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Sustainable diversion limits

The Basin Plan sets sustainable diversion limits or SDLs, which is how much water can be used in the Murray–Darling Basin. This limit then ensures there is enough water remaining in the environment to keep the river system and groundwater resources in a healthy state. Enforcement of sustainable diversion limits commences in July 2019, through accredited water resource plans (WRPs) prepared by Basin states.

SDLs are established for a set area for both surface water and groundwater. There are 29 surface water SDL resource units and 81 groundwater SDL resource units across the Basin.

Surface water SDLs

Surface water SDLs are specified as a long-term annual average water use at a water management unit (SDL resource unit) level and at a Basin-wide level.

The first step in establishing the SDLs was a process to determine how much water was currently being used by industries and communities annually. It was estimated, on average, that 13,623 gigalitres per year of surface water was being taken from the system for consumptive use. This is now known as the baseline diversion limit (BDL).

Baseline diversion limit: An estimate of how much water (on average) was used in the Murray–Darling Basin prior to the development of the Basin Plan.

Water recovery

It has been determined that if 2750 gigalitres per year of the 13,623 gigalitres per year of surface water can be recovered and retained in the system, this will be enough to improve the health of the river system. This is known as the water recovery target, and is also a long-term average.

The recovery volume is based on scientific method and judgement. It was determined based on an analysis of historical data, environmental science, and social and economic analysis, as well as the modelling of different future scenarios.

The Basin Plan’s water recovery efforts build on previous water recovery that has already occurred, through state and Australian Government based initiatives such as The Living Murray program.

Groundwater SDLS

An assessment of how much groundwater was being used, was also a factor in developing the Basin Plan. It was determined that the baseline diversion limit for groundwater was 2,385 gigalitres per year.

The Basin-wide SDL value for groundwater was then increased by 949 gigalitres per year to 3,334 gigalitres per year. This means more groundwater could be used each water year in some parts of the Basin. In two groundwater SDL units, a recovery amount was specified. In making this decision the MDBA considered various environmental objectives, the ecosystems reliant on groundwater, the relationship between surface water and groundwater, the ability of aquifers to continue to be productively used and water quality and salinity.

A long-term average

Surface water sustainable diversion limits are an average, representing the average limits on water diversions over the long-term. Water use will vary according to water availability in that year, in line with state water resource plans. This ensures sufficient flexibility to adapt with Australia's highly variable climatic conditions.

For example, under the Basin Plan, the Basin-wide long-term average limit on surface water use is 10,873 gigalitres (billion litres) per year. In very dry (low inflow) conditions, or times when water storage levels are low, much less water will be available for use than the average. In contrast, when water is more abundant water use may exceed the average.

Sustainable diversion limits: how much water can be used in the Murray–Darling Basin. This limit then ensures there is enough water remaining in the environment to keep the river system healthy.

Water recovery – a long-term average

In recovering water for the Basin Plan, the Australian Government has acquired a variety of classes of water entitlements from across the Basin. Each Basin state has its own entitlement system which means there are over 150 different classes of water entitlements in the Murray–Darling Basin. These entitlements range from having a high degree of reliability, to general security, to various supplementary classes that only receive a water allocation in more limited circumstances. Water has been collected across many different types of entitlements, meaning the Australian Government needs a way of knowing it has the right amount of water recovered for the Basin Plan.

To accurately measure how much water has been recovered, each sustainable diversion limit resource unit entitlement class is given a long term diversion limit equivalent factor to translate through to expected use so they can be counted on equal terms.

Long-term diversion limit equivalent factors are determined by Basin state governments, based on an assessment of how much water in a SDL resource unit is likely to be used. Basin state governments consider historical data, trends and forecasts to anticipate behaviour in each SDL resource unit and then decide how much of the water allocated to each entitlement class is likely to be used once a water resource plan is accredited. Once this factor is applied, it will be clear if more water needs to be recovered or if over recovery has occurred.

Long-term diversion limit equivalent factor: a type of conversion that allows you to compare different types of water entitlements across the Basin. Each class of entitlement has its own factor.

Limits can change

Any change in the baseline diversion limit will result in a change to surface water sustainable diversion limits. The baseline diversion limit is likely to change as each water resource plan is accredited, as new information becomes available on previous water use patterns and usage.

Due to this new information, along with the Sustainable Diversion Limit Adjustment Mechanism, it is expected surface water sustainable diversion limits will continue to change over the coming years.

As water resource plans are developed, new information about water use and management will become available. It is expected through this process, that the estimate of the baseline diversion limit will change and this will then change the sustainable diversion limit.