What are the reductions in the Regulated Lower Namoi Water Source?
The available water determination (AWD) for Regulated Lower Namoi supplementary water access licences will likely be reduced at the commencement of the 2023/24 water year to 0.36 megalitres (ML)/share. This AWD applies to all supplementary water access licence holders in the Regulated Lower Namoi water source. The AWD defines the maximum volume that can be taken in that year prior to trades and is normally 1ML/share.
Why is a reduced supplementary water allocation required?
The reduced allocation is required to address non-compliance with the long-term average annual extraction limit (LTAAEL). The Water Sharing Plan for the Upper Namoi and Lower Namoi Regulated River Water Sources specifies how this limit is calculated, how compliance is assessed and what action is required if there is non-compliance.
Results from modelling published under the healthy floodplains program show that the Namoi regulated river has exceeded the LTAAEL and that compliance action is required. These exceedances are described in the model scenario reports published during the water sharing plan consultation and can be found on the website and through the following links:
- Floodplain Harvesting Entitlements for Namoi Regulated River System: Model Scenarios Report (PDF, 742.37 KB)
The modelling reports for the Namoi are being independently peer-reviewed. A report on this review will be published once it’s finalised.
Since publication of this report, we have continued to make improvements to the model. This has led to an updated compliance assessment, which will be published on our website as soon as possible.
Growth in farm infrastructure has resulted in growth in total diversions. The water sharing plans specify triggers for when this growth needs to be acted upon; generally, when total diversions exceed the LTAAEL by more than 3%. This trigger has been exceeded in the Namoi. The current water sharing plan provisions require that the compliance action is to reduce the maximum available water determination (AWD) made on 1 July for supplementary water access licences.
The required compliance action is to reduce total diversions to the LTAAEL. Modelling is needed to determine the required action, as the AWD impacts actual supplementary access diversions differently each year, based on flow conditions and capability to store water. In many years there is no impact at all. See “What are the likely impacts?” for further information.
Will we update our assessment after commencement of floodplain harvesting licences?
Licensing of floodplain harvesting will address growth in floodplain harvesting. We will revise the compliance assessment after the commencement of these licences and publish an updated AWD if changes to allocations are required.
What are the likely impacts?
There is no guarantee that the supplementary access AWD volume can be taken, as licence holders can only take water when a supplementary flow event is declared. This means that the maximum volume has no effect in the many years that have insufficient supplementary access opportunities for an entitlement holder to use their water and reach the limit. Supplementary events are declared when flows exceed the requirements set out in water sharing plans - typically when all other water requirements have been met. The reduced AWD does not impact on how these events are declared. Instead, it reduces the maximum potential volume that might otherwise have been taken in the water year. Trading of water can continue as usual.
The reduced AWD will not have any impact on licence holders in years when there are few supplementary flow events declared and hence little opportunity to take the maximum annual limit. The figure below uses modelled results to show this. The orange bars show years that are affected by the reduced supplementary access AWD.
We can't predict what the actual impact will be in the 2023/24 water year, as we don't know what the supplementary access opportunities will be.
How do we assess compliance with the limits?
It is important to note that legal limits and our assessments against them are based on long term capability to take water. The way that we share water and check for compliance takes into account variable climatic conditions. This means that a growth-in-use management action can occur in wet or dry years. In any one year, water use can be above or below the long-term average limits. We assess compliance with the LTAAEL by comparing a model which reflects the limit to a model which reflects current conditions.
The models allow us to test what impact changes in infrastructure, rules or behaviour have on long term water use. Where we have metered water use, this is used to compare to the model. For LTAAEL compliance, what matters is that the valley scale total water use is reasonably reflected over multiple years; we don’t expect the model to match every year and we don’t expect the model to perfectly reflect each individual farm.
Simple tests for trend in metered use are not sufficient to assess for compliance with the LTAAEL. This is because unmetered water use is not included and also because the growth-in-use assessment method needs to take into account how permitted take would vary given climate, water availability and water sharing plan rules. This variation can be complex. There is not a simple relationship between rainfall or flow and the expected permitted use in that year. For example, in a wet period there might be less metered water use as rainfall and unmetered use such as floodplain harvesting fulfil needs to a greater extent. Going into a dry period, there might be large amounts of metered water use if headwater storages have good supplies.
We have assessed growth by representing changes in the valley. The most significant change that drives the growth is increased infrastructure. For example, on farm storage capacity has grown by over 50% since 1994, which the Cap is based on.
What about compliance against the Sustainable Diversion Limit (SDL)?
We use a different method to check for compliance with the sustainable diversion limit (SDL) under the Basin Plan framework. The SDL compliance method compares modelled or estimated allowable diversions for any one year, based on its observed climatic conditions to the actual diversions in that year. A running total is kept of the differences between expected and actual take until the cumulative total exceeds a threshold.
In some cases, we do not have metered data for the actual and an estimate is used. The current SDL compliance assessment method for the Namoi does not take into account the latest estimate for floodplain harvesting. It assumes no growth in this component. If the latest information were used, this would increase the risk of SDL non-compliance in the Namoi.
There are additional reasons why SDL compliance and LTAAEL compliance may result in different outcomes. For example, SDL uses a different trigger to determine when there is non-compliance. For further information on how the limits work, view Extraction limits.