²⁴ and SKM 2007²⁵). The accuracy of these estimates is limited due to the lack of available water-use data, however these estimates are the best available.

Specifying the SDL as a formula (SDL is the BDL minus any local and/or shared reduction amounts) provides scope for any improvements in estimating interception activity take or changes across various forms of take while maintaining an SDL that accurately reflects an ESLT.

128.Issue

Submissions expressed the opinion that further modelling of SDLs and the ESLT, including various scenarios above and below 2,750 GL/y, should be undertaken.

‘It is also important that urgent work is conducted on the hydrological models to remove or relax the effect of the constraints on modelled outcomes to enable a wider range of environmental water scenarios to be studied...’

Some submissions argued that modelling should be undertaken based on overcoming constraints, rather than being hampered by them; specifically that MDBA should model the outcomes for 4,000 GL/y reduction in diversions with constraints managed or removed.

Some submissions suggested that higher water recovery scenarios should be modelled in the northern basin, because the rivers in the northern basin are less regulated, and there are fewer operational constraints on environmental water delivery.

RESPONSE

MDBA undertook comprehensive modelling to inform the ESLT and SDLs in the proposed Basin Plan. This included the modelling of three Basin-wide ESLT options, representing reductions in diversions of 2,400, 2,800 and 3,200 GL/y. This modelling showed that a number of key environmental objectives were not achieved with the 2,400 GL/y scenario. The 3,200 GL/y scenario achieved marginally improved environmental outcomes compared to the 2,800 GL/y scenario that did not justify the increased social and economic impact associated with an extra 400 GL/y of water recovery. On this basis, and after some additional exploration of options in the Condamine-Balonne region, MDBA determined a proposed ESLT representing a reduction in diversions of 2,750 GL/y. See also the response to issue No. 118.

The experience gained from various model runs showed that, due to existing physical and operational constraints, the improvement in achievement of high-flow targets with increasing water recovery for environmental purposes is marginal.

Sufficient modelling has been undertaken to prepare the proposed Basin Plan in the context of the adaptive management approach. This adaptive management approach includes a review of SDLs in 2015, and ongoing reviews of the Basin Plan in accordance with the requirements of the Act. MDBA undertook extensive modelling, based on existing understanding of environmental water requirements and options to achieve those outcomes, but new information will come to light over time. MDBA has set up appropriate mechanisms to allow this information to be considered in the future. For example, section 6.06 of the proposed Basin Plan sets out a number of matters that will be considered in the 2015 review of SDLs. Assessment of those issues will involve additional modelling.

As discussed in the responses to issue No. 172 and 173, MDBA will continue to work with Basin governments on identifying key system constraints and the implications of removing these constraints

129.Issue

Submitters indicated that the most recent two years of climate data should be included in the modelling assessments for the Basin Plan.

RESPONSE

The SDLs were determined using detailed and sophisticated modelling techniques. Determining SDLs is not a simple averaging and subtraction exercise.

MDBA used the historic climate record (the 114-year period 1895 and 2009), which included a wide range of climatic conditions, to model the environmental outcomes of different SDL reduction scenarios. This variability allows the testing of the performance of each scenario in the very dry, the very wet and all the times in between. However, it is the relative differences between the various scenarios and Baseline Diversion Limit (BDL) conditions and the extent to which each scenario meets the environmental objectives that is important, not the climate baseline. If the MDBA changed the climate baseline to include 2010 and 2011 data, the frequency with which environmental targets are met between the SDL scenarios and BDL conditions would not change. The last two years have been very wet, but no more wet than the very wet periods already included in the 114-year period we have used to test the scenarios.

130.Issue

Submissions claimed MDBA had used an inconsistent approach with regard to constraints in its modelling.

RESPONSE

In the modelling for the proposed Basin Plan, environmental flow releases were limited by the existing operational constraints in various river valley models, except in the case of the River Murray system, for which the flow constraint at the Barmah Choke was relaxed to 40,000 ML/d for meeting environmental water requirements of downstream sites. This is considered feasible and would be needed to achieve some of the environmental outcomes for the downstream sites. The nature of the constraints at the Barmah Choke is well understood, and can be readily overcome, and the environmental outcomes that can be achieved by doing so are considerable. Consequently the modelling was undertaken with the expectation that the constraints would be overcome in the future.

MDBA is aware of some concerns on the relaxation of the channel capacity at Barmah Choke for the purposes of modelling the ESLT. The following points provide further explanation as to the choice of this modelling assumption:

• 
  In the upper Murray, flows are constrained at two points. Firstly, regulated flows are typically constrained to 25,000ML/d at Doctors Point to minimise flooding of private land. Then during the summer regulated period downstream of Yarrawonga, regulated flows are constrained at the Barmah Choke to minimise unseasonal flooding of the forest and for efficient delivery of consumptive water. Under current conditions, flows through the Barmah Choke are modelled as a maximum flow constraint downstream of Yarrawonga of 10,600 ML/d during summer and 22,000 ML/d during spring when flooding of Barmah forest may be desirable.
  
• 
  For the purposes of modelling the Basin Plan scenario, the Doctors Point constraint is maintained at 25,000ML/d and the constraint of 10,600 ML/d downstream of Yarrawonga during summer is also maintained. However, the Barmah Choke constraint was relaxed to 40,000 ML/d during winter/spring, allowing for some contribution from tributaries like the Ovens to increase downstream flows above 25,000 ML/d. Environmental flows target the winter/spring period, so unseasonal flooding of the forest is not an issue. 
  
• 
  However, MDBA is undertaking more detailed work on this, to look at the potential third-party impacts in line with concerns raised in the submissions.
  

The focus of MDBA modelling for the proposed Basin Plan was to assess environmental outcomes possible due to change in flow regime as a consequence of different levels of reduction in consumptive use.