Groundwater Mapping and Transition Zones, Namoi Catchment

Client: Namoi Catchment Management Authority
Year: 2011-2012
Project Reference: 2011004
WRL Technical Report: Namoi Groundwater Mapping and Transition Zones (2012/01)

Groundwater in the Namoi catchment supports an irrigation industry worth in excess of $380 million per annum, and is the water supply for many towns and intensive industries such as feedlots.

The draft Namoi Catchment Action Plan (CAP) was developed by the Namoi CMA to give strategic direction to natural resource management in the region. The targets in the CAP were developed based on critical thresholds identified via a resilience assessment of the catchment. These included the critical threshold that “alluvial aquifers are not drawn down below historical maximum drawdown levels” (Critical Threshold 5) and the action of identifying areas of disconnected and semi-connected aquifers (or transition zones). Catchment Target Water 2 of the CAP is “by 2020 there is an improvement in the ability of groundwater systems to support groundwater dependent ecosystems and designated beneficial uses” (Namoi CMA, 2011). To achieve this target, the Namoi CMA developed a range of required actions. WRL’s Projects Team was awarded the contract to map the historical maximum drawdowns and areas of disconnected and semi-connected aquifers throughout the catchment.

WRL assessed groundwater level trends using hydrographs from representative monitoring bores. This historical analysis identified hotspots of drawdown and produced the first maps of maximum drawdown prior to 2011 across the catchment. These were compared with recent drawdown in 2011 to determine if the historical levels had been exceeded.

Mapping aquifer areas that were either connected to, disconnected from or semi-connected (in transition) to modern recharge required a number of steps. Firstly, connectivity between groundwater was assessed using a clustering analysis based on groundwater hydrograph shapes. Secondly, the connection of each cluster to recharge was analysed using past assessments and streamflow correlation with groundwater levels in each pipe. Finally, matrices of drawdown, long-term decline and streamflow correlation of pipes in each cluster were used to categorise the recharge connection status of each cluster as connected, transition or disconnected.

Much of the catchment would appear to be in transition or disconnected due to the large stresses and long-term decline occurring. This body of work will help the Namoi CMA further efforts in relation to groundwater as outlined in the Namoi CAP, thus ensuring groundwater systems are able to support groundwater dependent ecosystems and designated beneficial uses for future generations.