Revision of sustainable allocation is needed
Our climate is changing, and this is impacting our water resources. Some groundwater declines that the Plan identified in the lead up to adoption have continued. Changes is rainfall amount and rainfall patterns are changing how much water is making it back into the groundwater system.
Water use is approximately 50% of allocation in the confined and unconfined aquifers. If water use increases in combination with a changing climate groundwater level declines will continue.
Declining groundwater levels means decreased recharge into the system, increased extraction or both. This is evidence that the sustainable limits cannot manage the resource for continued social, economic, cultural and environmental benefit of current and future generations.
Changes in groundwater levels over the 30 years to 2017 range from a decline of 6.62 m in some areas to a rise 0.93 m in others. The median decline is 0.72 m. The five-year trend in winter-recovered water levels (2017 to 2021) in the Coastal Flats has 82% of observation wells showing a declining trend in groundwater levels with a mean decline of 0.12 m/year. In the same area in 2021 the winter recovered water levels in 49% of 217 monitoring wells were classified as 'below average' or lower.
A small number of wells (6%) showed their lowest winter-recovered water level on record. These were generally located south of Mount Gambier near areas of intensive irrigation, forestry plantations or near drainage networks.
There are some rising trends with 6% of observation wells showing a rising trend in groundwater levels. Some of these are in the management areas of Coles and Short. Rising trends are from changes in land use as a result of a reductions to allocation applied to forestry. While there are rising trends in these areas' groundwater levels have not recovered to previous levels.
The Plan sought to manage groundwater sustainably by setting a target management level for each management area of the confined and unconfined aquifers. The target management level is based on a proportion of recharge in the management area. The target management level for each management area is defined as the level of system loss due to water extraction and recharge interception (for both licensed and unlicensed uses) that is sustainable in the management area. The Plan therefore considers the target management level as the sustainable limit per management area.
In the unconfined aquifer the method for determining the proportion of recharge varied between management areas. It was based on the level of risk to the water resources and the dependent community, industries and ecosystems.
For low and moderate risk management areas the sustainable limit was set as 90% of the mean annual vertical recharge or at 2013 allocations, whichever was higher.
For high and very high risk management areas the sustainable limit was set at 90% of the mean annual vertical recharge and where 2013 allocations exceeded sustainable limit a schedule of reductions was introduced to bring allocations back to sustainable limits.
The allocation of a proportion of recharge is an equitable and logical approach. Particularly as at the time of developing the Plan a regional groundwater model was not available to test use scenarios. But the proportion of recharge allocated for use was not a precautionary approach and lacked consistency with the objectives of the Plan.
The allocation of 90% of mean annual vertical recharge is a very high proportion. It relies on an accurate estimate of recharge and allowing no room for error. The recharge values for the Plan were determined from a variety of available sources, some dated to 1978 and 1995. Allocation of 90% of mean annual vertical recharge failed to consider that recharge was known to have declined since those values were determined. Although the Plan acknowledged climate change it made no allowances for potential changes in rainfall and consequent impacts on recharge.
In low and moderate risk management areas the Plan allowed for greater than 90% of recharge to be allocated where allocations exceeded the sustainable limit. Allowing for over-allocation in any management area is contradictory to managing risk to the resource.
The figure of 90% of recharge is an arbitrary figure that was not justified by the science underpinning the Plan. It is at the high end of allocations as a fraction of recharge. Lower allocation fractions are usual where there are high value groundwater dependent ecosystems.
Areas with shallow groundwater dependent wetlands are very vulnerable to small declines in groundwater levels. They are at particularly high risk from the approach of an allocation of a maximum of 10% of annual average vertical recharge for the environment.