A long-term water assessment has never been done before. This means there is no existing process – or methodology - we can use.
We are using the best available data and sophisticated modelling techniques to determine whether long‑term water availability has changed, and if so, whether there has been a disproportionate impact on consumers or the environment. The best available data and statistical analysis will also be used to determine if waterway health has declined because there isn’t enough water at the right times to maintain a health river ecosystem.
We have led the development of the methodology and have incorporated contributions from industry specialists, a technical advisory group and stakeholders.
What the process needs to do
To be suitable across Victoria now and into the future, the method needs to:
- enable any decline in long-term water availability to be detected, including for allocation to consumptive users (such as businesses, irrigators and the tap water to your house) and the environment
- identify observed declines in waterway health that are a result of flow-related issues
- be applicable to the many, diverse water supply systems across the State
- use the best available data
- be fit-for-purpose given the time and resources available to complete the assessment.
Monitoring fish populations.
The long-term water resource assessment will estimate the long-term availability of both surface water and groundwater.
A variety of methods will be used to estimate surface water availability across Victorian catchments, including: gauged data, water balances, and modelling. The method selected for a particular catchment will depend on the availability of the underlying data.
The assessment will be transparent about what sources of water are included, while making it clear which water users and catchment activities fall within the scope of the assessment.
The assessment will also consider groundwater availability using groundwater level data from more than 2000 monitoring bores in the State Observation Bore Network.
The intention of the long-term water resource assessment is to test the assumptions upon which the water sharing arrangements are based, including if there has been a long-term reduction in resource availability. The assessment will determine if the estimates of long-term water availability used when the current water sharing arrangements were last reviewed are still valid.
Sustainable water strategies were produced for Victoria’s central, northern, western and Gippsland regions between 2006 and 2011. The strategies:
- identified threats to water availability
- reviewed water sharing arrangements
- produced actions to improve the reliability of water supplies for consumptive users and where to prioritise more water for the environment.
Sustainable water strategies used estimates of long-term surface water availability based on all the available recorded or modelled streamflows.
The latest research from the Victorian Climate Initiative [link] show that current climate conditions are different from earlier decades of the 20th century. Victoria’s climate has been warmer and drier for several decades, with many catchments recording reduced streamflows.
The period from 1975 to the present is more representative of our current climate under today’s greenhouse gas concentrations.
This period also combines a wide range of natural climate variability, including the severe Millennium Drought [link] and several relatively wet years, and reflects our best estimate of current long-term surface water availability for the aim of this assessment.
The long-term water resource assessment will identify declines in groundwater levels since caps on groundwater entitlements were introduced between 1997 and 2000. The period since 1997 reflects groundwater levels which have adjusted to changes in climate, groundwater development and resource management.
Changes in rainfall is expected to be the main cause of any decline in water availability across Victoria. Other potential causes of decline could include the construction of farm dams and large-scale changes in land use. Groundwater pumping can also lower levels in aquifers and reduce groundwater flows to creeks, rivers and wetlands.
The long-term water resource assessment will report on the relative influence these factors may have on resource availability to inform any subsequent decisions made in response to the assessment.
If the assessment finds the amount of water resources available in Victoria has decreased over the long term, it will also find out whether there has been a disproportionate decline in water available for the environment or for consumptive use.
Water resource models provide the best estimate of the volume of surface water that can be taken for consumptive use because they incorporate the rules that govern water use and model the available water at the point of diversion, the ability to store water and the timing of the demand for water.
Water resource models do not currently exist for some smaller unregulated rivers. In these systems, it is proposed to estimate the allocation of water using gauged or modelled streamflow and the rules that govern when water can be harvested or diverted.
The long-term water resource assessment will undertake detailed groundwater assessments in regions where declines in groundwater levels have been observed and where there are groundwater users and connection between the groundwater and waterways. These local assessments will provide more detailed understanding of the long-term changes in groundwater availability.
The species that live in and around our waterways rely on well-established flow patterns to successfully feed, breed and allow movement through the landscape.
Changes in streamflow patterns have knock-on effects to both the health of waterways, aquatic ecosystems and ecosystems that rely on groundwater.
Hydrological analysis will identify which rivers have an increased health threat because of changes in flow patterns, then more work will be done to look at whether the ecological health in the waterways has deteriorated.
The Index of Stream Condition (ISC) provided information on components of the flow regime that are important for waterway health. This assessment will identify rivers where there has been a change in these flow components. Where environmental flow studies have identified flow components that are important for a specific river, the assessment will also determine if there has been a change in how often these flow components are provided. The flow components that have changed most significantly will guide the assessment in each system. For example; if there has been an increase in the frequency of cease-to-flows, the assessment might look for an impact on macro-invertebrates or fish.
Long-term monitoring data sets that are available for waterway health will be examined and statistical methods used to identify whether deterioration has occurred and how likely the deterioration is related to flow. This part of the assessment will depend on the availability of ecological data collected over a long-period, and may be limited to macroinvertebrates and water quality. Information collected about waterway health over shorter intervals, such as fish surveys, will also be incorporated into the assessment.